ListView的深入探讨
ListView的深入探讨
Glory_Bossy 发表于1年前
ListView的深入探讨
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摘要: 从源码层面对ListView进行深入探讨,对其工作原理进行进一步的剖析

ListView的工作原理

一. 前沿分析 1.1 ListView直接继承自AsbListView,而AbsListView有2个子显示类,一个是ListView,另一个是GridView,因此两者在工作原理和实现上都有诸多共同点的。然后AbsListView又继承自AdapterView,AdapterView又继承自ViewGroup 1.2 ListView只承担交互和提示而已,至于数据来源于哪里,在ListView里面是不关心的。因此,最基础ListView的工作模式只要有一个ListView空间和一个数据源。 1.3 若要让ListView终结于数据打交道,那样ListView就会变得臃肿,于是便有了Adapter这样的机制出现。正如Adapter名字“适配器”一样,它在ListView和数据源之间起一个桥梁作用。

RecycleBin机制

那么在开始分析ListView的源码之前,还有一个东西是我们提前需要了解的,就是RecycleBin机制,这个机制也是ListView能够实现成百上千条数据都不会OOM最重要的一个原因。其实RecycleBin的代码并不多,只有300行左右,它是写在AbsListView中的一个内部类,所以所有继承自AbsListView的子类,也就是ListView和GridView,都可以使用这个机制。主要源码如下所示:

/** 
 * The RecycleBin facilitates reuse of views across layouts. The RecycleBin 
 * has two levels of storage: ActiveViews and ScrapViews. ActiveViews are 
 * those views which were onscreen at the start of a layout. By 
 * construction, they are displaying current information. At the end of 
 * layout, all views in ActiveViews are demoted to ScrapViews. ScrapViews 
 * are old views that could potentially be used by the adapter to avoid 
 * allocating views unnecessarily. 
 *  
 * @see android.widget.AbsListView#setRecyclerListener(android.widget.AbsListView.RecyclerListener) 
 * @see android.widget.AbsListView.RecyclerListener 
 */  
class RecycleBin {  
    private RecyclerListener mRecyclerListener;  
  
    /** 
     * The position of the first view stored in mActiveViews. 
     */  
    private int mFirstActivePosition;  
  
    /** 
     * Views that were on screen at the start of layout. This array is 
     * populated at the start of layout, and at the end of layout all view 
     * in mActiveViews are moved to mScrapViews. Views in mActiveViews 
     * represent a contiguous range of Views, with position of the first 
     * view store in mFirstActivePosition. 
     */  
    private View[] mActiveViews = new View[0];  
  
    /** 
     * Unsorted views that can be used by the adapter as a convert view. 
     */  
    private ArrayList<View>[] mScrapViews;  
  
    private int mViewTypeCount;  
  
    private ArrayList<View> mCurrentScrap;  
  
    /** 
     * Fill ActiveViews with all of the children of the AbsListView. 
     *  
     * @param childCount 
     *            The minimum number of views mActiveViews should hold 
     * @param firstActivePosition 
     *            The position of the first view that will be stored in 
     *            mActiveViews 
     */  
    void fillActiveViews(int childCount, int firstActivePosition) {  
        if (mActiveViews.length < childCount) {  
            mActiveViews = new View[childCount];  
        }  
        mFirstActivePosition = firstActivePosition;  
        final View[] activeViews = mActiveViews;  
        for (int i = 0; i < childCount; i++) {  
            View child = getChildAt(i);  
            AbsListView.LayoutParams lp = (AbsListView.LayoutParams) child.getLayoutParams();  
            // Don't put header or footer views into the scrap heap  
            if (lp != null && lp.viewType != ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {  
                // Note: We do place AdapterView.ITEM_VIEW_TYPE_IGNORE in  
                // active views.  
                // However, we will NOT place them into scrap views.  
                activeViews[i] = child;  
            }  
        }  
    }  
  
    /** 
     * Get the view corresponding to the specified position. The view will 
     * be removed from mActiveViews if it is found. 
     *  
     * @param position 
     *            The position to look up in mActiveViews 
     * @return The view if it is found, null otherwise 
     */  
    View getActiveView(int position) {  
        int index = position - mFirstActivePosition;  
        final View[] activeViews = mActiveViews;  
        if (index >= 0 && index < activeViews.length) {  
            final View match = activeViews[index];  
            activeViews[index] = null;  
            return match;  
        }  
        return null;  
    }  
  
    /** 
     * Put a view into the ScapViews list. These views are unordered. 
     *  
     * @param scrap 
     *            The view to add 
     */  
    void addScrapView(View scrap) {  
        AbsListView.LayoutParams lp = (AbsListView.LayoutParams) scrap.getLayoutParams();  
        if (lp == null) {  
            return;  
        }  
        // Don't put header or footer views or views that should be ignored  
        // into the scrap heap  
        int viewType = lp.viewType;  
        if (!shouldRecycleViewType(viewType)) {  
            if (viewType != ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {  
                removeDetachedView(scrap, false);  
            }  
            return;  
        }  
        if (mViewTypeCount == 1) {  
            dispatchFinishTemporaryDetach(scrap);  
            mCurrentScrap.add(scrap);  
        } else {  
            dispatchFinishTemporaryDetach(scrap);  
            mScrapViews[viewType].add(scrap);  
        }  
  
        if (mRecyclerListener != null) {  
            mRecyclerListener.onMovedToScrapHeap(scrap);  
        }  
    }  
  
    /** 
     * @return A view from the ScrapViews collection. These are unordered. 
     */  
    View getScrapView(int position) {  
        ArrayList<View> scrapViews;  
        if (mViewTypeCount == 1) {  
            scrapViews = mCurrentScrap;  
            int size = scrapViews.size();  
            if (size > 0) {  
                return scrapViews.remove(size - 1);  
            } else {  
                return null;  
            }  
        } else {  
            int whichScrap = mAdapter.getItemViewType(position);  
            if (whichScrap >= 0 && whichScrap < mScrapViews.length) {  
                scrapViews = mScrapViews[whichScrap];  
                int size = scrapViews.size();  
                if (size > 0) {  
                    return scrapViews.remove(size - 1);  
                }  
            }  
        }  
        return null;  
    }  
  
    public void setViewTypeCount(int viewTypeCount) {  
        if (viewTypeCount < 1) {  
            throw new IllegalArgumentException("Can't have a viewTypeCount < 1");  
        }  
        // noinspection unchecked  
        ArrayList<View>[] scrapViews = new ArrayList[viewTypeCount];  
        for (int i = 0; i < viewTypeCount; i++) {  
            scrapViews[i] = new ArrayList<View>();  
        }  
        mViewTypeCount = viewTypeCount;  
        mCurrentScrap = scrapViews[0];  
        mScrapViews = scrapViews;  
    }  
  
}  

这里面只是主要的几个方法,并不很全。我们先对着几个方法进行简单介绍: fillActiveViews() 这个方法接收两个参数,第一个参数表示要存储的view的数量,第二个参数表示ListView中第一个可见元素的position值。RecycleBin当中使用mActiveViews这个数组来存储View,调用这个方法后就会根据传入的参数来将ListView中的指定元素存储到mActiveViews数组当中。

getActiveView() 这个方法和fillActiveViews()是对应的,用于从mActiveViews数组当中获取数据。该方法接收一个position参数,表示元素在ListView当中的位置,方法内部会自动将position值转换成mActiveViews数组对应的下标值。需要注意的是,mActiveViews当中所存储的View,一旦被获取了之后就会从mActiveViews当中移除,下次获取同样位置的View将会返回null,也就是说mActiveViews不能被重复利用。

addScrapView() 用于将一个废弃的View进行缓存,该方法接收一个View参数,当有某个View确定要废弃掉的时候(比如滚动出了屏幕),就应该调用这个方法来对View进行缓存,RecycleBin当中使用mScrapViews和mCurrentScrap这两个List来存储废弃View。

getScrapView 用于从废弃缓存中取出一个View,这些废弃缓存中的View是没有顺序可言的,因此getScrapView()方法中的算法也非常简单,就是直接从mCurrentScrap当中获取尾部的一个scrap view进行返回。

setViewTypeCount() 我们都知道Adapter当中可以重写一个getViewTypeCount()来表示ListView中有几种类型的数据项,而setViewTypeCount()方法的作用就是为每种类型的数据项都单独启用一个RecycleBin缓存机制。实际上,getViewTypeCount()方法通常情况下使用的并不是很多,所以我们只要知道RecycleBin当中有这样一个功能就行了。

二. 下面就可以分析ListView的原理了。

1. 第一次Layout

不管怎么说,ListView即使再特殊最终还是继承自View的,因此它的执行流程还将会按照View的规则来执行,对于这方面不太熟悉的朋友可以参考csdn上面的郭霖大神写的Android视图绘制流程完全解析,带你一步步深入了解View(二)http://blog.csdn.net/guolin_blog/article/details/16330267 。 View的执行流程无非就分为三步,onMeasure()用于测量View的大小,onLayout()用于确定View的布局,onDraw()用于将View绘制到界面上。而在ListView当中,onMeasure()并没有什么特殊的地方,因为它终归是一个View,占用的空间最多并且通常也就是整个屏幕。onDraw()在ListView当中也没有什么意义,因为ListView本身并不负责绘制,而是由ListView当中的子元素来进行绘制的。那么ListView大部分的神奇功能其实都是在onLayout()方法中进行的了,因此我们本篇文章也是主要分析的这个方法里的内容。

如果你到ListView源码中去找一找,你会发现ListView中是没有onLayout()这个方法的,这是因为这个方法是在ListView的父类AbsListView中实现的,代码如下所示:

/** 
 */ Subclasses should NOT override this method but {@link #layoutChildren()} 
 *instead. 
 */  
@Override  
protected void onLayout(boolean changed, int l, int t, int r, int b) {  
    super.onLayout(changed, l, t, r, b);  
    mInLayout = true;  
    if (changed) {  
        int childCount = getChildCount();  
        for (int i = 0; i < childCount; i++) {  
            getChildAt(i).forceLayout();  
        }  
        mRecycler.markChildrenDirty();  
    }  
    layoutChildren();  
    mInLayout = false;  
}  

可以看到,onLayout()方法中并没有做什么复杂的逻辑操作,主要就是一个判断,如果ListView的大小或者位置发生了变化,那么changed变量就会变成true,此时会要求所有的子布局都强制进行重绘。除此之外倒没有什么难理解的地方了,不过我们注意到,在第16行调用了layoutChildren()这个方法,从方法名上我们就可以猜出这个方法是用来进行子元素布局的,不过进入到这个方法当中你会发现这是个空方法,没有一行代码。这当然是可以理解的了,因为子元素的布局应该是由具体的实现类来负责完成的,而不是由父类完成。所以这里我们要进入ListView里面去看layoutChildren()方法,代码如下所示:

@Override  
protected void layoutChildren() {  
    final boolean blockLayoutRequests = mBlockLayoutRequests;  
    if (!blockLayoutRequests) {  
        mBlockLayoutRequests = true;  
    } else {  
        return;  
    }  
    try {  
        super.layoutChildren();  
        invalidate();  
        if (mAdapter == null) {  
            resetList();  
            invokeOnItemScrollListener();  
            return;  
        }  
        int childrenTop = mListPadding.top;  
        int childrenBottom = getBottom() - getTop() - mListPadding.bottom;  
        int childCount = getChildCount();  
        int index = 0;  
        int delta = 0;  
        View sel;  
        View oldSel = null;  
        View oldFirst = null;  
        View newSel = null;  
        View focusLayoutRestoreView = null;  
        // Remember stuff we will need down below  
        switch (mLayoutMode) {  
        case LAYOUT_SET_SELECTION:  
            index = mNextSelectedPosition - mFirstPosition;  
            if (index >= 0 && index < childCount) {  
                newSel = getChildAt(index);  
            }  
            break;  
        case LAYOUT_FORCE_TOP:  
        case LAYOUT_FORCE_BOTTOM:  
        case LAYOUT_SPECIFIC:  
        case LAYOUT_SYNC:  
            break;  
        case LAYOUT_MOVE_SELECTION:  
        default:  
            // Remember the previously selected view  
            index = mSelectedPosition - mFirstPosition;  
            if (index >= 0 && index < childCount) {  
                oldSel = getChildAt(index);  
            }  
            // Remember the previous first child  
            oldFirst = getChildAt(0);  
            if (mNextSelectedPosition >= 0) {  
                delta = mNextSelectedPosition - mSelectedPosition;  
            }  
            // Caution: newSel might be null  
            newSel = getChildAt(index + delta);  
        }  
        boolean dataChanged = mDataChanged;  
        if (dataChanged) {  
            handleDataChanged();  
        }  
        // Handle the empty set by removing all views that are visible  
        // and calling it a day  
        if (mItemCount == 0) {  
            resetList();  
            invokeOnItemScrollListener();  
            return;  
        } else if (mItemCount != mAdapter.getCount()) {  
            throw new IllegalStateException("The content of the adapter has changed but "  
                    + "ListView did not receive a notification. Make sure the content of "  
                    + "your adapter is not modified from a background thread, but only "  
                    + "from the UI thread. [in ListView(" + getId() + ", " + getClass()   
                    + ") with Adapter(" + mAdapter.getClass() + ")]");  
        }  
        setSelectedPositionInt(mNextSelectedPosition);  
        // Pull all children into the RecycleBin.  
        // These views will be reused if possible  
        final int firstPosition = mFirstPosition;  
        final RecycleBin recycleBin = mRecycler;  
        // reset the focus restoration  
        View focusLayoutRestoreDirectChild = null;  
        // Don't put header or footer views into the Recycler. Those are  
        // already cached in mHeaderViews;  
        if (dataChanged) {  
            for (int i = 0; i < childCount; i++) {  
                recycleBin.addScrapView(getChildAt(i));  
                if (ViewDebug.TRACE_RECYCLER) {  
                    ViewDebug.trace(getChildAt(i),  
                            ViewDebug.RecyclerTraceType.MOVE_TO_SCRAP_HEAP, index, i);  
                }  
            }  
        } else {  
            recycleBin.fillActiveViews(childCount, firstPosition);  
        }  
        // take focus back to us temporarily to avoid the eventual  
        // call to clear focus when removing the focused child below  
        // from messing things up when ViewRoot assigns focus back  
        // to someone else  
        final View focusedChild = getFocusedChild();  
        if (focusedChild != null) {  
            // TODO: in some cases focusedChild.getParent() == null  
            // we can remember the focused view to restore after relayout if the  
            // data hasn't changed, or if the focused position is a header or footer  
            if (!dataChanged || isDirectChildHeaderOrFooter(focusedChild)) {  
                focusLayoutRestoreDirectChild = focusedChild;  
                // remember the specific view that had focus  
                focusLayoutRestoreView = findFocus();  
                if (focusLayoutRestoreView != null) {  
                    // tell it we are going to mess with it  
                    focusLayoutRestoreView.onStartTemporaryDetach();  
                }  
            }  
            requestFocus();  
        }  
        // Clear out old views  
        detachAllViewsFromParent();  
        switch (mLayoutMode) {  
        case LAYOUT_SET_SELECTION:  
            if (newSel != null) {  
                sel = fillFromSelection(newSel.getTop(), childrenTop, childrenBottom);  
            } else {  
                sel = fillFromMiddle(childrenTop, childrenBottom);  
            }  
            break;  
        case LAYOUT_SYNC:  
            sel = fillSpecific(mSyncPosition, mSpecificTop);  
            break;  
        case LAYOUT_FORCE_BOTTOM:  
            sel = fillUp(mItemCount - 1, childrenBottom);  
            adjustViewsUpOrDown();  
            break;  
        case LAYOUT_FORCE_TOP:  
            mFirstPosition = 0;  
            sel = fillFromTop(childrenTop);  
            adjustViewsUpOrDown();  
            break;  
        case LAYOUT_SPECIFIC:  
            sel = fillSpecific(reconcileSelectedPosition(), mSpecificTop);  
            break;  
        case LAYOUT_MOVE_SELECTION:  
            sel = moveSelection(oldSel, newSel, delta, childrenTop, childrenBottom);  
            break;  
        default:  
            if (childCount == 0) {  
                if (!mStackFromBottom) {  
                    final int position = lookForSelectablePosition(0, true);  
                    setSelectedPositionInt(position);  
                    sel = fillFromTop(childrenTop);  
                } else {  
                    final int position = lookForSelectablePosition(mItemCount - 1, false);  
                    setSelectedPositionInt(position);  
                    sel = fillUp(mItemCount - 1, childrenBottom);  
                }  
            } else {  
                if (mSelectedPosition >= 0 && mSelectedPosition < mItemCount) {  
                    sel = fillSpecific(mSelectedPosition,  
                            oldSel == null ? childrenTop : oldSel.getTop());  
                } else if (mFirstPosition < mItemCount) {  
                    sel = fillSpecific(mFirstPosition,  
                            oldFirst == null ? childrenTop : oldFirst.getTop());  
                } else {  
                    sel = fillSpecific(0, childrenTop);  
                }  
            }  
            break;  
        }  
        // Flush any cached views that did not get reused above  
        recycleBin.scrapActiveViews();  
        if (sel != null) {  
            // the current selected item should get focus if items  
            // are focusable  
            if (mItemsCanFocus && hasFocus() && !sel.hasFocus()) {  
                final boolean focusWasTaken = (sel == focusLayoutRestoreDirectChild &&  
                        focusLayoutRestoreView.requestFocus()) || sel.requestFocus();  
                if (!focusWasTaken) {  
                    // selected item didn't take focus, fine, but still want  
                    // to make sure something else outside of the selected view  
                    // has focus  
                    final View focused = getFocusedChild();  
                    if (focused != null) {  
                        focused.clearFocus();  
                    }  
                    positionSelector(sel);  
                } else {  
                    sel.setSelected(false);  
                    mSelectorRect.setEmpty();  
                }  
            } else {  
                positionSelector(sel);  
            }  
            mSelectedTop = sel.getTop();  
        } else {  
            if (mTouchMode > TOUCH_MODE_DOWN && mTouchMode < TOUCH_MODE_SCROLL) {  
                View child = getChildAt(mMotionPosition - mFirstPosition);  
                if (child != null) positionSelector(child);  
            } else {  
                mSelectedTop = 0;  
                mSelectorRect.setEmpty();  
            }  
            // even if there is not selected position, we may need to restore  
            // focus (i.e. something focusable in touch mode)  
            if (hasFocus() && focusLayoutRestoreView != null) {  
                focusLayoutRestoreView.requestFocus();  
            }  
        }  
        // tell focus view we are done mucking with it, if it is still in  
        // our view hierarchy.  
        if (focusLayoutRestoreView != null  
                && focusLayoutRestoreView.getWindowToken() != null) {  
            focusLayoutRestoreView.onFinishTemporaryDetach();  
        }  
        mLayoutMode = LAYOUT_NORMAL;  
        mDataChanged = false;  
        mNeedSync = false;  
        setNextSelectedPositionInt(mSelectedPosition);  
        updateScrollIndicators();  
        if (mItemCount > 0) {  
            checkSelectionChanged();  
        }  
        invokeOnItemScrollListener();  
    } finally {  
        if (!blockLayoutRequests) {  
            mBlockLayoutRequests = false;  
        }  
    }  
}  

这段代码比较长,我们挑重点的看。首先可以确定的是,ListView当中目前还没有任何子View,数据都还是由Adapter管理的,并没有展示到界面上,因此getChildCount()方法得到的值肯定是0。接着会根据dataChanged这个布尔型的值来判断执行逻辑,dataChanged只有在数据源发生改变的情况下才会变成true,其它情况都是false,因此这里会进入执行逻辑,调用RecycleBin的fillActiveViews()方法。按理来说,调用fillActiveViews()方法是为了将ListView的子View进行缓存的,可是目前ListView中还没有任何的子View,因此这一行暂时还起不了任何作用。 接下来在switch会根据mLayoutMode的值来决定布局模式,默认情况下都是普通模式LAYOUT_NORMAL,因此会进入到后面的default语句当中。而下面又会紧接着进行两次if判断,childCount目前是等于0的,并且默认的布局顺序是从上往下,因此会进入到fillFromTop()方法,我们跟进去瞧一瞧:

/** 
 * Fills the list from top to bottom, starting with mFirstPosition 
 * 
 * @param nextTop The location where the top of the first item should be 
 *        drawn 
 * 
 * @return The view that is currently selected 
 */  
private View fillFromTop(int nextTop) {  
    mFirstPosition = Math.min(mFirstPosition, mSelectedPosition);  
    mFirstPosition = Math.min(mFirstPosition, mItemCount - 1);  
    if (mFirstPosition < 0) {  
        mFirstPosition = 0;  
    }  
    return fillDown(mFirstPosition, nextTop);  
} 

从这个方法的注释中可以看出,它所负责的主要任务就是从mFirstPosition开始,自顶至底去填充ListView。而这个方法本身并没有什么逻辑,就是判断了一下mFirstPosition值的合法性,然后调用fillDown()方法,那么我们就有理由可以猜测,填充ListView的操作是在fillDown()方法中完成的。进入fillDown()方法,代码如下所示:

/** 
 * Fills the list from pos down to the end of the list view. 
 * 
 * @param pos The first position to put in the list 
 * 
 * @param nextTop The location where the top of the item associated with pos 
 *        should be drawn 
 * 
 * @return The view that is currently selected, if it happens to be in the 
 *         range that we draw. 
 */  
private View fillDown(int pos, int nextTop) {  
    View selectedView = null;  
    int end = (getBottom() - getTop()) - mListPadding.bottom;  
    while (nextTop < end && pos < mItemCount) {  
        // is this the selected item?  
        boolean selected = pos == mSelectedPosition;  
        View child = makeAndAddView(pos, nextTop, true, mListPadding.left, selected);  
        nextTop = child.getBottom() + mDividerHeight;  
        if (selected) {  
            selectedView = child;  
        }  
        pos++;  
    }  
    return selectedView;  
} 

可以看到,这里使用了一个while循环来执行重复逻辑,一开始nextTop的值是第一个子元素顶部距离整个ListView顶部的像素值,pos则是刚刚传入的mFirstPosition的值,而end是ListView底部减去顶部所得的像素值,mItemCount则是Adapter中的元素数量。因此一开始的情况下nextTop必定是小于end值的,并且pos也是小于mItemCount值的。那么每执行一次while循环,pos的值都会加1,并且nextTop也会增加,当nextTop大于等于end时,也就是子元素已经超出当前屏幕了,或者pos大于等于mItemCount时,也就是所有Adapter中的元素都被遍历结束****了,就会跳出while循环。 注意这里面有个makeAndAddView()方法,进入这个方法,代码如下所示:

/** 
 * Obtain the view and add it to our list of children. The view can be made 
 * fresh, converted from an unused view, or used as is if it was in the 
 * recycle bin. 
 * 
 * @param position Logical position in the list 
 * @param y Top or bottom edge of the view to add 
 * @param flow If flow is true, align top edge to y. If false, align bottom 
 *        edge to y. 
 * @param childrenLeft Left edge where children should be positioned 
 * @param selected Is this position selected? 
 * @return View that was added 
 */  
private View makeAndAddView(int position, int y, boolean flow, int childrenLeft,  
        boolean selected) {  
    View child;  
    if (!mDataChanged) {  
        // Try to use an exsiting view for this position  
        child = mRecycler.getActiveView(position);  
        if (child != null) {  
            // Found it -- we're using an existing child  
            // This just needs to be positioned  
            setupChild(child, position, y, flow, childrenLeft, selected, true);  
            return child;  
        }  
    }  
    // Make a new view for this position, or convert an unused view if possible  
    child = obtainView(position, mIsScrap);  
    // This needs to be positioned and measured  
    setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]);  
    return child;  
}  

这里在第19行尝试从RecycleBin当中快速获取一个active view,不过很遗憾的是目前RecycleBin当中还没有缓存任何的View,所以这里得到的值肯定是null。那么取得了null之后就会继续向下运行,到第28行会调用obtainView()方法来再次尝试获取一个View,这次的obtainView()方法是可以保证一定返回一个View的,于是下面立刻将获取到的View传入到了setupChild()方法当中。那么obtainView()内部到底是怎么工作的呢?我们先进入到这个方法里面看一下(注意,这里是个重点!):

/** 
 * Get a view and have it show the data associated with the specified 
 * position. This is called when we have already discovered that the view is 
 * not available for reuse in the recycle bin. The only choices left are 
 * converting an old view or making a new one. 
 *  
 * @param position 
 *            The position to display 
 * @param isScrap 
 *            Array of at least 1 boolean, the first entry will become true 
 *            if the returned view was taken from the scrap heap, false if 
 *            otherwise. 
 *  
 * @return A view displaying the data associated with the specified position 
 */  
View obtainView(int position, boolean[] isScrap) {  
    isScrap[0] = false;  
    View scrapView;  
    scrapView = mRecycler.getScrapView(position);  
    View child;  
    if (scrapView != null) {  
        child = mAdapter.getView(position, scrapView, this);  
        if (child != scrapView) {  
            mRecycler.addScrapView(scrapView);  
            if (mCacheColorHint != 0) {  
                child.setDrawingCacheBackgroundColor(mCacheColorHint);  
            }  
        } else {  
            isScrap[0] = true;  
            dispatchFinishTemporaryDetach(child);  
        }  
    } else {  
        child = mAdapter.getView(position, null, this);  
        if (mCacheColorHint != 0) {  
            child.setDrawingCacheBackgroundColor(mCacheColorHint);  
        }  
    }  
    return child;  
}  

obtainView()方法中的代码并不多,但却包含了非常非常重要的逻辑,不夸张的说,整个ListView中最重要的内容可能就在这个方法里了。那么我们还是按照执行流程来看,在第19行代码中调用了RecycleBin的getScrapView()方法来尝试获取一个废弃缓存中的View,同样的道理,这里肯定是获取不到的,getScrapView()方法会返回一个null。这时该怎么办呢?没有关系,代码会执行到第33行,调用mAdapter的getView()方法来去获取一个View。那么mAdapter是什么呢?当然就是当前ListView关联的适配器了。而getView()方法又是什么呢?还用说吗,这个就是我们平时使用ListView时最最经常重写的一个方法了,这里getView()方法中传入了三个参数,分别是position,null和this。 那么我们平时写ListView的Adapter时,getView()方法通常会怎么写呢?这里我举个简单的例子:

@Override  
public View getView(int position, View convertView, ViewGroup parent) {  
    Fruit fruit = getItem(position);  
    View view;  
    if (convertView == null) {  
        view = LayoutInflater.from(getContext()).inflate(resourceId, null);  
    } else {  
        view = convertView;  
    }  
    ImageView fruitImage = (ImageView) view.findViewById(R.id.fruit_image);  
    TextView fruitName = (TextView) view.findViewById(R.id.fruit_name);  
    fruitImage.setImageResource(fruit.getImageId());  
    fruitName.setText(fruit.getName());  
    return view;  
}  

getView()方法接受的三个参数,第一个参数position代表当前子元素的的位置,我们可以通过具体的位置来获取与其相关的数据。第二个参数convertView,刚才传入的是null,说明没有convertView可以利用,因此我们会调用LayoutInflater的inflate()方法来去加载一个布局。接下来会对这个view进行一些属性和值的设定,最后将view返回。 那么这个View也会作为obtainView()的结果进行返回,并最终传入到setupChild()方法当中。其实也就是说**,第一次layout过程当中,所有的子View都是调用LayoutInflater的inflate()方法加载出来的**,这样就会相对比较耗时,但是不用担心,后面就不会再有这种情况了,那么我们继续往下看:

/** 
 * Add a view as a child and make sure it is measured (if necessary) and 
 * positioned properly. 
 * 
 * @param child The view to add 
 * @param position The position of this child 
 * @param y The y position relative to which this view will be positioned 
 * @param flowDown If true, align top edge to y. If false, align bottom 
 *        edge to y. 
 * @param childrenLeft Left edge where children should be positioned 
 * @param selected Is this position selected? 
 * @param recycled Has this view been pulled from the recycle bin? If so it 
 *        does not need to be remeasured. 
 */  
private void setupChild(View child, int position, int y, boolean flowDown, int childrenLeft,  
        boolean selected, boolean recycled) {  
    final boolean isSelected = selected && shouldShowSelector();  
    final boolean updateChildSelected = isSelected != child.isSelected();  
    final int mode = mTouchMode;  
    final boolean isPressed = mode > TOUCH_MODE_DOWN && mode < TOUCH_MODE_SCROLL &&  
            mMotionPosition == position;  
    final boolean updateChildPressed = isPressed != child.isPressed();  
    final boolean needToMeasure = !recycled || updateChildSelected || child.isLayoutRequested();  
    // Respect layout params that are already in the view. Otherwise make some up...  
    // noinspection unchecked  
    AbsListView.LayoutParams p = (AbsListView.LayoutParams) child.getLayoutParams();  
    if (p == null) {  
        p = new AbsListView.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT,  
                ViewGroup.LayoutParams.WRAP_CONTENT, 0);  
    }  
    p.viewType = mAdapter.getItemViewType(position);  
    if ((recycled && !p.forceAdd) || (p.recycledHeaderFooter &&  
            p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER)) {  
        attachViewToParent(child, flowDown ? -1 : 0, p);  
    } else {  
        p.forceAdd = false;  
        if (p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {  
            p.recycledHeaderFooter = true;  
        }  
        addViewInLayout(child, flowDown ? -1 : 0, p, true);  
    }  
    if (updateChildSelected) {  
        child.setSelected(isSelected);  
    }  
    if (updateChildPressed) {  
        child.setPressed(isPressed);  
    }  
    if (needToMeasure) {  
        int childWidthSpec = ViewGroup.getChildMeasureSpec(mWidthMeasureSpec,  
                mListPadding.left + mListPadding.right, p.width);  
        int lpHeight = p.height;  
        int childHeightSpec;  
        if (lpHeight > 0) {  
            childHeightSpec = MeasureSpec.makeMeasureSpec(lpHeight, MeasureSpec.EXACTLY);  
        } else {  
            childHeightSpec = MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED);  
        }  
        child.measure(childWidthSpec, childHeightSpec);  
    } else {  
        cleanupLayoutState(child);  
    }  
    final int w = child.getMeasuredWidth();  
    final int h = child.getMeasuredHeight();  
    final int childTop = flowDown ? y : y - h;  
    if (needToMeasure) {  
        final int childRight = childrenLeft + w;  
        final int childBottom = childTop + h;  
        child.layout(childrenLeft, childTop, childRight, childBottom);  
    } else {  
        child.offsetLeftAndRight(childrenLeft - child.getLeft());  
        child.offsetTopAndBottom(childTop - child.getTop());  
    }  
    if (mCachingStarted && !child.isDrawingCacheEnabled()) {  
        child.setDrawingCacheEnabled(true);  
    }  
}  

setupChild()方法当中的代码虽然比较多,但是我们只看核心代码的话就非常简单了,刚才调用obtainView()方法获取到的子元素View,这里在第40行调用了addViewInLayout()方法将它添加到了ListView当中。那么根据fillDown()方法中的while循环,会让子元素View将整个ListView控件填满然后就跳出,也就是说即使我们的Adapter中有一千条数据,ListView也只会加载第一屏的数据,剩下的数据反正目前在屏幕上也看不到,所以不会去做多余的加载工作,这样就可以保证ListView中的内容能够迅速展示到屏幕上。 那么到此为止,第一次Layout过程结束。

第二次Layout

如果你自己做一下实验的话就会发现,即使是一个再简单的View,在展示到界面上之前都会经历至少两次onMeasure()和两次onLayout()的过程。其实这只是一个很小的细节,平时对我们影响并不大,因为不管是onMeasure()或者onLayout()几次,反正都是执行的相同的逻辑,我们并不需要进行过多关心。 但是在ListView中情况就不一样了,因为这就意味着layoutChildren()过程会执行两次,而这个过程当中涉及到向ListView中添加子元素,如果相同的逻辑执行两遍的话,那么ListView中就会存在一份重复的数据了。因此ListView在layoutChildren()过程当中做了第二次Layout的逻辑处理,非常巧妙地解决了这个问题,下面我们就来分析一下第二次Layout的过程。 我们还是从layoutChildren()方法开始看起:

@Override  
protected void layoutChildren() {  
    final boolean blockLayoutRequests = mBlockLayoutRequests;  
    if (!blockLayoutRequests) {  
        mBlockLayoutRequests = true;  
    } else {  
        return;  
    }  
    try {  
        super.layoutChildren();  
        invalidate();  
        if (mAdapter == null) {  
            resetList();  
            invokeOnItemScrollListener();  
            return;  
        }  
        int childrenTop = mListPadding.top;  
        int childrenBottom = getBottom() - getTop() - mListPadding.bottom;  
        int childCount = getChildCount();  
        int index = 0;  
        int delta = 0;  
        View sel;  
        View oldSel = null;  
        View oldFirst = null;  
        View newSel = null;  
        View focusLayoutRestoreView = null;  
        // Remember stuff we will need down below  
        switch (mLayoutMode) {  
        case LAYOUT_SET_SELECTION:  
            index = mNextSelectedPosition - mFirstPosition;  
            if (index >= 0 && index < childCount) {  
                newSel = getChildAt(index);  
            }  
            break;  
        case LAYOUT_FORCE_TOP:  
        case LAYOUT_FORCE_BOTTOM:  
        case LAYOUT_SPECIFIC:  
        case LAYOUT_SYNC:  
            break;  
        case LAYOUT_MOVE_SELECTION:  
        default:  
            // Remember the previously selected view  
            index = mSelectedPosition - mFirstPosition;  
            if (index >= 0 && index < childCount) {  
                oldSel = getChildAt(index);  
            }  
            // Remember the previous first child  
            oldFirst = getChildAt(0);  
            if (mNextSelectedPosition >= 0) {  
                delta = mNextSelectedPosition - mSelectedPosition;  
            }  
            // Caution: newSel might be null  
            newSel = getChildAt(index + delta);  
        }  
        boolean dataChanged = mDataChanged;  
        if (dataChanged) {  
            handleDataChanged();  
        }  
        // Handle the empty set by removing all views that are visible  
        // and calling it a day  
        if (mItemCount == 0) {  
            resetList();  
            invokeOnItemScrollListener();  
            return;  
        } else if (mItemCount != mAdapter.getCount()) {  
            throw new IllegalStateException("The content of the adapter has changed but "  
                    + "ListView did not receive a notification. Make sure the content of "  
                    + "your adapter is not modified from a background thread, but only "  
                    + "from the UI thread. [in ListView(" + getId() + ", " + getClass()   
                    + ") with Adapter(" + mAdapter.getClass() + ")]");  
        }  
        setSelectedPositionInt(mNextSelectedPosition);  
        // Pull all children into the RecycleBin.  
        // These views will be reused if possible  
        final int firstPosition = mFirstPosition;  
        final RecycleBin recycleBin = mRecycler;  
        // reset the focus restoration  
        View focusLayoutRestoreDirectChild = null;  
        // Don't put header or footer views into the Recycler. Those are  
        // already cached in mHeaderViews;  
        if (dataChanged) {  
            for (int i = 0; i < childCount; i++) {  
                recycleBin.addScrapView(getChildAt(i));  
                if (ViewDebug.TRACE_RECYCLER) {  
                    ViewDebug.trace(getChildAt(i),  
                            ViewDebug.RecyclerTraceType.MOVE_TO_SCRAP_HEAP, index, i);  
                }  
            }  
        } else {  
            recycleBin.fillActiveViews(childCount, firstPosition);  
        }  
        // take focus back to us temporarily to avoid the eventual  
        // call to clear focus when removing the focused child below  
        // from messing things up when ViewRoot assigns focus back  
        // to someone else  
        final View focusedChild = getFocusedChild();  
        if (focusedChild != null) {  
            // TODO: in some cases focusedChild.getParent() == null  
            // we can remember the focused view to restore after relayout if the  
            // data hasn't changed, or if the focused position is a header or footer  
            if (!dataChanged || isDirectChildHeaderOrFooter(focusedChild)) {  
                focusLayoutRestoreDirectChild = focusedChild;  
                // remember the specific view that had focus  
                focusLayoutRestoreView = findFocus();  
                if (focusLayoutRestoreView != null) {  
                    // tell it we are going to mess with it  
                    focusLayoutRestoreView.onStartTemporaryDetach();  
                }  
            }  
            requestFocus();  
        }  
        // Clear out old views  
        detachAllViewsFromParent();  
        switch (mLayoutMode) {  
        case LAYOUT_SET_SELECTION:  
            if (newSel != null) {  
                sel = fillFromSelection(newSel.getTop(), childrenTop, childrenBottom);  
            } else {  
                sel = fillFromMiddle(childrenTop, childrenBottom);  
            }  
            break;  
        case LAYOUT_SYNC:  
            sel = fillSpecific(mSyncPosition, mSpecificTop);  
            break;  
        case LAYOUT_FORCE_BOTTOM:  
            sel = fillUp(mItemCount - 1, childrenBottom);  
            adjustViewsUpOrDown();  
            break;  
        case LAYOUT_FORCE_TOP:  
            mFirstPosition = 0;  
            sel = fillFromTop(childrenTop);  
            adjustViewsUpOrDown();  
            break;  
        case LAYOUT_SPECIFIC:  
            sel = fillSpecific(reconcileSelectedPosition(), mSpecificTop);  
            break;  
        case LAYOUT_MOVE_SELECTION:  
            sel = moveSelection(oldSel, newSel, delta, childrenTop, childrenBottom);  
            break;  
        default:  
            if (childCount == 0) {  
                if (!mStackFromBottom) {  
                    final int position = lookForSelectablePosition(0, true);  
                    setSelectedPositionInt(position);  
                    sel = fillFromTop(childrenTop);  
                } else {  
                    final int position = lookForSelectablePosition(mItemCount - 1, false);  
                    setSelectedPositionInt(position);  
                    sel = fillUp(mItemCount - 1, childrenBottom);  
                }  
            } else {  
                if (mSelectedPosition >= 0 && mSelectedPosition < mItemCount) {  
                    sel = fillSpecific(mSelectedPosition,  
                            oldSel == null ? childrenTop : oldSel.getTop());  
                } else if (mFirstPosition < mItemCount) {  
                    sel = fillSpecific(mFirstPosition,  
                            oldFirst == null ? childrenTop : oldFirst.getTop());  
                } else {  
                    sel = fillSpecific(0, childrenTop);  
                }  
            }  
            break;  
        }  
        // Flush any cached views that did not get reused above  
        recycleBin.scrapActiveViews();  
        if (sel != null) {  
            // the current selected item should get focus if items  
            // are focusable  
            if (mItemsCanFocus && hasFocus() && !sel.hasFocus()) {  
                final boolean focusWasTaken = (sel == focusLayoutRestoreDirectChild &&  
                        focusLayoutRestoreView.requestFocus()) || sel.requestFocus();  
                if (!focusWasTaken) {  
                    // selected item didn't take focus, fine, but still want  
                    // to make sure something else outside of the selected view  
                    // has focus  
                    final View focused = getFocusedChild();  
                    if (focused != null) {  
                        focused.clearFocus();  
                    }  
                    positionSelector(sel);  
                } else {  
                    sel.setSelected(false);  
                    mSelectorRect.setEmpty();  
                }  
            } else {  
                positionSelector(sel);  
            }  
            mSelectedTop = sel.getTop();  
        } else {  
            if (mTouchMode > TOUCH_MODE_DOWN && mTouchMode < TOUCH_MODE_SCROLL) {  
                View child = getChildAt(mMotionPosition - mFirstPosition);  
                if (child != null) positionSelector(child);  
            } else {  
                mSelectedTop = 0;  
                mSelectorRect.setEmpty();  
            }  
            // even if there is not selected position, we may need to restore  
            // focus (i.e. something focusable in touch mode)  
            if (hasFocus() && focusLayoutRestoreView != null) {  
                focusLayoutRestoreView.requestFocus();  
            }  
        }  
        // tell focus view we are done mucking with it, if it is still in  
        // our view hierarchy.  
        if (focusLayoutRestoreView != null  
                && focusLayoutRestoreView.getWindowToken() != null) {  
            focusLayoutRestoreView.onFinishTemporaryDetach();  
        }  
        mLayoutMode = LAYOUT_NORMAL;  
        mDataChanged = false;  
        mNeedSync = false;  
        setNextSelectedPositionInt(mSelectedPosition);  
        updateScrollIndicators();  
        if (mItemCount > 0) {  
            checkSelectionChanged();  
        }  
        invokeOnItemScrollListener();  
    } finally {  
        if (!blockLayoutRequests) {  
            mBlockLayoutRequests = false;  
        }  
    }  
} 

同样还是在第19行,调用getChildCount()方法来获取子View的数量,只不过现在得到的值不会再是0了,而是ListView中一屏可以显示的子View数量,因为我们刚刚在第一次Layout过程当中向ListView添加了这么多的子View。下面在第90行调用了RecycleBin的fillActiveViews()方法,这次效果可就不一样了,因为目前ListView中已经有子View了,这样所有的子View都会被缓存到RecycleBin的mActiveViews数组当中,后面将会用到它们。 接下来将会是非常非常重要的一个操作,在第113行调用了detachAllViewsFromParent()方法。这个方法会将所有ListView当中的子View全部清除掉,从而保证第二次Layout过程不会产生一份重复的数据。那有的朋友可能会问了,这样把已经加载好的View又清除掉,待会还要再重新加载一遍,这不是严重影响效率吗?不用担心,还记得我们刚刚调用了RecycleBin的fillActiveViews()方法来缓存子View吗,待会儿将会直接使用这些缓存好的View来进行加载,而并不会重新执行一遍inflate过程,因此效率方面并不会有什么明显的影响。 那么我们接着看,在第141行的判断逻辑当中,由于不再等于0了,因此会进入到else语句当中。而else语句中又有三个逻辑判断,第一个逻辑判断不成立,因为默认情况下我们没有选中任何子元素,mSelectedPosition应该等于-1。第二个逻辑判断通常是成立的,因为mFirstPosition的值一开始是等于0的,只要adapter中的数据大于0条件就成立。那么进入到fillSpecific()方法当中,代码如下所示:

/** 
 * Put a specific item at a specific location on the screen and then build 
 * up and down from there. 
 * 
 * @param position The reference view to use as the starting point 
 * @param top Pixel offset from the top of this view to the top of the 
 *        reference view. 
 * 
 * @return The selected view, or null if the selected view is outside the 
 *         visible area. 
 */  
private View fillSpecific(int position, int top) {  
    boolean tempIsSelected = position == mSelectedPosition;  
    View temp = makeAndAddView(position, top, true, mListPadding.left, tempIsSelected);  
    // Possibly changed again in fillUp if we add rows above this one.  
    mFirstPosition = position;  
    View above;  
    View below;  
    final int dividerHeight = mDividerHeight;  
    if (!mStackFromBottom) {  
        above = fillUp(position - 1, temp.getTop() - dividerHeight);  
        // This will correct for the top of the first view not touching the top of the list  
        adjustViewsUpOrDown();  
        below = fillDown(position + 1, temp.getBottom() + dividerHeight);  
        int childCount = getChildCount();  
        if (childCount > 0) {  
            correctTooHigh(childCount);  
        }  
    } else {  
        below = fillDown(position + 1, temp.getBottom() + dividerHeight);  
        // This will correct for the bottom of the last view not touching the bottom of the list  
        adjustViewsUpOrDown();  
        above = fillUp(position - 1, temp.getTop() - dividerHeight);  
        int childCount = getChildCount();  
        if (childCount > 0) {  
             correctTooLow(childCount);  
        }  
    }  
    if (tempIsSelected) {  
        return temp;  
    } else if (above != null) {  
        return above;  
    } else {  
        return below;  
    }  
}  

fillSpecific()这算是一个新方法了,不过其实它和fillUp()、fillDown()方法功能也是差不多的,主要的区别在于,fillSpecific()方法会优先将指定位置的子View先加载到屏幕上,然后再加载该子View往上以及往下的其它子View。那么由于这里我们传入的position就是第一个子View的位置,于是fillSpecific()方法的作用就基本上和fillDown()方法是差不多的了,这里我们就不去关注太多它的细节,而是将精力放在makeAndAddView()方法上面。再次回到makeAndAddView()方法,代码如下所示:

/** 
 * Obtain the view and add it to our list of children. The view can be made 
 * fresh, converted from an unused view, or used as is if it was in the 
 * recycle bin. 
 * 
 * @param position Logical position in the list 
 * @param y Top or bottom edge of the view to add 
 * @param flow If flow is true, align top edge to y. If false, align bottom 
 *        edge to y. 
 * @param childrenLeft Left edge where children should be positioned 
 * @param selected Is this position selected? 
 * @return View that was added 
 */  
private View makeAndAddView(int position, int y, boolean flow, int childrenLeft,  
        boolean selected) {  
    View child;  
    if (!mDataChanged) {  
        // Try to use an exsiting view for this position  
        child = mRecycler.getActiveView(position);  
        if (child != null) {  
            // Found it -- we're using an existing child  
            // This just needs to be positioned  
            setupChild(child, position, y, flow, childrenLeft, selected, true);  
            return child;  
        }  
    }  
    // Make a new view for this position, or convert an unused view if possible  
    child = obtainView(position, mIsScrap);  
    // This needs to be positioned and measured  
    setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]);  
    return child;  
} 

仍然还是在第19行尝试从RecycleBin当中获取Active View,然而这次就一定可以获取到了,因为前面我们调用了RecycleBin的fillActiveViews()方法来缓存子View。那么既然如此,就不会再进入到第28行的obtainView()方法,而是会直接进入setupChild()方法当中,这样也省去了很多时间,因为如果在obtainView()方法中又要去infalte布局的话,那么ListView的初始加载效率就大大降低了。

注意在第23行,setupChild()方法的最后一个参数传入的是true,这个参数表明当前的View是之前被回收过的,那么我们再次回到setupChild()方法当中:

/** 
 * Add a view as a child and make sure it is measured (if necessary) and 
 * positioned properly. 
 * 
 * @param child The view to add 
 * @param position The position of this child 
 * @param y The y position relative to which this view will be positioned 
 * @param flowDown If true, align top edge to y. If false, align bottom 
 *        edge to y. 
 * @param childrenLeft Left edge where children should be positioned 
 * @param selected Is this position selected? 
 * @param recycled Has this view been pulled from the recycle bin? If so it 
 *        does not need to be remeasured. 
 */  
private void setupChild(View child, int position, int y, boolean flowDown, int childrenLeft,  
        boolean selected, boolean recycled) {  
    final boolean isSelected = selected && shouldShowSelector();  
    final boolean updateChildSelected = isSelected != child.isSelected();  
    final int mode = mTouchMode;  
    final boolean isPressed = mode > TOUCH_MODE_DOWN && mode < TOUCH_MODE_SCROLL &&  
            mMotionPosition == position;  
    final boolean updateChildPressed = isPressed != child.isPressed();  
    final boolean needToMeasure = !recycled || updateChildSelected || child.isLayoutRequested();  
    // Respect layout params that are already in the view. Otherwise make some up...  
    // noinspection unchecked  
    AbsListView.LayoutParams p = (AbsListView.LayoutParams) child.getLayoutParams();  
    if (p == null) {  
        p = new AbsListView.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT,  
                ViewGroup.LayoutParams.WRAP_CONTENT, 0);  
    }  
    p.viewType = mAdapter.getItemViewType(position);  
    if ((recycled && !p.forceAdd) || (p.recycledHeaderFooter &&  
            p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER)) {  
        attachViewToParent(child, flowDown ? -1 : 0, p);  
    } else {  
        p.forceAdd = false;  
        if (p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {  
            p.recycledHeaderFooter = true;  
        }  
        addViewInLayout(child, flowDown ? -1 : 0, p, true);  
    }  
    if (updateChildSelected) {  
        child.setSelected(isSelected);  
    }  
    if (updateChildPressed) {  
        child.setPressed(isPressed);  
    }  
    if (needToMeasure) {  
        int childWidthSpec = ViewGroup.getChildMeasureSpec(mWidthMeasureSpec,  
                mListPadding.left + mListPadding.right, p.width);  
        int lpHeight = p.height;  
        int childHeightSpec;  
        if (lpHeight > 0) {  
            childHeightSpec = MeasureSpec.makeMeasureSpec(lpHeight, MeasureSpec.EXACTLY);  
        } else {  
            childHeightSpec = MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED);  
        }  
        child.measure(childWidthSpec, childHeightSpec);  
    } else {  
        cleanupLayoutState(child);  
    }  
    final int w = child.getMeasuredWidth();  
    final int h = child.getMeasuredHeight();  
    final int childTop = flowDown ? y : y - h;  
    if (needToMeasure) {  
        final int childRight = childrenLeft + w;  
        final int childBottom = childTop + h;  
        child.layout(childrenLeft, childTop, childRight, childBottom);  
    } else {  
        child.offsetLeftAndRight(childrenLeft - child.getLeft());  
        child.offsetTopAndBottom(childTop - child.getTop());  
    }  
    if (mCachingStarted && !child.isDrawingCacheEnabled()) {  
        child.setDrawingCacheEnabled(true);  
    }  
}  

可以看到,setupChild()方法的最后一个参数是recycled,然后在第32行会对这个变量进行判断,由于recycled现在是true,所以会执行attachViewToParent()方法,而第一次Layout过程则是执行的else语句中的addViewInLayout()方法。这两个方法最大的区别在于,如果我们需要向ViewGroup中添加一个新的子View,应该调用addViewInLayout()方法,而如果是想要将一个之前detach的View重新attach到ViewGroup上,就应该调用attachViewToParent()方法。那么由于前面在layoutChildren()方法当中调用了detachAllViewsFromParent()方法,这样ListView中所有的子View都是处于detach状态的,所以这里attachViewToParent()方法是正确的选择。 经历了这样一个detach又attach的过程,ListView中所有的子View又都可以正常显示出来了,那么第二次Layout过程结束。

这次就先写到这边,之后会继续更新的。

这篇文章是在郭霖大神博客: Android ListView工作原理完全解析,带你从源码的角度彻底理解 http://blog.csdn.net/guolin_blog/article/details/44996879 上面截取学习而得,希望对自己和你们都有所帮助。

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