Android之数据库异步加载利器--Loaders

原创
2014/01/25 09:40
阅读数 70

英文文档出处:http://developer.android.com/guide/components/loaders.html

Loaders,中文可理解为“加载器”,在Android3.0中新增。从字面含义可见其功能,即提供数据加载。特别地,加载数据的方式为异步。它具有以下特点:

  • Loaders用于所有的Activity和Fragment;
  •  提供异步数据装载机制;
  • 监控他们的来源数据变化情况,在数据发生变化的时候传递新的结果;
  • 自动重连到最后一个数据加载器游标,因此不需要重新查询数据


如何在应用中使用Loaders

使用Loaders的先决条件:

  • 需要一个Activity 或者 Fragmnet
  • 一个LoaderManager实例
  • 一个用于加载数据的的CursorLoader对象(依赖于ContentProvider)
  • 一个LoaderManager.LoaderCallbacks的实现类.
  • 一个数据展现适配器,比如SimpleCursorAdapter
  • 一个数据源,比如ContentProvider

启动数据加载器Loaders

LoaderManager管理者一个Activity或者Fragment中的一个或多个Loader实例,每个Activity或者Fragment只有对应一个LoaserManager。
         一般在Activity的onCreate方法或者Fragment的onActivityCreated方法中初始化一个Loader:
getLoaderManager().initLoader(0, null, this);
参数:

1、  第一个参数:0 为Loader的唯一标识ID;
2、  第二个参数: 为Loader的构造器可选参数,这里为null;
3、  第三个参数:this,这里表示当前Activity对象或者Fragment对象,提供给LoaderManager对象进行数据汇报。
InitLoader()方法保证了Loader初始化及对象激活,执行这个方法有2个可能的结果:

1、  如果ID存在,则重复利用;
2、  如果ID不存在,则出发LoaderManager.LoaderCallbacks的onCreateLoader()方法新创建一个Loader并返回;


不管在什么情况下,只有Loader状态发生了变化,与之关联的LoaderManager.LoaderCallbacks实现类都会被告知;

你可能注意到了,initLoader返回的Loader对象并未与任何变量关联,那是因为LoaderManager有自动的Loader管理功能;LoaderManager在必要的时候自动启动及停止数据加载操作,并且维护者Loader的状态;这就意味着,你很少直接与Loader进行交互。一般地,使用LoaderManager.LoaderCallbacks的onCreateLoader()方法去干预数据加载的过程中的特殊事件。


如何重启数据加载器Loaders

在上面创建Loaders时,如果ID不存在则创建,否则使用旧的Loader,但有些时候,我们需要清理掉旧的数据重新开始。
使用restartLoaser()可以做到。比如,SearchView.OnQueryTextListener的实现类,在查询条件发生改变时重启Loaders,以便获取最新的查询结果。

public boolean onQueryTextChanged(String newText) {

    // Called when the action bar search text has changed.  Update

    // the search filter, and restart the loader to do a new query

    // with this filter.

    mCurFilter = !TextUtils.isEmpty(newText) ? newText : null;

    getLoaderManager().restartLoader(0, null, this);

    return true;
}

如何使用LoaderManager的回调方法

LoaderManager.LoaderCallbacks 接口是客户端与LoaderManager进行交互的腰带。
Loader ,特别是CursorLoader,期望在停止状态后保存它们的状态。这样的话,用户在交互过程中,就避免了数据的重新加载而导致UI卡死的局面。使用回调函数,就可以知道什么时候去创建一个新的Loader,并且告知应用程序什么时候停止使用Loader加载的数据。

回调方法有:

  • onCreateLoader():根据给定的ID创建新的Loader;
  • onLoaderFinished():当Loader完成数据加载后调用;
  • onLoaderReset():Loader重置,使之前的数据无效;
 
onCreateLoader使用实例:

// If non-null, this is the current filter the user has provided.

String mCurFilter;

...

public Loader<Cursor> onCreateLoader(int id, Bundle args) {

    // This is called when a new Loader needs to be created.  This

    // sample only has one Loader, so we don't care about the ID.

    // First, pick the base URI to use depending on whether we are

    // currently filtering.

    Uri baseUri;

    if (mCurFilter != null) {

        baseUri = Uri.withAppendedPath(Contacts.CONTENT_FILTER_URI,

                  Uri.encode(mCurFilter));

    } else {

        baseUri = Contacts.CONTENT_URI;

    }

 

    // Now create and return a CursorLoader that will take care of

    // creating a Cursor for the data being displayed.

    String select = "((" + Contacts.DISPLAY_NAME + " NOTNULL) AND ("

            + Contacts.HAS_PHONE_NUMBER + "=1) AND ("

            + Contacts.DISPLAY_NAME + " != '' ))";

    return new CursorLoader(getActivity(), baseUri,

            CONTACTS_SUMMARY_PROJECTION, select, null,

            Contacts.DISPLAY_NAME + " COLLATE LOCALIZED ASC");
}

CursorLoader(Context context, Uri uri, String[] projection, String selection, String[] selectionArgs, String sortOrder)构造器参数解释:
Context:上下文,这里是Activity对象或者Fragment对象
Uri:内容检索地址
Projection:要显示的列,传null表示查询所有的列
Selection:查询过滤语句,类似SQL WHERE ,传null,表示查询所有
selectionArgs:查询参数,替换在selection中定义的  ?
sortOrder:排序定义,类似SQL ORDER BY


完整的实例

public static class CursorLoaderListFragment extends ListFragment

        implements OnQueryTextListener, LoaderManager.LoaderCallbacks<Cursor> {

 

    // This is the Adapter being used to display the list's data.

    SimpleCursorAdapter mAdapter;

 

    // If non-null, this is the current filter the user has provided.

    String mCurFilter;

 

    @Override public void onActivityCreated(Bundle savedInstanceState) {

        super.onActivityCreated(savedInstanceState);

 

        // Give some text to display if there is no data.  In a real

        // application this would come from a resource.

        setEmptyText("No phone numbers");

 

        // We have a menu item to show in action bar.

        setHasOptionsMenu(true);

 

        // Create an empty adapter we will use to display the loaded data.

        mAdapter = new SimpleCursorAdapter(getActivity(),

                android.R.layout.simple_list_item_2, null,

                new String[] { Contacts.DISPLAY_NAME, Contacts.CONTACT_STATUS },

                new int[] { android.R.id.text1, android.R.id.text2 }, 0);

        setListAdapter(mAdapter);

 

        // Prepare the loader.  Either re-connect with an existing one,

        // or start a new one.

        getLoaderManager().initLoader(0, null, this);

    }

 

    @Override public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) {

        // Place an action bar item for searching.

        MenuItem item = menu.add("Search");

        item.setIcon(android.R.drawable.ic_menu_search);

        item.setShowAsAction(MenuItem.SHOW_AS_ACTION_IF_ROOM);

        SearchView sv = new SearchView(getActivity());

        sv.setOnQueryTextListener(this);

        item.setActionView(sv);

    }

 

    public boolean onQueryTextChange(String newText) {

        // Called when the action bar search text has changed.  Update

        // the search filter, and restart the loader to do a new query

        // with this filter.

        mCurFilter = !TextUtils.isEmpty(newText) ? newText : null;

        getLoaderManager().restartLoader(0, null, this);

        return true;

    }

 

    @Override public boolean onQueryTextSubmit(String query) {

        // Don't care about this.

        return true;

    }

 

    @Override public void onListItemClick(ListView l, View v, int position, long id) {

        // Insert desired behavior here.

        Log.i("FragmentComplexList", "Item clicked: " + id);

    }

 

    // These are the Contacts rows that we will retrieve.

    static final String[] CONTACTS_SUMMARY_PROJECTION = new String[] {

        Contacts._ID,

        Contacts.DISPLAY_NAME,

        Contacts.CONTACT_STATUS,

        Contacts.CONTACT_PRESENCE,

        Contacts.PHOTO_ID,

        Contacts.LOOKUP_KEY,

    };

    public Loader<Cursor> onCreateLoader(int id, Bundle args) {

        // This is called when a new Loader needs to be created.  This

        // sample only has one Loader, so we don't care about the ID.

        // First, pick the base URI to use depending on whether we are

        // currently filtering.

        Uri baseUri;

        if (mCurFilter != null) {

            baseUri = Uri.withAppendedPath(Contacts.CONTENT_FILTER_URI,

                    Uri.encode(mCurFilter));

        } else {

            baseUri = Contacts.CONTENT_URI;

        }

 

        // Now create and return a CursorLoader that will take care of

        // creating a Cursor for the data being displayed.

        String select = "((" + Contacts.DISPLAY_NAME + " NOTNULL) AND ("

                + Contacts.HAS_PHONE_NUMBER + "=1) AND ("

                + Contacts.DISPLAY_NAME + " != '' ))";

        return new CursorLoader(getActivity(), baseUri,

                CONTACTS_SUMMARY_PROJECTION, select, null,

                Contacts.DISPLAY_NAME + " COLLATE LOCALIZED ASC");

    }

 

    public void onLoadFinished(Loader<Cursor> loader, Cursor data) {

        // Swap the new cursor in.  (The framework will take care of closing the

        // old cursor once we return.)

        mAdapter.swapCursor(data);

    }

 

    public void onLoaderReset(Loader<Cursor> loader) {

        // This is called when the last Cursor provided to onLoadFinished()

        // above is about to be closed.  We need to make sure we are no

        // longer using it.

        mAdapter.swapCursor(null);

    }
}










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