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Android Canvas绘图详解(图文)

董家二少
 董家二少
发布于 2015/04/30 14:40
字数 2193
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Android中使用图形处理引擎,2D部分是android SDK内部自己提供,3D部分是用Open GL ES 1.0。今天我们主要要了解的是2D相关的,如果你想看3D的话那么可以跳过这篇文章。

大部分2D使用的api都在android.graphics和android.graphics.drawable包中。他们提供了图形处理相关的: Canvas、ColorFilter、Point(点)和RetcF(矩形)等,还有一些动画相关的:AnimationDrawable、 BitmapDrawable和TransitionDrawable等。以图形处理来说,我们最常用到的就是在一个View上画一些图片、形状或者自定义的文本内容,这里我们都是使用Canvas来实现的。你可以获取View中的Canvas对象,绘制一些自定义形状,然后调用View. invalidate方法让View重新刷新,然后绘制一个新的形状,这样达到2D动画效果。下面我们就主要来了解下Canvas的使用方法。

Canvas对象的获取方式有两种:一种我们通过重写View.onDraw方法,View中的Canvas对象会被当做参数传递过来,我们操作这个Canvas,效果会直接反应在View中。另一种就是当你想创建一个Canvas对象时使用的方法:

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Bitmap b = Bitmap.createBitmap(100, 100, Bitmap.Config.ARGB_8888);  
Canvas c =newCanvas(b);

上面代码创建了一个尺寸是100*100的Bitmap,使用它作为Canvas操作的对象,这时候的Canvas就是使用创建的方式。当你使用创建的Canvas在bitmap上执行绘制方法后,你还可以将绘制的结果提交给另外一个Canvas,这样就可以达到两个Canvas协作完成的效果,简化逻辑。但是android SDK建议使用View.onDraw参数里提供的Canvas就好,没必要自己创建一个新的Canvas对象。接下来我们看看Canvas提供我们哪些绘制图形的方法。我们创建一个自定义View对象,使用onDraw方法提供的Canvas进行绘制图形。

CanvasDemoActivity.java:

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package com.android777.demo.uicontroller.graphics;  
                                                                                                                                  
import android.app.Activity;  
import android.content.Context;  
import android.graphics.Canvas;  
import android.graphics.Color;  
import android.graphics.Paint;  
import android.os.Bundle;  
import android.view.View;  
                                                                                                                                  
public class CanvasDemoActivity extends Activity {  
                                                                                                                                  
    @Override  
    protected void onCreate(Bundle savedInstanceState) {  
        super.onCreate(savedInstanceState);  
                                                                                                                                  
        setContentView(newCustomView1(this));  
                                                                                                                                  
    }  
                                                                                                                                  
    /**  
     * 使用内部类 自定义一个简单的View  
     * @author Administrator  
     *  
     */
    class CustomView1 extends View{  
                                                                                                                                  
        Paint paint;  
                                                                                                                                  
        public CustomView1(Context context) {  
            super(context);  
            paint =newPaint();//设置一个笔刷大小是3的黄色的画笔  
            paint.setColor(Color.YELLOW);  
            paint.setStrokeJoin(Paint.Join.ROUND);  
            paint.setStrokeCap(Paint.Cap.ROUND);  
            paint.setStrokeWidth(3);  
        }  
                                                                                                                                  
        //在这里我们将测试canvas提供的绘制图形方法  
        @Override  
        protected void onDraw(Canvas canvas) {  
                                                                                                                                  
        }  
                                                                                                                                  
    }  
                                                                                                                                  
}


执行结果是一片黑色的区域,因为在自定义的CustomView1中,我们没有做任何的绘制操作。canvas提供的绘制图形的方法都是以draw开头的,我们可以查看api:

Css_bugtester

从上面方法的名字看来我们可以知道Canvas可以绘制的对象有:弧线(arcs)、填充颜色(argb和color)、 Bitmap、圆(circle和oval)、点(point)、线(line)、矩形(Rect)、图片(Picture)、圆角矩形 (RoundRect)、文本(text)、顶点(Vertices)、路径(path)。通过组合这些对象我们可以画出一些简单有趣的界面出来,但是光有这些功能还是不够的,如果我要画一个仪表盘(数字围绕显示在一个圆圈中)呢? 幸好Android还提供了一些对Canvas位置转换的方法:rorate、scale、translate、skew(扭曲)等,而且它允许你通过获得它的转换矩阵对象(getMatrix方法,不知道什么是转换矩阵?看这里) 直接操作它。这些操作就像是虽然你的笔还是原来的地方画,但是画纸旋转或者移动了,所以你画的东西的方位就产生变化。为了方便一些转换操作,Canvas 还提供了保存和回滚属性的方法(save和restore),比如你可以先保存目前画纸的位置(save),然后旋转90度,向下移动100像素后画一些图形,画完后调用restore方法返回到刚才保存的位置。下面我们就演示下canvas的一些简单用法:

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protected void onDraw(Canvas canvas) {  
                                                                                                                                  
    canvas.drawCircle(100, 100, 90, paint);  
}


效果是:

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@Override  
protected void onDraw(Canvas canvas) {  
                                                                                                                                  
    //绘制弧线区域  
                                                                                                                                  
    RectF rect =newRectF(0, 0, 100, 100);  
                                                                                                                                  
    canvas.drawArc(rect,//弧线所使用的矩形区域大小  
            0, //开始角度  
            90,//扫过的角度  
            false,//是否使用中心  
            paint);  
                                                                                                                                  
}

使用下面的代码:

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protected void onDraw(Canvas canvas) {  
                                                                                                                                  
    //绘制弧线区域  
                                                                                                                                  
    RectF rect =newRectF(0, 0, 100, 100);  
                                                                                                                                  
    canvas.drawArc(rect,//弧线所使用的矩形区域大小  
            0, //开始角度  
            90,//扫过的角度  
            true,//是否使用中心  
            paint);  
                                                                                                                                  
}


两图对比我们可以发现,当 drawArcs(rect,startAngel,sweepAngel,useCenter,paint)中的useCenter为false时,弧线区域是用弧线开始角度和结束角度直接连接起来的,当useCenter为true时,是弧线开始角度和结束角度都与中心点连接,形成一个扇形。

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protected void onDraw(Canvas canvas) {  
                                                                                                                                  
    canvas.drawColor(Color.BLUE);  
                                                                                                                                  
}

canvas.drawColor是直接将View显示区域用某个颜色填充满。

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@Override  
protected void onDraw(Canvas canvas) {  
                                                                                                                                  
    //画一条线  
    canvas.drawLine(10, 10, 100, 100, paint);  
                                                                                                                                  
}


Canvas.drawOval:

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@Override  
protected void onDraw(Canvas canvas) {  
                                                                                                                                  
    //定义一个矩形区域  
    RectF oval =newRectF(0,0,200,300);  
    //矩形区域内切椭圆  
    canvas.drawOval(oval, paint);  
                                                                                                                                  
}


canvas.drawPosText:

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@Override  
protected void onDraw(Canvas canvas) {  
                                                                                                                                  
    //按照既定点 绘制文本内容  
    canvas.drawPosText("Android777",newfloat[]{  
            10,10,//第一个字母在坐标10,10  
            20,20,//第二个字母在坐标20,20  
            30,30,//....  
            40,40,  
            50,50,  
            60,60,  
            70,70,  
            80,80,  
            90,90,  
            100,100  
    }, paint);  
                                                                                                                                  
}


canvas.drawRect:

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@Override  
    protected void onDraw(Canvas canvas) {  
                                                                                                                                  
        RectF rect =newRectF(50, 50, 200, 200);  
                                                                                                                                  
        canvas.drawRect(rect, paint);  
                                                                                                                                  
    }  
                                                                                                                                  
}


canvas.drawRoundRect:

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@Override  
protected void onDraw(Canvas canvas) {  
                                                                                                                                  
    RectF rect =newRectF(50, 50, 200, 200);  
                                                                                                                                  
    canvas.drawRoundRect(rect,  
                        30,//x轴的半径  
                        30,//y轴的半径  
                        paint);  
                                                                                                                                  
}


canvas.drawPath:

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@Override  
protected void onDraw(Canvas canvas) {  
                                                                                                                                  
    Path path =newPath();//定义一条路径  
    path.moveTo(10, 10);//移动到 坐标10,10  
    path.lineTo(50, 60);  
    path.lineTo(200,80);  
    path.lineTo(10, 10);  
                                                                                                                                  
    canvas.drawPath(path, paint);  
                                                                                                                                  
}

canvas.drawTextOnPath:

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@Override  
        protected void onDraw(Canvas canvas) {  
                                                                                                                                  
            Path path =newPath();//定义一条路径  
            path.moveTo(10, 10);//移动到 坐标10,10  
            path.lineTo(50, 60);  
            path.lineTo(200,80);  
            path.lineTo(10, 10);  
                                                                                                                                  
//          canvas.drawPath(path, paint);  
            canvas.drawTextOnPath("Android777开发者博客", path, 10, 10, paint);  
                                                                                                                                  
        }


位置转换方法,canvas.rorate和canvas.translate:


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@Override  
protected void onDraw(Canvas canvas) {  
                                                                                                                                  
    paint.setAntiAlias(true);  
    paint.setStyle(Style.STROKE);  
    canvas.translate(canvas.getWidth()/2, 200);//将位置移动画纸的坐标点:150,150  
    canvas.drawCircle(0, 0, 100, paint);//画圆圈  
                                                                                                                                  
    //使用path绘制路径文字  
    canvas.save();  
    canvas.translate(-75, -75);  
    Path path =newPath();  
    path.addArc(newRectF(0,0,150,150), -180, 180);  
    Paint citePaint =newPaint(paint);  
    citePaint.setTextSize(14);  
    citePaint.setStrokeWidth(1);  
    canvas.drawTextOnPath(" http://www.android777.com", path, 28, 0, citePaint);  
    canvas.restore();  
                                                                                                                                  
    Paint tmpPaint =newPaint(paint);//小刻度画笔对象  
    tmpPaint.setStrokeWidth(1);  
                                                                                                                                  
    float  y=100;  
    int count = 60;//总刻度数  
                                                                                                                                  
    for(int i=0 ; i <count ; i++){  
        if(i%5 == 0){  
            canvas.drawLine(0f, y, 0, y+12f, paint);  
            canvas.drawText(String.valueOf(i/5+1), -4f, y+25f, tmpPaint);  
                                                                                                                                  
        }else{  
            canvas.drawLine(0f, y, 0f, y +5f, tmpPaint);  
        }  
        canvas.rotate(360/count,0f,0f);//旋转画纸  
    }  
                                                                                                                                  
    //绘制指针  
    tmpPaint.setColor(Color.GRAY);  
    tmpPaint.setStrokeWidth(4);  
    canvas.drawCircle(0, 0, 7, tmpPaint);  
    tmpPaint.setStyle(Style.FILL);  
    tmpPaint.setColor(Color.YELLOW);  
    canvas.drawCircle(0, 0, 5, tmpPaint);  
    canvas.drawLine(0, 10, 0, -65, paint);  
                                                                                                                                  
}


上面几个例子基本已经将常用的canvas.draw*方法测试过了,我们结合一些事件,做一些有用户交互的应用:



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package com.android777.demo.uicontroller.graphics;  
                                                                                                                                  
import java.util.ArrayList;  
                                                                                                                                  
import android.app.Activity;  
import android.content.Context;  
import android.graphics.Canvas;  
import android.graphics.Color;  
import android.graphics.Paint;  
import android.graphics.PointF;  
import android.os.Bundle;  
import android.view.MotionEvent;  
import android.view.View;  
                                                                                                                                  
public class CanvasDemoActivity extends Activity {  
                                                                                                                                  
    @Override  
    protected void onCreate(Bundle savedInstanceState) {  
        super.onCreate(savedInstanceState);  
                                                                                                                                  
        setContentView(newCustomView1(this));  
                                                                                                                                  
    }  
                                                                                                                                  
    /**  
     * 使用内部类 自定义一个简单的View  
     * @author Administrator  
     *  
     */
    class CustomView1 extends View{  
                                                                                                                                  
        Paint paint;  
        private ArrayList<PointF> graphics =newArrayList<PointF>();  
        PointF point;  
                                                                                                                                  
        public CustomView1(Context context) {  
            super(context);  
            paint =newPaint();//设置一个笔刷大小是3的黄色的画笔  
            paint.setColor(Color.YELLOW);  
            paint.setStrokeJoin(Paint.Join.ROUND);  
            paint.setStrokeCap(Paint.Cap.ROUND);  
            paint.setStrokeWidth(3);  
                                                                                                                                  
        }  
                                                                                                                                  
        @Override  
        public boolean onTouchEvent(MotionEvent event) {  
                                                                                                                                  
            graphics.add(newPointF(event.getX(),event.getY()));  
                                                                                                                                  
            invalidate();//重新绘制区域  
                                                                                                                                  
            returntrue;  
        }  
                                                                                                                                  
        //在这里我们将测试canvas提供的绘制图形方法  
        @Override  
        protected void onDraw(Canvas canvas) {  
            for(PointF point : graphics) {  
                canvas.drawPoint(point.x, point.y, paint);  
            }  
//          super.onDraw(canvas);  
                                                                                                                                  
        }  
    }  
                                                                                                                                  
}

当用户点击时将出现一个小点,拖动时将画出一条用细点组成的虚线:


canvas的应用


canva还可以制作很多自定义控件,比如google日历的monthview就是用canvas绘制出来的,github上有很多使用canva的项目,所有的图表库都是用canvas绘制的。

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