Android自定义view双缓存技术

双缓存说白了就是有两个绘图区域，一个是bitmap的canvas另一个是当前view的canvas。先将图形绘制在bitmap上，然后将bitmap绘制在view上，也就是说，我们在view上看到的效果其实就是bitmap上的内容。这样做的好处有以下几点：
提高绘图性能
可以在屏幕上展示绘图的过程
保存绘制的历史

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自定义一个写字板来帮助理解双缓存。如果不使用双缓存那么用户只能看到绘制的最后一笔的效果。因为在不断调用invalidate()，未保存的绘制过程会消失。

package newviews;import android.content.Context;import android.graphics.Bitmap;import android.graphics.Canvas;import android.graphics.Color;import android.graphics.Paint;import android.graphics.Path;import android.util.AttributeSet;import android.view.MotionEvent;import android.view.View;/** * Created by yuanteng on 2017/9/11. */public class DrawNsmeView extends View {;    private Paint paint;    private Path path;    //绘图是用于缓存绘制过程的bitmap    private Bitmap bufferBitmap;    private Canvas bufferCanvas;    private int preX,preY;    public DrawNsmeView(Context context, AttributeSet attrs) {        super(context, attrs);        path = new Path();        paint = new Paint(Paint.ANTI_ALIAS_FLAG);        paint.setColor(Color.RED);        paint.setStrokeWidth(5);        paint.setStyle(Paint.Style.STROKE);    }    @Override    protected void onSizeChanged(int w, int h, int oldw, int oldh) {        super.onSizeChanged(w, h, oldw, oldh);        int width = getMeasuredWidth();        int height = getMeasuredHeight();        bufferBitmap = Bitmap.createBitmap(width,height,Bitmap.Config.ARGB_8888);        if (bufferCanvas == null){            bufferCanvas = new Canvas(bufferBitmap);        }    }    @Override    protected void onDraw(Canvas canvas) {        super.onDraw(canvas);        canvas.drawBitmap(bufferBitmap,0,0,null);       // canvas.drawPath(path,paint);    }    @Override    public boolean onTouchEvent(MotionEvent event) {        int x = (int)event.getX();        int y = (int)event.getY();        switch (event.getAction()){            case MotionEvent.ACTION_DOWN:                //重置path 清除path中保存的数据                path.reset();                preX = x;                preY = y;                //将起点移动到此处                path.moveTo(x,y);                break;            case MotionEvent.ACTION_MOVE:                //画贝塞尔曲线，贝塞尔曲线比bufferCanvas.drawLine();画出来的曲线更平滑                path.quadTo(preX,preY,x,y);                //通知重绘制view                bufferCanvas.drawPath(path,paint);                invalidate();                preY = y;                preX = x;                break;            case MotionEvent.ACTION_UP:                //抬起时将path绘制在buffer中保存，并崇绘制view,崇绘制的时候回绘制buffer                bufferCanvas.drawPath(path,paint);                invalidate();                break;        }        return true;    }}

效果图：用模拟器画的效果没有真机好