Android自定义view双缓存技术

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