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