双缓冲技术局部更新原理之派生自View

package com.loaderman.customviewdemo;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Rect;
import android.util.AttributeSet;
import android.view.View;


public class RectView extends View {
    private Paint mPaint;
    public RectView(Context context) {
        super(context);
        init();
    }

    public RectView(Context context, AttributeSet attrs) {
        super(context, attrs);
        init();
    }

    public RectView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
        init();
    }

    private void init() {
        mPaint = new Paint();
        mPaint.setTextSize(30);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);

        //画大方
        mPaint.setColor(Color.RED);
        canvas.drawRect(new Rect(10, 10, 600, 600),mPaint);

        //画中方
        mPaint.setColor(Color.GREEN);
        canvas.drawRect(new Rect(30, 30, 570, 570),mPaint);

        //画小方
        mPaint.setColor(Color.BLUE);
        canvas.drawRect(new Rect(60, 60, 540, 540),mPaint);

        //画小小圆
        mPaint.setColor(Color.argb(0x3F, 0xFF, 0xFF, 0xFF));
        canvas.drawCircle(300, 300, 100, mPaint);

        //写字
        mPaint.setColor(Color.GREEN);
        canvas.drawText( "6", 300, 300, mPaint);
    }
}

效果: