使用雙緩沖技術實現Android畫板應用

什么是雙緩沖技術？雙緩沖技術就是當用戶操作界面完成后，會有一個緩沖區保存用戶操作的結果。

為什么要使用雙緩沖技術？拿Android 游戲開發來說，界面貞每次都是全部重畫的，也就說畫了新的，舊的就沒了，所以需要使用雙緩沖技術保存之前的內容。

如何實現雙緩沖？使用一個Bitmap對象保留之前的畫布即可。

package com.example.phonegaptest;

import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.Bitmap.Config;
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;

public class DrawView extends View {
    float preX;
    float preY;
    private Path path;
    public Paint paint = null;
    final int VIEW_WIDTH = 320;
    final int VIEW_HEIGHT = 480;
    Bitmap cacheBitmap = null;
    Canvas cacheCanvas = null;

    public DrawView(Context context, AttributeSet set) {
        super(context, set);
        cacheBitmap = Bitmap.createBitmap(VIEW_WIDTH, VIEW_HEIGHT,
                Config.ARGB_8888);
        cacheCanvas = new Canvas();

        path = new Path();
        cacheCanvas.setBitmap(cacheBitmap);

        paint = new Paint(Paint.DITHER_FLAG);
        paint.setColor(Color.RED);
        paint.setStyle(Paint.Style.STROKE);
        paint.setStrokeWidth(1);
        paint.setAntiAlias(true);
        paint.setDither(true);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        float x = event.getX();
        float y = event.getY();

        switch (event.getAction()) {
        case MotionEvent.ACTION_DOWN:
            path.moveTo(x, y);
            preX = x;
            preY = y;
            break;
        case MotionEvent.ACTION_MOVE:
            path.quadTo(preX, preY, x, y);
            preX = x;
            preY = y;
            break;
        case MotionEvent.ACTION_UP:
            cacheCanvas.drawPath(path, paint);
            path.reset();
            break;
        }
        invalidate();
        return true;
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        Paint bmpPaint = new Paint();
        canvas.drawBitmap(cacheBitmap, 0, 0, bmpPaint);
        canvas.drawPath(path, paint);
    }

}