android SurfaceView黑屏问题

说一个真实的案例。其中需求要做一个绘图功能，一听到绘图，自然而然就像到了SurfaceView这个类。所以我就用了。

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >
    <com.example.test.MySurfaceVivew 
        android:id="@+id/edit"
        android:layout_width="match_parent"
        android:layout_height="match_parent"/>

</LinearLayout>

然后就是MySurfaceView继承SurfaceView，然后我们就会定义一个drawCanvas方法去绘图，通常的做法就是这样

package com.example.test;

import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.Canvas;
import android.util.AttributeSet;
import android.view.SurfaceHolder;
import android.view.SurfaceView;

public class MySurfaceVivew extends SurfaceView{
	private SurfaceHolder holder;
	private Bitmap bitmap;

	public MySurfaceVivew(Context context, AttributeSet attrs) {
		super(context, attrs);
		// TODO Auto-generated constructor stub
		holder = this.getHolder();
	}
	
	private void drawCanvas(Bitmap bitmap){
		Canvas canvas = holder.lockCanvas();
		if(canvas != null){
			canvas.drawBitmap(bitmap, 0, 0, null);
			holder.unlockCanvasAndPost(canvas);
		}
	}
	
	public void setBitmap(Bitmap bitmap){
		this.bitmap = bitmap;
		this.drawCanvas(bitmap);
	}

}

通过setBitmap去加载一张图片，然后在用画布画出来。但是很不幸，现实是残酷的，当Activity加载布局时图片闪一下就黑屏了。为什么呢？查了一下资料，原来Activity在加载时后会卸载SurfaceView，在创建后很快就被卸载掉了，所以只能见到闪的一下就黑屏了。有些资料会说在onResume方法中设，我也试了一下，发现不行，这下连闪都不闪一下。调试发现，canvas画布为空。自然，黑屏也就理所当然了。那怎么办，解决问题的办法才是问题的关键。那么，我们就要在画布创建时把图片画出来。

有了突破点就好办了。canvas画布在什么时候创建，就的看他谁提供的。canvas = holder.lockCanvas(),不难看出宿主是surfaceholder。那么是不是surfaceholder被创建的同时创建了canvas呢？我做了一次尝试。

package com.example.test;

import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.Canvas;
import android.util.AttributeSet;
import android.view.SurfaceHolder;
import android.view.SurfaceView;

public class MySurfaceVivew extends SurfaceView implements SurfaceHolder.Callback{
	private SurfaceHolder holder;
	private Bitmap bitmap;

	public MySurfaceVivew(Context context, AttributeSet attrs) {
		super(context, attrs);
		// TODO Auto-generated constructor stub
		holder = this.getHolder();
	}
	
	private void drawCanvas(Bitmap bitmap){
		Canvas canvas = holder.lockCanvas();
		if(canvas != null){
			canvas.drawBitmap(bitmap, 0, 0, null);
			holder.unlockCanvasAndPost(canvas);
		}
	}
	
	public void setBitmap(Bitmap bitmap){
		this.bitmap = bitmap;
		this.drawCanvas(bitmap);
	}

	@Override
	public void surfaceChanged(SurfaceHolder holder, int format, int width,
			int height) {
		// TODO Auto-generated method stub
		
	}

	@Override
	public void surfaceCreated(SurfaceHolder holder) {
		// TODO Auto-generated method stub
		this.drawCanvas(bitmap);
	}

	@Override
	public void surfaceDestroyed(SurfaceHolder holder) {
		// TODO Auto-generated method stub
		
	}

}

果然，绘画成功了，拒绝黑屏