1. 听说游戏开发都用Surfaceview，上网搜了下，说是SurfaceView在更新视图时，采用了双缓存机制，可以提高更新效率，加强用户体验。下面两段粘贴自别人的博客，说的还挺清楚的。
Note: On each pass you retrieve the Canvas from the SurfaceHolder, the previous state of the Canvas will be retained. In order to properly animate your graphics, you must re-paint the entire surface. For example, you can clear the previous state of the Canvas by filling in a color with drawColor() or setting a background image with drawBitmap(). Otherwise, you will see traces of the drawings you previously performed.
在运用时可以理解为：SurfaceView在更新视图时用到了两张Canvas，一张frontCanvas和一张backCanvas，每次实际显示的是frontCanvas，backCanvas存储的是上一次更改前的视图，当使用lockCanvas（）获取画布时，得到的实际上是backCanvas而不是正在显示的frontCanvas，之后你在获取到的backCanvas上绘制新视图，再unlockCanvasAndPost（canvas）此视图，那么上传的这张canvas将替换原来的frontCanvas作为新的frontCanvas，原来的frontCanvas将切换到后台作为backCanvas。例如，如果你已经先后两次绘制了视图A和B，那么你再调用lockCanvas（）获取视图，获得的将是A而不是正在显示的B，之后你讲重绘的C视图上传，那么C将取代B作为新的frontCanvas显示在SurfaceView上，原来的B则转换为backCanvas。
2. 在网上依样画葫芦写了短代码，结果send to background的时候会crash。Debug后发现原因是没检查SurfaceHolder获得的Canvas是不是为空。在surfaceDestroyed之后，Canvas肯定没有了，所以要检查。
public class DemoSurfaceView extends SurfaceView implements Callback{
LoopThread thread;
public DemoSurfaceView(Context context) {
super(context);
init();
}
private void init(){
SurfaceHolder holder = getHolder();
thread = new LoopThread(holder, getContext());
holder.addCallback(this);
}
@Override
public void surfaceChanged(SurfaceHolder holder, int format, int width,
int height) {
}
@Override
public void surfaceCreated(SurfaceHolder holder) {
thread.isRunning = true;
thread.start();
}
@Override
public void surfaceDestroyed(SurfaceHolder holder) {
Log.i("Hailin", "surfaceDestroyed");
thread.isRunning = false;
try {
thread.join();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
class LoopThread extends Thread{
SurfaceHolder surfaceHolder;
Context context;
boolean isRunning;
float radius = 10f;
Paint paint;
public LoopThread(SurfaceHolder surfaceHolder,Context context){
this.surfaceHolder = surfaceHolder;
this.context = context;
isRunning = false;
paint = new Paint();
paint.setColor(Color.YELLOW);
paint.setStyle(Paint.Style.STROKE);
}
@Override
public void run() {
Canvas c = null;
while(isRunning){
try{
synchronized (surfaceHolder) {
if(surfaceHolder == null) {
Log.i("Hailin", "surfaceHolder == null");
}
c = surfaceHolder.lockCanvas(null);
if(c == null) {
Log.i("Hailin", "c == null");
return;
}
doDraw(c);
Thread.sleep(50);
surfaceHolder.unlockCanvasAndPost(c);
}
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
}
}
}
public void doDraw(Canvas c){
c.drawColor(Color.BLACK);
c.translate(200, 200);
c.drawCircle(0,0, radius++, paint);
if(radius > 100){
radius = 10f;
}
}
}
}
3. 照着《疯狂Android讲义》敲了断代码，发现在点击屏幕后在surfaceCreated里面画的背景图老是丢失。如果直接用Surfaceview.setBackground来设置背景，点击屏幕画的图会被背景图非黑色部分给遮住（背景图本来是白色背景，但是画上去都变黑色背景了）。最后只能在onTouch里面重新画一遍背景图。
public class SVTest extends Activity {
private SurfaceHolder holder;
private Paint paint;
private SurfaceView surface;
private Canvas canvas;
private boolean isTouched;
private Bitmap mBitmap;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
this.setContentView(R.layout.svtest);
paint = new Paint();
surface = (SurfaceView) this.findViewById(R.id.svTest);
final Bitmap back = BitmapFactory.decodeResource(SVTest.this.getResources(), R.drawable.ic_launcher);
DisplayMetrics dm = getResources().getDisplayMetrics();
int mScreenWidth = dm.widthPixels;
int mScreenHeight = dm.heightPixels;
mBitmap = Bitmap.createScaledBitmap(back, mScreenWidth, mScreenHeight, true);
// actually it set a Drawable with pure color as background
// surface.setBackgroundColor(Color.WHITE);
// set background Drawable
// surface.setBackgroundResource(R.drawable.ic_launcher);
holder = surface.getHolder();
holder.addCallback(new Callback() {
@Override
public void surfaceCreated(SurfaceHolder holder) {
// TODO Auto-generated method stub
Log.i("Hailin", "surfaceCreated");
// set background
canvas = holder.lockCanvas();
canvas.drawBitmap(mBitmap, 0, 0, null);
holder.unlockCanvasAndPost(canvas);
}
@Override
public void surfaceChanged(SurfaceHolder holder, int format,
int width, int height) {
// TODO Auto-generated method stub
Log.i("Hailin", "surfaceChanged");
}
@Override
public void surfaceDestroyed(SurfaceHolder holder) {
// TODO Auto-generated method stub
Log.i("Hailin", "surfaceDestroyed");
}
});
surface.setOnTouchListener(new OnTouchListener() {
@Override
public boolean onTouch(View v, MotionEvent event) {
// TODO Auto-generated method stub
if(event.getAction() == MotionEvent.ACTION_DOWN) {
if(!isTouched) {
// set background again
canvas = holder.lockCanvas();
canvas.drawBitmap(mBitmap, 0, 0, null);
holder.unlockCanvasAndPost(canvas);
isTouched = true;
}
int cx = (int) event.getX();
int cy = (int) event.getY();
Canvas canvas = holder.lockCanvas(new Rect(cx - 50, cy - 50, cx + 50, cy + 50));
canvas.save();
canvas.rotate(30, cx, cy);
paint.setColor(Color.RED);
canvas.drawRect(cx - 50, cy - 50, cx, cy, paint);
canvas.restore();
paint.setColor(Color.GREEN);
canvas.drawRect(cx, cy, cx + 50, cy + 50, paint);
holder.unlockCanvasAndPost(canvas);
}
return false;
}
});
}
@Override
public void onResume() {
super.onResume();
Log.i("Hailin", "onResume");
}
@Override
public void onPause() {
super.onPause();
Log.i("Hailin", "onPause");
}
@Override
public void onDestroy() {
super.onDestroy();
Log.i("Hailin", "onDestroy");
}
}