这段代码会令视图的内容被拉伸，但是如果drawRect:被再次调用，比如通过setNeedsDisplay，内容又会恢复原状。避免这类问题是因为绘制系统并不会因为bounds的改变而从头绘制，这个机制的行为是通过contentMode来配置的。

contentMode.mm

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    self.window = [[[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]] autorelease];
    
    MyView* mv = [[MyView alloc] initWithFrame:CGRectMake(0, 0, self.window.bounds.size.width - 50, 150)];
    mv.center = self.window.center;
    [self.window addSubview:mv];
    mv.opaque = NO;
    mv.tag = 111;
    mv.backgroundColor = [UIColor whiteColor];
    //mv.contentMode = UIViewContentModeScaleToFill; // This is default value.
    //mv.contentMode = UIViewContentModeTopLeft;
    [mv release];
    
    [self.window makeKeyAndVisible];
    
    [self performSelector:@selector(resize:) withObject:nil afterDelay:0.1];
    
    return YES;
}

- (void)resize:(id)dummy
{
    // 注意这段代码没有调用setNeedsDisplay
    UIView* mv = [self.window viewWithTag:111];
    CGRect f = mv.bounds;
    f.size.height *= 2;
    mv.bounds = f;
}