UIControl派生类＆amp;需要调用drawRect两次才能绘制正确的控件状态

Question

我有一个继承自UIControl的类来处理touchEvents，我不明白为什么'setNeedsDisplay'方法需要在touchEvents上调用两次才能在UI中绘制正确的状态。我在一些示例中看到了这种模式，其中各种触摸[Began | Ended]方法都直接调用setNeedsDisplay，然后使用performSelector再次调用延迟。

我可以看到未调用第二个setNeedsDisplay时的结果 - 控件首先呈现在选定状态但未恢复到正常状态，因此背景保持为选中状态。

为什么会出现这种情况？

// a class that inherits from UIControl
- (void)drawRect:(CGRect)rect
{
    NSLog(@"drawRect, control state: %d", self.state);
    if (self.state == UIControlStateNormal)
    {
        [self setContentToNormalState];
        [self setBackgroundColor:[UIColor whiteColor]];
        NSLog(@"Set background color: White");
    }
    else 
    {
        [self setContentToSelectedState];
        [self setBackgroundColor:[UIColor lightGrayColor]];
        NSLog(@"Set background color: Grey");
    }

    [super drawRect:rect];
}

- (void)hesitateUpdate
{
    [self setNeedsDisplay];
}

#pragma mark - Touch event handling

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    [self setHighlighted:YES];
    [self setNeedsDisplay];
    [super touchesBegan:touches withEvent:event];
}

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
    [self setHighlighted:NO];    
    [super touchesMoved:touches withEvent:event];
//    [self setNeedsDisplay];    
}

- (void)touchesCancelled:(NSSet *)touches withEvent:(UIEvent *)event {
    [super touchesCancelled:touches withEvent:event];
    [self setHighlighted:NO];

    [self setNeedsDisplay];
    [self performSelector:@selector(hesitateUpdate) withObject:nil afterDelay:0.1];
}

- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
{
    [super touchesEnded:touches withEvent:event];
    [self setHighlighted:NO];

    [self setNeedsDisplay];
    // Even with the following, if I tap and hold momentarily in the simulator, 
    // sometimes, the control has the gray background instead of white at the end
    [self performSelector:@selector(hesitateUpdate) withObject:nil afterDelay:0.1];     
}