UIButton在突出显示时忽略contentMode（adjustsImageWhenHighlighted）

UIButton *imageBtn = [UIButton ...
imageBtn.adjustsImageWhenHighlighted = NO;

[imageBtn addTarget:self action:@selector(doSomething:) forControlEvents:UIControlEventTouchUpInside];

[imageBtn addTarget:self action:@selector(doHighlighted:) forControlEvents:UIControlEventTouchDown];
[imageBtn addTarget:self action:@selector(doHighlighted:) forControlEvents:UIControlEventTouchDragEnter];
    [imageBtn addTarget:self action:@selector(doCancelHighlighted:) forControlEvents:UIControlEventTouchDragExit];

-(void)doSomething:(UIButton *)button{
    ...
    [self performSelector:@selector(doCancelHighlighted:) withObject:button afterDelay:0.2f];
}

-(void)doHighlighted:(UIButton *)button{
    UIImageView *imageView = [[UIImageView alloc] initWithFrame:CGRectMake(5, 5, 300, 300)];
    imageView.backgroundColor = [UIColor blackColor];
    imageView.alpha = 0.7;
    imageView.tag = 1000;
    [button addSubview:imageView];
}

-(void)doCancelHighlighted:(UIButton *)button{
    UIView *view = [button subviewWithTag:1000];
    [UIView animateWithDuration:0.2f animations:^{
        view.alpha = 0;
    } completion:^(BOOL finished) {
        [view removeFromSuperview];        
    }];
}

[self setImage:image forState:UIControlStateNormal];
[self setAdjustsImageWhenHighlighted:NO];

- (void)setHighlighted:(BOOL)highlighted {
    if (!highlightView) {
        self.highlightView = [[[UIView alloc] initWithFrame:self.bounds] autorelease];
        self.highlightView.backgroundColor = [UIColor darkGrayColor];
        self.highlightView.alpha = 0.0;
        [self addSubview:self.highlightView];
    }
    if (highlighted) {
        [UIView beginAnimations:@"highlight" context:nil];
        [UIView setAnimationDuration:0.2];
        highlightView.alpha = 0.5;
        [UIView commitAnimations];
    }
    else {
        [UIView beginAnimations:@"highlight" context:nil];
        [UIView setAnimationDuration:0.2];
        highlightView.alpha = 0.0;
        [UIView commitAnimations];
    }
}