UIButton and UIControlEventState issue

Question

I'm having a very specific "bug" in my iPhone application. I'm setting two images for the highlighted and normal states of a button. It works as expected when you "press" and then "touch up" at a slow pace, but if you click/tap it quickly, there's a noticeable flicker between states. Is this a known bug or am I setting the states incorrectly?

Here's the code that creates the buttons:

UIImage *normalImage = [[UIImage imageNamed:@"btn-small.png"] stretchableImageWithLeftCapWidth:10.0f topCapHeight:0.0f];
UIImage *highlightedImage = [[UIImage imageNamed:@"btn-small-down.png"] stretchableImageWithLeftCapWidth:10.0f topCapHeight:0.0f];

[self setBackgroundColor:[UIColor clearColor]];

[self setBackgroundImage:normalImage forState:UIControlStateNormal];
[self setBackgroundImage:highlightedImage forState:UIControlStateDisabled];
[self setBackgroundImage:highlightedImage forState:UIControlStateHighlighted];

[self setAdjustsImageWhenDisabled:FALSE];
[self setAdjustsImageWhenHighlighted:FALSE];

When a button is tapped it simply disables itself and enables the other button:

- (IBAction)aboutButtonTouched:(id)sender
{
    aboutButton.enabled = FALSE;
    rulesButton.enabled = TRUE;
}

- (IBAction)rulesButtonTouched:(id)sender
{
    rulesButton.enabled = FALSE;
    aboutButton.enabled = TRUE;
}

Any thoughts on this quick-click flicker?

Answer 1

Maybe you should reverse the sequence of changing the buttons

- (IBAction)aboutButtonTouched:(id)sender
{
    rulesButton.enabled = TRUE;
    aboutButton.enabled = FALSE;
}

When you hide the one button first and then show the other, there's maybe a little gap, which creates the flicker. I think it's better to show the other button first.

Answer 2

Ok, I resolved this. Took a bit of reverse engineering what I was trying to do, but I thought I'd post what I did in case it helps someone else.

The first thing I did was modify the aboutButtonTouched method to log the button's state property which is a bit-mask NSUInteger:

- (IBAction)aboutButtonTouched:(id)sender
{
    rulesButton.enabled = TRUE;
    [sender setEnabled:FALSE];    

    NSLog(@"%d", [sender state]);
}

At this point, the button is disabled through setEnabled, and the log reported that the state was "3". Looking at the bit-mask type for UIControlState:

enum {
   UIControlStateNormal               = 0,            // 0
   UIControlStateHighlighted          = 1 << 0,       // 1
   UIControlStateDisabled             = 1 << 1,       // 2
   UIControlStateSelected             = 1 << 2,       // 4
   UIControlStateApplication          = 0x00FF0000,
   UIControlStateReserved             = 0xFF000000
};

(Notes added since I can never remember bitwise). We can see that to get "3" (0011) we should use UIControlStateHighlighted | UIControlStateDisabled (0001|0010 or 1|2), something which I did not have as a state in my original button definition. The key here that there's a brief time when the state is both before just being disabled ("A control enters this state when a touch enters and exits during tracking and and when there is a touch up" -- from the docs). So the final state settings for the button where it does not flicker are:

[self setBackgroundImage:normalImage forState:UIControlStateNormal];
[self setBackgroundImage:highlightedImage forState:UIControlStateDisabled];
[self setBackgroundImage:highlightedImage forState:UIControlStateHighlighted];
[self setBackgroundImage:highlightedImage forState:UIControlStateHighlighted|UIControlStateDisabled];