issue with changing UIButton background image

I have the following code which basically just initialize a UIButton.

self.button = [[UIButton alloc] init];
    self.button.autoresizingMask = UIViewAutoresizingFlexibleRightMargin | UIViewAutoresizingFlexibleLeftMargin | UIViewAutoresizingFlexibleBottomMargin | UIViewAutoresizingFlexibleTopMargin;
    self.button.frame = CGRectMake(0.0, 0.0, buttonImage.size.width, buttonImage.size.height);
   [self.voteSpot setBackgroundImage:buttonImage forState:UIControlStateNormal];
[self.voteSpot setBackgroundImage:[UIImage imageNamed:@"MainButton-Selected.png"] forState:UIControlStateSelected];
    [self.button addTarget:self action:@selector(button) forControlEvents:UIControlEventTouchUpInside];

 开发者_如何学Python   CGFloat heightDifference = buttonImage.size.height - self.tabBar.frame.size.height;
    if (heightDifference < 0)
        self.button.center = self.tabBar.center;
    else
    {
        CGPoint center = self.tabBar.center;
        center.y = center.y - heightDifference/2.0;
        self.button.center = center;
    }

    [self.view addSubview:self.button];

When I press on the button I want the button background to be changed, so in the target I have the following:

- (void) button
{

    [self setSelectedViewController:[self.viewControllers objectAtIndex:1]];
    [self setTabBarWithImage:[UIImage imageNamed:@"Map-Profile.png"]];
}

The issue is that when the app first initially loads, I have to press the button twice to make the background change.. why is this? the first time I tap the button this doesn't change the button background image. Any idea?

I'm not sure what the problem is, but I'd start by creating the button using +buttonWithType instead of +alloc and -init. The buttonType property is read only, and initializing the button with -init doesn't let you set the type.

You should set the highlighted/selected backgrounds before the user can tap the button. In other words, move the first two lines from the action method to where you create the button.