ViewController do not update before Sleep - iphone

Question

I wrote the following code :
Code:
[MyImage setHighlighted:true];
Sleep(1);
[MyImage setHighlighted:false];
i need to highlight my image for one second and after that get it back , but X-Code when arrive to [MyImage setHighlighted:true] do not any thing ( i know it do it ) and after that Sleep executed and main thread of application goes to sleep for 1 second and after that the last line executed , but my question is why the first line do not update the UI and i can,t see any thing in user interface .
In first line of code the Sleep method do not executed and main thread is in normal state and it should set my image to highlight and after that goes to sleep for 1 second and after that set my image to normal state , but why not ?
I know that i can use NSTimer to do it but why the following code do not work ?
Thanks .

Answer 1

jackslash explains well what is happening.
You can change your code into something like this:
[image setHighlighted:true];
[self performSelector:#selector(aMethodThatChangesTheHighlight) withObject:self afterDelay:1];
Finally note that freezing the main thread is generally a bad idea.

Answer 2

changes to the screen are only made once your method returns. Changes are then all coalesced and made at once. In this case as you are sleeping the main thread your method does not return until you un highlight the image. After your method returns UIKit looks to see if it needs to draw any changes. As you have undone your changes before your method returned there is nothing to do so no change happens on the display.
The best thing to do would be to see if you can use the UIView class animation methods.
[UIView animateWithDuration:0.5
delay:0
options:UIViewAnimationCurveEaseInOut
animations:^{
//animation code goes here
//note that you have to change something that UIView considers to be animatable
//e.g.
self.myView.alpha = 0;
} completion:^(BOOL done){
//code to execute once the animation has completed goes here
}];

Answer 3

I like sch's answer better, but if you wanted to use animation as jackslash suggested, you would have to make the animation change something visual so that the completion block does not run immediately. Not even adding a "delay" will fix that.
MyImage.alpha = 0.99f;
[UIView animateWithDuration:1.0f delay:0.0f options:UIViewAnimationOptionAllowUserInteraction animations:^{
MyImage.alpha = 1.0f;
[MyImage setHighlighted:YES];
} completion:^(BOOL finished) {
[MyImage setHighlighted:NO];
}];