In objective c Xcode Changing UIImages at the push of a button - iphone

Question

When i push the button I would like the first image to show in the UIImageView, stay for a short period of time and then the next image to show. Only the second image shows after a period of time. The first image never comes on.
// TestProjectViewController.m
// Created by Jack Handy on 3/8/12.
#import "TestProjectViewController.h"
#implementation TestProjectViewController
#synthesize View1= _view1;
#synthesize yellowColor = _yellowColor;
#synthesize greenColor = _greenColor;
- (IBAction)button:(id)sender {
_greenColor = [UIImage imageNamed: #"green.png"];
_view1.image = _greenColor;
[NSThread sleepForTimeInterval:2];
_yellowColor = [UIImage imageNamed: #"yellow.png"];
_view1.image = _yellowColor;
}
#end

Answer 1

U can try and place the
_yellowColor = [UIImage imageNamed: #"yellow.png"];
_view1.image = _yellowColor;
and instead of the
[NSThread sleepForTimeInterval:2];
call this one
[self performSelector:#selector(changeColor) withObject:nil afterDelay:2];

Answer 2

The problem here is that you are replacing the image before the OS gets a chance to draw. Since all of these three operations: change the image, wait 2 seconds, change the image again) occur before your button action returns, you are preventing the Main thread from executing and thus the screen from refreshing. So, what's happening is that after 2 seconds, the screen draws with the image you most recently put in its place.
You need to have the waiting happen separately. There are three typical ways to do this, each of which have their benefits:
- send yourself a delayed message using -performSelector:withObject:afterDelay:
- spawn another thread or use a dispatch queue to run a thread in the background for the sleep and then send the message to the main thread from there
- or, use a timer.
My suggestion would be to use the timer, because it is easily cancelable if you need to do something like move to another screen.
NSTimer *timer = [NSTimer scheduledTimerWithTimeInterval:2.0 target: self selector: #selector(updateColor:) userInfo: nil repeats: NO];
// store the timer somewhere, so that you can cancel it with
// [timer invalidate];
// later as necessary
and then later:
-(void)updateColor:(NSTimer*)timer
{
_yellowColor = [UIImage imageNamed: #"yellow.png"];
_view1.image = _yellowColor;
}
If you want the colors to alternate, you can pass YES for the repeats: value in the creation code and then change -updateColor: to alternate... or move to the next color.