Altenating images in image view using thread - ios

Question

I have the following code to toggle between two images in a image view. I am not using GCD because this code is part of an already existing system that has been coded this way.
- (void)updateimage {
dispatch_async(dispatch_get_main_queue(), ^{
if (self.fileno % 2 == 0) {
self.imageViewTest.image = [UIImage imageNamed:#"Image7.png"];
}
else {
self.imageViewTest.image = [UIImage imageNamed:#"Image8.png"];
}
[self.imageViewTest setNeedsDisplay];
self.filenolabel.text = [NSString stringWithFormat:#"%d", self.fileno];
});
}
- (void)calculateFileNo {
while (1) {
self.fileno ++;
sleep(1);
[self updateimage];
}
}
- (void)viewDidLoad {
[super viewDidLoad];
self.detectionThread = [[NSThread alloc]initWithTarget:self selector:#selector(calculateFileNo) object:nil] ;
[self.detectionThread setName:#"NewThread"];
[self.detectionThread start];
}
The filenolabel label displays the right no for each loop. But at times the images do not toggle properly. The same image is displayed for 2 or 3 iterations of the loop. I want the image to be toggled for each iteration. Please help. Thanks in advance

Answer 1

Increment place is a problem 'self.fileno ++;' -> move it to dispatcher.
- (void)updateimage {
dispatch_async(dispatch_get_main_queue(), ^{
if (self.fileno % 2 == 0) {
self.imageViewTest.image = [UIImage imageNamed:#"Image7.png"];
}
else {
self.imageViewTest.image = [UIImage imageNamed:#"Image8.png"];
}
[self.imageViewTest setNeedsDisplay];
self.filenolabel.text = [NSString stringWithFormat:#"%d", self.fileno];
self.fileno ++;
});
}
- (void)calculateFileNo {
while (1) {
sleep(1);
[self updateimage];
}
}

Answer 2

Increase your file no inside the main thread.
self.fileno++;
You should only be calling setNeedsDisplay if you override drawRect in a subclass of UImageView which is basically a custom view drawing something on the screen, like lines, images, or shapes like a rectangle.
So you should call setNeedsDisplay when you make changes to few variables on which this drawing depends and for view to represent that change , you need to call this method which internally will give a call to drawRect and redraw the components.
When you change an imageView of image or make changes to any subview, you need call this method.
- (void)updateimage {
dispatch_async(dispatch_get_main_queue(), ^{
if (self.fileno % 2 == 0) {
self.imageView.image = [UIImage imageNamed:#"Image7.png"];
}
else {
self.imageView.image = [UIImage imageNamed:#"Image8.png"];
}
[self.imageView setNeedsDisplay];
self.fileno ++; // Increament your file no inside the main thread.
self.textView.text = [NSString stringWithFormat:#"%d", self.fileno];
});
}