Question

I would like to update a progressbar while i am instantiating some ui elements that takes some times. I first create my view during the viewLoad method and add my progress bar there. Once my view appeared in the viewDidAppear method I am making several uikit objects instantiation but i would like to update the progress bar in the mean time. I am not sure how to proceed since everything should happen in the main thread as it is ui elements.

Here is part of my code:

-(void) viewDidAppear:(BOOL)animated
{
    // precompute the source and destination view screenshots for the custom segue
    self.sourceScreenshotView = [[UIImageView alloc] initWithImage:[self.view pw_imageSnapshot]];

    [self.progressBar setProgress:.3];


    SCLViewController *rvc = [[SCLViewController alloc] init];
    UIView *destinationView = rvc.view;
    destinationView.frame = CGRectMake(0, 0, kWidthLandscape, kHeightLandscape);


    self.destinationScreenshotView = [[UIImageView alloc] initWithImage:[destinationView pw_imageSnapshot]];

    [self.progressBar setProgress:.5];

}

In the above code I just need to create two screenshots of views to use them later on. The problem is that i only see the last update (.5) when setting the progress to the progress bar. What is the proper way to do this update?

Answer 1

You can use the performSelectorInBackground:withObject: method in order to instantiate your heavy views. That method (the one that instantiates your views) will have to set your progress bar progress in the main thread.

So your code would look something like this:

- (void)viewDidAppear:(BOOL)animated
{
    [self performSelectorInBackground:@selector(instantiateHeavyViews) withObject:nil];
}

- (void)instantiateHeavyViews
{
    self.sourceScreenshotView = [[UIImageView alloc] initWithImage:[self.view pw_imageSnapshot]];
    [self performSelectorOnMainThread:@selector(updateMyProgressView:) withObject:[NSNumber numberWithFloat:0.3f] waitUntilDone:YES];

    SCLViewController *rvc = [[SCLViewController alloc] init];
    UIView *destinationView = rvc.view;
    destinationView.frame = CGRectMake(0, 0, kWidthLandscape, kHeightLandscape);

    self.destinationScreenshotView = [[UIImageView alloc] initWithImage:[destinationView pw_imageSnapshot]];

    [self performSelectorOnMainThread:@selector(updateMyProgressView:) withObject:[NSNumber numberWithFloat:0.5f] waitUntilDone:YES];
}

- (void)updateMyProgressView:(NSNumber *)progress
{
    [self.progressBar setProgress:[progress floatValue]];
}

Edit: of course, it won’t animate your progress bar (I don’t know if that is what you wanted). If you want it to move on while your views are being created, you should use a delegate to be notified of the progress, and this can be a bit harder. This way you would be able to update the progress bar every time the delegate is notified.