UiAlertView with UIProgressView for copying files locally

I've been trying to get this working for a day now and I'm still failing. I want to copy a number of files into the Documents folder of my app from the bundle when the app is installed, but this makes the user wait for a long time with the app showing the splash screen.

So I thought I'd make an initial UIAlertView with a UIProgressView as a subview that gets updated every time a file is copied into the documents folder. However the alert shows and the progress bar never gets updated. My logic was:

Set up the UIProgressView and UIAlertView as instance variables of my ViewController.
In ViewDidLoad, present the alert and set the delegate

In - (void)didPresentAlertView:(UIAlertView *)alertView perform a for loop that copies the files and updates the UI. the code was :

- (void)didPresentAlertView:(UIAlertView *)alertView{
    NSString *src, *path;
    src = // path to the Bundle folder where the docs are stored //
    NSArray *docs = [[NSFileManager defaultManager]contentsOfDirectoryAtPath:src error:nil];

    float total = (float)[docs count];
    float index = 1;

    for (NSString *filename in docs){
        path = [src stringByAppendingPathComponent:filename];
        if ([[NSFileManager defaultManager]fileExistsAtPath:path]) {
            ... // Copy files into documents folder
            [self performSelectorOnMainThread:@selector(changeUI:) withObject:[NSNumber numberWithFloat:index/total] waitUntilDone:YES];                
            index++;
        }
    }
[alertView dismissWithClickedButtonIndex:-1 animated:YES];
}

And the code for ChangeUI is

- (void) changeUI: (NSNumber*)value{
    NSLog(@"change ui %f", value.floatValue);
    [progressBar setProgress:value.floatValue];
}

However this just updates the UI from 0 to 1, although the NSLog prints all the intermediate values. Does anyone here know what I'm doing wrong?

Thanks in advance.

Solution

The problem is that your loop is on the main thread, and thus the UI has no chance to update until the very end. Try doing the work on a background thread using GCD:

dispatch_async(DISPATCH_QUEUE_PRIORITY_DEFAULT, ^
    {
        NSString *src, *path;
        src = // path to the Bundle folder where the docs are stored //
        NSArray *docs = [[NSFileManager defaultManager]contentsOfDirectoryAtPath:src error:nil];

        float total = (float)[docs count];
        float index = 1;

        for (NSString *filename in docs){
            path = [src stringByAppendingPathComponent:filename];
            if ([[NSFileManager defaultManager]fileExistsAtPath:path]) {
                ... // Copy files into documents folder
                dispatch_async(dispatch_get_main_queue(), ^{ [self changeUI:[NSNumber numberWithFloat:index/total]]; } );

                index++;
            }
        }
        dispatch_async(dispatch_get_main_queue(), ^{ [alertView dismissWithClickedButtonIndex:-1 animated:YES]; } );
    } );