Managing a view while another process runs on iPhone

Question

Hello,

I am having a bit of a problem here, I am let the user upload some video to the server however I am having some difficulties managing a view that I am using to illustrate its progress, I know why the problem is happening and i found a way around it (sort of) here my problem

So if one tries to make some code that looks something like this (in a UIViewController

-(void)uploadMovie
{
   UIActivityView indicator=new...
  [self.view addSubview:indicator]
   [uploader UploadMyMovie:data]

 }

This code wont work, the uploader will lock the controller and will not allow time for the indicator to come on screen in time, i found waiting for a few seconds before calling the uploader works but i took another approach.

The approach was to start a new thread for the uploader and have a protocol in which the uploader object informs the UIViewController (or some delegate) when it starts uploading, its progress, and when it finishes uploading. Something like

     -(void)uploadMovie
    {
       UIActivityView indicator=new...
      [self.view addSubview:indicator]
       NSThread *thread=...
       [thread start]
     }

the delegate methods look something like this

    #pragma mark UploadProgressDelegate

-(void)didStartUploading
{

    progressLabel= [[UILabel alloc] initWithFrame:CGRectMake(93, 240, 116, 32)];
    ind= [[UIActivityIndicatorView alloc]   initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleWhiteLarge];
    ind.center=self.view.center;
    [progressLabel setTextColor:[UIColor blackColor]];
    [progressLabel setText:@"TEST"];
    [self.view addSubview:progressLabel];
    [self.view addSubview:ind];
    [ind startAnimating];
    [self.view bringSubviewToFront:progressLabel];
    [ind setHidesWhenStopped:TRUE]; 
    [self.view setUserInteractionEnabled:FALSE];
}
-(void)progressUpdate:(NSString*)progress
{
     [progressLabel setText:progress];
}
-(void)didEndUploading;
{
    [progressLabel removeFromSuperview];
    [ind stopAnimating];
    [ind removeFromSuperview];
    [progressLabel release];
    [ind release];
    [self.view setUserInteractionEnabled:TRUE];
}

This works great and the indicator shows and everything, then i decided to let the user see the progress by adding a UILabel (reflected in the code above), howeverr for this the solution does not work, the label does not display and ofcourse no updates...

I was wondering if anyone has encounter this situation and has come up with a solution for it? or if you can possibly see from the code above why the label isnt showing...

Thanks

Answer 1

in some cases, I've found that I need to go back to the main thread to do certain things...

so in your delegate methods you'd do

[self performSelectorOnMainThread: @selector(updateLabel:) withObject:newLabelText waitUntilDone:NO];

and then

- (void) updateLabel:(NSString *)newLabelText
{
    [progressLabel setText:newLabelText];
}

I'm not sure what the rules are for what things have to be done on the main thread rather than in the background, though.

Answer 2

UIKit is not thread-safe. If you're updating a UI element, you need to sync back into the main thread or all bets are off.