在viewWillAppear之后无法以编程方式更改UILabel文本

Question

I have a routine in my iOS program that imports and manipulates a file from Dropbox. This can take some time (5-10 seconds) and it doesn't make sense to return the user to the normal UI while it's doing it, so I want to present a view letting the user know what the progress is.

From one VC, I use Dropbox's drop-in file picker, then load up a presented (modal VC) thus:

        ZSImportVC *importVC = [[ZSImportVC alloc] init];
        importVC.results = results;
        [importVC setModalPresentationStyle:UIModalPresentationFormSheet];
        [self presentViewController:importVC animated:YES completion:^{
            [self performFetch];
        }];

The VC (a bog-standard UIViewController), has a UILabel property:

@property (weak, nonatomic) IBOutlet UILabel *statusMessage;

In viewWillAppear: I can set the text of this label without any problem. The thing is, I want to keep changing this text as the process of manipulating the file continues.

The method that manipulates the file is called from viewDidAppear:

- (void)viewDidAppear:(BOOL)animated
{
    [super viewDidAppear:animated];
    [self processImport];
}

However, within the processImport method, the following has no effect:

self.statusMessage.text = @"Some text to update the user.";

So I created a method:

- (IBAction)updateStatus:(NSString *)message
{
    [self.statusMessage setText:message];
    NSLog(@"%@", message);
}

just to check what's going on. The NSLog shows that the method is being called okay, but the label text doesn't change. I tried adding:

[self.statusMessage setNeedsDisplay];

to the method, but that didn't help. I'm not using any private queues or background threads. I read somewhere that using NSNotification helps, so I tried adding this to viewDidLoad:

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(updateStatus:) name:@"updateStatus" object:nil];

Then changed the called method to:

- (void)updateStatus:(NSNotification *)message
{
    [self.statusMessage setText:message.object];
    NSLog(@"%@", message.object);
}

and called this from the main method with:

[[NSNotificationCenter defaultCenter] postNotificationName:@"updateStatus" object:@"Retrieving file from Dropbox" userInfo:nil];

I could see from the console messages that the updateStatus method is getting called, but still the text doesn't change. Clearly I'm missing something here. Any thoughts?

Answer 1

Check your ZSImportVC instances on Debug perspective.

I think u are sending [self.statusMessage setText:message.object]; to another instance, that it is not shown on the screen.

I mean,

put a debug point here:

- (void)viewDidAppear:(BOOL)animated
{
    [super viewDidAppear:animated];
    [self processImport];     /* Debug point here*/
}

and check what´s the hexadecimal direction for self .

Now , do the same here:

- (void)updateStatus:(NSNotification *)message
{
    [self.statusMessage setText:message.object];  /* Debug point here*/
    NSLog(@"%@", message.object);
}

Btw, I had some problems with

 [[NSNotificationCenter defaultCenter] postNotificationName:@"updateStatus" object:@"Retrieving file from Dropbox" userInfo:nil];

because the instance that receive this notification was not the correct.

I fixed that calling a normal method, but u maybe could not do that.

Sorry for my english :S