Set UILabel text within code block

问题

I have a code block for my NSURLConnection sendAsynchronousRequest, and when I try to set a UILabel's text, the text is never set, even though the values are there. Here's my code:

    NSString *address = [addresses objectAtIndex:indexPath.row];
    NSURLRequest *urlRequest = [NSURLRequest requestWithURL:[NSURL URLWithString:[NSString stringWithFormat:@"http://myurl.com/%@", address]]];
    [NSURLConnection sendAsynchronousRequest:urlRequest queue:[[NSOperationQueue alloc] init] completionHandler:^(NSURLResponse *response, NSData *data, NSError *error) {
        if ([data length] > 0 && error == nil)
        {
            NSString *dataOfData = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
            if(![dataOfData isEqualToString:@"ERROR: address invalid"]) {
                [balanceLabel setText:[NSString stringWithFormat:@"Balance: %@", dataOfData]];
                if(data) {
                    qrCodeButton.alpha = 1;
                }
            } else {
                errorLabel.text = @"This address is invalid.";
            }
        }
        else if ([data length] == 0 && error == nil)
        {
            NSLog(@"Nothing was downloaded.");
            [balanceLabel setText:@"Server Error, Please Try Again"];
        }
        else if (error != nil){
            NSLog(@"Error = %@", error);
        }
    }];

Why is the UILabel's text never set? Is there a limitation to code blocks? If so, how would I fix my problem? Cheers!

回答1:

It is because an NSOperationQueue is not the main thread. What you're doing is illegal. And the sad thing is that there is no need for it! Just say:

[NSURLConnection sendAsynchronousRequest:urlRequest 
     queue:[NSOperationQueue mainQueue]
     completionHandler:^(NSURLResponse *response, NSData *data, NSError *error) {
        // ... and the rest exactly as you have it now

All fixed. Your request is asynchronous on a background thread, but when it comes back to you on the completion handler, you'll be on the main thread and able to talk to the interface etc.

回答2:

Your code operates UI element should execute on main thread.

dispatch_async(dispatch_get_main_queue(), ^{
    if ([data length] > 0 && error == nil)
    {
        NSString *dataOfData = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
        if(![dataOfData isEqualToString:@"ERROR: address invalid"]) {
            [balanceLabel setText:[NSString stringWithFormat:@"Balance: %@", dataOfData]];
            if(data) {
                qrCodeButton.alpha = 1;
            }
        } else {
            errorLabel.text = @"This address is invalid.";
        }
    }
    else if ([data length] == 0 && error == nil)
    {
        NSLog(@"Nothing was downloaded.");
        [balanceLabel setText:@"Server Error, Please Try Again"];
    }
    else if (error != nil){
        NSLog(@"Error = %@", error);
    }
}) ;

Make sure errorLabel is not nil and the UILabel is visible (It is added in the view hierarchy and its frame is appropriate).

来源：https://stackoverflow.com/questions/21769545/set-uilabel-text-within-code-block

标签

ios

objective-c

nsurlrequest