Question

I have been trying to play around with this for a long time and I can’t seem to find the best approach. I am getting confused because there seem to be different answers/opinions on how to accomplish this seemingly simple task.

I want to be able to have a reusable class called ActivityIndicatorController. This controller has two main methods: activateIndicator and deactivateIndicator. It takes a UIView as an argument/property as well as an NSString for a label. Upon activation, it will turn off user interaction in the UIView and add a rectangle subview (with alpha and rounded corners), a UIActivityIndicator control and a UILabel for the status text. This is desirable because that way I don’t have to have custom UIActivityIndicatorView code in each view controller or to have to set up an ActivityIndicator in each NIB.

The problem I am fundamentally having is how to kick off this process of adding and animating the ActivityIndicator. Some methods I have tried don’t display the new view at all. Others work, but the ActivityIndicator doesn’t animate.

I have tried using [NSThread detachNewThreadSelector:@selector(startAnimating) toTarget:activityIndicator withObject:nil] inside the activateIndicator method, but that doesn’t display the new UIView.

I have tried using [NSThread detachNewThreadSelector:@selector(activateIndicator) toTarget:activityIndicatorController withObject:nil] from a calling method, but this would put the whole creation of the new UIView in a separate thread.

Now to the question:

Part 1: I understand that all UI should be handled on the main thread, is that correct?

Part 2: What is the difference/advantage/disadvantage of using [NSThread detachThreadSelector] versus NSOperation?

Part 3: Is it better to:

(a) send the lengthy operation to a new background thread with a callback to the main thread OR

(b) send the UIActivityIndicatorView startAnimating method to a separate thread and run the lengthy process on the main thread

AND why?

Here is my current code:

ActivityViewController class:

-(void)activateIndicator {
NSLog(@"activateIndicator called");
if (isActivated || !delegateView)
    return;
NSLog(@"activateIndicator started");

[delegateView.view setUserInteractionEnabled:NO];
[delegateView.navigationController.view setUserInteractionEnabled:NO];
[delegateView.tabBarController.view setUserInteractionEnabled:NO];

float w = [[UIScreen mainScreen] bounds].size.width;
float h = [[UIScreen mainScreen] bounds].size.height;

NSLog(@"Width = %f\nHeight = %f", w, h);

if (!disabledView) {
    disabledView = [[[UIView alloc] initWithFrame:CGRectMake((w - kNormalWidth) / 2.0, (h - kNormalHeight) / 2.0, kNormalWidth, kNormalHeight)] autorelease];
    disabledView.center = [[[delegateView.view superview] superview] center];
    [disabledView setBackgroundColor:[UIColor colorWithRed:0.0 green:0.0 blue:0.0 alpha:0.85]];

CALayer *layer = [disabledView layer];
NSLog(@"layer=%@",layer);
NSLog(@"delegate=%@",[layer delegate]);
layer.cornerRadius = 12.0f;
}

if (!activityIndicator) {
    activityIndicator = [[[UIActivityIndicatorView alloc] initWithFrame:CGRectMake(kNormalWidth / 2, 10.0f, 40.0f, 40.0f)] autorelease];
    [activityIndicator setActivityIndicatorViewStyle:UIActivityIndicatorViewStyleWhiteLarge];
    activityIndicator.center = disabledView.center;
}

if (!activityLabel) {
    activityLabel = [[[UILabel alloc] initWithFrame:CGRectMake(10.0f, 100.0f, kNormalWidth - 20, 38)] autorelease];
    activityLabel.text = labelText;
    activityLabel.textAlignment = UITextAlignmentCenter;
    activityLabel.backgroundColor = [UIColor colorWithWhite:0.0f alpha:0.0f];
    activityLabel.textColor = [UIColor colorWithWhite:1.0f alpha:1.0f];
    activityLabel.center = disabledView.center;
}

[[[delegateView.view superview] superview] addSubview:disabledView];

[[[delegateView.view superview] superview] addSubview:activityIndicator];
[[[delegateView.view superview] superview] addSubview:activityLabel];


[NSThread detachNewThreadSelector:@selector(startAnimating) toTarget:activityIndicator withObject:nil];
}

Calling Code from multiple places in the app:

    ActivityIndicatorController *aic = [[ActivityIndicatorController alloc] init];
aic.delegateView = self;
aic.labelText = @"Test...";
[aic activateIndicator];

//DO LENGTHY WORK ON MAIN THREAD

[aic deactivateIndicator];
[aic release], aic = nil;

Answer 1

Part 1: I understand that all UI should be handled on the main thread, is that correct?

Correct.

Part 2: What is the difference/advantage/disadvantage of using [NSThread detachThreadSelector] versus NSOperation?

NSOperation is a higher-level interface that allows you to queue operations, create several operations that depend on each other, etc. Other options to work with tasks in the background are performSelectorOnMainThread:.../performSelectorInBackground:... and Grand Central Dispatch.

Part 3: Is it better to:

(a) send the lengthy operation to a new background thread with a callback to the main thread OR

(b) send the UIActivityIndicatorView startAnimating method to a separate thread and run the lengthy process on the main thread

Because of the answer to question 1, (a) is your only option.