Unfortunately no, this (probably) isn't safe:

RACCommand.executing is documented to send the initial value on its first subscription. If it is NO, then you'd try to dispose of the disposable, but it'd still be nil since -subscribeNext: hasn't returned. You could solve this by using a RACSerialDisposable:

- (void)viewDidAppear:(BOOL)animated {
   RACDisposable *disposable = [Singleton.command.executing subscribeNext:^(id x) {
        BOOL isExecuting = [x boolValue];
        if (isExecuting) {
            // The command is currently executing.
            // Display progress indicator.
        } else {
            // The command has finished executing.
            // Remove progress indicator (if shown) and do something with the command's data.

            // Unsubscribe from the command's executing signal.
            [serialDisposable dispose];
            serialDisposable = nil;
        }
    }

    serialDisposable.disposable = disposable;
}

Ah, ok... so, to be sure I understand, serialDisposable = disposable outside of the block allows the subscribeNext block to finish before disposing of the subscription. Is that correct?

I didn't know about the RACSerialDisposable class. In your example above, how/where is serialDisposable declared?

Thanks!

RACSerialDisposable will dispose of disposable if the serial disposable has been disposed of. So it's eliminating the potential ordering problems with the __block disposable.

serialDisposable should be a property on the class.

Gotcha. Thanks!!

@joshaber I think you meant serialDisposable.disposable = disposable at the end there.

I did indeed! That's important. Thanks @Coneko.

If I'm understanding correctly, you could get the same effect with:

[[[[Singleton.command.executing
    ignore:@NO]
    doNext:^(id _) {
        // The command is currently executing.
        // Display progress indicator.
    }]
    takeUntil:[Singleton.command.executing ignore:@YES]]
    subscribeCompleted:^{
        // The command has finished executing.
        // Remove progress indicator (if shown) and do something with the command's data.
    }];

This avoids any need to manage disposables by hand.

Brilliant! This works perfectly in my scenario.

I am kicking off the Singleton's RACCommand in a table view controller's viewDidLoad method to download data (if needed). By the time viewDidAppear is called, if the command has not yet finished, then I want to display an indeterminate progress HUD. Once the data is fetched, then I remove the HUD and reload the table view.

At this point, the table view controller's UIRefreshControl can be used to reload the data, at which point I do not want the progress HUD to be displayed again, because the refresh control is visible while the data is refreshed. (I tried displaying the UIRefreshControl manually, rather than using a custom HUD indicator, but I encountered strange side effects.)

The RACCommand and data are stored in a Singleton because it is shared between multiple view controllers.

I guess I should have explained all that to begin with... Sorry!