Question

In Grand Central Dispatch I want to start a spinner – UIActivityIndicatorView – spinning prior to beginning long running task:

dispatch_async(cloudQueue, ^{

    dispatch_async(dispatch_get_main_queue(),
            ^{
                [self spinnerSpin:YES];
            });

    [self performLongRunningTask];

    dispatch_async(dispatch_get_main_queue(),
            ^{
                [self spinnerSpin:NO];
             });


});

Here is the spinnerSpin method:

- (void)spinnerSpin:(BOOL)spin {

    ALog(@"spinner %@", (YES == spin) ? @"spin" : @"stop");

    if (spin == [self.spinner isAnimating]) return;

    if (YES == spin) {

        self.hidden = NO;
        [self.spinner startAnimating];
    } else {

        [self.spinner stopAnimating];
        self.hidden = YES;
    }

}

One thing I have never seen discussed is the difference – if any – between [myView setNeedsDisplay] and [myActivityIndicatorView startAnimating]. Do they behave the same?

Thanks,
Doug

Answer 1

The [UIView setNeedsDisplay] method has nothing to do with a UIActivityIndicatorView‘s animation state.

setNeedsDisplay simply informs the system that this view’s state has changed in a way that invalidates its currently drawn representation. In other words, it asks the system to invoke that view’s drawRect method on the next drawing cycle.

You very rarely need to invoke setNeedsDisplay from outside of a view, from code that is consuming the view. This method is meant to be invoked by the view’s internal logic code, whenever something changes in its internal state that requires a redraw of the view.

The [UIActivityIndicatorView startAnimating] method is specific to the UIActivityIndicatorView class and simply asks the indicator to start animating (e.g. spinning). This method is instant, without requiring you to call any other method.

On a side note, you could simplify your code by simply calling startAnimating or stopAnimating without manually showing/hiding it. The UIActivityIndicatorView class has a hidesWhenStopped boolean property that defaults to YES, which means that the spinner will show itself as soon as it starts animating, and hide itself when it stops animating.

So your spinnerSpin: method could be refactored like this (as long as you haven’t set the hidesWhenStopped property to NO):

- (void)spinnerSpin:(BOOL)spin {
    if (YES == spin) {
        [self.spinner startAnimating];
    } else {
        [self.spinner stopAnimating];
    }
}