异步 function 执行？

Question

in my iOS app I do the following.

viewDidAppear(){

   // Load a spinner in a view on the top
   [DSBezelActivityView newActivityViewForView:self.view]; 
   // Execute code that require 3 seconds
   ...
   // Stop the spinner
   [DSBezelActivityView removeViewAnimated:YES];
}

The problem is that the spinner doesn't appear, because the the cpu is working hard (something similar). It's like that the code betweek the start and stop has precedence on the rendering of the view.

I would love to find a way to show effectively the start of the spinner, without using a timer to delay the code execution.

Thanks

Answer 1

If you have a method like

-(void) showSpinner:(UIView*)view {
    dispatch_async(dispatch_get_main_queue(), ^{
        [DSBezelActivityView newActivityViewForView:view];
    });
}

there are several ways to call it from a different thread. Choose one from the following:

[NSThread detachNewThreadSelector:@selector(showSpinner:) toTarget:self withObject:self.view];
// or 
[self performSelectorInBackground:@selector(showSpinner:) withObject:self.view];
// or 
NSInvocationOperation *invOperation = [[NSInvocationOperation alloc] initWithTarget:self selector:@selector(showSpinner:) object:self.view];
NSOperationQueue *opQueue = [[NSOperationQueue alloc] init];
[opQueue addOperation:invOperation];
// or 
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    [self showSpinner:self.view];
});

Alt + click for details.

Answer 2

Move code between start and stop activity indicator into separate thread because it's blocking main thread. That's why activity indicator is not showing.

Edit: Example

Answer 3

I agree with the 1st answer with a couple of modifications. I just went through this exact same problem. The problem is that anything graphical is going to automatically move to the background updating when you have code that takes time to get through. Throwing the spinner to the background is what it essantially doing anyway. What you want is (sadly) for you main code to run in the background and the spinner to run in the foreground. I know this sounds bad, but in some cases allowing your code to run a bit slower to give indication that the app is doing something useful is beneficial to the user.

In order to get the spinner to work: 1) Take all the code that takes the 3 seconds to run, and put that into a function that is a void function 2) Instantiate your spinner but store it to a variable that is accessible outside your viewDidAppear routine. 3) Startup a new NSTimer with that runs continuously with an increment of about every quarter second or so. I will define what goes into the routine that gets called every cycle later. 4) Call the routine you created in step 1 using the performSelectorInBackground capability. This essentially is now going to run your startup (3 seconds worth) in the background which is really the only way to allow the animated spinner to show up and truly animate. 5) In the routine you created in step 1, add a line of code right at the top that updates a (global to the object) boolean to true, stating that we are in the middle of our main 3 second routine. 6) At the end of the routine defined in step 1 add a line of code setting the same global defined in step 5 to false indicating that our 3 second routine is completed. 7) In the timer routine, we now want to do something that looks like the following:

// If busy that start the spinner
if(YES == busy){
    [spinner startAnimating];
}else{
    [spinner stopAnimating];

    // Here we can also stop and deallocate the timer
}

If you need more aid on this subject, I can indeed provide exact code. Take a look at the example app that I have developed for the Pepperdine News Group. When you press a button, the spinner comes up on the top right of the screen.

http://itunes.apple.com/us/app/pepperdine-graphic-for-iphone/id516343215?mt=8