Make UIView Appear Before Network Operation

Question

I have a seemingly simple problem that I cannot for the life of me seem to figure out. In my iOS App, I have a UICollectionView that triggers network operation upon tapping it that can take a few seconds to complete. While the information is being downloaded, I want to display a UIView that fills the cell with a UIActivityIndicatorView that sits in the square until the loading is done, and the segue triggered. The problem is that it never appears. Right now my code looks like:

myLoadView.hidden = NO;
//Network Operation
myLoadView.hidden = YES;

The App simply stops for a couple seconds, and then moves on the the next view. I'd imagine Grand Central Dispatch has somthing to do with the solution, however please keep in mind that this code takes place in prepareForSegue , and the network info needs to be passed to the next View. For this reason not finishing the download before switching scenes has an obvious problem. Any help would be VASTLY appreciated. Thanks!

Answer 1

iOS commits changes in the interfaces after working out a routine. Hence you should perform your network operation in a background thread and then get back back on the main and perform the "show my view now thing". Have a look the below code for reference.

myLoadView.hidden = NO;
dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{
    //Network Operation
    dispatch_async(dispatch_get_main_queue(), ^{
        myLoadView.hidden = YES;
    });
});

Answer 2

Your network operation seems to be carried out on the main thread, aka UI thread. This blocks all further UI calls, including the call to unhide a view, until completion.

To resolve this, make your call asynchronous.

You should read this in full, if you haven't already.

Answer 3

As mentioned by other answers, the problem is that the UIView change doesn't happen until the current method finishes running, which is where you are blocking. Before GCD was available I would split methods in two and use performSelector:withObject:afterDelay (to run the second part also on the UI loop) or performSelectorInBackground:withObject: at the end of the first method. This would commit all the waiting animaations first, then do the actual tasks in the second method.

Answer 4

对于这种指示，更好的选择是使用自定义HUD库，例如SVProgressHUD或MBProgressHUD