Wait for result without stopping UI - iphone

Question

I have an iPad app, and I want to do this
-(IBAction) clicked {
image=download(#"http://....."); // this is on the main thread
}
The download function is going to call a whole bunch of non blocking functions to download a file from the internet, but download itself shouldn't return until the image is downloaded.
While the program is waiting for the download at the image=download(...) line above, I want the UI to be able to still function, for example be able to scroll a UITableView, click another button etc.
So what I did was this inside the download function I used a RunLoop
-(void) download:(NSString *)url
{
BOOL stillDownloading=TRUE;
while(stillDownloading) {
stillDownloading=downloadAFwBytes(...);
CFRunLoopRunInMode(kCFRunLoopCommonModes, 0, YES);
}
}
I thought the CFRunLoopRunInMode function will keep pumping UI messages, touches, scrolls through the main UI thread so that the UI will keep working and not freeze until the download finished, but for some reason, it only works for a short time, and eventually the UI freezes.
Do you know why, or how to fix?
The download function is called everywhere in the program, that expects it to wait for the download, so I can't change it to non blocking at the moment.

Answer 1

The direct answer to your question is, no, this is not what CFRunLoopRunInMode does. What you are effectively trying to do is have the current run loop "yield" so execution can continue while the loading operation continues. This is not how iOS and run loops work. Your download function blocks the thread it is on until downloading is complete so the only solution to your issue is to change the implementation so that downloading occurs on a background thread and the objects that care are notified when it is complete. Here's a relatively small change that can get you on the right track. This overall topic (concurrency, managing background tasks) is a bigger discussion and there are different considerations/tradeoffs. I'll cut to the chase and hopefully get you on the right track.
Define a couple NSNotification's that your download method can post for interested objects to observe:
// in the .h file of the class performing the download
extern NSString * const MyClassLoadingDidStartNotification;
extern NSString * const MyClassLoadingDidFinishNotification;
// in the .m file of the class performing the download
NSString * const MyClassLoadingDidStartNotification = #"MyClassLoadingDidStart";
NSString * const MyClassLoadingDidFinishNotification = #"MyClassLoadingDidFinish";
In your download routine, do the download in the background and post the appropriate notifications:
-(void) download:(NSString *)url
{
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
[[NSNotificationCenter defaultCenter] postNotificationName:MyClassLoadingDidStartNotification object:self];
BOOL stillDownloading=TRUE;
while(stillDownloading) {
stillDownloading=downloadAFwBytes(...);
}
[[NSNotificationCenter defaultCenter] postNotificationName:MyClassLoadingDidFinishNotification object:self];
});
}
In any object that initiates a download, observe and handle the notifications
// in any class that initiates a download
- (void)init...
{
self = [super init...];
if (self) {
// other initialization
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(didStartLoading:) name:MyClassLoadingDidStartNotification object:nil];
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(didFinishLoading:) name:MyClassLoadingDidFinishNotification object:nil];
}
return self;
}
- (void)dealloc
{
[[NSNotificationCenter defaultCenter] removeObserver:self name:MyClassLoadingDidStartNotification object:nil];
[[NSNotificationCenter defaultCenter] removeObserver:self name:MyClassLoadingDidFinishNotification object:nil];
}
- (void)didStartLoading:(NSNotification *)notification
{
// update UI to show loading status (make sure you do UI changes on main thread)
// optionally check notification.object to ensure it's the loader class instance you care about
}
- (void)didFinishLoading:(NSNotification *)notification
{
// update UI to show loading status (make sure you do UI changes on main thread)
// optionally check notification.object to ensure it's the loader class instance you care about
}
Keep in mind that this is a very basic starting point. As you learn more and decide what you need you will definitely customize it. For example, you may want to add error handling, limit concurrent loading operations, provide other loading status, etc.

Answer 2

There are some architecture concerns with your question, but to properly download an image in the background and load it into a UIImageView or use it in any other way, I'd suggest taking a look at AFNetworking and read through their sample code for downloading resources.

Answer 3

Can you request to download the image on the background and then, return a static image asap in your method?
This way, the main thread returns fast and depending on your architecture, your background thread can update the image once it got it?
I think we need more details on your code (your download method is void so how is the downloaded data used?) to be able to really help.

Answer 4

Along with the 3 other guys telling you the same thing, I'll tell you to forget about your own run loop model, and just use the async download capabilities (assuming you're downloading over the net). You don't even have to build a thread, just start the async downloads and they will tell you when they're done. If you don't have the time to code it right, when will you have the time to fix your code?

Answer 5

Not sure if this is solved yet but can't you use?
[self.operationQueue addOperationWithBlock:^{
[self MethodName];
}];
and when you want something to happen in the UI like table updates then put this in the method code:
[[NSOperationQueue mainQueue] addOperationWithBlock:^{
////perform something here ////
}];
hope this helps.