Wait for Xcode method to Finish - ios

Question

I am using generateCGImagesAsynchronouslyForTimes to make some images and save them to a NSMutableArray, now when the function generateCGImagesAsynchronouslyForTimes finishes I want to use the image in this array, how can I have the code I want to exectue after all the images have been generated to finish. I would just put it in the completionHandler code block, but I don't want it run multiple times I just want to run it once, after this method has finished.
EDIT
This is all inside - (BFTask *)createImage:(NSInteger)someParameter {
AVAssetImageGenerator *imageGenerator = [AVAssetImageGenerator assetImageGeneratorWithAsset:passsedAsset];
[imageGenerator generateCGImagesAsynchronouslyForTimes:times
completionHandler:^(CMTime requestedTime, CGImageRef image, CMTime actualTime,
AVAssetImageGeneratorResult result, NSError *error) {
if (result == AVAssetImageGeneratorSucceeded) {
UIImage *img = [UIImage imageWithCGImage:image];
NSData *imgData = UIImageJPEGRepresentation(img, 1.0);
UIImage *saveImage = [[UIImage alloc] initWithData:imgData];
[mutaleArray addObject:saveImage];
//I get Assigment to read only property error on line below
completionSource.task = saveImage;
}
]};
What should i be assigning that to?

Answer 1

The two approaches I would consider first are NSOperationQueue (you can detect when it's empty) or the easier choice of using the Bolts framework.
Bolts allows you to create an array of tasks that all run asynchronously and then once they're finished it goes on to the next bit.
Let me get a link...
Here you go... https://github.com/BoltsFramework
You can also get this through cocoapods which makes everything much easier.
An example of how bolts works...
At the moment you will have a function that creates an image asynchronously. Something like... - (UIImage *)createImage: (id)someParameter; well now you can do this...
- (BFTask *)createImage:(NSInteger)someParameter
{
BFTaskCompletionSource *completionSource = [BFTaskCompletionSource taskCompletionSource];
//create your image asynchronously and then set the result of the task
someAsyncMethodToCreateYourImageWithACompletionBlock...^(UIImage *createdImage){
// add the images here...
[self.imageArray addObject:createdImage];
// the result doesn't need to be the image it just informs
// that this one task is complete.
completionSource.result = createdImage;
}
return completionSource.task;
}
Now you have to run the tasks in parallel...
- (void)createAllTheImagesAsyncAndThenDoSomething
{
// create the empty image array here
self.imageArray = [NSMutableArray array];
NSMutableArray *tasks = [NSMutableArray array];
for (NSInteger i=0 ; i<100 ; ++i) {
// Start this creation immediately and add its task to the list.
[tasks addObject:[self createImage:i]];
}
// Return a new task that will be marked as completed when all of the created images are finished.
[[BFTask taskForCompletionOfAllTasks:tasks] continueWithBlock:^id(BFTask *task){
// this code will only run once all the images are created.
// in here self.imageArray is populated with all the images.
}
}

Answer 2

Assuming that generateCGImagesAsynchronouslyForTimes:completionHandler: calls its completion handlers sequentially (that seems reasonable, but the docs don't explicitly promise), then this is very simple. Just set a __block variable to the count of your times and decrement it once per completion. When it's zero, call your other function.
__block NSInteger count = [times count];
[imageGenerator generateCGImagesAsynchronouslyForTimes:times
completionHandler:^(CMTime requestedTime, CGImageRef image, CMTime actualTime,
AVAssetImageGeneratorResult result, NSError *error) {
... Do all the stuff ...
if (--count <= 0) {
finalize()
}
If generateCGImagesAsynchronouslyForTimes: actually does work in parallel and so might call the completion handlers in parallel, then you can handle all of this with dispatch groups.
dispatch_group_t group = dispatch_group_create();
//
// Enter the group once for each time
//
[times enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
dispatch_group_enter(group);
}];
//
// This local variable will be captured, so you don't need a property for it.
//
NSMutableArray *results = [NSMutableArray new];
//
// Register a block to fire when it's all done
//
dispatch_group_notify(group, dispatch_get_main_queue(), ^{
NSLog(#"Whatever you want to do when everything is done.");
NSLog(#"results is captured by this: %#", results);
});
AVAssetImageGenerator *imageGenerator = [AVAssetImageGenerator assetImageGeneratorWithAsset:nil];
[imageGenerator generateCGImagesAsynchronouslyForTimes:times
completionHandler:^(CMTime requestedTime, CGImageRef image, CMTime actualTime,
AVAssetImageGeneratorResult result, NSError *error)
{
if (result == AVAssetImageGeneratorSucceeded) {
//
// Create saveImage
//
id saveImage = #"";
//
// Update external things on a serial queue.
// You may use your own serial queue if you like.
//
dispatch_sync(dispatch_get_main_queue(), ^{
[results addObject:saveImage];
});
//
// Signal we're done
//
dispatch_group_leave(group);
}
}];