Passing blocks between view controllers - ios

Question

Iv searched a lot for this and cant find an answer to my specific question. But basically my question is, can i pass a completion block into another view controller and redefine it in the new view controller.
So for example in view controller A i have a method to perform a download with a completion block. First i create my block property in view controller A.
#property (copy)void (^downloadCompleteBlock)(NSArray *downloadItems);
I tried changing this to strong as opposed to copy but this did not solve my problem.
Then i define the completion block as follows.
self.downloadCompleteBlock = ^(NSArray *downloadItems) {
NSLOG(#"download complete in view controller A";
};
Then i call my download method passing in this completion block.
[self download:self.downloadCompleteBlock];
However, if this completion handler is not called by the time i leave this view controller (if the download isn't complete) I would want the completion block to perfrom something different on the next view controller. So in my prepare for segue i attempted to pass in this block to view controller B.
[controllerB setCompletionBlock:self.downloadCompleteBlock];
And this method in view controller B then redefines what happens when this completion block gets called.
- (void)setCompletionBlock:(void(^)(NSArray *downloadItems))downloadFinishedBlock {
downloadFinishedBlock = ^(NSArray *downloadItems) {
self.collectionData = downloadItems;
[self.collectionView reloadData];
};
}
However, the original block in view controller a still gets called when the download finishes as opposed to the block in view controller B. Anyone know how to have the completion block in view controller B called if that view is loaded before the download completes? I know i could use a notifier but i'm curious if i can do this with blocks.
Thanks

Answer 1

This is kind of a tough problem. At its heart is the problem of how to keep the block around after the first view controller goes away. Your current code solves that problem unwittingly by having the block refer to self. The vc is retained by that reference, which is good news if it needs to be around when the request finishes, but it's bad news because now the vc and the block will retain each other forever. (Google 'retain cycle'.)
So how do we get a long-running process that runs a block on completion and might outlive two or more view controllers? For starters, break that process into its own object. The interface of that object would look like:
#interface DownloadThingy
#property (copy)void (^downloadCompleteBlock)(NSArray *); // note, no need for dummy param names here
- (id)initWithRequestParams:(id)whateverIsNeededToStart;
- (void)start;
#end
Now, the view controller that want to start this can declare a strong property to it, create one, give it a completion block (see below**), and start it. When it's time for a segue, it can pass the downloadThingy to another vc, who can give it a different completion block.
** Since the request object is being kept as a property in one or more vcs, and since it retains the block, you still need to look out for a retain cycle:
(vc->downloadThingy->block->vc)
In VcA, do this:
- (void)startADownloadThingy {
self.downloadThingy = [[DownloadThingy alloc] initWithRequestParams:someParams];
__weak VcA *weakSelf = self;
self.downloadThingy.downloadCompleteBlock = ^(NSArray *downloadItems) {
// don't use self in here, use weakSelf
}
}
VcB will get called on the segue; it might or might not need to follow the same precaution. The distinction is whether this second vc retains a downloadThingy property. If it doesn't plan to hand it off to any other vc, it can skip the property, and thereby skip the worry about a retain cycle.
// another vc is handing off a running downloadThingy
- (void)heresARunningDownloadThingy:(DownloadThingy *)downloadThingy {
// if we have our own property, then
self.downloadThingy = downloadThingy;
// and we need to do the weakSelf trick
__weak VcA *weakSelf = self;
self.downloadThingy.downloadCompleteBlock = ^(NSArray *downloadItems) {
// don't use self in here, use weakSelf
}
}
Or...
// another vc is handing off a running downloadThingy
- (void)heresARunningDownloadThingy:(DownloadThingy *)downloadThingy {
// we do not have our own property
downloadThingy.downloadCompleteBlock = ^(NSArray *downloadItems) {
// feel free to use self in here
}
}
One last thing: it's a good practice for the DownloadThingy to aggressively nil out its block after it's through invoking it. So when the request is done, have it do this...
// DownloadThingy.m
// request is complete
self.downloadCompleteBlock(arrayFullOfResults);
self.downloadCompleteBlock = nil;