Nesting methods with completion blocks - objective-c

Question

I have several methods that have the following structure:
- (void) doSomethingWithCompletion: (void (^)(NSError *error)) completion {
__block NSError *fetchError = nil;
dispatch_group_t dispatchGroup = dispatch_group_create();
for (Item* item in self.items)
{
dispatch_group_enter(dispatchGroup);
// fetchError = fetch online data
}
dispatch_group_notify(dispatchGroup, dispatch_get_main_queue(),^{
if (completion)
completion(fetchError);
});
}
My goal is to run several doSomethings after each other, so I could so something like this:
[self doSomethingAWithCompletion: ^(NSArray *results NSError *error) {
if (error == nil) {
[self doSomethingBWithArray: results withCompletion: ^(NSError *error) {
if (error == nil) {
[self doSomethingCWithCompletion: ^(NSError *error) {
if (error == nil) {
// done!!
}
}];
}];
}];
What I am struggling with is the second code block (no pun); is nesting all the methods the way to go, or are there other solutions?
The important thing is, is that doSomethingBWithCompletion cannot begin before doSomethingAWithCompletion is done, and doSomethingCWithCompletion needs to wait until doSomethingBWithCompletion is complete, etc.
Also, doSomethingBWithCompletion uses data that is generated in doSomethingAWithCompletion, etc.
EDIT: After a lot of thinking, refactoring, and simplifying my code, I was able to end up with only two functions, using the nested approach as I outlined above and with a #property for the results array.

Answer 1

The important thing is, is that doSomethingBWithCompletion cannot begin before doSomethingAWithCompletion is done, and doSomethingCWithCompletion needs to wait until doSomethingBWithCompletion is complete, etc.
According to the comments:
The Results of the block are not depending on the result of the first aren't they?
And
Yes they are. For instance, in the first doSomething I determine which items are outdated, in the second doSomething I download and parse the updated items, and in the third doSomething I save them to the store.
(BTW: You should really add this information to your Q.)
If an action depends on the result (not only execution) of a previous action, you have to nest the blocks. Your code does not look like this, because there is no data passed to the completion blocks.
If you do not have such a dependency, you could use a private serial dispatch queue. However, this is a solution in your case, too, if you have akin of a manager class holding the data passed from block to block. But this seems to be highly anticonceptual.

Answer 2

There may be community attempt to add promises to objective-c, and it would be nice to have, because that's just what's needed here. Without committing to a whole new library, you can handle the nesting (which I agree is a bummer) by doing the async tasks recursively... something like this for your example code:
Start with an operation that takes no params and results in an array...
- (void)firstOpWithCompletion:(void (^)(NSArray *, NSError *))completion {
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(queue, ^{
NSArray *components = [#"this is an array of strings from the FIRST op" componentsSeparatedByString:#" "];
if (completion) {
completion(components, nil);
}
});
}
Here are a couple that take an array param and result in an array...
- (void)secondOpWithParam:(NSArray *)array completion:(void (^)(NSArray *, NSError *))completion {
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(queue, ^{
if (completion) {
NSArray *components = [#"these strings are from the SECOND op" componentsSeparatedByString:#" "];
NSArray *result = [array arrayByAddingObjectsFromArray:components];
if (completion) {
completion(result, nil);
}
}
});
}
- (void)thirdOpWithParam:(NSArray *)array completion:(void (^)(NSArray *, NSError *))completion {
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(queue, ^{
if (completion) {
NSArray *components = [#"these strings are from the THIRD op" componentsSeparatedByString:#" "];
NSArray *result = [array arrayByAddingObjectsFromArray:components];
if (completion) {
NSLog(#"we did it. returning %#", result);
completion(result, nil);
}
}
});
}
// ...as many as these as you need
Now, as in my answer prior to this edit, we just add a param pass initially and in the intermediate calls...
- (void)doSeveralThingsInSequence:(NSArray *)todo param:(NSArray *)param {
if (todo.count == 0) return;
// you could generalize further here, by passing a "final" block and run that before the return
NSString *nextTodo = todo[0];
SEL sel = NSSelectorFromString(nextTodo);
IMP imp = [self methodForSelector:sel];
void (*func)(id, SEL, NSArray *, void (^)(NSArray *, NSError *)) = (void *)imp;
func(self, sel, param, ^(NSArray *result, NSError *error) {
if (!error) {
NSArray *remainingTodo = [todo subarrayWithRange:NSMakeRange(1, todo.count-1)];
[self doSeveralThingsInSequence:remainingTodo param:result];
}
});
}
Stepping through the code: this method bails if there's nothing to do, otherwise it takes the next selector name from the passed array, gets the C function implementation for it and invokes it, placing a completion block on the call stack that starts the process over for the remaining selectors.
Finally, doEverything calls the first operation to get started, then starts running a list of operations (which can be an arbitrarily long list) passing the array output from one as the array input to the next. (You could generalize this further by passing id's along the chain
- (void)doEverything {
[self firstOpWithCompletion:^(NSArray *array, NSError *error) {
NSArray *todo = #[ #"secondOpWithParam:completion:", #"thirdOpWithParam:completion:" ];
[self doSeveralThingsInSequence:todo param:array];
}];
}
I tested this exactly as posted and saw the expected output:
(
this,
is,
an,
array,
of,
strings,
from,
the,
FIRST,
op,
these,
strings,
are,
from,
the,
SECOND,
op,
these,
strings,
are,
from,
the,
THIRD,
op
)