How can I use retry/polling in ObjC?

[thomers]

How would I write this attempt method from the docs in ObjC?

func attempt<T>(maximumRetryCount: Int = 3, delayBeforeRetry: DispatchTimeInterval = .seconds(2), _ body: @escaping () -> Promise<T>) -> Promise<T> {
    var attempts = 0
    func attempt() -> Promise<T> {
        attempts += 1
        return body().recover { error -> Promise<T> in
            guard attempts < maximumRetryCount else { throw error }
            return after(delayBeforeRetry).then(on: nil, attempt)
        }
    }
    return attempt()
}

attempt(maximumRetryCount: 3) {
    flakeyTask(parameters: foo)
}.then {
    //…
}.catch { _ in
    // we attempted three times but still failed
}

I have read the comments in #350 (comment) about catch behaving the same as recover, but I'm struggling with translating it into ObjC.

Thanks!

---

• Please specify the PromiseKit major version you are using

PromiseKit 4.5.2 (mainly because I'm not sure about changes to Obj-C support in later versions)

• Please specify how you installed PromiseKit

Cocoapods 1.5.3

[mxcl]

- (void)attempt:(int)maximumRetryCount, delay:(NSTimeInterval)delay provider:(PMKPromise* (^)()) {
    __block int attempts = 0;
    PMKPromise *(^attempt)() = ^{
        attempts++;
        return provide().catch(^(id error) {
            if (attempts >= maximmumRetryCount) @throw error;
            return PMKAfter(delay).then(attempt);
        })
    };
    return attempt()
} 

My objc is rusty so may not compile.

[mxcl]

mainly because I'm not sure about changes to Obj-C support in later versions

6 is the same as 4 for objc. No need to upgrade, but we don't typically back port fixes to v4.

[thomers]

Wow, that is some serious block syntax action going on.

I got it to compile with these modifications:

- (AnyPromise *) attempt:(NSUInteger) maximumRetryCount delayBeforeRetry:(NSTimeInterval) delay provider:(AnyPromise* (^)()) provide {
	__block NSUInteger attempts = 0;
	AnyPromise *(^attempt)() = ^{
		attempts++;
		return provide().catch(^(id error) {
			if (attempts >= maximumRetryCount) @throw error;
			return PMKAfter(delay).then(attempt);
		});
	};
	return attempt();
}

Does this look right?

I got these warnings:

On the empty () in the method signature provider:(AnyPromise* (^)()) provide {
"This block declaration is not a prototype."

On AnyPromise *(^attempt)() = ^{ :
"Block pointer variable 'attempt' is unintialized when captured by block."

EDIT: Also, how do I call the attempt method correctly with this method:

- (AnyPromise *) fetchIdentityId {
	return [AnyPromise promiseWithResolverBlock:^(PMKResolver resolve) {
		[[[AWSMobileClientHelper sharedAWSMobileClient] getIdentityId] continueWithBlock:^id _Nullable(AWSTask<NSString *> * _Nonnull task) {
			if(!task.error){
				resolve(nil);
			} else {
				resolve(task.error);
			}
			return nil;
		}];
	}];
}

Thanks so much for your help and your work on such an awesome library!

[mxcl]

[self attempt:3 delayBeforeRetry:2 provider:^{ return [self fetchIdentityId]; }]

[mxcl]

Like you could probably use a selector rather than a block. It's been a long time since I did any objc.

[thomers]

Perfect! Again, thank you!