[Question] RetryOperation paramters

[woodydaniel]

I'm really confused about the retryOperation retry block parameter. Is there an example someone could give me or point me to. My current usage of the retryOperation doesn't require the retry block handler - what would I pass through in this case. Would also be great to know what to pass through if I did want to utilise the retry handler.

Thanks

[danthorpe]

Hi @woodydaniel - sure, I can help.

The retry block is not technically optional, however it can be given a default value of "pass through" like this: = { $1 } - not terribly obvious I know.

There are not yet any example of RetryOperation in usage beyond CloudKitOperation. But, if you read the docs in the source you can see how it can be initialised. Note that there are a number of different ways to initialise RetryOperation and RepeatedOperation all of which are subclass-able.

So, I suggest you subclass it to provide your own initialisation API.

The retry block can be used to do a few things

1. Custom logging or reporting of failures
2. Adjusting how the "suggested" retry operation is configured
3. Return nil to stop attempting retry and instead finish with an error

If you do not want to do either 1, or 2, then you can pass through the "suggested" retry payload (includes the next() instance from the operation generator, and an optional Delay). So, the retry block function has this signature:

public typealias Handler = (RetryFailureInfo<T>, Payload) -> Payload?

which means, just return the 2nd argument to "pass through": = { $1 }.

But what is this Delay all about - well, the generators which are used to construct RetryOperation are a delay generator and the operation generator. The delay generator can be used to optionally add delays between retries. This is handy for exponential back-off, random delays etc. There is built in support for creating these. See WaitStrategy.

Lastly, there is the concept of does this retry forever? - which can be solved in a number of ways.

1. The operation generator (i.e. the generator which creates new NSOperation instances) can return nil - this means no more retries.
2. The retry block can return nil
3. The maxCount argument can be provided - this is the maximum number of retry attempts to make before finishing with errors.

[woodydaniel]

@danthorpe thanks for the quick response and detailed explanation.
Unfortunately I'm still fairly new at Swift dev and am struggling a bit with one or two things mentioned, specifically - still getting the retry parameter correct. If you wouldn;t mind seeing below and commenting, hopefully I'll get it right after this:

Currently I have a class PostAttachmentOperation of type Operation which uploads an attachment.
For my RetryOperation I have this:

RetryOperation(
maxCount: 3, 
strategy: WaitStrategy.Fixed(3), 
postAttachmentOperation, 
retry: { (RetryFailureInfo<NSOperation>, RepeatedPayload<NSOperation>) ->     
RepeatedPayload<NSOperation>? in
    //retrun something here
 })

I understand what you said about passing the second argument - the retry payload - through, I'm just stuck putting that into code.

I saw in the RetyOoperation class you gave this example:

let operation = RetryOperation(           
maxCount: 3,
strategy: .Random((0.1, 1.0)),
generator: MyOperationGenerator()
 ) { info, delay, op in
     // inspect failure info
     return (delay, op)
 }

I tried to reconcile that example with mine but with no success.
Apologies again if I'm not seeing something really obvious, your exaplnaiton above was really succinct and well explained.

[danthorpe]

Yep, okay, so in the header docs - there is a mistake, the delay, op is now a single argument (a tuple). So you need to do this:

RetryOperation(maxCount: 3, strategy: .Fixed(3), postAttachmentOperation, retry: { $1 })

Except... that your postAttachmentOperation looks as though it is an operation, not a generator of operations. NSOperation is a "one shot" class, you cannot retry it as it. Instead we use a GeneratorType to create a new instance when we want to do a retry.

Therefore you need to do something like this:

let retry = RetryOperation(maxCount: 3, strategy: .Fixed(3), AnyGenerator { PostAttachmentOperation() }, retry: { $1 })

But obviously you might want to configure the PostAttachmentOperation - so you could either have a function of this type: () -> PostAttachmentOperation? which you can pass directly into AnyGenerator. Or you do the configuration afterwards using the mechanics of RetryOperation and RepeatedOperation

retry.addConfigureBlock { postAttachmentOperation in
    postAttachmentOperation.post = thePost
   // etc
}

This block gets run on every new operation which is retried.

[woodydaniel]

Thanks @danthorpe, this makes a huge amount of sense. I'm going to look in to the AnyGenreator struct as i'm not familiar with it but overall your last explanation was awesome!
Thank you

[danthorpe]

Hi, @woodydaniel I'm going to close this ticket now - feel free to to re-open it or a create a new issue if you have any follow up question. 😀

[woodydaniel]

Sure @danthorpe thanks for all the help!