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Taming Callbacks in iOS: Bolts Framework

17 Dec 2014

Managing many asynchronous events in iOS can get hairy, but it’s a fact of life when working with remote services. So how do we avoid callback hell?

Promises.

Below I’ll explain why I chose the Bolts Framework (BF) over PromiseKit and ReactiveCocoa.

A naive way to handle asynchronous code in Swift is with standard blocks. Let’s say we need to change the orientation of a video, save a thumbnail, and then save the video for a post. We could end up with something like this:

videoAsset.fixOrientation({ (response: MFVideoAssetResponse!) -> Void in
    if(response.success) {
        let imageData = UIImageJPEGRepresentation(response.thumbnail, 1)
        let imageFile = PFFile(data: imageData, contentType: "image/jpg")

        imageFile.saveInBackgroundWithBlock({ (succeeded: Bool, error: NSError!) -> Void in
            if(succeeded) {
                let videoData = NSData.dataWithContentsOfMappedFile(response.url.path!) as NSData
                let videoFile = PFFile(data: videoData, contentType: "video/quicktime")

                videoFile.saveInBackgroundWithBlock({ (succeeded: Bool, error: NSError!) -> Void in
                    if(succeeded) {
                        post.saveEventually()
                    } else {
                        println("## \(error.description)") // Horrible Error Handler 1!
                    }
                })
            }
        })
    } else {
        println("## FAILED \(response.error.description)") // Horrible Error Handler 2!
    }
})

Here’s the callback pyramid of doom. If you’re unfamiliar with this, you can read more about it here (article is on javascript, but concept still applies).

We don’t want this, and promises are the way to fix it. But which framework should we use?

Reactive Cocoa?

  • Based on the principals of Functional Reactive Programming.
  • Required one to wrap everything with RAC types.
  • Focused on handling streams, not my primary use case.
  • When all I need are promises, RAC was too heavy and obtrusive for my tastes.

PromiseKit?

  • Inspired by Bolts.
  • Separate library for Objective C and Swift, so it can take advantage of Swift generics.
  • Comes with Objective C categories for many built in iOS types, like NSURLConnection.
  • At the time of this writing, it does not have a pod for the Swift library because of CocoaPod’s limitations.
  • Not straightforward when switching between Objective C and Swift.

Bolts Framework’s BFTask? Yes.

  • The simplest of the three.
  • Works seamlessly across Objective C and Swift.
  • Does not support Swift generics which can lead to some verbosity.
  • Supported natively by the Parse Framework, which is what I’m using.

I actually started with PromiseKit and just went to Bolts because I found it the simplest of the three. At first, I was all about the Swift support, but found myself going back and forth between Obj C and Swift, making Bolts a better match. As CocoaPods and Swift matures, I will definitely take another glance at PromiseKit. Right now, I’m extremely happy with BFTask:

class func createAsync(videoAsset: MFVideoAsset) -> BFTask! {
    return videoAsset.fixOrientation().continueWithSuccessBlock { (task) -> AnyObject! in
        let response: MFVideoAssetResponse! = task.result as MFVideoAssetResponse;
        println("## Video Orientation Fixed to \(response.url)")

        let imageData = UIImageJPEGRepresentation(response.thumbnail, 1)
        let imageFile = PFFile(data: imageData, contentType: "image/jpg")

        let videoData = NSData.dataWithContentsOfMappedFile(response.url.path!) as NSData
        let videoFile = PFFile(data: videoData, contentType: "video/quicktime")

        let parallelTasks: NSMutableArray = [imageFile.saveInBackground(), videoFile.saveInBackground()]
        return BFTask(forCompletionOfAllTasks:parallelTasks).continueWithSuccessBlock({(task)-> AnyObject! in
            var post = PFObject(className: "Post")
            post["type"] = "video"
            post["image"] = imageFile
            post["video"] = videoFile
            return post.saveEventuallyAsTask()
        })
    }
}

And better error handling code because in true promise fashion, the caller can receive the error at the end regardless of where it happens:

PostRepository.createAsync(videoAsset).continueWithBlock(task: BFTask!) -> AnyObject! {
    if(task.success) {
        var post = task.result as PFObject
        println(post)
    } else {
        // Better Error Handler!
        self.presentViewController(
            UIAlertControllerFactory.ok("Error Saving Post", message: task.error.description),
            animated: true,
            completion: nil)
    }
    return nil
}

Takeaway

  • Smash your iOS pyramids of doom with Bolts or PromiseKit.
  • Handle to streams of potentially infinite data with ReactiveCocoa.
  • See the source code for all these samples here

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