Bolts in Swift

Bolts is a collection of low-level libraries designed to make developing mobile apps easier. Bolts was designed by Parse and Facebook for our own internal use, and we have decided to open source these libraries to make them available to others.

Tasks

Bolts Tasks is a complete implementation of futures/promises for iOS/OS X/watchOS/tvOS and any platform that supports Swift. A task represents the result of an asynchronous operation, which typically would be returned from a function. In addition to being able to have different states completed/faulted/cancelled they provide these benefits:

• Tasks consume fewer system resources, since they don't occupy a thread while waiting on other Tasks.
• Tasks could be performed/chained in a row which will not create nested "pyramid" code as you would get when using only callbacks.
• Tasks are fully composable, allowing you to perform branching, parallelism, and complex error handling, without the spaghetti code of having many named callbacks.
• Tasks allow you to arrange code in the order that it executes, rather than having to split your logic across scattered callback functions.
• Tasks don't depend on any particular threading model. So you can use concepts like operation queues/dispatch queues or even thread executors.
• Tasks could be used synchronously or asynchronously, providing the same benefit of different results of any function/operation.

Getting Started

• CocoaPods

Add the following line to your Podfile:

pod 'Bolts-Swift'

Run pod install, and you should now have the latest parse release.

• Carthage

Add the following line to your Cartfile:

github "BoltsFramework/Bolts-Swift"

Run carthage update, and you should now have the latest version of Bolts in your Carthage folder.

• Using Bolts as a sub-project

  You can also include parse as a subproject inside of your application if you'd prefer, although we do not recommend this, as it will increase your indexing time significantly. To do so, just drag and drop the BoltsSwift.xcodeproj file into your workspace.

• Import Bolts

  Now that you have the framework linked to your application - add the folowing line in every .swift that you want to use Bolts from:

  import BoltsSwift
  

Chaining Tasks

Every Task has a function named continueWith(), which takes a continuation closure. A continuation will be executed when the task is complete. You can the inspect the task to check if it was successful and to get its result.

save(object).continueWith { task in 
  if task.cancelled {
    // Save was cancelled
  } else if task.faulted {
    // Save failed
  } else {
    // Object was successfully saved
    let result = task.result
  }
}

In many cases, you only want to do more work if the previous task was successful, and propagate any error or cancellation to be dealt with later. To do this, use continueOnSuccessWith function:

save(object).continueOnSuccessWith { task in 
  // The object was saved succesfully
  let result = task.result
}

If you return any object from continueWith function - this will become a result of the new function. And if you want to call into more tasks and return those results - you can use continueWithTask. This gives you an ability to chain more asynchronous work together. In the following example we want to fetch a user profile, then fetch a profile image, and if any of these operations failed - we still want to display an placeholder image:

fetchProfile(user).continueOnSuccessWithTask { task in 
  return fetchProfileImage(task.result);
}.continueWith { task in 
  if let image = task.result {
    return image
  }
  return ProfileImagePlaceholder()
}

Creating Tasks

To create a task - you would need a TaskCompletionSource, which is a consumer end of any Task, which gives you an ability to control whether the task is completed/faulted or cancelled. After you create a TaskCompletionSource, you need to call setResult()/setError()/cancel() to trigger its continuations and change its state.

func fetch(object: PFObject) -> Task<PFObject> {
  let taskCompletionSource = TaskCompletionSource<PFObject>()
  object.fetchInBackgroundWithBlock() { (object: PFObject?, error: NSError?) in
    if let error = error {
      taskCompletionSource.setError(error)
    } else if let object = object {
      taskCompletionSource.setResult(object)
    } else {
      taskCompletionSource.cancel()
    }
  }
  return taskCompletionSource.task
}

Tasks in Parallel

You can also perform several tasks in parallel and chain the result of all of them using whenAll() function.

let query = PFQuery(className: "Comments")
find(query).continueWithTask { task in
  var tasks: [Task<PFObject>] = []
  task.result?.forEach { comment in
    tasks.append(self.deleteComment(comment))
  }
  return Task.whenAll(tasks)
}.continueOnSuccessWith { task in
  // All comments were deleted
}

Task Executors

Both continueWith() and continueWithTask() functions accept an optional executor parameter. These allow you to control how the continuation is executed. The default executor will dispatch to global dispatch queue, but you can provide your own executor to schedule work in a specific way. For example, if you want to continue with work on the main thread:

fetch(object).continueWith(Executor.mainThread) { task in 
  // This closure will be executor on the main application's thread
}

How Do I Contribute?

We want to make contributing to this project as easy and transparent as possible. Please refer to the Contribution Guidelines.