[swift-evolution] [Draft Proposal] A simplified notation for avoiding the weak/strong dance with closure capture lists

[Evan Maloney]

Hello,

This draft proposal is a follow-up to the previous threads: "Wanted: syntactic sugar for [weak self] callbacks" and "Allowing guard let self = self else { … } for weakly captured self in a closure."

A gist for this draft exists here <https://gist.github.com/emaloney/d34ac9b134ece7c60440>.

Comments welcome!

E.

---

Simplified notation for avoiding the [weak self]/strongSelf dance with closures

Proposal: TBD
Author: Evan Maloney <https://github.com/emaloney>
Status: Draft
Review manager: TBD
 <https://gist.github.com/emaloney/d34ac9b134ece7c60440#introduction>Introduction

Frequently, closures are used as completion callbacks for asynchronous operations, such as when dealing with network requests. It is quite common to model these sorts of operations in such a way that an object instance represents a request/response transaction, for example:

protocol NetworkTransaction: class
{
    enum Result {
        case Succeeded(NSData)
        case Failed(ErrorType)
    }

    func execute(completion: (Result) -> Void)
}
Here, the NetworkTransaction protocol declares the interface by which an asynchronous transaction occurs. The user of a NetworkTransaction calls the execute() function, passing in a completion function that is called at some time in the future, when the transaction completes.

For example, imagine a hypothetical DataConsumer class that uses a transaction to try to fetch some network data and process it:

class DataConsumer
{
    let transaction: NetworkTransaction

    init(transaction: NetworkTransaction)
    {
        self.transaction = transaction
    }

    func fetchData()
    {
        transaction.execute() { [weak self] result in
            guard let strongSelf = self else { return }

            switch result {
            case .Succeeded(let data):
                strongSelf.processData(data)

            case .Failed(let err):
                strongSelf.handleError(err)
            }
        }
    }

    func processData(data: NSData) 
    {
        // process the data
    }

    func handleError(error: ErrorType)
    {
        // handle the error
    }
}
You'll notice the [weak self]/strongSelf dance in the fetchData() function. This is a common pattern with asynchronously-executed closures, and it signals the possibility that a closure might outlive its usefulness.

Because the NetworkTransaction may complete at any time, it is possible that the closure will execute after the DataConsumer that initiated the transaction has been deallocated. Perhaps the user has navigated elsewhere in the application and whatever data was to be fetched by DataConsumer is no longer needed.

In this case, after a DataConsumer instance goes away, we don't really want the closure doing anything. So, we capture self weakly to ensure that the closure doesn't hold a reference to the owning DataConsumer. That prevents a reference cycle and ensures that DataConsumer can be deallocated when no longer in use.

When it comes time to execute the closure, the guard statement effectively asks the question, "Is self still alive?" If the answer is no, the guard forces a return and the rest of the closure does not execute.

If self is still alive, then the weakly-captured self will be non-nil and it will be converted into a strong reference held by strongSelf for the duration of the closure's execution.

When the closure is done executing, strongSelf goes away, once again making the DataConsumer eligible for deallocation when no other references are held.

 <https://gist.github.com/emaloney/d34ac9b134ece7c60440#the-problem>The Problem

The [weak self]/strongSelf dance requires common boilerplate wherever it is used, and the fact that a self-like variable with an arbitrary name adds noise within the closure. The more strongSelf is needed within the closure, the more noise there is.

Further, using a consistent name like strongSelf is by convention only; it can't be enforced by the compiler, so searching your codebase for a given keyword won't be exhaustive if team members use different names.

 <https://gist.github.com/emaloney/d34ac9b134ece7c60440#proposed-solution>Proposed Solution

The proposed solution adds a new capture type by repurposing the guard keyword for another use, which would look like:

transaction.execute() { [guard self] result in
    switch result {
    case .Succeeded(let data):
        self.processData(data)

    case .Failed(let err):
        self.handleError(err)
    }
}
Here, the [guard self] capture list serves as a signal that the compiler should handle the weak/strong dance itself. When encountering [guard self], the compiler should emit code that does the following:

Captures self in a weak reference on behalf of the closure
Whenever the closure is about to be executed, the weak reference is checked to see if self is still alive
If self is not alive, the closure returns immediately and nothing within the braces is executed
If self is alive, it is upgraded to a strong reference for the lifetime of the closure's execution. Within the closure, self is non-optional, unlike how it would be with a [weak self] capture. When the closure is done executing, the strong reference will be cleared and only the weak reference will be held on behalf of the closure.
 <https://gist.github.com/emaloney/d34ac9b134ece7c60440#closures-with-return-values>Closures with Return Values

For closures with a Void return type, the notation is simple, because no explicit value is expected as a result of executing the closure.

Closures with a return value will require an else clause:

let pollHappiness: () -> Bool = { [guard self else false] in
    return self.isHealthy && !self.isHungry && !self.isFearful && self.hasLove
}
Here, the else clause provides a value to return in cases where self has gone away and the guard fails.

In this example, if you call pollHappiness() after self has been deallocated, false will always be returned.

 <https://gist.github.com/emaloney/d34ac9b134ece7c60440#capturing-other-references>Capturing Other References

This notation is not limited to handling self. These capture lists are valid, too:

let capturingTwo = { [guard self, button] in
    // weakly capture self and button
    // but execute the closure with strong references
    // if and only if self and button still exist
    // when the closure is about to execute
}

let captureWithReturn: () -> UIView = {
    [guard button, window = button!.window else nil] in ...

    // window is captured and stored weakly at the time the
    // closure declaration is encountered during execution;
    // button is guaranteed to be non-nil at that time, but
    // it may go away later, so we guard on button too
} 
 <https://gist.github.com/emaloney/d34ac9b134ece7c60440#caveats>Caveats

This notation is not intended to be a full-fledged replacement for guard statements within the closure. We are only using guard here as a way to declare a specific memory-management behavior for references.

As a result, we are not attempting to support a where clause or boolean expressions within this notation. 

Rather, we're simply adding a new capture behavior and providing a means to specify an early exit if the behavior couldn't be fulfilled because one or more of the required objects was deallocated before the closure was executed.

 <https://gist.github.com/emaloney/d34ac9b134ece7c60440#impact-on-existing-code>Impact on Existing Code

None, since this does not affect any existing constructs. Implementation of this proposal will not result in any code breakage.

 <https://gist.github.com/emaloney/d34ac9b134ece7c60440#alternatives-considered>Alternatives Considered

 <https://gist.github.com/emaloney/d34ac9b134ece7c60440#status-quo>Status Quo

The primary alternative is to do nothing, requiring developers to add boilerplate guard code and handle upgrading the weak-to-strong references manually.

As stated above, this leads to needless boilerplate that can easily be factored out by the compiler. Also, the use of a self-like variable with an arbitrary name makes it more difficult to exhaustively find such uses in large projects. With this proposal, searching for the text "[guard" is all that's necessary to find all instances of this memory management technique.

Finally, the need to declare and use alternate names to capture values that already have existing names adds visual clutter to code and serves to obscure the code's original intent, making it harder to reason about.

 <https://gist.github.com/emaloney/d34ac9b134ece7c60440#alternate-notations>Alternate Notations

The notation proposed above was chosen for concision. Two variations were considered, but rejected, because they add words to the statements without adding meaning.

 <https://gist.github.com/emaloney/d34ac9b134ece7c60440#requiring-weak-after-guard>Requiring weak after guard

This alternate notation used a weak keyword after the guard keyword in the capture list, eg. [guard weak self] instead of [guard self].

The use of the weak keyword did not modify behavior in any way, so it was removed from the proposal to be less verbose.

 <https://gist.github.com/emaloney/d34ac9b134ece7c60440#requiring-return-after-else>Requiring return after else

A longer notation requiring the return keyword for closures specifying return values was considered, but rejected, for similar reasons.

Such a notation would make [guard self else nil] look like [guard self else return nil].

 <https://gist.github.com/emaloney/d34ac9b134ece7c60440#requiring-both>Requiring both

If both keywords above were required, the verbiage would become quite unwieldy in the closure's capture list, which often appears on the same line after the opening parenthesis, which itself is often indented:

let toggleHappyButton: () -> Bool? = { [guard weak self, button else return nil] in
    let state = button.highlighted
    button.highlighted = self.pollHappiness()
    return state
}
That seems longer than necessary.

For the reasons above, the two keywords above were not included in this proposal.

 <https://gist.github.com/emaloney/d34ac9b134ece7c60440#citations>Citations

Variations on this proposal were discussed earlier in the swift-evolution threads "Wanted: syntactic sugar for [weak self] callbacks <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160201/008713.html>" and "Allowing guard let self = self else { … } for weakly captured self in a closure. <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160201/009023.html>".
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