On Fri, Feb 12, 2016 at 10:44 AM, Erica Sadun via swift-evolution < swift-evolution@swift.org> wrote:
So long as it's technically possible, this would rock.
-- E
On Feb 12, 2016, at 11:23 AM, Evan Maloney <emaloney@gilt.com> wrote:
Hello,
I sent out an earlier draft of this proposal last week, and have updated
it based on feedback from the list.
The newer proposal now has a much smaller conceptual footprint to address
concerns that the previous iteration was too complex and would lead to
verbose notation within the capture lists.
You can find the updated gist here
<https://gist.github.com/emaloney/d34ac9b134ece7c60440>, as well as
pasted below.
Evan
---
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
<A simplified notation for avoiding the weak/strong dance with closure capture lists · GitHub;
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.
<A simplified notation for avoiding the weak/strong dance with closure capture lists · GitHub
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 the wrong name.
<A simplified notation for avoiding the weak/strong dance with closure capture lists · GitHub
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 becomes a no-op
<https://en.wikipedia.org/wiki/NOP>; calling the closure returns
immediately without anything inside the braces being 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.
<A simplified notation for avoiding the weak/strong dance with closure capture lists · GitHub;
Non-self References
Because guard is an additional capture type, like weak and unowned, it
can also be used to capture references other than self:
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 being asked to execute
}
When encountering multiple references being captured via guard, the
closure will execute *only* when *all* references are still alive when
the closure is being asked to execute.
<A simplified notation for avoiding the weak/strong dance with closure capture lists · GitHub;
Limitations
Because guard is a special capture type that causes the closure to become
a no-op once a referenced object deallocates, it is only designed to be
used with closures returning Void.
This limitation was deemed acceptable because it would cover the vast
majority of cases, and those that it didn't cover can still fall back on
the existing technique.
The compiler should emit an error if this notation is used in conjunction
with a closure that has a non-Void return type.
<A simplified notation for avoiding the weak/strong dance with closure capture lists · GitHub
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. Therefore,
guard within [square brackets ] should be seen as a capture type on par
with weak or unowned.
Unlike with a typical guard statement, we are not attempting to support
an else or where clause, or any 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 objects was deallocated.
The word guard was chosen as the capture type because (1) it functions as
a guard, ensuring that the closure doesn't execute unless the specified
objects are still alive, and (2) it obviates the need for the full-fledged
guard statement that would otherwise be required to achieve the same
result.
<A simplified notation for avoiding the weak/strong dance with closure capture lists · GitHub
on Existing Code
None, since this does not affect any existing constructs. Implementation
of this proposal will not result in any code breakage.
<A simplified notation for avoiding the weak/strong dance with closure capture lists · GitHub
Considered
<A simplified notation for avoiding the weak/strong dance with closure capture lists · GitHub
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.
<A simplified notation for avoiding the weak/strong dance with closure capture lists · GitHub
with an Optional Return Type
One possible addition to this proposal would extend support to any closure
returning an Optional of some kind.
If one of the objects in a guard capture list has been deallocated,
executing the closure will always result in an immediate nil return.
This idea was excluded from this iteration of the proposal due to a
concern that it relied on a "magic return value" (albeit a reasonable one)
and a perception that the community favored a solution with a smaller
conceptual footprint.
<A simplified notation for avoiding the weak/strong dance with closure capture lists · GitHub
with a Bool Return Type
One possible addition to this proposal would extend support to any closure
returning a Bool.
If one of the objects in a guard capture list has been deallocated,
executing the closure will always result in an immediate false return.
This idea was excluded from this iteration of the proposal due to a
concern that it relied on a "magic return value" (albeit a reasonable one)
and a perception that the community favored a solution with a smaller
conceptual footprint.
<A simplified notation for avoiding the weak/strong dance with closure capture lists · GitHub
with Arbitrary Return Types
An earlier iteration of this proposal included support for closures with
arbitrary return values. The community consensus was that the proposal was
too heavy-weight and tried to do too much, and would lead to verbosity
within the capture declaration. As a result, this idea was removed from the
proposal.
The ability to handle non-Void return values relied on supporting an else clause
within a guard-based capture list:
let happinessLevel: () -> Int = { [guard self else -1] in
var level = 0
level += self.isHealthy ? 25 : 0
level += !self.isHungry ? 25 : 0
level += !self.isFearful ? 25 : 0
level += self.hasLove ? 25 : 0
return level
}
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 happinessLevel() after self has been
deallocated, the value -1 will always be returned.
<A simplified notation for avoiding the weak/strong dance with closure capture lists · GitHub
Variations on this proposal were discussed earlier in the following
swift-evolution <https://lists.swift.org/mailman/listinfo/swift-evolution>
threads:
- Wanted: syntactic sugar for [weak self] callbacks
<https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160201/008713.html>
- 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>
- [Draft Proposal] A simplified notation for avoiding the weak/strong
dance with closure capture lists
<https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160201/009241.html>
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