> On Feb 23, 2017, at 4:40 AM, Anton Mironov <antonvmiro...@gmail.com> wrote:
>
>
>> On Feb 23, 2017, at 4:28 AM, Matthew Johnson <matt...@anandabits.com
>> <mailto:matt...@anandabits.com>> wrote:
>>
>>>
>>> On Feb 22, 2017, at 7:13 PM, Anton Mironov <antonvmiro...@gmail.com
>>> <mailto:antonvmiro...@gmail.com>> wrote:
>>>
>>>>
>>>> On Feb 23, 2017, at 02:19, Matthew Johnson <matt...@anandabits.com
>>>> <mailto:matt...@anandabits.com>> wrote:
>>>>
>>>>>
>>>>> On Feb 22, 2017, at 6:06 PM, Anton Mironov <antonvmiro...@gmail.com
>>>>> <mailto:antonvmiro...@gmail.com>> wrote:
>>>>>
>>>>>
>>>>>> On Feb 23, 2017, at 01:18, Matthew Johnson <matt...@anandabits.com
>>>>>> <mailto:matt...@anandabits.com>> wrote:
>>>>>>
>>>>>>>
>>>>>>> On Feb 22, 2017, at 5:06 PM, Anton Mironov <antonvmiro...@gmail.com
>>>>>>> <mailto:antonvmiro...@gmail.com>> wrote:
>>>>>>>
>>>>>>> -1
>>>>>>> I support improvements in this area but I do not think that adding
>>>>>>> guarded closures will fix the case.
>>>>>>> It raises multiple concerns:
>>>>>>> - prepending ? to the closure declaration is as forgettable as `[weak
>>>>>>> self]`
>>>>>>
>>>>>> No, this is why I included the `@guarded` parameter annotation. This
>>>>>> allows an API to require its callers to use a guarded closure. Strong
>>>>>> references would have to be explicit in the capture list.
>>>>>>
>>>>>>> - reactive programming often assumes chaining of operations. How
>>>>>>> guarded closures affect next operations in the chain?
>>>>>>
>>>>>> Can you provide a concrete example of real world code you wrote
>>>>>> manually? I will convert it to use guarded closures to show how it is
>>>>>> affected.
>>>>>
>>>>> You can use the second piece of code I’ve provided before.
>>>>
>>>> How do `map` and `onUpdate` store the reference to the context? Is it
>>>> weak? If so, what happens when the context is released? After you answer
>>>> that I will be able to show how it would look under this proposal.
>>>
>>> Yes, they store a weak reference to the context.
>>>
>>> This code implies using primitive that has multiple update events (button
>>> actions in this case) followed by a single completion event (can only be an
>>> error of context deallocation in this case). I call it `Channel` (because
>>> name `Stream` is already taken by `Foundation.Stream` aka `NSStream`).
>>>
>>> A release of context will lead to a release of closure and all operations
>>> it depends on.
>>>
>>> `Channel.map`’s context does not exist anymore so closure will be released
>>> and `Channel` returned from `map` will be immediately completed with
>>> failure.
>>> This will also lead to a release of `Channel` returned by
>>> `Channel.debounce()`. `Channel.debounce()` will lead to release of
>>> `Channel` returned by `actions(forEvents: [.touchUpInside])`.
>>>
>>> `Channel.onUpdate`’s context does not exist anymore so closure will be
>>> released.
>>> This will also lead to a release of `Channel` returned by
>>> `Channel.distinct()`.
>>>
>>> It might look complex but it is based on a simple idea: release all
>>> retained resources if your context is gone.
>>
>> I’m very familiar with patterns like this but didn’t want to make any
>> assumptions about how your code works. How does this code currently detect
>> that the context has been released? Does it wait until an event is pushed
>> through the channel and see that the context is now nil?
>>
>
> No, it does not wait. It releases all retained resources (closure and
> channels it depends on) when context drains it’s `disposableBag`.
Can you explain further? You gave it a context of `self`. Is the context
required to be a generic argument that conforms to a protocol which exposes a
`disposableBag` member? What does it mean to drain the `disposableBag`? What
causes it to get drained? How does the library detect that the context has
drained it’s `disposableBag`? I need to understand all of the details to show
how your use case can be modeled with guarded closures.
>
>> If this system was using guarded closures the `map` and `onUpdate` methods
>> would specify their function argument `@guarded` which would automatically
>> make all captures guarded. You are not limited to a single context object,
>> but if that is the need of your code you of course can capture a single
>> object.
>>
> I did not think of multiple contexts before. Maybe I should. I thought that
> two (or more) context you want to interact with are either:
> - have a simple relation (child-parent or a regular ownership) so there is no
> need for the weak reference for the second context
> - are operating on unrelated `DispatchQueue`s so mutation of an internal
> state of both contexts on the same queue may not exist
Many times you won’t need more than one context, all you need is `self`. But
sometimes you write a closure that captures more than one object. Both of
these objects are part of the closure context and with a guarded closure they
are all guarded by default. It’s always *possible* to write code explicitly in
the style of a single context object like you are, but it’s often far more
convenient to just let a closure capture everything into an implicit single
context (the closure’s context that the compiler creates when capture happens).
With guarded closures we have the ability to do this and be ensured that we
don’t extend the lifetime of anything without explicitly stating a strong
capture in the capture list.
>
>> The calling code would look like this:
>>
>> self.button.actions(forEvents: [.touchUpInside])
>> .debounce(interval: 3.0)
>> .map ?{
>> return self.searchField.text
>> }
>> .distinct()
>> .onUpdate ?{ (searchQuery) in
>> self.performSearch(query: searchQuery)
>> }
>>
>> In the implementation of the library where you used to store a weak
>> reference to the context and a strong reference to the closure you would
>> just store a strong reference to the guarded closure. The guarded closure
>> itself manages the weak / strong dance. Where you used to check the weak
>> reference to the context for nil to see if it is alive or not you would
>> check the `isAlive` property on the closure reference to determine if the
>> closure is still alive or not. When it is no longer alive you tear down
>> exactly the same as you do today when you detect that the context reference
>> is nil.
>
> That is a point. Next check `isAlive` will be performed right before the next
> call of the closure. This call could be performed in a minute or in an hour
> or even never performed. Guarded closure will capture variables until then.
Can you explain how your mechanism for detecting earlier release works? Weak
references get nil`d lazily upon access so you must have some other kind of
plumbing that detects this.
If we can identify a way to support the immediate release use case with guarded
closures I would be very happy. After you explain how your system handles this
I will try to think of a way to support it well.
Even if we can’t, I think guarded closures would still benefit your use case.
The don’t prevent you from building an API that accepts a separate lifetime
object and releasing your reference to the closure immediately when that
lifetime object drains its `disposeBag`. What they would give you in that
scenario is a guarantee that the closures passed to `map` and `onUpdate` don’t
capture anything strongly without an explicit capture list. You can get this
guarantee while still using a manual context that controls the lifetime of the
closure.
This design would look something like this:
self.button.actions(forEvents: [.touchUpInside])
.debounce(interval: 3.0)
.map(lifetime: self) ?{
return self.searchField.text
}
.distinct()
.onUpdate(lifetime: self) ?{
self.performSearch(query: $0)
}
You pass an independent lifetime object that controls the duration of the
library’s reference to the closure. The difference here is that you don’t pass
the lifetime object to the closure as a context. Instead, you require a
guarded closure to be used to capture whatever context is necessary. The
lifetime and context are completely independent. Neither extends the lifetime
of any objects, with the exception of any strong captures in the capture list
of the guarded closure.
Separating lifetime from context gives you the ability to have finer grained
control over lifetime (they are often the same, but there are probably good use
cases where you would like them to be different). Using guarded closures
allows you to reference names from the surrounding scope without have to
re-declare them in the closures’s signature.
This approach does add an additional weak reference. If the advantages above
aren’t worth the extra weak reference you could simply stick with the design
you already have, but adopt `@guarded` to caution users against strong captures.
It’s also worth considering the use case of passing instance methods to `map`
and `onUpdate`. In your current design it might looks something like this
(assuming you have overloads that handle unbound instance methods):
self.button.actions(forEvents: [.touchUpInside])
.debounce(interval: 3.0)
.map(context: self, MyType.instanceMethod)
.distinct()
.onUpdate(context: self, MyType.otherInstanceMethod)
With this proposal, if you switch to the idea of a lifetime instead of a
context you could write the above as:
self.button.actions(forEvents: [.touchUpInside])
.debounce(interval: 3.0)
.map(lifetime: self, ?instanceMethod)
.distinct()
.onUpdate(lifetime: self, ?otherInstanceMethod)
You no longer have to explicitly state your type name repeatedly. You just
pass a guarded bound instance method.
>
>>
>>>
>>>>>
>>>>>>
>>>>>>> - the closure must exist until either the control deallocates (source
>>>>>>> of actions) or self deallocates (destination of actions). Guarded
>>>>>>> closure will not provide an expected behavior
>>>>>>
>>>>>> Yes they will. The guarded closure lives until the control releases it.
>>>>>> But it becomes a no-op if any of the references captured with a guard
>>>>>> are released before that happens. This is much like the behavior of the
>>>>>> target / action pattern but generalized to support closures.
>>>>>
>>>>> I doubt that turning closure into no-op is a simple thing to do. It will
>>>>> require having a registry of closures that depend on an instance. A
>>>>> runtime will have to go through the registry and turn closures into
>>>>> no-op. Or there is an another solution that I do not see.
>>>>
>>>> What I mean when I say no-op is that the code the user places in the
>>>> closure would be prefixed by a `guard` clause with an early return. The
>>>> no-op is when the guard clause is triggered, just as if you had written it
>>>> manually.
>>>
>>> I think that this is an important part. Using no-op (or avoiding execution
>>> of closure as I understand it) leads to another unwanted retain of
>>> variables captured by the closure.
>>
>> There is no additional retain of the variables over a solution that captures
>> the variables independently of the closure and passes them as arguments if
>> they’re alive when called (like the context in your example). Where do you
>> think you see an extra retain of the variables?
>
> I meant that keeping the closure alive when a context is dead is an extra
> retain of a captured variables.
>
> Look, I’m not saying that extra care about retain cycles is bad. I’m just
> saying that making an implicit weak reference does not solve the whole
> problem. I think that adding guarded closures is not a step towards a
> complete solution.
>
>>
>>>
>>>>
>>>> I imagine the compiler might also place an additional check inside the
>>>> `else` of the `guard` which would release any context it was still hanging
>>>> on to as it would know that the context was no longer needed. Imagine
>>>> this as if the compiler synthesized code had access to an optional strong
>>>> reference to the context and set it to nil in the else clause. It would
>>>> also be possible for this closure to expose an `isAlive: Bool` property
>>>> that would check whether or not the context was still around. This is a
>>>> bit of hand waving - I’m sure the real implementation would be more
>>>> sophisticated. But I think that conveys the basic idea.
>>>>
>>>> Here is some pseudocode:
>>>>
>>>> let foo: Foo = Foo()
>>>> let bar: Bar = Bar()
>>>>
>>>> // hand out references to the owner that controls the lifetime of foo and
>>>> bar
>>>>
>>>> ?{
>>>> // compiler synthesized code
>>>> // this is a property on the closure object itself, not visible
>>>> within the scope of the user code in the closure
>>>> // it is used by libraries to detect when they can safely discard
>>>> their reference to the closure because it has become a no-op
>>>> // context is a super secret compiler reference to the context of
>>>> the closure
>>>> var isActive: Bool { return context != nil }
>>>>
>>>> // compiler synthesized code that prefixes the user code
>>>> guard let foo = foo, let bar = bar else {
>>>> context = nil
>>>> return
>>>> }
>>>> // end compiler synthesized code
>>>>
>>>> // begin user code
>>>> // do something with foo and bar
>>>> }
>>>>
>>>>>
>>>>>>
>>>>>>> - managing lifecycle of nested guarded closures could be complex to
>>>>>>> understand and implement into the language
>>>>>>
>>>>>> I’m glad you brought this up. I’ll give it some thought. If there does
>>>>>> turn out to be complexity involved I wouldn’t have a problem prohibiting
>>>>>> that.
>>>>>>
>>>>>>> - why would you consider using @escaping instead of @guarded?
>>>>>>
>>>>>> Because sometimes the right default for a function taking an escaping
>>>>>> closure is a strong reference. I wouldn't want `DispatchQueue.async` to
>>>>>> take a guarded closure. That API doesn’t contain any semantic content
>>>>>> around *why* you dispatched async. It’s not a callback, but instead a
>>>>>> way of moving work around.
>>>>>>
>>>>>>>
>>>>>>> I personally prefer doing something like this:
>>>>>>>
>>>>>>> ```swift
>>>>>>> self.button.onAction(forEvents: [.touchUpInside], context: self) {
>>>>>>> (self, sender, event) in
>>>>>>> self.performSearch(query: self.searchField.text)
>>>>>>> }
>>>>>>> ```
>>>>>>>
>>>>>>> or
>>>>>>>
>>>>>>> ```swift
>>>>>>> self.button.actions(forEvents: [.touchUpInside])
>>>>>>> .debounce(interval: 3.0)
>>>>>>> .map(context: self) { (self, _) in
>>>>>>> return self.searchField.text
>>>>>>> }
>>>>>>> .distinct()
>>>>>>> .onUpdate(context: self) { (self, searchQuery) in
>>>>>>> self.performSearch(query: searchQuery)
>>>>>>> }
>>>>>>> ```
>>>>>>>
>>>>>>> This code neither requires an addition of language features nor
>>>>>>> contains retain cycles. All closures will be released as soon as source
>>>>>>> or destination deallocates.
>>>>>>
>>>>>> This isn’t too bad but it does require manually threading the context.
>>>>>> This is more work for both the library and the client than necessary.
>>>>>> It also does not help users avoid an accidental strong reference in the
>>>>>> closure. It nudges them not to by offering to thread the context but it
>>>>>> doesn’t do anything to prevent it. You can still create a strong
>>>>>> reference (event to self) without specifying it in the capture list.
>>>>>
>>>>> You are correct. This will code will not help to avoid accidental strong
>>>>> reference. But it gives an opportunity to do things without explicit weak
>>>>> references just as guarded closures do. It also adds an ability to avoid
>>>>> an execution of pure (or just not bound to context) operations you
>>>>> depends on. Deallocation of context will lead to cancellation of full
>>>>> chain of operations and unsubscription from button event.
>>>>>
>>>>>> I think there is a place for a language solution here.
>>>>>
>>>>> The only language solution I expect is a static analyzer warning about
>>>>> retain cycle (like in ObjC).
>>>>>
>>>>> I’m starting to think that my solution is similar to yours. I’ve done
>>>>> these things with a library rather than with language support. I will
>>>>> definitely take advantage of guarded self in closures as soon as the
>>>>> proposal will be accepted. But I would prefer and suggest using my
>>>>> solution for now.
>>>>>
>>>>>>
>>>>>>>
>>>>>>>> On Feb 22, 2017, at 22:57, Matthew Johnson via swift-evolution
>>>>>>>> <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote:
>>>>>>>>
>>>>>>>> Hi David,
>>>>>>>>
>>>>>>>> I just shared a draft proposal to introduce guarded closures last
>>>>>>>> week:
>>>>>>>> https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20170213/032478.html
>>>>>>>>
>>>>>>>> <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20170213/032478.html>.
>>>>>>>> I think you would find it very interesting.
>>>>>>>>
>>>>>>>> I considered including a new capture list specifier `guard` in this
>>>>>>>> proposal but decided against it. Guarded behavior requires prefixing
>>>>>>>> the contents of the closure with a guard clause that returns
>>>>>>>> immediately if the guard is tripped. This is a property of the
>>>>>>>> closure as a whole, not of an individual capture. For that reason, I
>>>>>>>> decided that allowing a `guard` specifier for an individual capture
>>>>>>>> would be inappropriate.
>>>>>>>>
>>>>>>>> Instead, a guarded closure has a guarded by default capture behavior
>>>>>>>> which can be overridden with `weak`, `unowned` or `strong` in the
>>>>>>>> capture list. The thread on this proposal was relatively brief. I
>>>>>>>> plan to open a PR soon after making a few minor modifications.
>>>>>>>>
>>>>>>>> Matthew
>>>>>>>>
>>>>>>>>> On Feb 22, 2017, at 2:48 PM, David Hedbor via swift-evolution
>>>>>>>>> <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote:
>>>>>>>>>
>>>>>>>>> Hello,
>>>>>>>>>
>>>>>>>>> (apologies if this got sent twice - gmail and Apple mail seems to
>>>>>>>>> confused as to what account the first mail was sent from)
>>>>>>>>>
>>>>>>>>> I’m new to this mailing list, but have read some archived messages,
>>>>>>>>> and felt that this would be a reasonable subject to discuss. It’s
>>>>>>>>> somewhat related to the recent posts about @selfsafae/@guarded but
>>>>>>>>> distinctly different regardless.
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> Problem:
>>>>>>>>>
>>>>>>>>> It’s often desirable not to capture self in closures, but the syntax
>>>>>>>>> for doing so adds significant boilerplate code for [weak self] or us
>>>>>>>>> unsafe when used with [unowned self]. Typically you’d do something
>>>>>>>>> like this:
>>>>>>>>>
>>>>>>>>> { [weak self] in self?.execute() }
>>>>>>>>>
>>>>>>>>> This is simple enough but often doesn’t work:
>>>>>>>>>
>>>>>>>>> { [weak self] in self?.boolean = self?.calculateBoolean() ]
>>>>>>>>>
>>>>>>>>> This fails because boolean is not an optional. This in turn leads to
>>>>>>>>> code like this:
>>>>>>>>>
>>>>>>>>> { [weak self] in
>>>>>>>>> guard let strongSelf = self else { return }
>>>>>>>>> strongSelf.boolean = self.calculateBoolean() }
>>>>>>>>>
>>>>>>>>> And this is the boilerplate code. My suggestion is to add a syntax
>>>>>>>>> that works the same as the third syntax, yet doesn’t require the
>>>>>>>>> boilerplate code.
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> Solution:
>>>>>>>>>
>>>>>>>>> Instead of using unowned or weak, let’s use guard/guarded syntax:
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> { [guard self] in
>>>>>>>>> self.isExecuted = self.onlyIfWeakSelfWasCaptured()
>>>>>>>>> }
>>>>>>>>>
>>>>>>>>> In essence, guarded self is equivalent to a weak self, that’s
>>>>>>>>> captured when the closure is executed. If it was already released at
>>>>>>>>> that point, the closure is simply not executed. It’s equivalent to:
>>>>>>>>>
>>>>>>>>> { [weak self] in
>>>>>>>>> guard let strongSelf = self else { return }
>>>>>>>>> strongSelf.isExecuted = strongSelf.onlyIfWeakSelfWasCaptured()
>>>>>>>>> }
>>>>>>>>>
>>>>>>>>> Except with a lot less boilerplate code, while not losing any clarify
>>>>>>>>> in what it does.
>>>>>>>>>
>>>>>>>>> Impact / compatibility:
>>>>>>>>>
>>>>>>>>> This is simply additive syntax, and wouldn’t affect any existing code.
>>>>>>>>> _______________________________________________
>>>>>>>>> swift-evolution mailing list
>>>>>>>>> swift-evolution@swift.org <mailto:swift-evolution@swift.org>
>>>>>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>>>>>>> <https://lists.swift.org/mailman/listinfo/swift-evolution>
>>>>>>>>
>>>>>>>> _______________________________________________
>>>>>>>> swift-evolution mailing list
>>>>>>>> swift-evolution@swift.org <mailto:swift-evolution@swift.org>
>>>>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>>>>>> <https://lists.swift.org/mailman/listinfo/swift-evolution>
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