I am using Firebase to observe event and then setting an image inside completion handler
FirebaseRef.observeSingleEvent(of: .value, with: { (snapshot) in
if let _ = snapshot.value as? NSNull {
self.img = UIImage(named:"Some-image")!
} else {
self.img = UIImage(named: "some-other-image")!
}
})
However I am getting this error
Closure cannot implicitly capture a mutating self parameter
I am not sure what this error is about and searching for solutions hasn't helped
The short version
The type owning your call to FirebaseRef.observeSingleEvent(of:with:)
is most likely a value type (a struct
?), in which case a mutating context may not explicitly capture self
in an @escaping
closure.
The simple solution is to update your owning type to a reference once (class
).
The longer version
The observeSingleEvent(of:with:)
method of Firebase is declared as follows
func observeSingleEvent(of eventType: FIRDataEventType, with block: @escaping (FIRDataSnapshot) -> Void)
The block
closure is marked with the @escaping
parameter attribute, which means it may escape the body of its function, and even the lifetime of self
(in your context). Using this knowledge, we construct a more minimal example which we may analyze:
struct Foo {
private func bar(with block: @escaping () -> ()) { block() }
mutating func bax() {
bar { print(self) } // this closure may outlive 'self'
/* error: closure cannot implicitly capture a
mutating self parameter */
}
}
Now, the error message becomes more telling, and we turn to the following evolution proposal was implemented in Swift 3:
@noescape
contextsStating [emphasis mine]:
Capturing an
inout
parameter, includingself
in a mutating method, becomes an error in an escapable closure literal, unless the capture is made explicit (and thereby immutable).
Now, this is a key point. For a value type (e.g. struct
), which I believe is also the case for the type that owns the call to observeSingleEvent(...)
in your example, such an explicit capture is not possible, afaik (since we are working with a value type, and not a reference one).
The simplest solution to this issue would be making the type owning the observeSingleEvent(...)
a reference type, e.g. a class
, rather than a struct
:
class Foo {
init() {}
private func bar(with block: @escaping () -> ()) { block() }
func bax() {
bar { print(self) }
}
}
Just beware that this will capture self
by a strong reference; depending on your context (I haven't used Firebase myself, so I wouldn't know), you might want to explicitly capture self
weakly, e.g.
FirebaseRef.observeSingleEvent(of: .value, with: { [weak self] (snapshot) in ...
If you need to mutate a value type (struct
) in a closure, that may only work synchronously, but not for async calls, if you write it like this:
struct Banana {
var isPeeled = false
mutating func peel() {
var result = self
SomeService.synchronousClosure { foo in
result.isPeeled = foo.peelingSuccess
}
self = result
}
}
You cannot otherwise capture a "mutating self" with value types except by providing a mutable (hence var
) copy.
The reason this does not work in async contexts is: you can still mutate result
without compiler error, but you cannot assign the mutated result back to self
. Still, there'll be no error, but self
will never change because the method (peel()
) exits before the closure is even dispatched.
To circumvent this, you may try to change your code to change the async call to synchronous execution by waiting for it to finish. While technically possible, this probably defeats the purpose of the async API you're interacting with, and you'd be better off changing your approach.
Changing struct
to class
is a technically sound option, but doesn't address the real problem. In our example, now being a class Banana
, its property can be changed asynchronously who-knows-when. That will cause trouble because it's hard to understand. You're better off writing an API handler outside the model itself and upon finished execution fetch and change the model object. Without more context, it is hard to give a fitting example. (I assume this is model code because self.img
is mutated in the OP's code.)
I'm thinking about something among the lines of this:
BananaNetworkRequestHandler
executes requests asynchronously and then reports the resulting BananaPeelingResult
back to a BananaStore
BananaStore
then takes the appropriate Banana
from its inside by looking for peelingResult.bananaID
banana.bananaID == peelingResult.bananaID
, it then sets banana.isPeeled = peelingResult.isPeeled
,You see, from the quest to find a simple fix it can become quite involved easily, especially if the necessary changes include changing the architecture of the app.
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