[swift-users] Swift 4 "Cannot use mutating member on immutable value: 'self' is immutable"

[Roderick Mann]

Also, it works fine at a couple of other call sites. Why is this one special?

> On Sep 13, 2017, at 18:11 , Zhao Xin <owenzx at gmail.com> wrote:
> 
> Change `self` to `ModelFetcher`. You are calling a class static property, not a class instance property.
> 
> Zhao Xin
> 
> On Thu, Sep 14, 2017 at 8:37 AM, Rick Mann via swift-users <swift-users at swift.org> wrote:
> Moving to Swift 4, I'm running into an issue for which I can't seem to find an answer in google:
> 
> "Cannot use mutating member on immutable value: 'self' is immutable"
> 
> The code looks like:
> 
> class
> ModelFetcher : NSObject, URLSessionDelegate
> {
>     ...
>     static  let     managerDispatchQueue                    =   DispatchQueue(label: "Model Download Manager Queue")
>     static  var     pendingFetchers                         =   [ModelFetcher]()
>     static  var     currentFetcher:         ModelFetcher?
> 
>     class
>     func
>     startNextFetcher()
>     {
>         self.managerDispatchQueue.async
>         {
>             guard
>                 self.currentFetcher == nil,
>                 let mf = self.pendingFetchers.popFirst()
>                          ~~~~ ^                             error: cannot use mutating member on immutable value: 'self' is immutable
>             else
>             {
>                 return
>             }
> 
>             self.currentFetcher = mf
>             mf.start()
>         }
>     }
>     ...
> }
> 
> This code compiled fine in Xcode 8, or in Xcode 9/Swift 3.2 as a monolithic app (the error shows up when this code is factored into a framework). Other mutating references to self seem to compile okay (e.g. "self.currentFetcher = nil" or "self.pendingFetchers.remove(at: idx)"). Not sure what's special about this one.
> 
> 
> --
> Rick Mann
> rmann at latencyzero.com
> 
> 
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-- 
Rick Mann
rmann at latencyzero.com