updateValue not working for Dictionary
dictionary, ios, swift, xcode
Solution
this is not a bug or a quirk in the compiler.
it is how `Optional` implemented (which may be flawed or not)
what happened is that the `Optional` store the `Dictionary` as immutable object (with `let` perhaps). So even `Optional` it is mutable, you can't modify the underlying `Dictionary`object directly (without reassign the Optional object).
`updateValue(forKey:)` is mutating method, you can't call it on immutable object and hence the error.
you can workaround it by doing
var d = dict!
d.updateValue(data, forKey: id)
because you copy the dictionary to another mutable variable, which then is mutable and able to call mutating method on it
but without `dict = d`, your change won't be applied on `dict` because `Dictionary` is value type, it makes copy on every assignment
related answer
Problem
I'm creating a test app using Swift in Xcode, and I've run into an annoying issue. I'm writing a simple class that will act as a cache using a Dictionary object. My implementation is below: ``` import Foundation import UIKit class ImageCache { var dict:Dictionary<String,NSData>?; init() { dict = Dictionary<String,NSData>(); } func exists(id:String) -> Bool { return dict!.indexForKey(id)!==nil; } func getImage(id:String) -> UIImage? { if(!exists(id)) { return nil; } return UIImage(data: (dict!)[id]); } func setData(id:String, data:NSData) { dict!.updateValue(data, forKey: id); } } ``` The issue is in the last method, with Xcode stating "Could not find member 'UpdateValue'". This is weird, because the code hint seems to show it just fine: But when I try to compile: Could this potentially be a bug in Xcode? Or am I missing something super-obvious?