Create NSMutableArray from Array<T> in Swift

Question

I am trying to write an extension where I define a new method for sorting the contained objects in-place by their property using NSSortDescriptorclass. However, I failed to populate the mutable version of self for some reason and getting the following error:

Cannot find an initializer for type NSMutableArray that accepts an argument list of type (array: Array<T>)

Here is my method:

extension Array {

    public mutating func sortInPlace(field: String, ascending: Bool) {

        let mutableArray = NSMutableArray(array: self) // error here.

        let sortDescriptor = NSSortDescriptor(key: field, ascending: ascending)     
        mutableArray.sortUsingDescriptors([sortDescriptor])

        self = mutableArray as AnyObject as! [Generator.Element]
    }
}

I also tried the following but no luck:

let mutableArray = NSMutableArray(array: self as! [AnyObject])

Lance · Accepted Answer · 2015-07-02 23:54:16Z

1

Without seeing your extension definition it's hard to say, but this works for me:

extension Array where T : AnyObject {
    func toMutableArray() -> NSMutableArray {
        return NSMutableArray(array: self)
    }
}

let array: [NSString] = ["A", "B"]
let mArray = array.toMutableArray()

print(mArray) // ( A, B )

The problem here is you need to constrain the extension on Array to only take in an array of AnyObject. String is a struct, not a class (AnyObject is constrained to reference types). This is also why I needed the type specifier of [NSString] on my array. Otherwise, it would be an array of structs, not an array of String values.

edited Jul 2, 2015 at 23:54

answered Jul 2, 2015 at 23:47

Lance

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1 Comment

Lance Over a year ago

great! I've added a better explanation to my answer.

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