Swift: Cannot assign to immutable expression of type 'AnyObject?!'

Question

I searched, but I didn't find a familiar answer, so...

I am about to program a class to handle parse methods like updating, adding, fetching and deleting.

func updateParse(className:String, whereKey:String, equalTo:String, updateData:Dictionary<String, String>) {

    let query = PFQuery(className: className)

    query.whereKey(whereKey, equalTo: equalTo)
    query.findObjectsInBackgroundWithBlock {(objects, error) -> Void in
        if error == nil {
            //this will always have one single object
            for user in objects! {
                //user.count would be always 1
                for (key, value) in updateData {

                    user[key] = value //Cannot assign to immutable expression of type 'AnyObject?!'

                }

                user.saveInBackground()
            } 

        } else {
            print("Fehler beim Update der Klasse \(className) where \(whereKey) = \(equalTo)")
        }
    }

}

As I am about to learn swift at the moment, I would love to get an answer with a little declaration, so that I would learn a little bit more.

btw: I later call this method like this:

parseAdd.updateParse("UserProfile", whereKey: "username", equalTo: "Phil", updateData: ["vorname":self.vornameTextField!.text!,"nachname":self.nachnameTextField!.text!,"telefonnummer":self.telefonnummerTextField!.text!])

Solution 1

The error message says, you're trying to change an immutable object, which is not possible.

Objects declared as method parameters or return values in closures are immutable by default.

To make the object mutable either add the keyword var in the method declaration or add a line to create a mutable object.

Also index variables in repeat loops are immutable by default.

In this case a line is inserted to create a mutable copy and the index variable is declared as mutable.

Be careful to change objects while being enumerated, this could cause unexpected behavior

...
query.findObjectsInBackgroundWithBlock {(objects, error) -> Void in
    if error == nil {
        //this will always have one single object
        var mutableObjects = objects
        for var user in mutableObjects! {
            //user.count would be always 1
            for (key, value) in updateData {

                user[key] = value
...

Solution 2

In swift a lot of types are defined as structs, which are immutable by default.

I had the same error doing this :

protocol MyProtocol {
    var anInt: Int {get set}
}

class A {

}

class B: A, MyProtocol {
    var anInt: Int = 0
}

and in another class :

class X {

   var myA: A

   ... 
   (self.myA as! MyProtocol).anInt = 1  //compile error here
   //because MyProtocol can be a struct
   //so it is inferred immutable
   //since the protocol declaration is 
   protocol MyProtocol {...
   //and not 
   protocol MyProtocol: class {...
   ...
}

so be sure to have

protocol MyProtocol: class {

when doing such casting

Comments

[vadian]

It affects also the index variable in the repeat loop. I changed the post

[Chris Birch]

Nice find - saved me a lot of time ta.

[Tino]

This has been changed - it's recommended to use protocol MyProtocol: AnyObject { instead

[Warpling]

This is the only explanation I've found that made sense. Thank you!!

[Abhishek Thapliyal]

@ZpaceZombor: Thanks my issue is resolved with you comments :)