Cannot assign to 'X' in 'Y' in Swift

Question

I have a dictionary with Structs in it. I am trying to assign the values of the struct when I loop through the dictionary. Swift is tellin

Answer 1

Your guess is true.

By accessing blockStatus, you are creating a copy of it, in this case, it's a constant copy (iterators are always constant).

This is similar to the following:

var numbers = [1, 2, 3]

for i in numbers {
   i = 10  //cannot assign here
}

References:

Control Flow

In the example above, index is a constant whose value is automatically set at the start of each iteration of the loop.

Classes and Structures

A value type is a type that is copied when it is assigned to a variable or constant, or when it is passed to a function. [...] All structures and enumerations are value types in Swift

Methods

Structures and enumerations are value types. By default, the properties of a value type cannot be modified from within its instance methods.

However, if you need to modify the properties of your structure or enumeration within a particular method, you can opt in to mutating behavior for that method. The method can then mutate (that is, change) its properties from within the method, and any changes that it makes are written back to the original structure when the method ends. The method can also assign a completely new instance to its implicit self property, and this new instance will replace the existing one when the method ends.

You can opt in to this behavior by placing the mutating keyword before the func keyword for that method:

Answer 2

You could loop through the array with an index

for index in 0..<statuses.count {
    // Use your array - statuses[index]
}

that should work without getting "cannot assign"

Answer 3

If 'Y' in this case is a protocol, subclass your protocol to class. I had a protocol:

 protocol PlayerMediatorElementProtocol {
      var playerMediator:PlayerMediator { get }
 }

and tried to set playerMediator from within my player mediator:

element.playerMediator = self

Which turned into the error cannot asign 'playerMediator' in 'element'

Changing my protocol to inherit from class fixed this issue:

 protocol PlayerMediatorElementProtocol : class {
      var playerMediator:PlayerMediator { get }
 }

Why should it inherit from class?

The reason it should inherit from class is because the compiler doesn't know what kind your protocol is inherited by. Structs could also inherit this protocol and you can't assign to a property of a constant struct.