In Swift, Does Resetting the Property Inside Didset Trigger Another Didset

In Swift, does resetting the property inside didSet trigger another didSet?

I also thought, that this is not possible (maybe it wasn't in Swift 2), but I tested it and found an example where Apple uses this. (At "Querying and Setting Type Properties")

struct AudioChannel {
    static let thresholdLevel = 10
    static var maxInputLevelForAllChannels = 0
    var currentLevel: Int = 0 {
        didSet {
            if currentLevel > AudioChannel.thresholdLevel {
                // cap the new audio level to the threshold level
                currentLevel = AudioChannel.thresholdLevel
            }
            if currentLevel > AudioChannel.maxInputLevelForAllChannels {
                // store this as the new overall maximum input level
                AudioChannel.maxInputLevelForAllChannels = currentLevel
            }
        }
    }
}

And below this piece of code, there is the following note:

In the first of these two checks, the didSet observer sets currentLevel to a different value. This does not, however, cause the observer to be called again.

How is didSet called again when setting inside through a function?

After checking with Swift github and asking questions about this problem, I find out that this problem is more complex as it seems. But there is a specific rule about this problem:

didSet observer will not trigger only if access to property within
its own didSet observer can be done through direct memory access.

Problem is that it is a little ambiguous when access to property will be direct(unless probably if you are developer of Swift). An important feature that has an effect on my question is this:

Class instance method never access class properties directly.

This quote shows problem with my code, even though I can argue that when an instance member should be able to access property directly whenever you call it in didSet observe. When I have a code like this:

class B {
    var i = 0 {
        didSet {
            print("called")            
            doit()
        }
    }

    func doit() {
        self.i += 1
    }
}

doit() function cannot access i directly which triggers didSet again causing infinite loop.

Now what is the workaround?

You can use inout for passing properties from its own didSet to a instance function without triggering didSet. Something like this:

class B {
    var i = 0 {
        didSet {
            print("called")            
            doit(&i)
        }
    }

    func doit(_ i: inout Int) {
        i += 1
    }
}

And one last thing. Starting Swift 5, conditions for selecting direct memory access for properties within its own didSet will become more restricted. Based on github, only conditions that will use direct memory access is are the following:

 Within a variable's own didSet/willSet specifier, access its storage
 directly if either:
 1) It's a 'plain variable' (i.e a variable that's not a member).
 2) It's an access to the member on the implicit 'self' declaration.
     If it's a member access on some other base, we want to call the setter
     as we might be accessing the member on a *different* instance.

This means codes like following will trigger infinite loop while it does not right now:

class B {
    var i = 0 {
        didSet {
            print("called")            
            var s = self
            s.i += 1
        }
    }
}

Is there a way to get didSet to work when changing a property in a class?

Nothing happens because the observer is on test, which is a Foo instance. But you changed test.number, not test itself. Foo is a class, and a class is a reference type, so its instances are mutable in place.

If you want to see the log message, set test itself to a different value (e.g. a different Foo()).

Or, add the println statement to the other didSet, the one you've already got on Foo's number property.

Or, make Foo a struct instead of a class; changing a struct property does replace the struct, because a struct is a value type, not a reference type.

Inner didSet protection bizarrely extends to the whole class?

This is bug SR-419.

From the comment on the bug:

Ugh. We really need to check that the base of the property access is statically self.

and from my experiments it seems that the didSet observer is not invoked only if you set the same property on any object. If you set any other property (even on the same object), the observer is invoked correctly.

class A {
    var name: String
    var related: A?
    var property1: Int = 0 {
        didSet {
            print("\(name), setting property 1: \(property1)")

            self.property2 = 100 * property1
            related?.property1 = 10 * property1
            related?.property2 = 100 * property1
        }
    }
    var property2: Int = 0 {
        didSet {
            print("\(name), setting property 2: \(property2)")
        }
    }

    init(name: String) {
        self.name = name
    }
}

let a = A(name: "Base")
a.related = A(name: "Related")
a.property1 = 2

Output:

Base, setting property 1: 2

Base, setting property 2: 200

Related, setting property 2: 200

when the expected output should be:

Base, setting property 1: 2

Base, setting property 2: 200

Related, setting property 1: 20

Related, setting property 2: 2000

Related, setting property 2: 200

It seems you also need to assign that property directly from the observer. Once you enter another function (or observer), the observers start working again:

var property1: Int = 0 {
    didSet {
        print("\(name), setting property 1: \(property1)")

        onSet()
    }
}

...    
func onSet() {
    self.property2 = 100 * property1
    related?.property1 = 10 * property1
    related?.property2 = 100 * property1
}

And that is the best workaround.

Another workaround (thanks @Hamish) is to wrap nested assignments into an immediately executed closure:

var property1: Int = 0 {
    didSet {
       {
           self.property2 = 100 * property1
           related?.property1 = 10 * property1
           related?.property2 = 100 * property1
       }()
    }
}

Depending on code before the closure, you might have to wrap it into parenthesis or insert a semicolon after the preceding statement.

Swift didSet does not fire

Why is it so?

didSet fires when you change value of the property, in example it would be when array itself changes.

Here

exerciseSets[index].isEnabled = !exerciseSets[index].isEnabled

you change property of some object in the array, but array stays the same, and contains same objects. So didSet won't be called.

Can I somehow use the former option?

No, you can't. Use latter.

EDIT: I checked @martin-r advice to use structs for ExerciseSet, and it works, didSet set is called. Because when property value change for structure basically means creation of new structure with all but changed field being copied.

Is it possible to allow didSet to be called during initialization in Swift?

Create an own set-Method and use it within your init-Method:

class SomeClass {
    var someProperty: AnyObject! {
        didSet {
            //do some Stuff
        }
    }

    init(someProperty: AnyObject) {
        setSomeProperty(someProperty)
    }

    func setSomeProperty(newValue:AnyObject) {
        self.someProperty = newValue
    }
}

By declaring someProperty as type: AnyObject! (an implicitly
unwrapped optional), you allow self to fully initialize without
someProperty being set. When you call
setSomeProperty(someProperty) you're calling an equivalent of
self.setSomeProperty(someProperty). Normally you wouldn't be able to
do this because self hasn't been fully initialized. Since
someProperty doesn't require initialization and you are calling a
method dependent on self, Swift leaves the initialization context and
didSet will run.

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