为什么不在函数外部进行引用会创建保留周期？

Question

Sorry, but I know this is a really dumb question, and I already kind of 'know' the answer, but I need someone to clearly explain to me WHY the answer is what it is.

Lately, I've become a bit obsessed/paranoid about retain cycles and memory leaks in my code, after going through some nightmarish debugging with various memory issues, so in the future I want to nip them in the bud. But after reading and learning a lot about ARC and retain cycles in Swift, although it makes sense, I still don't really have enough of an "intuitive" or natural feel for it, to feel confident that I could spot one, or the lack of one, as I'm coding. So I'm starting to become a little paranoid that I'm creating retain cycles with even basic stuff without realizing it.

So, with that in mind, why doesn't any ordinary function that uses a variable declared outside of it create a retain cycle? For example:

    class someClass {
        let a = "I'm letter a"
        let moreLetters = addLetters()
        func addLetters () -> String { 
            let newString = a + "bcdefg"
            return newString
        }
    }

In this case, self.moreLetters references the function addLetters, and then the constant self.a is references from within the function addLetters. So would this create a retain cycle if I don't capture weak/unowned self? It seems absurd to me that something this simple would cause a problem...or is it? What about in a nested function, like this:

    func someFunction () -> String {
        let a = "I'm letter a"
        func addLetters () -> String { 
            let newString = a + "bcdefg"
            return newString
        }
        let moreLetters = addLetters()
        return moreLetters
    }

Would that also create a retain cycle? (Yeah I know this is a convoluted way of performing a simple task; I'm just using this code as an example to make my point).

Have I become super-paranoid and am severely overthinking things?

Answer 1

First, you need to understand how a basic retain cycle is formed. A retain cycle is formed when an object A refers to an object B strongly and at the same time. object B refers to object A strongly as well.

Let's look at your first bit of code.

class someClass {
    let a = "I'm letter a"
    let moreLetters = addLetters()
    func addLetters () -> String { 
        let newString = a + "bcdefg"
        return newString
    }
}

Actually, a class by itself can never create retain cycles, so let's add some code to create an object:

var obj = someClass()

First, a is initialized to "I'm letter a". After that, moreLetters is initialized by calling the method addLetters . After the method returns, moreLetters is initialized to "I'm letter abcdefg". So far so good.

Now we set obj to nil :

obj = nil

If a retain cycle were formed, obj would not be deinitialized. However, in actuality, obj is deinitialized because nothing holds a strong reference to obj !

"Wait a minute!" you say, "But the method addLetters still refers to someClass because it has a in it!" Well, in fact, addLetters has already returned! Therefore, everything in it doesn't matter anymore! In addition, addLetters belongs to obj , which you have already set to nil !

Now let's look at your second code:

func someFunction () -> String {
    let a = "I'm letter a"
    func addLetters () -> String { 
        let newString = a + "bcdefg"
        return newString
    }
    let moreLetters = addLetters()
    return moreLetters
}

A retain cycle does not form because there isn't a even a reference type! There is no objects to be created. All you did in the second code is playing with strings, which are value types. Even if there were a class, a retain cycle would not form because as I said, when you set obj to nil , all the methods in it "disappear" because methods belong to objects.

---

What about those closures where I must write [weak self] or a retain cycle forms?

Those closures are escaping closures . Normal closures are deinitialized after they return. However, escaping closures are retained by some object so they are not deinitialized immediately. For more info, see Escaping Closures in Swift