Why doesn't this Swift `defer` statement work as expected?

Question

I am trying to understand how the defer statement works in Swift 2 because I apparently do not understand it correctly.

I have a postprocess() method that should only be called when view is instantiated:

internal func postprocess() {
    assert(self.node.isViewInstantiated()) // <- this is failing using the first method
    ...
}

I was originally attempting to call it like so:

public var view: UIView {
    get {
        if !node.isViewInstantiated() {
            defer {
                postprocess()
            }
        }
        return node.view // node.view getter creates view
    }
}

But this was setting off the assert. When I changed it to this, it started working:

public var view: UIView {
    get {
        if node.isViewInstantiated() {
            return node.view
        } else {
            var result = node.view
            postprocess()
            return result
        }
}

(Note that node.view is self-instantiating, hence isViewIntantiated() .)

Can someone please explain why the defer doesn't actually defer?

Answer 1

Swift's defer keyword queues up a block to execute when the current scope is exited , which isn't necessarily the same as when the function returns.

A defer block inside an if block will be executed as you leave the scope of the if block.

You could rewrite your code as such:

public var view: UIView {
    get {
        let shouldPostProcess = !node.isViewInstantiated
        defer {
            if shouldPostProcess {
                postprocess()
            }
        }
        return node.view // node.view getter creates view
    }
}

And it should work perfectly fine. Now the defer -ed block is scoped to the get method call and executes as it returns, where before it was scoped to the if block and executed as the if block scope exited.