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The Swift 2.0 Defer Keyword

Couldn't think of a suitable image, so here is a kitten on a computer.At WWDC 2015, Apple announced some substantial updates to the Swift Programming Language for version 2.0. Since its announcement, Swift has been undergoing a lot of updates. In fact, version 1.2 of the language had been released in April 2015 and by June we had the first preview of 2.0.

I decided to blog about some of the changes to the language that really caught my attention. The one that I like the most is the “defer” keyword. In .Net (and other languages), we have this concept of try/finally. Sometimes you can include a catch in there, but it isn’t required. The idea is that after the code executes in the try portion, the code in finally is guaranteed to execute. That’s the perfect place to make sure that you’ve disposed of objects or otherwise cleaned up after yourself. Pretty much every tutorial that I’ve ever seen in .Net has something like this:

Sharp .Netters might point out that coding a “using” statement accomplishes essentially the same thing with anything that implements IDisposable, but I’m just trying to demonstrate a point 😉

There are a few problems with this try/finally style of coding, however. First, what tends to happen is that developers wrap large blocks of code in try blocks. The problem with that is that you don’t know what actually exists by the time you get to the finally. Maybe you’ve returned early from the method, or maybe an exception was thrown. Now, you’ve got to litter your entire finally block with “if this exists, set it to null” or “if this is open, close it” kind of stuff. That’s where “defer” comes in. Let’s take a look at some code that I ran in an Xcode 7 Beta 2 Playground:

Remember, the defer block isn’t called until the function is exited. So, what is written to the console is:

Do you see the power in that? Now, after I declare an object, I can write a deferred block and it is guaranteed to execute when the function exits. That can be any number of early return statements, that can be because of an exception, or it can be because the function just ran out of code and returned automatically (like mine here). These defer blocks are also managed like a stack, so the most recent defer statements are run first. Let’s see that in action:

This gives us

Hopefully, that example helps make some sense of it. So what did we get? First of all, we got our two print statements that we were doing other work and then even more work showing that the entirety of the function executes before any defer blocks are called. But then it executes newFoo’s defer block first, finishing with originalFoo’s block last.

That seems pretty awesome to me. I realize that Swift isn’t starting anything terribly new here. The Go Programming Language already had this exact concept. That doesn’t mean that it isn’t a good idea.

Swift was created from the very beginning to be a “safe” language and I think defer blocks accomplish this in a few ways. First of all, it ensures that code gets executed. Secondly, it makes sure that only appropriate code is executed (it won’t try to cleanup for objects not yet created). Thirdly, it keeps the cleanup right near the declaration, so readability and discoverability is improved. Why is that safe? If code is easy to read and understand, and the places for modification are well-known and well-understood, then the code is going to be higher quality.

I’m excited for what Swift 2.0 is bringing to the table.

2 Comments

Maury  on January 2nd, 2016

“Do you see the power in that?”

Sure, but I also see very confusing syntax. Here’s an alternate syntax with exactly the same functionality but in a form that doesn’t twist your brain:

func showOffDefer() {
var originalFoo: Foo?

originalFoo = Foo()
originalFoo!.Bar = "Lorem Ipsum"
originalFoo!.Baz = 7

always {
print ("Bar was \(originalFoo!.Bar)")
originalFoo = nil
print ("Now it is \(originalFoo?.Bar)")
}

print("We are doing other work")
}

Conceptually, the code in the defer “runs at the end”, so then by any logic it should be physically placed there.

(note from Pete: edited by me only to wrap code in code markup)

Pete  on January 5th, 2016

There are several advantages to putting the “cleanup” near the creation of the object. It could be considered a good practice for the same reason that declaring a variable near its use is better than just declaring every variable at the top.

Aside from that, though, the point is that you can’t always clean the code up at the end. What if your method needs to return early? Should you check and clean up every time you return early? In this case, you declare the clean up one time and then no matter where your method returns, you are covered.

NSHipster might do a better job of explaining usages than I have at his post on the same subject Guard and Defer.

Here is a relevant quote:

Between guard and the new throw statement for error handling, Swift 2.0 certainly seems to be encouraging a style of early return (an NSHipster favorite) rather than nested if statements. Returning early poses a distinct challenge, however, when resources that have been initialized (and may still be in use) must be cleaned up before returning.

The new defer keyword provides a safe and easy way to handle this challenge by declaring a block that will be executed only when execution leaves the current scope.

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