Swift @escaping and Completion Handler

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Swift Completion Handler Escaping & Non-Escaping:

As Bob Lee explains in his blog post Completion Handlers in Swift with Bob:

Assume the user is updating an app while using it. You definitely want
to notify the user when it is done. You possibly want to pop up a box
that says, “Congratulations, now, you may fully enjoy!”

So, how do you run a block of code only after the download has been
completed? Further, how do you animate certain objects only after a
view controller has been moved to the next? Well, we are going to find
out how to design one like a boss.

Based on my expansive vocabulary list, completion handlers stand for

Do stuff when things have been done

Bob’s post provides clarity about completion handlers (from a developer point of view it exactly defines what we need to understand).

@escaping closures:

When one passes a closure in function arguments, using it after the function’s body gets executed and returns the compiler back. When the function ends, the scope of the passed closure exist and have existence in memory, till the closure gets executed.

There are several ways to escaping the closure in containing function:

  • Storage: When you need to store the closure in the global variable, property or any other storage that exist in the memory past of the calling function get executed and return the compiler back.

  • Asynchronous execution: When you are executing the closure asynchronously on despatch queue, the queue will hold the closure in memory for you, can be used in future. In this case you have no idea when the closure will get executed.

When you try to use the closure in these scenarios the Swift compiler will show the error:

error screenshot

For more clarity about this topic you can check out this post on Medium.

Adding one more points , which every ios developer needs to understand :

  1. Escaping Closure : An escaping closure is a closure that’s called after the function it was passed to returns. In other words,
    it outlives the function it was passed to.
  2. Non-escaping closure : A closure that’s called within the function it was passed into, i.e. before it returns.

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