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Unlock the mysteries of Swift completion handlers. Learn how they work, what role closures play, and how to effectively manage asynchronous tasks.
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Understanding the Completion Handler in Swift: A Deep Dive into Closures and Callbacks

In the world of Swift programming, one of the concepts that often causes users confusion is the completion handler. If you've spent time tackling closures, you may have noticed how tricky it can be to understand exactly when certain parameters are passed in those handlers. In this guide, we’ll explore how completion handlers work, using a practical example to clarify your doubts and enhance your understanding.

The Problem: Confusion Around Completion Handlers

If you've been grappling with code like this:

[[See Video to Reveal this Text or Code Snippet]]

You may find yourself asking:

When do manager and file get passed to the completion closure?

How does the present() function tie into all this?

If these questions are keeping you up at night, you're not alone!

Breaking Down the Solution

1. Understanding @ escaping Closures

To start, take note that the closure is marked with @ escaping, which means it can outlive the lifespan of the function it’s passed to. This is crucial for asynchronous operations where you’ll want to return data after the function has completed.

2. Role of the viewer

Here’s where it gets interesting:

[[See Video to Reveal this Text or Code Snippet]]

By assigning the completion closure to viewer, we’re not calling the closure immediately. Instead, we’re storing it so that viewer can call it later, once certain events transpire—like a file being picked or an operation completing.

3. The present() Function

The call to vc?.present(self, animated: true) is vital but it does not call the completion closure. The present() function simply presents the current view controller, and it's in that presented view controller (viewer) where the closure will eventually be invoked.

4. When Are manager and file Passed?

So, how do manager and file get populated? After interacting with the viewer, once an action occurs—like selecting a file—the completion closure is called within viewer with appropriate arguments, which will be filled at that point.

Here's what happens step-by-step:

The user interacts with the viewer.

Once a file is picked, viewer calls the completion closure.

It passes the manager and file as arguments back to where picker.pick was called.

5. The Client Code

When you see the function call:

[[See Video to Reveal this Text or Code Snippet]]

You might expect manager and file from the picker directly. However, the parameters are delivered to the closure once the viewer decides to execute it after the selection process. The power of the closure is that it allows the client to define what happens when the file is picked without having to manage how the picking happens.

Conclusion

In Swift, understanding how completion handlers work with closures can greatly enhance your coding experience. With the breakdown above, you should now have a clearer view of how these components come together. The key takeaway is that while the present method plays a vital role in presenting the user interface, it's the viewer that ultimately handles calling the completion closure with the necessary arguments.

We hope this guide demystified the workings of completion handlers in Swift and empowered you to use them confidently in your programming endeavors!