Скачать или смотреть How to Fix an Empty Array in viewDidLoad() When Fetching Data in Swift

Learn why your array is empty in viewDidLoad() and how to properly append values using a callback function in Swift.
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Why Is My Array Empty in viewDidLoad() When Fetching Data?

If you're developing an iOS app using Swift, you might face situations where you expect an array to be filled with values when the view loads, but it turns out to be empty. This is a common issue many developers encounter, particularly when dealing with asynchronous data fetching. In this guide, we will explore the reasons why your array might not populate as expected and how to effectively resolve this problem.

The Problem: Empty Array in viewDidLoad()

Consider the following scenario: You want to populate an array (tagArr) with values fetched from a database when your view loads. However, when you print the array immediately after calling the fetching function in viewDidLoad(), it appears empty. Here's the basic outline of your code:

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In this situation, the output of print(AA) returns an empty array ([]), while print(self.tagArr) inside the callback correctly shows the populated array. The key here lies in asynchronous behavior of the observeTagPool method.

Understanding Asynchronous Calls

The call to API.Tag.observeTagPool is asynchronous, meaning it doesn't block the execution of the subsequent code. As the function fetchTagPool() runs, it immediately returns the current state of self.tagArr, which, at that moment, is still empty since the asynchronous operation has yet to complete.

Fixing the Code: Using a Callback Function

To resolve this issue, you need to modify your fetchTagPool function to return the populated array through a callback rather than directly from the function. Here's how you can implement it:

Updated Code

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Important Considerations

Callback Function: By passing a closure as a parameter to fetchTagPool, you can perform actions with the populated array once it is fully loaded.

Closure Syntax: Notice the use of @ escaping in the callback parameter declaration. This is required for closures that are called after the function that declared them has returned.

Run on Main Thread: If your API.Tag.observeTagPool runs on a background thread, it’s crucial to ensure that updates to any UI elements (like UILabel) happen on the main thread. Here’s how you can do that:

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Conclusion

In summary, the initial issue with your empty array arises from the asynchronous nature of fetching data. By using a callback function, you can make sure your UI reflects the updated data correctly and operates smoothly. If you ever find yourself in this situation again, remember to confirm the execution flow of asynchronous operations in Swift to avoid similar pitfalls.

Now you have the tools to fix your empty array dilemma and make your app more robust and user-friendly.