UITableView tableView.reloadData()调用时没有更新，请问是什么原因导致的？

Question

I have a table view that initially starts empty. I am trying to update it once an asynchronous method is called Networking().fetchRecipies(ingredients: searchText) . However, after this method is called, the recipieTableView remains unpopulated. What is causeing this error? Below is the code I am running that should be run everytime the text in the text field changes. I know this is working because I am also printing the results of the API call

@IBOutlet weak var recipieTableView: UITableView!
var recipies = LocalData.recipies.recipieList
var filteredRecipies = [Recipie]()
    
    
override func viewDidLoad() {
    filteredRecipies = recipies
    super.viewDidLoad()
}

func searchBar(_ searchBar: UISearchBar, textDidChange searchText: String) {
     Networking().fetchRecipies(ingredients: searchText)
     self.recipieTableView.reloadData()
     DispatchQueue.main.async {
         self.recipieTableView.reloadData()
         self.recipieTableView.beginUpdates()
         self.recipieTableView.endUpdates()
     }
}

Answer 1

However, after this method is called, the recipieTableView remains unpopulated. What is causeing this error

What's causing the error is that when you say "after" you should actually be saying "before". Your code does not run in the order it is written: It runs in this order:

 Networking().fetchRecipies(ingredients: searchText) // 2
 self.recipieTableView.reloadData() // 1...
 // ...

In other words, all your calls to reloadData are happening before you fetch the recipes.

You need to write fetchRecipes such that it reloads the table view after the fetching has finished.

Answer 2

You say the method Networking().fetchRecipies(ingredients:) is called asynchronously, which basically means your code steps into another thread and simultaneously continues to execute to the end of the method your asynchronous function was called from.

So the line self.recipieTableView.reloadData() is called when your data is not yet populated.

After that you call DispatchQueue.main.async which is also unnecessary. Since you're already on the main thread (because that's the convention for searchBar(_:textDidChange:) , as for the most of UIKit methods), the only thing your method does is runs the contents of the dispatched block of code some time later (but chances are still much sooner before your fetch request completes).

Read more on how concurrency works on iOS in the official guide . If you're a beginner, it would be just enough to understand the difference between serial and concurrent queues and sync and async dispatches.

Back to your problem. I suggest that you examine the API of your Networking class and check if there's any way to notify you when your data has been actually fetched. Most iOS APIs provide a completion block that is executed with your newly fetched data passed as an argument. For example, it could look like the following:

Networking().fetchRecipies(ingredients: searchText, completion: { fetchedIngredients in 
   // ... update your data source array
   // reload table view
})

Call tableView.updateData() directly from inside such a block if it's called on the main queue, or wrap it inside DispatchQueue.main.async { } if your completion block is invoked from a background queue (refer to the API to find it out).

And, most important, don't forget to update your array (I believe it's either recipies or filteredRecipies ) which serves as the source for your table's data BEFORE you call updateData() .