Understanding the Issue and Setting Up for Success

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In this article, we will delve into the world of view controllers in iOS development. Specifically, we will explore a common issue related to refreshing a view controller’s content. The question presented is straightforward: when creating a form with dynamic content pulled from a web server, how can you refresh the page without causing an app crash?

Background on Threads and Performance

One of the most critical concepts in iOS development is threading. The main thread, also known as the UI thread, is responsible for handling user interactions and updating the visual interface. When your app performs background tasks or makes network requests, it does so on separate threads (often referred to as background threads). This separation ensures that your app remains responsive and can handle multiple tasks simultaneously.

However, when you try to update the UI from a background thread, iOS will not allow it. According to Apple’s documentation, “Never perform UI-related operations from a background thread.” If you attempt to do so, you may encounter issues such as freezes, crashes, or unexpected behavior.

Refreshing a View Controller

In this scenario, we’re dealing with a view controller (ExercisesViewController) and its corresponding view (the form’s UI). The question asks about refreshing the page when a button is clicked. To do so, we need to update the view controller’s content.

Understanding setNeedsDisplay Method

The setNeedsDisplay method is a part of the UIView class in iOS development. When called on a view controller’s view object, this method informs the view that it needs to be redrawn or updated with new data.

However, here’s the catch: setNeedsDisplay can only be called from the main (UI) thread.

The Problem with Using setNeedsDisplay

The issue presented in the question is related to using setNeedsDisplay on a background thread. In the provided code snippet, we see:

- (IBAction)markButtonSelected:(id)sender{
    [self performSelectorInBackground:@selector(setNeedsDisplay) withObject:nil];
}

By calling performSelectorInBackground, we’re attempting to call the setNeedsDisplay method on a background thread. This is not allowed because setNeedsDisplay can only be called from the main thread.

The Solution

To fix this issue, you need to update the code that calls setNeedsDisplay to perform it on the main thread:

- (IBAction)markButtonSelected:(id)sender{
    [self.view setNeedsDisplay];
}

By using [self.view setNeedsDisplay];, we’re explicitly calling the method on the main thread, which is what iOS expects.

Conclusion and Example Use Cases

In this article, we explored a common issue related to refreshing a view controller’s content when creating dynamic forms. We also delved into the concept of threads in iOS development and how they relate to UI updates.

When working with dynamic forms or web servers, it’s essential to keep in mind that updating the UI requires careful consideration of threading and performance. By following best practices like using setNeedsDisplay on the main thread, you can ensure a smooth user experience and avoid potential crashes.

Example Use Case

Suppose we’re creating an app with a form that pulls data from a web server. When a button is clicked, we want to refresh the page by updating the form’s content:

// Add refreshing agenda
scrollViewFrame = CGRectMake(0, 200, 80, 40);
mark = [UIButton buttonWithType:UIButtonTypeRoundedRect];
mark.frame = scrollViewFrame;
[mark setTitle:@"Get Score" forState:UIControlStateNormal];
[mark setBackgroundColor:[UIColor clearColor]];

[mark addTarget:self 
     action:@selector(markButtonSelected:) 
 forControlEvents:UIControlEventTouchUpInside];
[scrollView addSubview:mark];

- (IBAction)markButtonSelected:(id)sender{
    [self.view setNeedsDisplay]; // Update this line to call setNeedsDisplay on the main thread
}

By making this change, we ensure that our app remains responsive and updates its UI correctly when the button is clicked.

Last modified on 2024-02-29