UISlider Subclass Causing UI Hang in Only One UIView of Universal App

Abstracting away platform-specific complexities is an essential aspect of developing universal iOS apps. In this article, we will explore a peculiar issue encountered by a developer while working on such an app. The problem revolves around a custom UISlider subclass causing a UI hang in one of its view controllers. We’ll delve into the code and discuss possible causes to help you identify similar issues in your own projects.

The Problem

A developer created a universal iOS app with separate iPhone and iPad view controllers as subclasses of a XIB-less view controller. Each subclassed view controller contains a device-specific XIB, with each XIB containing portrait and landscape UIViews that are switched upon orientation change. The controls common to all four UIViews are synchronized with an IBAction method linked through IBOutletCollections. Recently, the developer created a custom UISlider subclass for their own slider design, which appears to work fine in both portrait and landscape orientations, except when switching back from landscape to portrait.

In this scenario, the custom slider’s implementation code is provided, along with two action methods that are triggered by the slider: volumeChange and volumeChangeFinish. The issue manifests only on iPhone simulators running in portrait orientation and disappears when switching to landscape or back to portrait.

The Code

Here is the relevant part of the custom UISlider subclass implementation:

- (void)setValue:(CGFloat)value animated:(BOOL)animated {
    [super setValue:value animated:animated];
    
    // Update other sliders on touch inside up
    if ([self sender] == self.volumeSliders) {
        VolumeSlider *incomingVolume = sender;
        for (VolumeSlider *volumeSlider in self.volumeSliders) {
            volumeSlider.value = incomingVolume.value;
        }
    }
}

- (IBAction)volumeChangeFinish:(id)sender {
    NSLog(@"VOLUME change finished");
    
    // Update other sliders on touch inside up
    VolumeSlider *incomingVolume = sender;
    for (VolumeSlider *volumeSlider in self.volumeSliders) {
        volumeSlider.value = incomingVolume.value;
    }
}

In the setValue:animated: method, we update the value of all VolumeSlider instances within the view controller. However, this update is not atomic and can lead to unexpected behavior if called simultaneously.

Possible Causes

Several factors might contribute to the UI hang observed on iPhone simulators running in portrait orientation:

1. Non-atomic updates

As mentioned earlier, the setValue:animated: method updates multiple sliders within a single block of code. If this block is executed simultaneously with other concurrent updates, it can lead to inconsistent values and UI hangs.

2. Corrupted view

The answer provided by the developer suggests that instantiating a new, stock UISlider into the iPhone view resolved the issue. This implies that there might be a corrupted or partially loaded view in the XIB that’s causing the problem.

To confirm this hypothesis, you can try the following:

• Create a new XIB and add a single UISlider to it.
• Run your app on the iPhone simulator with the new XIB.
• If the issue persists, investigate the loading process of your original XIB to identify any potential issues.

3. iOS version-specific bug

Given that the problem only occurs on iPhone simulators running iOS 4.3, it’s possible that there’s a platform-specific bug in this version of iOS. You can try running your app on earlier or later versions of iOS to see if the issue persists.

4. Other concurrent updates

It’s also possible that other concurrent updates within your view controller are causing the UI hang. Make sure to analyze all relevant code and identify any potential sources of synchronization issues.

Conclusion

In conclusion, this article explored a peculiar issue encountered by a developer working on a universal iOS app. By examining the code and discussing possible causes, we can better understand how to diagnose and fix similar problems in our own projects. Remember that abstracting away platform-specific complexities is crucial for developing apps with minimum maintenance effort.

Recommendations

To avoid such issues in your future projects:

• Use atomic updates when updating multiple values concurrently.
• Regularly review and test the loading process of your XIBs to ensure they are not corrupted or partially loaded.
• Test your app on various versions of iOS to identify any platform-specific bugs.

By following these guidelines, you can develop more robust and reliable apps that minimize the risk of UI hangs and other unexpected behavior.

Last modified on 2024-12-02