Understanding UIApplicationWillResignActiveNotification and its Impact on UI Changes

In iOS development, notifications are used to inform applications about various system-level events. One such notification is UIApplicationWillResignActiveNotification, which is sent to an application when it is about to resign active state (i.e., the user is navigating away from the app or switching to another app). This notification provides an opportunity for developers to make changes to their UI before the app relinquishes control.

However, in many cases, this delay can lead to a frustrating experience for users. They may see the previous state of the app’s UI for a brief moment after they’ve left it, which can be annoying and unprofessional.

In this article, we’ll explore why UIApplicationWillResignActiveNotification causes delays in UI changes and discuss potential workarounds to achieve immediate UI updates.

Notification Delivery Mechanism

To understand how UIApplicationWillResignActiveNotification works, let’s dive into its notification delivery mechanism.

Notifications are sent by the system to registered observers using the NSNotificationCenter. When an application wants to become an observer of a specific notification type (in this case, UIApplicationWillResignActiveNotification), it must register itself with the NSNotificationCenter instance using the addObserver:selector:name:object: method.

Here’s an example code snippet that demonstrates how to add an observer for the UIApplicationWillResignActiveNotification:

[[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(applicationWillResignActive:)
                                             name:UIApplicationWillResignActiveNotification
                                           object:nil];

In this code:

• We call addObserver:selector:name:object: and pass the following parameters:
  • The default notification center instance (NSNotificationCenter defaultCenter).
  • The current implementation of the observer’s selector method (applicationWillResignActive:).
  • The name of the notification we’re interested in observing (UIApplicationWillResignActiveNotification).
  • nil as the object that sent the notification.

Once the observer is registered, the system will send notifications to our application when it encounters the specified event (in this case, an app resigning active state). Our implementation of the selector method will then be called to perform necessary actions before relinquishing control back to the system.

The Delay: Understanding When the Notification is Delivered

Now that we’ve covered how to register as a notification observer, let’s explore why there’s often a delay between when the notification is sent and when our code executes.

The reason for this delay lies in how notifications are delivered. In iOS, notifications are stored in a queue until they’re processed by the system. When a notification is received, it’s added to a queue (called a “notification queue”) and then scheduled to be dequeued at some point in the future.

When an app resigns active state, the system sends the UIApplicationWillResignActiveNotification to all observers who have registered for this event type. The system does not immediately notify our implementation of the selector method; instead, it stores the notification and schedules it to be processed later.

The processing time depends on various factors, such as:

• The number of active notifications in the queue.
• The app’s workload at the moment (e.g., if it’s performing background tasks).
• System resources available for handling user interactions.

Example Scenario: Delayed UI Update

Let’s consider a scenario where we want to update our app’s UI immediately after resigning active state. We’ve implemented our changes in the applicationWillResignActive: method:

- (void)applicationWillResignActive:(NSNotification *) notification {
    // my changes (hide some views, change bg color, change text, etc)
    self.view.backgroundColor = [UIColor darkGrayColor];
}

In this example, we’re updating the app’s background color when resigning active state. However, due to the notification queue, there might be a delay before our implementation is called.

For instance, suppose another task in our app has higher priority and consumes more system resources. In such cases, the applicationWillResignActiveNotification will remain queued until other tasks are completed or become less demanding on resources.

As a result, when we’re trying to update the UI after resigning active state, our implementation may not be called immediately. Instead, we might see the previous background color for a fraction of a second before it’s updated to darkGrayColor.

Potential Workarounds: Immediate UI Updates

Given the limitations with notification delivery and processing times, how can we ensure immediate UI updates when an app resigns active state? Here are some potential workarounds:

1. Implement Custom Gesture Recognizers or Actions

To bypass the delay in notifications, consider implementing custom gesture recognizers or actions that respond to user interactions (e.g., tapping on a view) instead of relying solely on UIApplicationWillResignActiveNotification.

Here’s an example code snippet for a custom gesture recognizer:

#import <UIKit/UIKit.h>

@interface MyView : UIView

@property (nonatomic, strong) UIButton *closeButton;

@end

@implementation MyView

- (instancetype)initWithFrame:(CGRect)frame {
    self = [super initWithFrame:frame];
    if (self) {
        // Initialize closeButton button and set delegate
        self.closeButton = [[UIButton alloc] initWithFrame:CGRectMake(0, 0, 50, 30)];
        [self.closeButton addTarget:self action:@selector(closeButtonTapped:) forControlEvents:UIControlEventTouchUpInside];

        // Configure closeButton to appear at specific location
        self.closeButton.backgroundColor = [UIColor redColor];
        [self addSubview:closeButton];
    }
    return self;
}

- (void)closeButtonTapped:(UIButton *)button {
    // Perform necessary actions when closeButton is tapped
    self.backgroundColor = [UIColor darkGrayColor]; // update background color
}

By implementing a custom gesture recognizer, we can ensure that our UI updates occur immediately without relying on notification delivery times.

2. Use a Separate Thread for Updating the UI

Another approach to achieve immediate UI updates is to use a separate thread for updating the UI after resigning active state. This method involves executing the update operations in the background, allowing the main application loop to remain responsive and minimizing delays.

Here’s an example of how you could implement this on a background thread:

- (void)applicationWillResignActive:(NSNotification *) notification {
    // Create a separate background queue if needed
    dispatch_queue_t bgQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

    // Execute the UI update operations in the background queue
    dispatch_async(bgQueue, ^{
        // Update your app's UI here (no delays expected)
        self.view.backgroundColor = [UIColor darkGrayColor];
    });
}

This approach ensures that UI updates occur concurrently with the main application loop, reducing potential delays and improving overall responsiveness.

3. Directly Modify the App’s Window Property

For a more direct approach, you can modify the app’s window property to immediately update its background color:

- (void)applicationWillResignActive:(NSNotification *) notification {
    // Update the app's window properties directly
    [[UIWindow *] setRootViewController:self];
    self.view.backgroundColor = [UIColor darkGrayColor]; // update background color
}

While this method allows for immediate UI updates, it can have unintended consequences on your app’s layout and behavior. Be cautious when using this approach.

Conclusion

In conclusion, UIApplicationWillResignActiveNotification causes delays in UI changes due to the notification queue and processing times. To overcome these limitations, you can explore workarounds such as implementing custom gesture recognizers or actions, using a separate thread for updating the UI, or directly modifying the app’s window properties.

By choosing an approach that suits your specific needs and design requirements, you can ensure immediate UI updates when an application resigns active state.

Last modified on 2024-11-14