Pan Gesture Used for Popping View Controller Crashing on iOS 7

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In this article, we’ll explore a common issue when using pan gestures to navigate through view controllers in an iOS application. Specifically, we’ll delve into why a simple pan gesture may crash your app on iOS 7 and how you can fix it.

Introduction

When building iOS applications, we often use gestures as a convenient way to trigger actions or switch between views. In this case, we’re using a pan gesture to pop view controllers from the navigation stack. While this works fine on older versions of iOS, it’s not compatible with iOS 7 due to its unique gesture recognition behavior.

Understanding Pan Gesture Recognition

Before we dive into the issue at hand, let’s quickly review how pan gestures work in iOS. When you create a UIPanGestureRecognizer, you specify an action that will be triggered when the user interacts with the view. In our case, this action is pop:, which will navigate to the previous view controller in the navigation stack.

By default, the gesture recognizer will recognize touch events and trigger the specified action when it detects a swipe gesture. However, the state of the gesture can change over time, indicating whether the user has released their finger or is still swiping.

The Problem with iOS 7

In iOS 7, Apple changed the behavior of gesture recognizers to better handle touch events and animations. Specifically, they introduced a new state called UIGestureRecognizerStateEnded, which indicates when the user releases their finger from the screen.

The issue arises because our pan gesture is still triggered even after the user has released their finger, leading to multiple pop gestures being fired in quick succession. This can cause problems with navigation and animation, as the next pop gesture may not have enough time to complete before a new one is triggered.

Debugging the Issue

To debug this issue, we need to examine our code and see where it’s going wrong. In our pop: method, we’re checking for two states:

if (pan.state == UIGestureRecognizerStateBegan || pan.state == UIGestureRecognizerStateChanged)

However, as our answer points out, this is not sufficient. We should be checking for the end state instead, which will ensure that we only trigger one pop gesture.

if (pan.state == UIGestureRecognizerStateEnded)

Solution

By making this simple change, we can prevent multiple pop gestures from being fired in quick succession, fixing the issue on iOS 7. Here’s our updated pop: method:

- (void)pop:(UIPanGestureRecognizer*)pan
{
    if (pan.state == UIGestureRecognizerStateEnded)
    {
        CGPoint vel = [pan velocityInView:self.view];

        if (vel.x > 1000)
        {
            [self.navigationController popViewControllerAnimated:YES];
        }
    }
}

Additional Considerations

In addition to the issue with gesture state, there’s another consideration worth mentioning: preventing popping into nowhere when the navigation controller is already at the root view. This is done automatically by the popViewControllerAnimated: method, which will not trigger a pop if the navigation controller is already at the root.

However, our pan gesture may still trigger a pop even when the navigation controller is at the root, causing problems with animation and navigation. To prevent this, we can add an additional check to ensure that the navigation controller is not already at the root before popping:

if (pan.state == UIGestureRecognizerStateEnded)
{
    CGPoint vel = [pan velocityInView:self.view];

    if (vel.x > 1000 && self.navigationController.topViewController != [[self.navigationController viewControllers] firstObject])
    {
        [self.navigationController popViewControllerAnimated:YES];
    }
}

Conclusion

In this article, we explored a common issue when using pan gestures to navigate through view controllers in an iOS application. Specifically, we looked at why a simple pan gesture may crash your app on iOS 7 and how you can fix it.

By making a few changes to our code, including checking for the end state of the gesture recognizer and preventing popping into nowhere when the navigation controller is already at the root, we can ensure that our application works smoothly even in iOS 7.

Last modified on 2024-06-23