Resizing UIView from Right to Left with Animation on iOS

In this article, we will explore how to resize a UIView from right to left with animation on iOS. This can be achieved by using the layoutSubviews method in conjunction with the animateWithDuration block.

Understanding the Problem

The problem at hand is that when animating the frame of a UIView, it sometimes behaves unexpectedly, bouncing or oscillating between two values instead of smoothly transitioning to its final position. We will explore why this happens and how we can avoid it by using the layoutSubviews method.

Background on UIView Animation

When you animate a view’s properties using the animateWithDuration block, UIKit internally uses a mechanism called “animation blocks” to perform the animation. The animation block is executed in the main thread, which allows for smooth animations and precise control over the animation’s timing.

However, when animating a view’s frame or bounds, there are some limitations and gotchas that can lead to unexpected behavior. One of these issues is related to how UIKit handles the layout of views within a superview.

The Issue with Animated Frame Updates

When you update a view’s frame programmatically using UIView animateWithDuration, UIKit doesn’t automatically adjust the frame of any subviews or other views that may overlap with the changing frame. This means that if your view has subviews, they might not be resized correctly when the parent view is animated.

To illustrate this issue, let’s consider an example:

Suppose we have a UIView called searchBar with a UILabel as its only child:

self.searchBar = [[UIView alloc] initWithFrame:CGRectMake(0.f, 0.f, 100.f, 44.f)];
self.searchBar.backgroundColor = [UIColor redColor];
self.searchBar.label = [[UILabel alloc] initWithFrame:CGRectMake(10.f, 10.f, 80.f, 20.f)];
self.searchBar.label.text = @"Search";

Now, when we animate the searchBar’s frame using UIView animateWithDuration, the animation might not work as expected:

[UIView animateWithDuration:1.0 animations:^{
    self.searchBar.frame = CGRectMake(200.f, 0.f, 100.f, 44.f);
}];

In this case, when we call self.searchBar.layoutSubviews, it will only update its own subviews (in this case, the label), but not the searchBar itself. This means that the label might be resized correctly, but the search bar’s background color and other properties will remain unchanged.

The Solution: Layout Subviews

To avoid this issue, we can call layoutSubviews after updating the view’s frame, as hinted in the original answer:

[UIView animateWithDuration:1.0 animations:^{
    self.searchBar.frame = CGRectMake(200.f, 0.f, 100.f, 44.f);
    [self.searchBar layoutSubviews];
}];

By calling layoutSubviews, we ensure that any subviews within the searchBar are properly resized and updated to match its new frame. This should fix the unexpected animation behavior.

More on Layout Subviews

Let’s dive deeper into what happens when you call layoutSubviews. When you create a view, UIKit automatically lays out your child views in a specific order using a mechanism called “layout”. The layout process involves several steps:

1. Layout Size: Calculate the size of each child view.
2. Layout Bounds: Calculate the bounds (i.e., position and size) of each child view relative to its parent view’s frame.
3. Layout Frame: Calculate the final frame for each child view by combining its layout size and bounds.

When you update a view’s frame using UIView animateWithDuration, UIKit doesn’t automatically update the layouts of your subviews. By calling layoutSubviews after updating the view’s frame, we ensure that any subviews are properly resized to match the new frame.

The Example: Resizing a SearchBar

Let’s put this solution into practice with an example:

- (IBAction)resizeButtonTapped:(id)sender {
    if (searchBar.frame.origin.x == 95) {
        [UIView animateWithDuration:1.0 animations:^{
            self.searchBar.frame = CGRectMake(0.f, 0.f, 280.f, 44.f);
            [self.searchBar layoutSubviews];
        }];
    } else {
        [UIView animateWithDuration:1.0 animations:^{
            self.searchBar.frame = CGRectMake(95.f, 0.f, 185.f, 44.f);
            [self.searchBar layoutSubviews];
        }];
    }
}

In this example, we update the searchBar’s frame using UIView animateWithDuration, and then call layoutSubviews after updating the frame. This ensures that any subviews (in this case, the label) are properly resized to match the new frame.

Conclusion

Resizing a UIView from right to left with animation on iOS can be challenging due to UIKit’s internal mechanisms and limitations. By understanding how to use the layoutSubviews method, we can avoid unexpected behavior and achieve smooth animations.

In this article, we explored the issue with animated frame updates, how it affects subviews, and how calling layoutSubviews after updating a view’s frame can fix the problem. We also provided an example demonstrating how to resize a search bar from right to left using this technique.

With this knowledge, you should be able to create smooth animations when resizing views on iOS. Remember to always call layoutSubviews after updating a view’s frame to ensure proper layout and animation behavior.

Last modified on 2023-09-29