Understanding the Problem: iOS UIView Derived View Instance DrawRect Being Called But Not Updating on Screen?

When working with custom views in iOS, it’s not uncommon to encounter issues where the drawRect method is called repeatedly, but the view itself doesn’t update on screen as expected. In this article, we’ll delve into the problem described by the user and explore possible solutions.

Problem Overview

The user is trying to animate a custom view by changing its color property over time. The view has an overridden drawRect method that clears the context and fills the view with the new color. However, despite calling setNeedsDisplay and updating the color property, the view doesn’t update on screen until it stops being manipulated.

Understanding setNeedsDisplay

When a view is modified, its layer needs to be updated to reflect the changes. This is where setNeedsDisplay comes into play. When a view’s layer needs to be updated, it sends a layoutIfNeeded message to its superview and then schedules a redraw of itself when the next time through the event loop.

However, in this case, the user is updating the color property every frame, but only calling setNeedsDisplay on their custom view periodically. This raises questions about why the view isn’t updating on screen as expected.

Understanding drawRect

The drawRect method is called when a view needs to be redrawn due to changes in its properties or layout. When this method is called, it’s responsible for drawing the contents of the view.

In this case, the custom view has an overridden drawRect method that clears the context and fills the view with the new color. This works fine initially, but when the user updates the color property, it seems like the changes are being lost until the view stops being manipulated.

Possible Causes

There are several possible causes for this issue:

1. Layer Repaint Issues: When a view’s layer needs to be updated, it can lead to repaint issues if the update is not properly scheduled.
2. Thread Safety: iOS uses a multi-threaded architecture, which means that views and their updates need to be thread-safe.
3. ** redraw cycles**: When setNeedsDisplay is called on an object which contains an UIImageView, the drawRect method of the UIImageView will also get called but for setNeedsDisplay to update the UIImageView we have to call it every frame.

Solution

To resolve this issue, there are several possible solutions:

1. Schedule Redraws: Instead of calling setNeedsDisplay periodically, schedule redraws using a timer or a delegate callback.
2. Use Core Animation: Consider using Apple’s built-in animation framework, Core Animation, to animate the color change.
3. Improve Layer Repaint Efficiency: Optimizing layer repaints can help improve performance and reduce lag.

Example Solution

Here’s an example of how you could use a timer to schedule redraws:

- (void)updateColor {
    // Update the color property here
    self.color = red;
    
    // Schedule a redraw
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(5 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        [self setNeedsDisplay];
    });
}

In this example, the updateColor method updates the color property and schedules a redraw using a timer.

Conclusion

The problem described by the user is a complex issue that involves understanding the intricacies of iOS’s view lifecycle and layer repaints. By examining the possible causes and solutions outlined above, developers can better understand how to animate custom views on screen without encountering issues with redraw cycles or thread safety concerns.

In conclusion, while this may seem like a complex issue, it is actually quite straightforward once you understand the underlying mechanisms of iOS’s view lifecycle and layer repaints. By following best practices for scheduling redraws and optimizing layer repaint efficiency, developers can create smooth and responsive animations for their custom views.

Last modified on 2024-08-07