Understanding the Issue with performSelectorOnMainThread:withObject:waitUntilDone:

When developing iOS apps, one of the most common threading challenges developers face is dealing with asynchronous execution. The performSelectorOnMainThread:withObject:waitUntilDone: method provides a convenient way to execute code on the main thread while ensuring that other tasks are not blocked by the operation.

However, as we delve into this method’s behavior, we’ll discover some quirks and potential pitfalls that can lead to unexpected issues. In this article, we’ll explore these problems in depth, provide practical solutions, and discuss alternative approaches using dispatch.

What is performSelectorOnMainThread:withObject:waitUntilDone:?

performSelectorOnMainThread:withObject:waitUntilDone: is a method available on Objective-C objects that allows you to execute another selector (method) on the main thread. The primary purpose of this method is to ensure that asynchronous execution is not used to block the UI or other tasks.

The basic syntax is as follows:

- (void)performSelectorOnMainThread:(SEL)selector withObject:(id)object waitUntilDone:(BOOL)wait;

Here, selector specifies the method to be executed on the main thread, while object is an optional parameter that can be used to pass data to the executed method. The waitUntilDone parameter determines whether the execution should block until the operation completes.

How Does it Work?

When you call performSelectorOnMainThread:withObject:waitUntilDone:, Apple’s runtime checks if the specified selector is already being executed on the main thread or not. If it is, the execution of your method will be merged with the existing selector invocation.

If the selector is not executing on the main thread, a new task is created to execute the selector on the main thread using dispatch_get_main_queue() and dispatch_async(). This ensures that the operation is performed in a separate queue, allowing other tasks to continue running without interruption.

Quirks and Pitfalls

Despite its convenience, performSelectorOnMainThread:withObject:waitUntilDone: has some quirks and pitfalls that can lead to unexpected behavior:

1. Interference with Automatic Reference Counting (ARC): ARC can sometimes interfere with the execution of selectors on the main thread using this method.

2. Blocking the Main Thread: If the waitUntilDone parameter is set to YES, your selector may block the main thread, which can lead to a freeze or hang in your app.

3. Task Creation: When performSelectorOnMainThread:withObject:waitUntilDone: creates a new task for executing a selector on the main thread, it uses dispatch_async() with an implicitly created serial queue. However, if multiple tasks are concurrently posted using this method, it can lead to unexpected behavior and performance issues.

4. iOS Betas: The question from the Stack Overflow post mentions that this issue starts occurring in iOS betas, which implies a potential bug or compatibility problem with Apple’s runtime on these versions of the operating system.

5. Device-Specific Issues: The original poster experienced issues with devices running specific iOS versions and models, which may be related to hardware or software-specific factors.

Alternative Approaches Using dispatch

One alternative approach to using performSelectorOnMainThread:withObject:waitUntilDone: is to create a block and post it on the main thread using dispatch_async():

dispatch_async(dispatch_get_main_queue(), ^{
    // Do work here
});

This approach provides similar functionality without the potential pitfalls associated with performSelectorOnMainThread:withObject:waitUntilDone:, such as blocking the main thread or interfering with ARC.

By understanding how these methods interact with each other and Apple’s runtime, developers can create more robust, efficient, and reliable iOS apps that take advantage of asynchronous execution and threading.

Last modified on 2024-03-10