Question

I am new to programming. I am porting cpp (WIN32) to cocoa framework. I have a method called start(process) from where 2 methods gets called. I want to do the operation in it parallely.I want to do InterThread communication.

This can be done by performSelectorOnMainThread:withObject:waitUntilDone.

Here I need to call the 2nd thread first and the 1st thread is called second.The 2nd thread waits for the 1st thread’s signal.(eg:1st thread does addition of two no’s and 2nd thread does the display and some other operations)

[receiverobj performSelectorOnMainThread:withObject:waitUntilDone] 

is the syntax for doing it.But both of them are instance methods of the same class.And the 1st threads return type is a void value and the 2nd threads return value is uint8_t. How to receive the signal from the 1st thread onto the second thread which has begun its execution just before the 1st Thread.

Answer 1

The first thing about Cocoa is that all display code should run on the main thread. So if you are asking how to let the main thread know it needs to do some display work, performSelectorOnMainThread:waitUntilDone: is the right answer. This method works by putting an artificial “event” in the main thread’s run loop (the loop that processes events from the UI and timers etc). The receiver will invoke the method exactly as if you had called it directly but it will happen on the main thread.

If you want to signal another thread that the display work has finished, you can do it synchronously like this:

[receiver performSelectorOnMainThread: @selector(mySelector) withObject: nil waitUntilDone: YES];

The calling thread will then pause until the method has finished.

If you just want to fire and forget it’s

[receiver performSelectorOnMainThread: @selector(mySelector) withObject: nil waitUntilDone: NO];

The pattern is generalisable to any thread with the method performSelectorOnThread:withObject:waitUntilDone:. However, if you do this, you must make sure the target thread is executing a run loop.