Objective c: How to use memory managment properly for asynchronous methods

Question

I need to call a method that starts some asynchronous code

MyClass* myClass = [[MyClass alloc] init];
[myClass startAsynchronousCode];

Now I cant simply release it as this would cause an error since the code is still running:

[myClass release];  // causes an error

What is the best way to deal with the memory?

Answer 1

You could have -[MyClass startAsynchronousCode] invoke a callback:

typedef void(*DoneCallback)(void *);

-(void) startAsynchronousCode {
  // Lots of stuff
  if (finishedCallback) {
    finishedCallback(self);
  }
}

and then instantiate a MyClass like this:

MyClass *myClass = [[MyClass alloc] initWith: myCallback];

myCallback might look like this:

void myCallback(void *userInfo) {
  MyClass *thing = (MyClass *)userInfo;
  // Do stuff
  [thing release];
}

Answer 2

What I've always done is maintained an instance variable that points to the asynchronous object.

- (id)init {
    myClass = [[MyClass alloc] init];
    [myClass startAsynchronousCode];
}

- (void)myClassDidFinish:(MyClass *)myClass {
    [myClass release];
}

Answer 3

You must retain your myClass object internally in startAsynchronousCode method. And release it internally too after it finished.

This behavior used in NSURLConnection, UIAlertView and other async objects.

Answer 4

How are you invoking the asynchronous code? If you use NSThread +detachNewThreadSelector:toTarget:withObject:, you'll find that the target object is retained by the thread until it terminates and then it is released. So you can release the object immediately after the asynchronous message.

e.g.

@implementation MyClass

-(void) startAsynchronousCode
{
    [NSThread detachNewThreadSelector: @selector(threadMain:) toTarget: self withObject: nil];
}

-(void) threadMain: (id) anObject
{
    NSAutoreleasePool* pool = [[NSAutoreleasePool alloc] init];
    // do stuff
    [pool drain];
}
@end

With the above, the following code is perfectly safe:

MyClass* myClass = [[MyClass alloc] init];
[myClass startAsynchronousCode];
[myClass release];