Question

What is the differences between these two?

 [NSThread detachNewThreadSelector:@selector(method) toTarget:self withObject:nil];
 [self performSelectorInBackground:@selector(method) withObject:nil];

I normally use the second method to spawn a new thread.
But I was wondering if I call this twice like shown below in a method then what is going to happen? Also If I have a tabmenu and each menu spawns a thread then which one I should use?

 [self performSelectorInBackground:@selector(method1) withObject:nil];
 [self performSelectorInBackground:@selector(method2) withObject:nil];

Answer 1

They are identical. Here is the what the official documentation has to say on this topic:

In
iOS and Mac OS X v10.5 and later, all
objects have the ability to spawn a
new thread and use it to execute one
of their methods. The
performSelectorInBackground:withObject:
method creates a new detached thread
and uses the specified method as the
entry point for the new thread. For
example, if you have some object
(represented by the variable myObj)
and that object has a method called
doSomething that you want to run in a
background thread, you could could use
the following code to do that:

[myObj
performSelectorInBackground:@selector(doSomething)
withObject:nil];

The effect of calling
this method is the same as if you
called the
detachNewThreadSelector:toTarget:withObject:
method of NSThread with the current
object, selector, and parameter object
as parameters. The new thread is
spawned immediately using the default
configuration and begins running.
Inside the selector, you must
configure the thread just as you would
any thread. For example, you would
need to set up an autorelease pool (if
you were not using garbage collection)
and configure the thread’s run loop if
you planned to use it. For information
on how to configure new threads, see
“Configuring Thread Attributes.”

As for what happens if you do:

[self performSelectorInBackground:@selector(method1) withObject:nil];
[self performSelectorInBackground:@selector(method2) withObject:nil];

…you will spawn two new threads, one of which starts executing at method1 and one of which starts executing at method2. The threads may execute concurrently (i.e. the second one will not wait for the first one to terminate before it starts executing).