Question

i want to block the main thread until something else is done in the background.

i used:

result=[self performSelectorInBackground:@selector(calculate:) withObject:expression];

just bellow this line I am using result:

[self use:result];

i dont want to use result until it is available.
to achieve this I implemented

-calculate:
{
    [[(AppDelegate *)[[UIApplication sharedApplication] delegate] queue] waitUntilAllOperationsAreFinished];
    calculating result...
}

and still, the result is used before it is calculated. so, i didnt block the main thread. pls help me do that.

thanks

Answer 1

You don’t want to block the main thread; it will prevent users from using the UI.

What you want is to use the result obtained in the background once it’s ready.

One method is to call -use: at the end of the method called in the background: you define

-(void)calculate:(NSString*)input 
{
    .... do the calculation ...
    [self performSelectorOnMainThread:@selector(use:) withObject:result];
}

then you just call from the main thread

[self performSelectorInBackground:@selector(calculate:) withObject:expression];

This way, you can achieve what you want without blocking the main thread.

By the way, -performSelectorInBackgronud has nothing to do with NSOperationQueue. So [queue waitUntilAllOperationsAreFinished] doesn’t wait the execution of the method invoked by -performSelectorInBackground.
And in any case, you shoudln’t call waitUntilAllOperationAreFinished unless absolutely necessary.

To know when an NSOperation is done, you KVC the property isFinished. See the NSOperation reference.