Question

I want to perform [document doSomething] as soon as [document isBusy] is false. What is the best way to do it? I tried it with a while-loop but that delays the following code, which is not what I want.

Answer 1

If the action a semantic of your document model

From your question both isBusy and doSomething are features of your document and the action to doSomething when isBusy goes false is appears to be a semantic of your document model and should therefore be implemented by your document model, i.e. something like:

- (void) setIsBusy:(BOOL)flag
{
   if(flag != _isBusy) // check if this is a change
   {
      _isBusy = flag;
      if(flag)
      {
         // doSomething, or schedule doSomething if it is a long operation etc., e.g.
         [self doSomething];
      }
   }
}

Using KVO to implement semantics within a single object is probably unusual, but there are cases where it is useful. In this case it would replace a direct action with an indirect one – KVO would execute doSomething at exactly the same point as the above sample code, there would just be a number of intermediate system methods between setIsBusy and doSomething plus the associated overhead of setting up the KVO.

If the action is a semantic of your document’s client

Of course, if the linkage between these two is independent of your document model, i.e. is a semantic of the client of your document, then KVO is appropriate and would be implemented in your client. Your client would register as an observer of your document, i.e. something like:

[document addObserver:self
           forKeyPath:@"isBusy"
              options:(NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld)
              context:NULL];

in a method of the client. Then in your client when a notification of the change is received take the appropriate action:

- (void)observeValueForKeyPath:(NSString *)keyPath
                      ofObject:(id)object
                        change:(NSDictionary *)change
                       context:(void *)context
{
   if ([keyPath isEqual:@"isBusy"] && object == document)
      [document doSomething;
   else
      [super observeValueForKeyPath:keyPath
                           ofObject:object
                             change:change
                            context:context];
}