How can I invoke a method as soon as a bool flag changes?

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 isBusy has a associated setter method, you can call doSomething in that setter. A better option would be to use Key Value Observing to observe the value of isBusy and take action when it changes.

Answer 2

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 , ie 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, ie 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, ie 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];
}

Answer 3

Use KVC/KVO (Key Value Coding and Key Value Observation). The subject is covered well in the Apple Documentation. KVO and KVC is a basic programming technique which provides may solutions to your problem.