I'm struggling implementing proper KVO for a number of Bool properties whose
state is retrieved from a bit field or a bit mask. These bits are part of a
single "_state" integer ivar, which will be modified with atomic operations:
enum StateT {
StateZero = 0,
State_X = 1 << 0,
State_1 = 1 << 1,
State_2 = 1 << 2,
StateBarMask = State_1 | State_2
};
@property (BOOL) isFoo; // shall be KVO compliant
@property (BOOL) isBar; // shall be KVO compliant
- (BOOL) isFoo {
return (_state & State_X) != 0;
}
- (BOOL) isBar {
return (_state & StateBarMask) != 0;
}
- (void) doFoo {
StateT old_state = OSAtomicOr32OrigBarrier(State_X, &_state);
if ( (old_state & State_X) != 0 ) {
return; // already in State_X
}
...
}
- (void) doSomething1 {
if (!OSAtomicCompareAndSwap32Barrier(StateZero, State_1, &_state)) {
return;
}
...
}
- (void) doSomething2 {
if (!OSAtomicCompareAndSwap32Barrier(State_1, State_2, &_state)) {
return;
}
...
}
So, in order to implement KVO, I would need to track changes of the ivar
_state, and invoke
-willChangeValueForKey:, and
-didChangeValueForKey:
appropriately.
But, how exactly can I accomplish this with the given code above?
Well, despite the documentation says nothing about the prerequisites when
invoking -willChangeValueForKey: -- but I guess, I should invoke this method
**before** the actual value changes. And likewise, I should invoke
-didChangeValueForKey: **after** the value has been changed.
With that code above, this seems difficult, though.
One approach I came up with (which I don't like though) is to use a "shadow
variable" for the state, where basically the KVO is implemented on, e.g.:
- (BOOL) isBar {
return (_state_shadow & StateBarMask) != 0;
}
When tracking the before and after value of the atomic _state variable, I could
then do something like:
- (void) doSomething1 {
if (!OSAtomicCompareAndSwap32Barrier(State_1, State_2, &_state)) {
return;
}
[self modifiedState:State_1 newState:State_2];
...
}
In modifiedState:newState: I would invoke the KVO methods and set the
_state_shadow accordingly.
Are there better ways?
Thanks for hints!
Andreas
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