Re: KVO using threads
Re: KVO using threads
- Subject: Re: KVO using threads
- From: "Paulo F. Andrade" <email@hidden>
- Date: Wed, 9 May 2007 11:57:14 +0100
Hi,
First of all, thank you very much for your help.
I think I have understood your Receptionist pattern. However because,
as you said, it is difficult to interpose the proxy-like object in
all situations, I was thinking of doing it slightly different.
I would subclass my controller (in my case an NSArrayController) and
override the observeValueForKeyPath:... to call the super version of
the method, using the perfomSelectorOnMainThread: .
Because I can only send one argument using
performSelectorOnMainThread: I would need a helper method to do this.
Maybe it's better to write some code, here goes:
@implementatiton ArrayControllerSubclass
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)
object change:(NSDictionary *)change context:(void *)context
{
NSArray *args = [NSArray arrayWithObjects: keyPath, object, change,
context, nil];
//call the helper method
[self performSelectorOnMainThread:@selector(auxiliaryMethod:)
withObject:args waitUntilDone:NO modes:[NSArray
arrayWithObject:NSDefaultRunLoopMode]];
}
- (void)auxiliaryMethod:(NSArray *)args
{
[super observeValueForKeyPath:[args objectAtIndex:0] ofObject:[args
objectAtIndex:1] change:[args objectAtIndex:2] context:[args
objectAtIndex:3]];
}
@end
Before going about testing this, does anybody see something terribly
wrong with this approach that I'm overlooking?
As for the sending KVO side, I'm already using Core Data with my
domain objects. So putting a synchronized(self){...} on the mutator
methods would suffice to keep willChange and didChange ordered.
Thank you for your time!
Paulo F. Andrade 52439@IST
mailto: email@hidden
On 2007/05/01, at 22:11, Chris Kane wrote:
Actually KVO and its messages are thread-safe. But the objects
that send (by having their properties changed, say) and receive
(observers) the KVO notifications may not be thread-safe, or expect
to receive the messages on different threads, which may require
special additional actions to handle. Such is the case with the
Bindings-related objects (like the controllers) in AppKit.
One general solution might be to interpose a proxy-like object
between the model objects (presumably thread-safe) and the
controllers. I called this a receptionist pattern in my WWDC 2006
talk. The receptionist object should implement
observeValueForKeyPath:..., you should initialize the receptionist
with the real object (say, a controller), and you should use the
receptionist wherever you would otherwise refer to the controller.
(This last step is sometimes difficult to do in all cases.) When
the receptionist receives a KVO observeValue... call, it should
save the parameters in a little private helper object, and send
itself a private message to be performed on the main thread
(performSelectorOnMainThread...) to deliver that message with its
parameters to the real object (which will be on the main thread
then). No, I don't have an example which does this.
That handles (if you can pull it off) the model -> controller ->
view data flow direction. Presumably if your model objects are
thread-safe, changes on the main thread coming down from the view
to the model objects will be fine.
On the sending (KVO-notification-causing) side of things, using
automatic KVO is not entirely thread-safe because the willChange/
change/didChange combination is not atomic. Using manual KVO
(sending the willChange... and didChange... methods yourself in all
the right places) you can make these atomic with your own lock (I
suggest using the object itself as the lock with @synchronized() is
a reasonable default choice of locking technique). Using automatic
KVO can, for example, make the change dictionary that observers get
completely wrong with respect to the change that occurred due to
execution ordering issues.
Chris Kane
Cocoa Frameworks, Apple
On May 1, 2007, at 12:24 PM, Scott Anguish wrote:
No. KVO messages are not threadsafe.
On May 1, 2007, at 2:40 PM, Paulo F. Andrade wrote:
Hi!
Does KVO work when the changes are made in different thread?
I'm changing a tooMany relationship using the add<Key>Object:
method generated by the Xcode modeling tool.
The changes however aren't reflected in my tableview. I've also
tried using the mutableSetForKey:.
I know my tableView is set up correctly because if I restart my
application (causing all changes do flush to the SQLite store),
every change I made shows up correctly!
I've search the net for this and found that Bindings don't work
with DO, does this means they don't work with threads?
Paulo F. Andrade 52439@IST
mailto: email@hidden
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