KVC / KVO and bindings: why am I only getting one change notification?
I see some quirky behavior with Cocoa KVC / KVO and bindings. I have an object NSArrayController
, its "content" is associated with NSMutableArray
, and I have a controller registered as a property observer arrangedObjects
on NSArrayController
. With this setup, I expect to get a KVO notification every time the array is changed. However, it appears that the KVO notification is sent only once; the first time the array was changed.
I created a new "Cocoa Application" project in Xcode to illustrate the problem. Here is my code:
BindingTesterAppDelegate.h
#import <Cocoa/Cocoa.h>
@interface BindingTesterAppDelegate : NSObject <NSApplicationDelegate>
{
NSWindow * window;
NSArrayController * arrayController;
NSMutableArray * mutableArray;
}
@property (assign) IBOutlet NSWindow * window;
@property (retain) NSArrayController * arrayController;
@property (retain) NSMutableArray * mutableArray;
- (void)changeArray:(id)sender;
@end
BindingTesterAppDelegate.m
#import "BindingTesterAppDelegate.h"
@implementation BindingTesterAppDelegate
@synthesize window;
@synthesize arrayController;
@synthesize mutableArray;
- (void)applicationDidFinishLaunching:(NSNotification *)notification
{
NSLog(@"load");
// create the array controller and the mutable array:
[self setArrayController:[[[NSArrayController alloc] init] autorelease]];
[self setMutableArray:[NSMutableArray arrayWithCapacity:0]];
// bind the arrayController to the array
[arrayController bind:@"content" // see update
toObject:self
withKeyPath:@"mutableArray"
options:0];
// set up an observer for arrangedObjects
[arrayController addObserver:self
forKeyPath:@"arrangedObjects"
options:0
context:nil];
// add a button to trigger events
NSButton * button = [[NSButton alloc]
initWithFrame:NSMakeRect(10, 10, 100, 30)];
[[window contentView] addSubview:button];
[button setTitle:@"change array"];
[button setTarget:self];
[button setAction:@selector(changeArray:)];
[button release];
NSLog(@"run");
}
- (void)changeArray:(id)sender
{
// modify the array (being sure to post KVO notifications):
[self willChangeValueForKey:@"mutableArray"];
[mutableArray addObject:[NSString stringWithString:@"something"]];
NSLog(@"changed the array: count = %d", [mutableArray count]);
[self didChangeValueForKey:@"mutableArray"];
}
- (void)observeValueForKeyPath:(NSString *)keyPath
ofObject:(id)object
change:(NSDictionary *)change
context:(void *)context
{
NSLog(@"%@ changed!", keyPath);
}
- (void)applicationWillTerminate:(NSNotification *)notification
{
NSLog(@"stop");
[self setMutableArray:nil];
[self setArrayController:nil];
NSLog(@"done");
}
@end
And here's the output:
load
run
changed the array: count = 1
arrangedObjects changed!
changed the array: count = 2
changed the array: count = 3
changed the array: count = 4
changed the array: count = 5
stop
arrangedObjects changed!
done
As you can see, the KVO notification is only sent the first time (and again when the app exits). Why is this so?
update:
Thanks to orque for pointing out that I should be tied to contentArray
mine NSArrayController
, not just content
. The above code works as soon as this change is made:
// bind the arrayController to the array
[arrayController bind:@"contentArray" // <-- the change was made here
toObject:self
withKeyPath:@"mutableArray"
options:0];
First, you must bind to contentArray (not content):
[arrayController bind:@"contentArray"
toObject:self
withKeyPath:@"mutableArray"
options:0];
Then an easy way is to just use arrayController to change the array:
- (void)changeArray:(id)sender
{
// modify the array (being sure to post KVO notifications):
[arrayController addObject:@"something"];
NSLog(@"changed the array: count = %d", [mutableArray count]);
}
(in a real-world scenario, you most likely just want the button action to call -addObject :)
Usage - [NSMutableArray addObject] will not automatically notify the controller. I see that you were trying to work around this by manually using willChange / didChange on mutableArray. It won't work because the array itself hasn't changed. That is, if the KVO system asks for a mutableArray before and after the change, it will still have the same address.
If you want to use - [NSMutableArray addObject], you can change the / didChange value on ordered objects:
- (void)changeArray:(id)sender
{
// modify the array (being sure to post KVO notifications):
[arrayController willChangeValueForKey:@"arrangedObjects"];
[mutableArray addObject:@"something"];
NSLog(@"changed the array: count = %d", [mutableArray count]);
[arrayController didChangeValueForKey:@"arrangedObjects"];
}
There may be a cheaper key that will give the same effect. If you have a choice, I would recommend just working through the controller and leaving notifications up to the base system.
A much better way than explicitly posting KVO notifications with integer values is to implement array accessors and use them. KVO then sends notifications for free.
So instead of this:
[self willChangeValueForKey:@"things"];
[_things addObject:[NSString stringWithString:@"something"]];
[self didChangeValueForKey:@"things"];
You would do this:
[self insertObject:[NSString stringWithString:@"something"] inThingsAtIndex:[self countOfThings]];
Not only will KVO post a change notice for you, but it will be a more specific notice, being an array insert change rather than a whole array change.
I usually add a method addThingsObject:
that does the above, so that I can do:
[self addThingsObject:[NSString stringWithString:@"something"]];
Note that add<Key>Object:
it is not currently a KVC-recognized selector format for array properties (only set properties), whereas insertObject:in<Key>AtIndex:
, therefore, your implementation should be the first (if you choose to) use the latter.
Oh, I've been looking for this solution for a long time! Thanks everyone! After getting the idea and playing around, I found another very fancy way:
Suppose I have a CubeFrames object:
@interface CubeFrames : NSObject {
NSInteger number;
NSInteger loops;
}
My Array contains Cubeframes objects, they are managed by (MVC) with an objectController and displayed in the tableView. The bindings are done in the usual way: the "Content Array" of the objectController is bound to my array. Important: set the "Class Name" of the Controller object to the CubeFrames class
If I add observers like this to my Appdelegate:
-(void)awakeFromNib {
//
// register ovbserver for array changes :
// the observer will observe each item of the array when it changes:
// + adding a cubFrames object
// + deleting a cubFrames object
// + changing values of loops or number in the tableview
[dataArrayCtrl addObserver:self forKeyPath:@"arrangedObjects.loops" options:0 context:nil];
[dataArrayCtrl addObserver:self forKeyPath:@"arrangedObjects.number" options:0 context:nil];
}
- (void)observeValueForKeyPath:(NSString *)keyPath
ofObject:(id)object
change:(NSDictionary *)change
context:(void *)context
{
NSLog(@"%@ changed!", keyPath);
}
Now I will actually catch all the changes: adding and removing lines, changing loops or numbers :-)