I'm stuck with following problem. I have a UIScrollView, _myScrollView, and I want to have another UIScrollView following it's movements. So I'm using key-value observing for the properties "zoomScale" and "contentOffset", but the observeValueForKeyPath:ofObject:change:context: method only report changes in "contentOffset", not in "zoomScale", though the zooming workes fine. (See code snippet below.) What could be the reason for this?
-(void)viewDidLoad {
...
[_myScrollView addObserver:self forKeyPath:#"contentOffset" options:NSKeyValueObservingOptionNew context:NULL];
[_myScrollView addObserver:self forKeyPath:#"zoomScale" options:NSKeyValueObservingOptionNew context:NULL];
}
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
if ([keyPath isEqualToString:#"zoomScale"]) {
NSLog(#"zoomScale: %#", change); // Never gets called
}
...
}
The zoomScale property isn't KVO compliant. But UIScrollViewDelegate has a scrollViewDidZoom method that you can use to track changes to zoomScale.
UIKit actually doesn't support KVO.
From the docs:
Note: Although the classes of the UIKit framework generally do not
support KVO you can still implement it in the custom objects of your application, including custom views.
It does work sometimes (as you've seen), but support for it is undocumented and inconsistent. Use the delegate methods instead.
Related
I am trying to implement a KVO example for clipsToBounds property of all subviews in my UIView. I do not quite understand how to change the value in observeValueForKeyPath method. I am using this code:
-(void)ViewDidLoad{
[self.navigationController.view addObserver:self forKeyPath:#"clipsToBounds" options:NSKeyValueObservingOptionNew |
NSKeyValueObservingOptionOld context:nil];
}
-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
NSLog(#"Triggered...")
}
It is triggered when ever i change the property clipToBounds of a subview that exists in the UIView i have. I need to change the value back to false for every trigger that happens. What should i write inside the observeValueForKeyPath to change the clipsToBounds property? Any help appreciated.
of course adding the Observer must be done before it works.
Guessing your typo in "ViewDidLoad" would just never be called because it should be "viewDidLoad".
Apart from that your KVO pattern could look like..
static void *kvoHelperClipsToBounds = &kvoHelperClipsToBounds;
-(void)viewDidLoad {
[self.navigationController.view addObserver:self forKeyPath:#"clipsToBounds" options:NSKeyValueObservingOptionNew context:&kvoHelperClipsToBounds];
}
-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
if (context == kvoHelperClipsToBounds) {
NSLog(#"context compared successful...");
//be careful what you cast to.. i dont check isKindOf here.
UINavigationBar* navbar = (UINavigationBar*)object;
if (navbar.subviews.count > 1) {
__kindof UIView *sub = navbar.subviews[1];
if (sub.clipsToBounds) {
dispatch_async(dispatch_get_main_queue(),^{
sub.clipsToBounds = NO;
[self.navigationItem.titleView layoutIfNeeded];
});
}
}
}
// or compare against the keyPath
else if ([keyPath isEqualToString:#"clipsToBounds"]) {
NSLog(#"classic Key compare Triggered...");
}
else
[super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
}
[super observeValueForKeyPath...] passes not recognized keyPath to super to let super's class KVO work, otherwise those would be ignored if super's implementation would rely on observing them. Which should also explain how you could observe all subviews if needed. But think about that there could be potentially hundrets of subviews triggering observeValueForKeyPath if a subclass of UIView would implement it and all subviews (or the ones you like) would be inherited also from this special subclass.
When you change clipsToBounds inside the KVO where you observe it, you possibly invoke a loop, specially when you watch both - old and new values. you would change the property, the property triggers kvo, kvo changes the property, the property triggers kvo and on and on.
set [self.navigationController.view setClipsToBounds:YES] to change the property. But if done inside KVO it will trigger KVO again as explained.
Usually you would set clipsToBounds in -initWithFrame: or in -initWithCoder: or via Interface Builder and maybe just observe if it gets changed to adapt some other code.
Sidenote: the context just needs to be unique to distinguish it from other KVO.. it could also be reference to a real objects pointer.
Don't forget added Observers must be removed before deallocation.
I have an observer on a UITextView to detect if its content size is changing:
[_textView addObserver:self forKeyPath:#"contentSize" options:NSKeyValueObservingOptionNew context:NULL];
This code always worked to call the following function, where I do resizing of the UITextView:
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
However, in iOS 9, this function is never getting called. What changed in iOS 9 and how do I fix this?
If you do a search for UIKit and KVO Compliance you will see that everyone says you can't rely on it. See this question and this question.
I don't know what changed, but I think you should just subclass UITextView and overload setContentSize: if you want to know when it changes.
I need notifications with textView.text, but the do not work.
My code:
Add a KVO:
[textView addObserver:self
forKeyPath:#"text"
options:NSKeyValueObservingOptionNew
context:nil];
Observing method
-(void)observeValueForKeyPath:(NSString *)keyPath
ofObject:(id)object
change:(NSDictionary *)change
context:(void *)context
I am trying change contentSize. KVO is working...
https://developer.apple.com/library/ios/documentation/General/Conceptual/DevPedia-CocoaCore/KVO.html
Note: Although the classes of the UIKit framework generally do not support KVO, you can still implement it in the custom objects of your application, including custom views.
There have another way to help you simply.
Conform UITextViewDelegate protocol methods.
Use method - (void)textViewDidChange:(UITextView *)textView; to get textView.text changed.
Hi I am developing IOS application in which I am using add observer with key value pair. But it's not working. I did following things:
[self.scrollView addObserver:self
forKeyPath:#"new"
options:(NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld)
context:nil];
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
// inside here ...
}
But its not calling the above method. If I change forKeyPath to contentOffset then its working fine. But I want to change that value. Am I doing something wrong? Need Help. Thank you.
[webView.scrollView addObserver:self forKeyPath:#"contentOffset" options:NSKeyValueObservingOptionNew context:nil];
You need to give the property of the scrollview here.
I have a view controller with a view that changes (for example), and I would like to observe the frame of any view that self.view is set to.
Is there any difference between:
[self.view addObserver:self forKeyPath:#"frame" options:0 context:nil];
and
[self addObserver:self forKeyPath:#"view.frame" options:0 context:nil];
For the second one, if the view changes will messages still be recieved when the new view's frame changes, or will it only send messages if the frame of the view that was set when the observer was added?
Is there any way to observe changes to the frame property even if the view of the view controller changes after adding the observer?
Use the second path. #"view.frame" will notify you about the frame changes even when the "view" itself is changed. Cocoa will add observers for every object in the keyPath "chain" for you automatically (which means every item in the keyPath must be KVO-compatible).
You asked if there is a difference between the two, The answer is yes, there is a difference between them:
The first one
says "me as a view", I add an observer named self (aka) viewControllerObject, if you invoked this in viewController.m whenever my property named "frame" is changed.
The Second one
Says "me as ViewController" I'm adding myselfAsAnObserver whenever theKeyPath named "view.frame" is changed.
Since every observer should implement
-(void)observeValueForKeyPath:(NSString *)keyPath
ofObject:(id)object
change:(NSDictionary *)change
context:(void *)context
For this case you won't notice much difference because you added a viewController as an observer in either of the method above, but it will make a difference when you are dealing with different objects. But the rule is simple, each added observer should implement the
-(void)observeValueForKeyPath:(NSString *)keyPath
ofObject:(id)object
change:(NSDictionary *)change
context:(void *)context
One more thing:
Its a good idea to create a context for observation
e.g
//In MyViewController.m
//..
static int observingViewFrameContext
// In ...
[self addObserver:self
forKeyPath:#"view.frame"
options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld
context:&observingViewFrameContext];
// .. don' forget to remove an observer ! too