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Is it possible to make the -init method private in Objective-C?
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how to block a superclass method to be called to a subclass
(5 answers)
Closed 9 years ago.
Suppose you have a UIView subclass. You define an init method "myInitWithFrame: ... andWhatNot:...". You know you won't be using the init method inherited from UIView ever and your custom init method does some vital custom initialising so that you want to force client classes to never use the inherited initWithFrame method.
Is it possible to hide the standard initWithFrame method that was inherited from UIView?
Actually, you can get compile-time warnings about calling a method on a subclass. Use the __attribute((deprecated)) attribute. If you want people to use -initWithPizza: instead of -initWithFrame:, do this:
#interface MyView : UIView
- (id)initWithPizza:(MyPizza *)pizza;
#end
#interface MyView (Deprecations)
- (id)initWithFrame:(CGRect)frame __attribute((deprecated("Use initWithPizza: instead")));
#end
Putting the -initWithFrame: declaration in a separate category is necessary to avoid Xcode complaining that you declared the method in the header but didn't implement it. Since you're just inheriting it from the superclass, that's fine; you don't have to implement it at all. But if you want to implement it to throw an exception, or call through to -initWithPizza: with a default argument, that's fine.
Of course, this won't stop UIKit from calling -initWithFrame: if it was already going to do so. But if you can guarantee that won't happen, then you're fine.
Actually, you CAN restrict with a subclass. You can override whichever methods you want to block in your subclass's .h file. You can make initWithFrame unavailable by placing the following in your .h file.
- (id) initWithFrame:(CGRect) frame __attribute__((unavailable("message")));
This will make the initWithFrame: method unavailable to anyone using your subclass.
To keep other code form calling this method, you can further restrict by putting this in your .m file.
- (id) initWithFrame:(CGRect) frame
{
return nil;
}
No. You can't prevent the users of your subclass from calling the methods of a superclass. You could override them and throw an exception inside, but that would just produce a broken subclass.
Remember that inheritance works as an "is a" extension, that is, instances of your subclasses should behave normally in any context that doesn't know about this particular subclass but knows about its superclass. It's only in places that have explicit knowledge about your subclass that you can benefit from adding extra initialization and other methods.
For example, UIKit has no knowledge of your subclass. So if you want to make your UIView subclass available from a NIB, you need to use the initialization methods that will be called by the NIB loading system, namely initWithCoder:. You can simply call your own initialization methods inside initWithCoder:. But if there are any additional parameters you would like to pass to the init method, you'll have to provide a way to configure them after initialization.
Related
I'm just curious, when it comes to initializing UIViews; I notice that when I call the instance constructor -(id)initWithFrame:(CGRect)frame, the primitive constructor -(id)init is never actually called, meaning that I would have to initialize instance variables in both constructors. Is this meant to be an optimization strategy such that calling -(id)initWithFrame:(CGRect)frame is computationally "faster" (encourage better performance) than calling UIView *view = [UIView new]; view.frame = frame;? Is the initWithFrame option quicker than creating a new instance and assigning its frame manually? I actually prefer to do the latter to maintain programmatic consistency.
No it's simply that initWithFrame is the designated initializer of UIView (from the class reference):
Discussion
The new view object must be inserted into the view
hierarchy of a window before it can be used. If you create a view
object programmatically, this method is the designated initializer for
the UIView class. Subclasses can override this method to perform any
custom initialization but must call super at the beginning of their
implementation.
Subclass of UIView
I have a subclass MyView of UIView.
This subclass has a #property UIView * realView.
What I want to do
Whenever a message is sent to MyView, I want to "forward it" to self.realView, excepted for few messages.
For instance, in the implementation of MyView, I would have this override:
- (void)setBackgroundColor:(UIColor *)color
{
[self.realView setBackgroundColor:color] ;
}
Instead of overriding explicitly all the methods, can I do it automatically, at the runtime?
Exceptions
For some methods, I want to have an explicit control. For instance:
- (void)setFrame:(CGRect)frame
{
/* do stuff */
[super setFrame:frame] ;
}
Instead of overriding explicitly all the methods, can I do it automatically, at the runtime?
You implement the -forwardInvocation: method to send any unrecognized messages to the other object. -forwardInvocation is called whenever an object doesn't implement the selector that's passed to it as a sort of second chance to handle a message. You can override it to send the message to another object (which is pretty much what NSProxy does), log the messages, etc.
As #cobbal points out below, -forwardInvocation will help you with methods not implemented in your superview, but it won't handle methods that are implemented int the superview because your MyView inherits implementations of those. For example, if you want to use a UIView as a proxy for a UIButton, all the methods specific to UIButton can be handled by -forwardInvocation:, but those defined by UIView cannot. In order to get a behavior other than the inherited method, you will of course need to override. In some situations you can get around that by deriving MyView from NSObject or UIResponder instead of from UIView, thus avoiding the inherited UIView implementations, but if MyView needs to be a real view you're stuck with overriding each method.
If you think about it, it's hard to imagine how your goal could be met without explicitly overriding each inherited method. You say that you only want to forward most messages, but how can the poor runtime tell which ones you do want to forward and which ones you don't? All it can do is look for a method for the given selector and call it if it finds one, or take some action (like calling -forwardInvocation:) if it doesn't.
Update: #robmayoff points out -forwardingTargetForSelector:, which didn't occur to me but is probably a better solution in your case. It still doesn't handle the situation where you need to redirect methods that you inherit from a superclass, though.
It's entirely possible.
First you need WZProtocolIntercepter. Then use the intercepter as the normal UIView:
WZProtocolInterceptor* fakeView = [[WZProtocolInterceptor alloc]
initWithInterceptedProtocol:#protocol(TheMethodsForTheMiddleManToHandle)];
fakeView.receiver = self.realView;
fakeView.middleMan = self;
[someViewController.view addSubview:fakeView];
Put the methods you want to control in TheMethodsForTheMiddleManToHandle:
#protocol TheMethodsForTheMiddleManToHandle
- (void)setFrame:(CGRect)frame;
#end
I'm working on an accessibility project for an iOS application. Because accessibility does not act quite as advertised, I have to override accessibilityFrame, accessibilityActivationPoint and pointInside:withEvent in a subclass in order to expand the region recognized by VoiceOver (for both drawing and touch recognition) beyond the "natural" bounds of the control view. So, in order to change the VoiceOver bounds of a UIButton I have to subclass that class and then add these three methods. In order to do this for a UILabel I have to add another subclass with the code, and so on.
I can refactor the code in these methods to a central location, but I was wondering if this can be done more elegantly with inheritance. I'd like to put this code into a subclass of UIView (maybe called UIViewAccessible) and then create a subclass of UIButton called UIButtonAccessible which inherits from UIButton which would in turn inherit from UIViewAccessible instead of UIView. Is this possible, or can something like this be done with a category?
Edit: According to the docs, you can't really achieve this with a category:
If the name of a method declared in a category is the same as a method in the original class, or a method in another category on the same class (or even a superclass), the behavior is undefined as to which method implementation is used at runtime.
Is there some other way to do this?
To answer your question, no, it can't, since your UIViewAccessible is a second degree sibling to UIButton in the inheritance chain (both inherit from UIView at some point). But I guess you already knew that. As for a solution, you could wrap around your UIView accessible classes a decorator and use protocols for strong typing. That way you'll keep the code in one place. I've described this technique here in more detail (although for a different purpose, it's the same situation).
For the views that would support accessibility you'll have to do this:
#property (nonatomic, strong) UIView<MyAccesibilityProtocol>* view;
//self.view can come from the nib or previously created in code
self.view = [[AccesibilityDecorator alloc] initWithDecoratedObject:self.view];
//you can then use self.view like any other UIView,
//and because it also implements an
//accessibility protocol, you can use the methods
//implemented in the wrapper as well.
//more than that, you can control which methods to override
//in the AccesibilityDecorator class
[self.view addSubview:otherView];//could be overridden or not
[self.view myAccesibilityMethod];//custom method declared in the protocol
I am learning to program the iphone and I wanted to do some drawing. I followed some example code and subclassed the viewcontroller and it worked fine. Now as I wanted to expand the program I came upon a design question that I could use a little help on.
I subclass myviewcontroller with mynewview. If I have any code in the myviewcontroller how do I call or reference it in mynewview and vice versa? I am not sure if I am asking this right but I am trying to understand the relationship between the class and subclass.
Objective-C objects benefit from inheritance. All classes are subclasses of NSObject, therefore you can call init on any object. If you created a custom class and gave it a method doSomethingAwesome, you are free to then implement doSomethingAwesome in any subclass of your custom class. However, declaring a method in a subclass does not add that method to the superclass. As an aside, I rarely find myself subclass sing my own custom classes. I believe that it is encouraged to maintain what is called a shallow object hierarchy. Usually I subclass the stock cocoa classes, customize to my needs and if I need custom methods in more than one subclass I will declare a category on the superclass rather than relying on inheritance to provide my custom behavior
The messaging system in Objective-C is dynamic. Every object includes a struct with information that the runtime use for introspection. Here the runtime will find a list of methods the object is able to respond. So, let's say you message an instance like this:
[mynewview someMethod];
The runtime will first check the object information to trying to find some method that will be able to respond the message. If nothing is found, then will query the super class, and so on. In fact, the runtime is much more complex, and will give any object more opportunities to respond (that's the dynamic part. For instance, mynewview might not have any method called someMethod and yet, might be able to satisfy the call, but that's something you might not want to worry right now).
From a child class you can call the superclass implementation of a given method with the keyboard super, so if mynewview is a subclass of myviewcontroller you can call myviewcontroller implementation from mynewview with:
[super someMethod];
If someMethod is both present in myviewcontroller and in mynewview, the runtime will automatically only call the child implementation, you have to call the parent implementation (if you have to) from the child implementation.
I recently did a programmatic alloc/init of a table cell subclass, and with some NSLog's, I was able to learn that pretty much all of the initializers were being called even though all I did was alloc/init, if I recall correctly.
init
initWithStyle
What is the logic to this?
Which one calls which?
Everything traces back to init. A UITableViewCell is a subclass of NSObject, so it has an init method.
initWithFrame is deprecated, and has been for some time (since iOS 3). You shouldn't be using it.
It was replaced in iOS 3 with initWithStyle, which you use to indicate what style of cell you'd like to create.
initWithCoder is another NSObject method, part of the NSCoding protocol. Again, you can see it in UITableViewCell because it is a sub-class of NSObject. initWithCoder is used to unarchive an object (perhaps you have saved an object directly to a file, for example).
You can tell which calls which by looking at the order in which the log messages appear.
A common pattern in Objective-C code is to have a "designated initialiser", which actually creates and returns a configured instance of the class. All of the other initialisers call the designated initialiser with fixed values for some of the parameters that weren't specified by the caller, or provide further configuration once the designated initialiser has returned.