I am trying to teach myself objective-c, but coming from a Python/Java background, it is proving very difficult. I tried to post a bunch of points I am confused on, but it was marked as too vague, so I'll break it into sections.
First, every example of a delegate and outlet I've found uses cocoa view code to get the idea across. Unfortunately, I don't yet understand enough of the code to grasp the example's point. So, can anyone provide a more basic example of a delegate? My understanding is that it is a way of subclassing; how is this better than traditional subclassing? Why does every cocoa project automatically include appDelegate.m? Can delegates be used for other purposes, not just GUI stuff?
Okay, I think I see. So a delegate is a class that conforms to the protocol of some other class. A protocol is simply a list of methods and variables that have to (or can, if set to optional) be implemented in the delegate class. To make a delegate, you have to use the #interface keyword, then the name of your delegate, then (in < > signs) the protocol name? So if class c1 wants to set itself up as a delegate of class c, class c must first specify a protocol, right? You would then be able to implement everything in c's protocol in c1:
#interface c1;
I feel like I'm missing some bits there, but hopefully I have the concepts right. :) This also explains the mysterious less- and greater-than signs; they declare what interface the delegate implements.
Outlets are similarly always tied to view code. They seem to be some kind of inter-object messaging system. Is that right? Again, an example of a basic outlet that is not mixed in with complex GUI statements would be great.
So outlets are never needed? I know that IBOutlet and IBAction are not needed except for use with Interface Builder, but I thought outlets were more general than that? The docs seemed to indicate that they are not even specifically for interfaces, but could be used for anything.
Thanks in advance for any help.
Update: Delegates don't have to conform to protocols. Protocols just make it easier to require some classes to have methods. It allows you to know for certain an object one has set as a delegate implements a certain method so you can call it safely, and allows the compiler to verify that method is indeed implemented (if you declare a delegate instance variable as id<SomeProtocol> delegate, the compiler will give a warning or error if you try to set delegate to an object of a class that doesn't conform to SomeProtocol.
Protocols help ensure safety, but they're not strictly necessary. A class can have a delegate (or multiple!), and they don't have to conform to any protocols at all.
As for outlets, no, they're specifically and only used with Interface Builder. The IBOutlet and IBAction keywords have no effect on code (they're even stripped out before compile time) - they're only markers for Interface Builder to look for so it knows which properties and methods should be accessible within the interface. The term 'outlet' is a direct reference to something marked as an IBOutlet, and is really not used in any other context that I can tell.
Again, it's okay if you don't understand this right away. Think it over a bit, and at some point, it'll just 'click'. I was caught up on delegates for a long time, just like this, before one day, I realized that delegates really aren't any special. They're regular objects referenced by other objects - it's just that this design pattern has a special name (delegation), and these objects are only called delegates. They could just as easily be called gyros or falafels, and the net effect would be the same. :P
You don't need to name an object delegate for it to be a delegate; it's just a convention.
About delegates: the first thing to understand, and this got me for a while until I had the proper "Aha!" moment, is that there is nothing special about a "delegate". The word "delegate" is just a title for a type of object that another class depends on, very often for content or decision-making. A developer will use a delegate when they don't want to (or can't) tie one of their classes to another class by name - it's an Object-Oriented way of decoupling and making classes more generic.
Now, very often, classes will require delegates to have specific methods they rely on, and one way to ensure that is with a protocol (more commonly known as an interface in Java). Protocols define a list of methods; classes "conform" to a protocol if they declare they do in their interface (e.g. #interface IFObject : NSObject <SomeProtocol> { ... }) and if they implement all the methods they're required to. Protocols can have optional methods as well.
This model is used often with view controllers, views, and the GUI in general because many AppKit and UIKit classes are written to be as generic as possible. NSTableView, for instance, implements the most basic behavior it can possibly implement without requiring any implementation-specific information; for the rest, it relies on other objects, ones that conform to the NSTableViewDelegate and NSTableViewDataSource protocols. Any object can conform to the protocols, as long as they implement the right methods (and in this case, a controller class will usually implement methods from both protocols, but it doesn't have to be so). In fact, one easy way to understand this topic better is to take a look at NSTableView - it's got a delegate property and a dataSource property, but in effect, they're no different. delegate could be called monkeyButt, and the concept would still work. The key is to not treat delegates as a black box - there's nothing special about them.
Delegates can be also used for non-GUI purposes; one concrete example, as you mention, is the app delegate. NSApplication sends a delegate notifications to let it know when the application has been launched (among other things), so it can set up shop. Again, any object can be a delegate to any other object, for any purpose. It's simply a convention.
Briefly about outlets: as others have mentioned, outlets are simply connections between an interface defined in an XIB and your code. They're a way of letting Xcode link up the interface to the appropriate elements so that when your application loads the interface file, it can load up the right pieces of code or execute them.
They're generally an easier way of setting up an interface - they're not strictly necessary (you can create an interface programmatically, without using an XIB file), but if you do decide to go the XIB route, they're how you relate your interface to your code.
A delegate is an object that is delegated some task by an object it declares itself the delegate of. Let's say an object does some task and then needs to notify the object that "owns" it, so to speak. In order to allow the object to work under any conditions, it cannot be allowed to know what type of object it should contact, so we set its delegate and adhere to its protocol. Setting the object's delegate is like saying "Here's the object I want you to use contact with the messages declared in your protocol. I promise the object actually implements the methods in the protocol." For example, you might see this.
#protocol SpriteDelegateProtocol
#required
- (void) projectionMatrix;
#optional
- (void) animation:(int)animationIndex willCompleteFrameNumber:(int)frame forSprite:(id)sender;
#end
#interface Sprite
#property (nonatomic, assign) id<SpriteDelegateProtocol> delegate;
#end
The interface for our sprite object declares that it has a property called a delegate, which is of type id. It can be any object, but it must adhere to the protocol in the triangle brackets. If it says it adheres to the protocol, it has to implement the methods declared under the #required and can implement the ones listed under the #optional. It's up to us to determine whether or not our delegate actually implements the optional methods using something like respondsToSelector:. We might even store the return value if we had a lot of methods to do that for.
The application delegate is special in that it is the delegate to our UIApplication object. It receives messages on application state, such as applicationDidEnterBackground:. You can see what methods are in the protocol adhered to by our application delegate here.
Delegates can be used with any object. Any object can say it has a delegate property to set, and that it must have the following methods as shown above. The protocol is essentially a portable interface that can be used to tell another object what we need it to implement so that we can call said methods to delegate some portion of functionality out to it. We can notify our delegate when a task has completed, ask it to give us information (typically called a data source instead of a delegate so that they can be different objects if you want) or ask it whether or not we should do some task at all.
An outlet is a way of connecting a view instantiated in a NIB or storyboard to a property in its controller. So, if you place a UIViewController into your main storyboard and change its class to MyGreatControllerSubclass then proceed to place a button in that controller's view, you might want to hook that button up to an "outlet" (property) in the controller's interface so that you can access the button from the controller later.
No delegates are not a way of subclassing. I also came from Java and have been doing O-C for almost 5 years now.
Delegates generally conform to Protocols, which are the equivalent of Interfaces. What's great about that is it leaves you a lot of flexibility in how you implement the things that are covered by the delegate. So for instance, you will see that Table Views can be implemented by extending UITableViewController or implementing the delegate. That way, if you have a composed view where a user is selecting items from one table and putting them into another, you can do a single controller and have it perform the delegation for both views.
The best way to think about delegates is as examples of message/event oriented callbacks. Have you ever heard the saying that what makes a Framework different from ordinary programming is, with a Framework, it tells you what it wants you to do to ensure the results you seek? That is what delegation is. I was doing a Collection View last night and could not get the edit menu to come up. Just provide delegation of three methods, know nothing about how the menu is invoked, where the Long Press gesture handler is, etc.
Note that Delegates are the glue that makes Dependency Injection unneeded in the O-C world. I prefer them.
Delegates are the common design pattern (http://en.wikipedia.org/wiki/Delegation_pattern), they are not objective-c specific.
Outlets make possibly the binding of Controller and View (GUI). May be you know the MVC paradigm (model-view-controller)? So you build your GUI (the View from MVC) with Interface Builder and bind the elements of this GUI such as Buttons, Text fields and so on to your Obj-C code (the Controller from MVC) using outlets.
Take a look at this post: Conforming Objective-C’s Protocols in C# Codes
Don't worry about the word "C#" in the title, you could just ignore that part. The main part of this post talks about protocols and delegates in Objective-C. This might be helpful.
Related
Is it a good practice or a bad practice to have same-named methods across classes, particularly where inheritance and/or protocols are involved?
For example, I have a protocol of some kind:
#protocol DataReceiver <NSObject>
- (void)handleNewData:(id)data;
#end
And I have a controller class that owns some stuff, such as
#interface OwnerOfAllThings <DataReceiver>
//this child does NOT conform to DataReceiver
#property (nonatomic, strong) SomeOwnedModel myChild;
#end
But the child is opaque, so my implementation of -handleNewData: is basically just a pass-through and calls a method in the child that takes care of the actual handling. So my question is, although it's not functionally so different, is it a bad idea to put an identically named method in SomeOwnedModel, being that it might be confused with the declaration in the protocol? Or should it in fact be exactly the same, for consistency?
i.e., which is better (assuming that the SomeOwnedModel.h declares the appropriate method):
#implementation OwnerOfAllThings
- (void)handleNewData:(id)data {
//Option 1:
[self.myChild handleNewData:data]; //OR...
//Option 2:
[self.myChild ownerReceivedNewData:data]; //or whatever else
}
#end
Since OwnerOfAllThings is simply invoking the method in the child, I might be inclined to keep the same method names, possibly even have SomeOwnedModel conform to the same protocol. If you use the "use unique names" pattern, I can only imagine how unwieldy it would become if you had three or four levels of classes or if you had a ton of methods in your protocol, being forced to come up with arbitrarily unique names for each.
By way of example, I have NSOperation based framework for NSURLSession, with a session manager object (a wrapper for NSURLSession), and a bunch of individual NSOperation classes (one for download tasks, one for data tasks, etc.). In this case, while NSURLSession conforms to not only NSURLSessionDelegate, it also conforms to NSURLSessionTaskDelegate, NSURLSessionDataDelegate, etc. (because that's how NSURLSession works). But when this session manager receives one of these task-related delegate calls, it simply routes the event to the appropriate operation object. So to make this clear, I made the various operation objects conform to the appropriate protocol (e.g. download operation conforms to NSURLSessionDownloadDelegate, data operation conforms to NSURLSessionDataDelegate, etc.). The end result is that despite a dizzying collection of classes and protocols, the intent is unambiguous.
So, bottom line, if the parent is simply routing the event to the appropriate child and offering no material contribution beyond that, I'd be inclined to use same methods (and possibly same protocol). But if the parent class is offering something substantively richer/different, functionally, over what the child is performing, then a unique name would probably be appropriate.
As an aside, when coming up with your method names, I might suggest a method name that distinguishes "I received a bit of data and more may be coming" (i.e. didReceive...) vs. "I received all of the data" (i.e. didComplete...). This is an informal convention Apple uses in NSURLConnection and NSURLSession, and following it might be helpful to make your intent clear.
The best way to name a method is always to try to accurately describe (beware of excessive verbosity) what the method does. If both your methods do exactly the same it's probably a good idea to extract that functionality to a self-contained class.
Often the convention is followed in iOS to name delegate methods like the one you have shown in the following form:
-(void)dataSource:(DataSource*)dataSource didReceiveNewData:(id)data {
//Call your "child" object here
[self.myChild doAWithData:data];
}
Where dataSource is the object that calls the delegate method.
In my opinion you're running into this problem because you're not accurately describing what the method actually does by using "handle" data.
Perhaps a good source for standards is Apple's documentation.
I am new at programming in general (though I have had a C class many, many years ago) and am learning Objective-C for programming on the iPhone. I have what I think is a simple question, but after looking for a while (days, off and on) I can't find the answer that I'm looking for explicitly.
I know that when subclassing an Objective-C class I should implement the initialize method along with the deallocate method (unless using ARC for the latter, if I am correct?). The questions are:
Are these the only two to worry about, or will other classes potentially have additional methods that can be required to be implemented?
If other classes might have methods that I am required to implement when subclassing them, where is that documentation typically found? (I don't seem to see that in the Apple framework docs, though that kind of information is there for protocols it appears)
Thanks for your help!
Technically, you are not required to implement even the init and dealloc if the inherited versions are sufficient. Also, ARC does not free you from having to write dealloc in all cases (but it certainly covers the overwhelming majority). For example, if you allocate memory for your object using malloc, you need to free it in the dealloc.
When you add instance variables to your class, you need to initialize them. Typically, you do that in a designated initializer. Again, if you do not to initialize anything, you do not have to code your own initializer; same goes for deinitializer.
The only case when you need to implement a method is when you adopt a protocol with one or more methods marked #requried. These methods are marked in the protocol reference. For example, tableView:cellForRowAtIndexPath: and tableView:numberOfRowsInSection: are marked with the "required method" tag in Apple's documentation.
No methods are required when subclassing an NSObject (or any of their subclasses, such as UIViewController, UIView, etc. etc.).
If you create a new, let's say UIViewController, it's generally a good idea to keep the methods you find in the newly created file as a guideline/template, but you're not really required to keep any of the methods. The super class will always call the methods on itself.
Be aware, though, some methods you have to call super, like viewWillAppear, etc.
OK I alllmost get it. I've done a bunch of reading in objective-c and I really want to understand delegation because it seems super important for cocoa-touch and I want to design an iPhone app.
So, a delegate is an object that can be given a certain task. It is said to follow a 'protocol' if it implements certain functions. So a view-controller, for example, can say "hey, I'm not sure where to get this data from..or hey, I'm not sure how to format this thing...can you do it for me?" and the delegate is like "sure I got you covered".
OK that makes sense to me. What doesn't make sense to me is how I get the delegate to return stuff to a view controller. Like say my delegate can go to a URL and read a sports score or something. So I say "delegate get me this score" ...how do I get the view controller to say "got it, here it is" and then have it inside the view controller. There might be a gap in my understanding here. Would I have to instantiate the view controller inside the delegate? That doesn't make sense to me...because then I'd have two view controllers...Feel free to change my analogy if you can make it clearer.
Thanks!
I think you're confused because a similar pattern is used for (but is not limited to!) two common tasks, both of which apply to your situation.
The patterns
Having an external object provide data for you (this is usually called a data source). See, for example, the UITableViewDataSource protocol.
This is implemented by a return value from the method: such as - (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath;
The object implementing the protocol returns some value (in this case, a cell) to the caller.
The thing I'll mention about data sources: the data source itself (the object implementing the protocol) usually contains more of your application's custom logic, while the controller which requires a data source can be more generic. For example, UITableView is a generic view controller that displays a table view, while a class implementing the UITableViewDataSource protocol needs to know the details of your application's database.
(However, to be thorough, you do often subclass UITableView for custom logic, but this is most often presentation logic and not business logic.)
These methods call in to your application logic, and are expected to return immediately.
Providing callbacks once you've finished loading data.
For example, the NSURLConnection class has the corresponding NSURLConnectionDelegate protocol. The most common use pattern is:
Your object creates a NSURLConnection, with itself as the delegate.
You configure and start the connection.
You receive progress and data via the delegate methods you implement.
In this case the object which requires a delegate is an auxiliary object that knows how to load data from a URL in the background. The delegate methods are callbacks to your application logic, and are called at any time after the object is told to start loading data (or whatever it's designed to do).
Delegation is also used for other things on iOS, such as the UI-related tasks performed by objects conforming to UITableViewDelegate.
Your situation
This all depends on what your application is, and what your view controller is responsible for — but it sounds like you want the view controller to delegate the loading of data — basically, it needs a data source. (You should also consider if the built-in UITableView & UITableViewDataSource might suit your needs.) But if your data source is going to asynchronously load data from the internet, it might need to implement some data-loading callbacks via something such as NSURLConnection.
These two methods don't necessarily go together well, because the view controller will expect its data source to immediately return data, but the data source might need to wait for data to load.
This is why UITableView has a method -reloadData, so the object which serves as the data source can tell it when data is available. You might want to use a pattern like this in your application.
(But again, in all likelihood you won't need to implement a fully custom stack — either you can combine some classes to reduce your use of delegation, or you can use more built-in classes to suit your needs.)
Define your protocol's methods with return values.
That said, getting a URL is a bad example, since waiting for a delegate to return the results would block the calling thread. In this case, you would have to have a way for the delegate to call back with the results when done. This can be achieved either by passing a delegate to the delegate, or passing one or more Objective-C blocks to the delegate (onSuccess, onError, …).
On that subject, blocks are much easier to code than delegates and protocols, and are gradually supplanting them in Apple's and third-party APIs.
I'm new to Mac programming and I'm working on a document-based application.
My NSDocument subclass creates a NSWindowController subclass. This window controller creates two NSViewController subclasses as well.
Sometimes, a change in one of the views of a NSViewController needs to notify the NSDocument and/or the main model class. Also, a change in the model needs to be notified to every/some view(s).
My question is: what is the best approach so that there is no (or minimum) coupling? I know there are several choices, but I'm not sure which one suits best for my application as I'm newbie not to programming but to Cocoa and especially NSDocument:
KVO. Looks nice and easy to implement, but I don't like the idea of not explicitly notifying the observer(s) about a change (AFAIK, self.someProperty = newValue does automagically notify observers), and don't like the fact that you have to register to property names which could change in time.
Notifications. I know what they are and I've used them for iOS. But I've read somewhere that they are not guaranteed to be sent immediately to observers. Is it true? If not, do you see them as a good approach for a document-based app?
Delegates. Yes, under normal conditions (or what I've usually seen), a class has one delegate. But creating an array of delegates works as well (just tested it). The problem I see here is that every time I need to notify the delegates I have to loop through them, make sure they respond to a method, and call that method.
Are there any other alternatives I'm missing?
KVO by a controller class is the most common way to do coupling between a model and its view(s). In fact, Cocoa Bindings, which are intended to mostly eliminate code in the controller layer, are based on KVO. It is true that KVO/KVC relies on property names, and that if those change, you'll have to change the bindings or KVO setup connecting your view. However, it's not usually feasible to make your views completely unaware of the underlying model specifics, so I don't see this as a problem.
My recommendation would be to use Cocoa Binding where you can, as they eliminate a lot of glue code. In places where they can't be used, your controllers (the middle layer in MVC) should use KVO to observe model changes and update the appropriate views. Changes in the views can be passed back to the model via property accessors and/or KVC by the controllers.
Yes, under normal conditions (or what I've usually seen), a class has
one delegate. But creating an array of delegates works as well (just
tested it).
Delegates are often used to modify the behavior of the delegating object. An application delegate is a good example: NSApplication by itself isn't very interesting; it relies on its delegate to define the interesting behavior of the application. Having multiple delegates all trying to modify the behavior of a single object could be a problem if the various delegates conflict with each other. What do you do if the delegates disagree?
There are some cases in Cocoa where a class uses more than one delegate, but each one has a separate role. For example, NSTableView has both a delegate and a data source, but both are really delegates of a sort.
The problem I see here is that every time I need to notify the
delegates I have to loop through them, make sure they respond to a
method, and call that method.
That's not hard to solve. For example, you could create an NSInvocation to encapsulate the call, and then send the invocation to each "delegate." However, if you do that you'll have nearly reinvented the notification system. If you need the one-to-many communication that you'd get with your multiple delegates proposal, you'll probably be better off using notifications or KVO.
I'm using the delegate pattern for one of my objects. My idea is that I will be able to swap the delegate out later for a different delegate implementing a different strategy. I suppose this is just as much the strategy pattern as the delegate pattern.
My question is, is it bad practice for my delegate to have a reference back to the object for which it is the delegate? There are a couple properties of that object that I need to have access to regardless of which delegate/strategy I use. If this is bad form, how should I get access to the properties?
Not necessarily. For example, it's quite common for a "controller"-type object to create data objects and views and make itself the delegate of some of those objects in order to properly control things.
It is not at all bad practice. The delegate pattern allows for a class to have a generic way to talk to any number of objects as long as the implement the same protocol. But the class on which you set the delegate also will usually have a number of public properties or methods that allow you to query or alter what the class is doing, in response to which the class may in turn trigger a number of the delegate calls. So you need a reference of the class to which you are the delegate, in order to tell the object to do something different than it is already, and of course to release it when you are through with it!
This is also why it's important to always have any delegate properties as assign, and not retain, properties. That way when the originating class is released it will actually be released, instead of having delegate objects it is holding onto causing a retain loop that keeps both around.
It's also why whenever you are being released, you should set the delegate reference to nil in anything you may have a delegate reference set. This way a class will not have an invalid delegate reference if the delegate is released before the class that uses a delegate.
Generally a dependency should not have a reference back to the dependent object, as this would be a classical circle reference. To avoid having a back-reference you can supply the needed properties as arguments in a delegate method, or you move the properties themselves into the delegate.
I'd say yes, it's bad practice. The idea behind a delegate is that it's effectively a standalone object that receives messages about the object for which it is the delegate (the "delegator"). The delegator should have a reference to the delegate, not the other way around, otherwise it's not a true delegation relationship anymore.
A preferred way to accomplish what you're asking is to provide the sending object along with whatever message your delegate receives. For example, on your delegate, instead of having a delegator property and then receiving, for example, a didDoSomething:(id)anObject method, you could remove the delegator property and send the message delegator:(id)anObject didDoSomething:(id)anotherObject. This way, you keep the delegate distinct from the delegator, but still get access to the delegator's properties when you need them.
This way also has the advantage of not providing access to the delegator in methods when you don't truly need it; for example, your delegate could have a didDoSomething method that takes no arguments, not even a delegator, and is just used for logging, as well as a delegator:(id)anObject didSomethingElse:(id)anotherObject method that calls some properties on the delegator and is much more involved.
Finally, this method allows you to use the same delegate for multiple delegators, since you don't need to update a delegator property for each delegate object.
For a good example of how this works, take a look at the NSURLConnection documentation, specifically its delegate methods - a lot of them take the form connection:didDoSomething:, where the first argument is the connection calling the delegator. Developers commonly define a single connection delegate for multiple connections, implementing their delegate methods to do different things depending on properties of the NSURLConnection object passed in.