In cocoa-touch development...
Use AppDelegate for delegate classes
Create separate delegate class and locate in new .h/.m for each class need to use delegate
Use view controller classes(whenever such exist) to do that job for all
classes managed by this controller
What would you recommend?
Well it depends. The entire concept for delegate protocols exist so that you can have a lot of flexibility. Sometimes you take the simply default route but sometimes you need to be able to have a lot of different delegate classes.
(1) App delegate -- the app delegate should only be used for UIApplicationDelegateProtocol methods or delegates for actual properties of the delegate instance itself. In other words, if the app delegate doesn't directly deal with an instance e.g. the application object, then the app delegate should serve as the instance's delegate. Piling up extranous methods in the app delegate will muddle the app and make it snarlingly interconnected and difficult to debug and maintain.
(2) Wholly separate delegate classes are usually used when you have (A) a large number of delegate protocols to implement or (B) you have the same protocol to implement for multiple instances but require a different behavior for each object's delegate. E.g. you have several UITextFields each of which behave differently. You create as separate delegate class for each so that each text field has it's own custom implementation of the delegate protocol's methods.
(3) Using the controller for delegates is the easiest, most logical and most modular way to go in the majority of cases. In many cases such as UI elements the delegate methods need an awareness of other UI elements which the controller can provide.
In sum, never do (1) as a general parking place for any random delegate methods and default to (3) in the majority of cases.
Related
Can anyone simply explain delegating pattern and callback function in objective C.
Or can point to some documents (with Easy and Basic Explanation) which can make these concepts clearer to me. As i am not getting any idea about it from any book or website or (Apple Developer)references.
All the resources i have come across to understand this, makes me more confuse by using the terminology which i find hard to digest.
Any Help Appreciated.
I will try to explain the value of delegation with one example.
Say you have a table; the table will show some rows. Suppose now, you want to customize the way this table react to some event, e.g., the selection of a specific row.
One common way of doing this in OOP is subclassing a table base class and overriding some methods there.
With delegation, you don't need subclassing the table base class; rather you use the base class and tell it to "forward" some messages to some other object. This is the basic idea.
In our example, when the row is clicked, the table base class does not know what do except sending a message to one object that you specify as the delegate to carry through that action.
So, one basic advantage of delegation is that you do not need to subclass. Another advantage you have is that a delegate can act as the delegate for several other objects. Indeed, if you have a look at the generic declaration of a delegate method, you will see that the first parameter is the object that is delegating:
- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath
so when the delegate receives the message, it knows who sent it and with which object it should interact.
The example I gave you about the table shows one kind of delegation, which I would not compare to callbacks. Anyway, delegation can be used as well as a kind of advanced calling-back-scheme.
Take the class NSURLConnection; it can be used to manage aynchronous communication.
Async communication is the typical case where callbacks are used.
With NSURLConnection, the delegate pattern is preferred; so, instead of specifying a callback (a function, which has to be a static function, or a static class method), you specify an object. This object implements the methods that a protocol defines (NSURLConnectionDelegate protocol); you can see those as a whole set of callback functions. When the NSURLConnection has some data ready, it will call a method of the interface, e.g., – connection:didReceiveResponse: to notify that it has received the response.
In such case, the point is not avoiding subclassing, rather it is a more flexible callback mechanisms (allowing for better encapsulation, basically).
I hope this helps clarifying the two concepts...
delegate noun |ˈdeligit|
A person sent or authorized to represent others, in particular an
elected representative sent to a conference.
A delegate in the Cocoa Touch frameworks such as Foundation or UIKit is a separate object instance that is entrusted to listen in to, and optionally decide behaviours on behalf of the delegator.
Take the example of UIWebView. The web view can entrust responsibility onto one delegate as long as that instance conforms to the UIWebViewDelegate protocol, that is promises to behave as the web view expects a delegate to behave.
The delegate gets called from time to time. Embedded in the delegate method names is information about what the call is about.
webView:shouldStartLoadWithRequest:navigationType: - Tells the delegate that the webview wants to load a request, and ask the delegate if it should do so.
webView:willStartLoad: - Tells the delegate that the web view will start loading a page, and it is too late to do anything (This is a hypothetical method that is not actually available, I use it as a consistent example).
webView:didFinishLoad: - Tells the delegate that the web view did load a page and it is now finished.
The value of using delegation is that you do not need to sub-class UIWebView in order to tweak how it behaves. One objkect can also serve as delegate to many different webviews, or even serve as delegate to many different kinds of classes.
A normal UIViewController subclass probably conforms to many delegate protocols, for example UITableViewDelegate to respond to selections in a table, UIActionSheetDelegate to respond to selection from an action sheet, etc.
In Objective-C they make a big deal about delegates and make it seem like they're somehow special, but they're simply objects that get called. The only difference is the way they're used: You pass a class instance pointer to some service, and it calls back through specified methods of that instance when it needs some action or data from your code (very much similar to the callback scheme used in other languages).
"Protocols" are just a way to define the methods used in a delegate arrangement. It's a "poor man's multiple inheritance" scheme, virtually identical to the "interface" concept in Java.
But because "protocols" are available, you can (but don't have to) make your delegate class the same class as, say, your view controller. This simplifies your design in many cases (data can be easily shared, etc), but it's not at all mandatory to "share" a class that way, vs using a unique class for the delegate.
I've read in a number of places that using the AppDelegate as the heart of the application is bad (apart from the actual UIApplication delegates themselves).
Would it be wise to create custom delegate classes to keep code modularized.
For example if I need to format various text inputs, create a Formatter delegate class and set the delegate on any text input object that needs to be formatted?
Does that make sense?
The point of the articles you've read is 'Do not keep all application logic in app delegate'. Responsibility of app delegate is to handle important events in app lifecycle in lightweight way by notifying appropriate parts of application, not to manage views or data flows or network calls. For example in application:didFinishLaunchingWithOptions: delegate usually adding root controller's view into window, makes this window active and leaves everything else to controller.
I have a UITextView that points to the File's Owner as its delegate, and I have a number of the delegate functions implemented. However, I would also like to point to a second object (in this case a TextExpander object, http://www.freshblocks.com/tutorials/how-to-add-textexpander-touch-sdk-to-iphone-apps/) also as a delegate. How might this be possible? As far as I know there can only be one delegate in objective-c.
I don't know the specific of TextExpander but he delegate design pattern assumes one and only one delegate. You can't have two delegates for one object.
There are ways around this. You could designate one of the delegates as the primary delegate and implement all the methods in this class. That class could then simple call the secondary delegate for the required methods. This is the simplest approach but does result in the primary delegate becoming tightly coupled with the secondary delegate.
Another approach would be to resolve the messages dynamically and use message forwarding. You would still require the a primary/secondary delegate pattern, but instead of the primary delegate needing to implement all the methods it would simply pass all messages it doesn't respond to onto the secondary delegate. This approach means that the secondary delegate can change by adding/removing additional delegate methods without having to change the primary delegate. The key method is - (id)forwardingTargetForSelector:(SEL)sel. See Objective-C Message Forwarding for a good explanation.
It seems that they solve this in step 5, by setting [textExpander setNextDelegate:self]; before setting the textExpander object as the "primary" delegate of the view.
I don't have the code myself so I haven't tried it, though, and their documentation is... well not.
Just for information: Delegates are not an Objective-C feature, but a design pattern using though Cocoa/Cocoa-touch.
Delegate pattern allow only one object to be notified of the changes of another. The solution to have an intermediate delegate object could be an easy way of doing things.
If you want to keep your code clean and reduce the coupling you might use another pattern known as KVO (Key Value Observing). Apple provides a good guide on how to use it.
In KVO a single object, that needs to be KVC (Key Value Coding) compliant, can notify multiple objects without even knowing about it. It uses an intermediate notifier object (singleton for you application).
Check the Apple doc on KVC + KVO and you shall be able to do what you want.
I have a class with the protocol NSXMLParserDelegate implemented. I call this ParserHelper. I parse a lot of XML which happen to have some tags in all of them and i have about 20 parsers (one for each type of XML), and i didnt want to implement it over and over again.
So, I subclassed my ParserHelper class 20 times, adding in each subclass the required tags/behaviours like CaseAParser, CaseBParser and so on...
I just implement the delegate methods i needed and called it on the superclass once i'm done with the method.
I assume it works on every delegate.
This way, all you need is to make your class a subclass of SMTEDelegateController and implement the methods you need, remembering to call the [super whatEverMethod:andParameters] at the end of your functions and you should be good to go.
I think I've understood what that Delegate is supposed to do. If a class conforms to that protocol, it tells the underlying system: "Hey man, I am the UIApplication object's delegate! Tell me what's up, and I may tell you what to do!".
What, if multiple classes implement that? Is that possible? Does that make any sense?
While you could implement multiple classes that conform to the UIApplicationDelegate protocol only one, the first, would receive these messages.
Implementing a protocol to create a delegate is only one part of the equation. That delegate then has to be registered with the code that's generating the messages and these systems generally only support one delegate.
In the case of UIApplication you can change the delegate using the 'delegate' property in the UIApplication shared class but this will replace the original delegate, not add an additional one.
If you need to broadcast UIApplication level messages to other systems then this is functionality you should add to your existing delegate.
You can implement multiple classes that adopt the UIApplicationDelegate protocol, but only one can be the actual delegate at any given time. It's set by [UIApplication sharedApplication].delegate, which is normally set up by the main NIB file by an outlet connection.
Just conforming to the protocol doesn't set your object as the delegate, you need to do that explicitly either in the nib or in code. As already mentioned, only one object can be a delegate at one time. Having multiple delegates may make sense in some cases-- for example if you have a table view that displays two sets of data, you could make two delegate and datasource objects for it, and switch between them as needed. It probably doesn't make sense to do this for the application's delegate though, since the code there is pretty specific.
Keep in mind that sometimes an object will send notifications in addition to calling delegate methods. A lot of time it looks like they're the same thing, since the object will automatically subscribe your delegate to the notification if it includes a certain method signature. The key difference though is that other objects besides the delegate can also subscribe to these notifications, so you can hook them up to multiple objects at once.
As Daniel Dickson stated:
You can implement multiple classes that adopt the UIApplicationDelegate protocol, but only one can be the actual delegate at any given time. It's set by [UIApplication sharedApplication].delegate, which is normally set up by the main NIB file by an outlet connection.
... but know that you can swap these out at runtime if you need to. I recently looked at using this technique as a way of merging two applications developed by different parties that could not share source code or refactor; yet needed to co-locate under a single icon on the device.
I'm new to development on the iPhone. Just about every sample project (as well as the default project templates) have one or more delegates and controllers. Can someone give me a breakdown of what the delegates are responsible for vs. what the controllers are supposed to do?
The simplest way I can think to differentiate the two are:
A delegate is a protocol (interface) that defines methods that an object implements in order to receive specific messages from other objects. Delegates objects are most often used to receive asynchronous callbacks such as user input, I/O.
A controller is an object that usually contains UI elements (views, controls, etc.) and data, and both receives and send messages to the various objects within it. In many cases, a controller is a delegate and can implement several delegate protocols to receive events from multiple objects.
Keep in mind that many UI elements and controls let you pass events back to the controller by linking them to an IBAction method in Interface Builder. This is very handy as it doesn't require extra code to implement delegates. However, some other APIs such as the ABPeoplePickerNavigationController or NSURLConnection have no visualization in Interface Builder and so must use delegates to handle their events.
A delegate is some object that implements a set of methods which either your application or the framework you link against depends on for functioning. It is a means of implementing a delegation based design pattern wherein the responsibility for performing an action is transferred from some root source to an interested third party. For instance, UIApplication has delegate methods that provide a third party with the ability to perform operations at certain times during the applications lifetime. It can be though of as a milestone in a timeline into which you can contribute to the story.
A controller is a totally different animal and is responsible for doing, well, the controlling. A ViewController is charged with managing views - for loading them into memory from disk when they are needed and unloading them when they are not. They transform content from some underlying model object into a form that is usable by your view objects, load content into your in-memory model from the disk or from the internet, and dump the contents back to disk when you save and/or quit.