Can someone give me a quick, one-to-few paragraph(s) overview of the structure of iPhone apps. I'm working my way through a book, but I can't really understand the purpose of App Delegates, MainWindow.xib vs. individual views' nib files, actions vs. outlets, etc.
I'm looking for a high-level description, because all I can find are really detailed accounts of this stuff.
You can find out more about Objective-C design patterns like delegation in Apple's documentation. Here's a quick overview of the things you mentioned:
Objects communicate with other options in various ways, one of which is delegation. An app delegate is an object that receives certain messages from the application.
XIB files contain a description of your user interface and connections. MainWindow.xib contains the UI information for your main window, but you should have individual XIB files for other view controllers for the sake of efficiency (you should load the XIBs lazily when the user tries to instantiate a certain kind of view controller, and cache the XIB to prevent reloading it; see UINib for detail).
Actions and outlets are ways of connecting code to UI elements in your XIB. An action is a message sent by a UI element to an object. For example, a button press can send an action to your view controller. An outlet would be a reference to that button.
For more about Objective-C in the context of iOS development, consult the documentation:
http://developer.apple.com/iphone/library/referencelibrary/GettingStarted/Learning_Objective-C_A_Primer/index.html
http://developer.apple.com/iphone/library/referencelibrary/GettingStarted/Creating_an_iPhone_App/index.html
The physical structure of an iPhone application consists of a directory that acts as a bundle for the main executable and all resources used (images, property lists, interface files, etc.).
When run, the executable first performs the main() function. That function either programmatically instantiates an application delegate or loads the main interface file (which specifies the application delegate). The application delegate acts in response to application-level events, such as termination or entering and exiting the background.
The construction of your interface can either occur programmatically or through the use of Interface Builder .xib files. If programmatically, your application delegate will construct the initial overall interface, then hand things off to individual view controllers to manage the display of specific views. If done via Interface Builder, you'll define interface files that compile into freeze-dried objects that are deserialized into your interface at runtime.
Overall, Cocoa applications tend to follow the Model-View-Controller design pattern, where these three areas of your application are separated in code. Views are generic reusable display elements, your model contains application data (often persisted using Core Data or SQLite), and your controllers provide the application-specific logic that glues everything together.
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What is the need of delegation in iphone/ipad development or objective C?
I read so many articles on it. All were telling how to implement the concept, but no one was telling why we need to implement that, in which case we should implement it.
Suppose you want to implement Login functionality in your app ... now you won't show Login screen every time you run your app.. only when it is first started and you don't have a login and password...
So in this case..
Your app starts :
View 1 loads (default view )
You check no Login name is there..
You load a new view..(Login View ) .
User enter his details..you get your login and password...
now you want to go back to default view and load the main app with
the names the user entered in Login View....
Now you will use delegate to pass these information(login details) back to default View..so that it knows..its details. now there are many different ways to do these things...like notification and singleton classes.. but when you want to sent more than 3-4 sets of data.. it is best to use delegates
Think of all the components that iOS and Cocoa provide you with. TableViews, TextFields, PopOvers...etc.
When the developers wrote these components, they couldn't possibly know all the various implementations that us developers were going to create using these components. But we need somehow to communicate with them in a generic way.
These components use delegates. The delegate is an implementation independent way of describing some behaviour that your component can conform to.
When UITableView need to find out what is the height of the rows, the UITableView only needs to know about UITableViewDelegate. It doesn't need to know about MyTableViewController, JohnsTableViewController, BobsTableViewController... etc.
So the delegate is decoupling the component from the implementation and the type.
Decoupling is a good thing. It makes maintaing and changing code a lot easier, and makes code reusable.
Delegation is a simple and powerful pattern in which one object in a
program acts on behalf of, or in coordination with, another object.
The delegating object keeps a reference to the other object—the
delegate—and at the appropriate time sends a message to it. The
message informs the delegate of an event that the delegating object is
about to handle or has just handled. The delegate may respond to the
message by updating the appearance or state of itself or other objects
in the application, and in some cases it can return a value that
affects how an impending event is handled. The main value of
delegation is that it allows you to easily customize the behavior of
several objects in one central object.
SOURCE
Use a delegate if you want to talk to only one object. For example, a
tableView has a delegate - only one object should be responsible for
dealing with it.
Use notifications if you want to tell everyone that something has
happened. For example in low memory situations a notification is sent
telling your app that there has been a memory warning. Because lots of
objects in your app might want to lower their memory usage it's a
notification.
this was an answer posted to my question here
There are two key benefits of delegation: customizing objects without subclassing, and improving encapsulation.
Customization without subclassing is a benefit you get from many of the Cocoa and Cocoa-Touch APIs using the delegate pattern. If they didn't do so, you might have to subclass a UITableView every time you wanted to change its behavior by using different types of cells or a different data source. Instead, you just set the table view's delegate and data source to customize its behavior.
As for encapsulation, the delegate pattern helps you keep the different components of your code separate. For example, if your custom View needs to have some data, it would be bad practice to simply give it access to your Model, or even full access to your Controller. Instead, you'd probably set up some kind of delegate protocol for your View that your Controller would implement. That way your classes know no more about each other than they need to, so that changes in one part would be less likely to break others.
Could anyone tell me when we use the AppDelegate.m and AppDelegate.h during iPhone programming? I have used only the ViewController.m and ViewController.h for basic learning. I just want to know when and why AppDelegate is used.
Both define classes, but the classes are used for different things. ViewController.h/m define a view controller class that manages a hierarchy of views -- basically, one screen of an application. You might have multiple screens that each have their own view controller.
AppDelegate.h/m define a class that manages the application overall. The app will create one instance of that class and send that object messages that let the delegate influence the app's behavior at well-defined times. For example, -application:didFinishLaunchingWithOptions: is sent when the app has finished launching and is ready to do something interesting. Take a look at the UIApplicationDelegate reference page for a list of messages that the app delegate can implement to modify the behavior of the application.
I would like to add the following to #Caleb's answer.
If care is not taken, the AppDelegate could easily become one of the most accessed objects in the application. I usually refrain from calling methods in the AppDelegate from any of my ViewControllers. Unless, something needs to be reported to the AppDelegate that would influence the behaviour of the whole application.
I keep my AppDelegate for the following:
initialization: whatever needs to be done on the very first launch (after an install or an update)
data migration from version to version (e.g. if you use CoreData and migrations)
configuration of objects linked via IBOutlets from MainWindow.xib
determining the initial orientation to launch in
saving uncommitted data / state prior to the application being terminated or entering background mode
registering for the Apple Push Notification Service and sending the device token to our server
opening one of the supported application URLs (e.g. maps://)
For other use case scenarios and a more thourough description of the AppDelegate, see the iOS Application Programming Guide.
The view-controller. h/m is responsible of controlling the connection between your model and your view (more on MVC here).
AppDelegate. h/m is responsible for the life-cycle of your application. What to do when the user press the home button and exit your app, what to do when the app enter background. Things like this.
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'm new to Objective-C and the iPhone and thought I was getting the hang of it until, after many play apps, I ran into a really basic problem around MVCs, NIBs and IB. Are there any really clear explained examples of how to follow this framework that I could go back to?
#interface test1ViewController : UIViewController {
IBOutlet myView *myview;
IBOutlet myModel *mymodel;
}
Both the views and models are linked in by IBOutlets but instantiating the model object either kills the application or produces an object which does not respond to any messages.
I am also unclear as to where to carry best out initialisations. I currently do this over viewDidLoad in the view controller. Is there a standard way to do this and does the simulator actually always start up in the same way? Should one use awakeFromNib? Does it make any difference if I use plain code or the IB? And if I have use the IB, should it include a model object?
The MVC idea would make good sense for me here because I have potentially several views and view controllers all feeding back into - and sharing - one common central data model. Any references or advance on this newbie problem would be more than welcome!
I wouldn't spend too much time worrying about the 'classic' definition of MVC. iOS follows it, but there's a lot of confusing terminology. ("View Controller")
You say trying to use model kills your app. Are you retaining myModel? You have to retain all IBOutlets.
nibs are collections of "Freeze-Dried" objects. When you load a nib, the objects in it are "rehydrated", if you will. This means they spring back to life with all of their properties set to whatever they were when you froze them. So you talk of "instantiating" and "initializing" but this does not apply to IB. The objects are ALREADY instantiated and initialized. Imagine that compiling the nib 'pauses' the objects. When you load the nib, the objects resume doing whatever they were doing when frozen. They will get an awakeFromNib message, so that's a good place to put some code to check on what the state of the app is, see if you have to do stuff with your object to bring it up to speed.
viewDidLoad seems like an "initialization" method but don't be fooled. It's part of the view controller life cycle and it can be called more than once! (If your controller's view is purged as part of a low memory warning, viewDidLoad might be called again if the view has to be... wait for it... reloaded.) So, it's appropriate to put view setup stuff in viewDidLoad, but not other initialization type things.
For the "common data" thing, I like to create a singleton data model class. Your various views can set properties on the model, or send notifications. You can also use KVO (key value observing) to watch for changes in the model.
IB makes functionality invisible. I don't like it and I don't use IB any more, preferring to have everything in code. Then when you look at code you see what is going on - all the navigation controllers, all the formatters etc. - without switching over to IB. Maybe Xcode4 will make it better with integrated IB but I probably won't go back. Lots of people do like IB so try both methods and see what you like best.
IBOutlet/IBAction actually mean nothing to the compiler but they let IB recognise the ivars it can send messages to or that will write to elements in a xib. Your use of it here is a bit simplistic unless you really do have a model that only communicates one way with the xib. What you more usually have is a range of controls in the xib linked to the view object, the view communicating directly with the controller. The controller communicates with the model. Very loosely speaking, model is your internal representation of data and logic, view is what you see, controller is the glue between them.
The MVC line can be fuzzy and you just have to get comfortable with it. If you have a slider control with a value representing some value in your model then it can be hard to think of it as part of the interface especially when you persist the value and use it as a central part of your model. I like the Stanford iPhone class introduction of it. They don't spend a heap of time on it because it can be difficult to follow exactly and there are situations where it isn't best.
Notes from that class - you can find the video on iTunes to follow along.
Your use of viewDidLoad is correct, that's your chance to perform initialization on views and their objects. If using IB you will probably not have much to do there because you can set most properties in the xib. If not using IB you will use it a lot more.
A lot of times something like your model would be wired at runtime by your application delegate or by the view controller itself.
IB is generally used more to link views and controllers together, with the application handing around model(s).
That said, you should be able to have IB create an instance of your model and assign it in an IBOutlet. Was your model OK with just being created without the classic init method being called? Did it implement NSCoding properly?
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.