As I understand, Pickers have an Delegate class. Why's there an additional Datasource needed?
There is a difference between a delegate and a datasource.
A datasource is typically used to configure what is displayed by a control. "How many rows do you have?", "What should i display in this row", etc.
A delegate is usually used to let the controlling code know that something happened. "Hey someone selected this row.", "Hey someone started editing this row."
Because the picker view needs information on how to populate the table inside each segment.
Sometimes it may be useful to have one class handling where the data comes from and another handling what to do with it (MVC pattern)
Like most supervisors, the picker is stupid. It can't figure out what to work on, so it keeps being given data and tasks(by the provider); but it needs to have someone who knows what to do with it. That's the delegate.
The API used to combine datasource and delegate methods into one protocol (at least for UITableViews), but it's inflexible - the way it is now you can have a totally different object provide the data to be displayed from the code that is responsible for handling the mechanics of using a picker.
Generally though you do end up wiring both to the same object.
Related
Yes, I'm a noob at Xcode. right now I'm working my way through the Big Nerd Ranch IOS programming book, and I just need a little clarification on the Model-View-Controller bit.
Model class is called 'CellData', an NSObject
View class 'CellView', a UIView
Controller class is called 'CellAppDelegate', a UIResponder.
The DrawRect method does a lot of drawing, with colors based upon the data from the CellData Class. Everything so far seems tells me that I should not call a 'CellData' method from my 'CellView' class. Normally yeah when your just using UIButtons with TestFields, yeah, makes perfect sense.
In this case, I have up to pass a value from 'CellData' to 'CellView' up to 6000 times to refresh a View. Does it still make sense to keep on calling back and forth between using the CellAppDelegate (seems like a lot more work for the computer), or am I really 'allowed' to retrieve a value from CellData?
You've answered your question yourself really. No, 6000 calls per render isn't the right thing to do, especially on a table view cell which will be redrawn pretty frequently.
Personally, I see the controller's job as asking for data objects from the model layer (where business logic etc happens) and passing them into views. So, in your case, I would just pass the data object into your cell. (I would have it as a property on your cell with a setter that also called [self setNeedsDisplay]; to trigger a redraw)
That being said, I also tend to favour the model layer giving out immutable data objects so the controller / view can't do anything wrong with them :) With this approach, if the controller wanted to edit the data object it would have to call a method in the model layer to do so.
Though, of course, this is just my personal opinion and choice of architecture. And obviously for existing UI objects (i.e. a UILabel etc) I can't pass in the data object, I have to set the text property directly from my view controller.
PS CellAppDelegate is a bad name for a view controller - the custom is to end the class name with ViewController so it would be CellAppViewController. This naming scheme makes it very easy to come back to your code in a week or so and still understand what everything does :)
Everything so far seems tells me that I should not call a 'CellData' method from my 'CellView' class.
This sounds like a misunderstanding. It is extremely common for something like CellView to be handed a single CellData by the controller. The view is free to query the model object it is responsible for displaying. This is not a violation of MVC. It should generally not write to the model object, nor should it talk to other model objects typically.
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.
I have been using Objective-C for a while and pretty much understand most of its features. However, the concept of delegates eludes me. Can someone please give a succinct and easy to comprehend explanation of what delegates are, how they are used in the iPhone SDK, and how I can best make use of them in my own code?
Thank you!
There are a couple main reasons to use delegates in Objective-C, which are subtly different:
Enhancing the base functionality of a framework class. For example, a UITableView is pretty boring on its own, so you can give it a delegate to handle the interesting bits (creating table cells, adding text to section headers, what have you). This way, UITableView never changes, but different table views can look and act very differently.
Communicating to parent objects in your dependency hierarchy. For example, you may have a view with a button that the user may push to do something that affects other views. The view will have to send a message to its parent view, or perhaps the view controller, so that it can create or destroy or modify other views. To do this you'd pass the parent object into your view, most likely through a protocol, as a weak reference (in Objective-C, an assign property). The view could then send any message declared in the protocol to the parent, or delegate, object.
This approach need not involve views. For example NSURLConnection passes event back to its delegate, which may be the object that created it, using this mechanism.
Essentially, all a delegate is, is an object that accepts feedback from another object. Put simply, when stuff happens to an object, it tells its delegate (assuming it has one).
For instance, lets say I have a UIViewController with a UITextView placed in the middle of the view. I set up my UIViewController to be the delegate of the UITextView. Then, when certain actions are performed on the text view (begin editing, text changes, end editing, etc), it tells it's delegate so it can do whatever logic it needs to do, like spell checking every time characters change, or dismissing the keyboard when it receives a return key press.
Delegate methods perform a similar function to callback functions in C.
Hope that makes sense :)
Best and simple concept I got from a Lynda.com Tutorial was: When you set a Delegate it means you have been given work to do. So, if you want to use methods that are written in a protocol method, you must implement them by searching in the Delegate Class Reference and using them. I hope it helped.
By the way, Delegates are excellents. They are your friends. They have been made to make your life as a programmer much easier.
I'm new to Objective-C and not a full time programmer. I'm beginning to understand the Model-View-Controller design pattern for differentiating the UI from the model. So the user takes an action and the view controller sends a message to the delegate (model). But I'm not sure what the best way to send actions from the delegate back to the view controller.
For example, the user pushes a button, the VC messages the Delegate. That part I understand. Then the delegate takes action, and following that the delegate wants to update the VC (e.g., update a label).
So what I missed (or have forgotten) is how this gets done, while maintaining separation between the UI and the model. I suppose I can use the notification center. Or I think I can just have the view controller pass a callback to the delegate. Or maybe there's another choice I don't know of. Can someone give me a recommendation, please?
I think you're slightly misunderstanding the MVC paradigm. Models should never be delegates of views, since models should have no dependencies or knowledge of any view classes. Typically, a view sends a message to its delegate or target (if you're using target/action), which is usually a controller (often a subclass of UIViewController on iOS). The controller then accesses data from the model and can update any views that need updating. I'd recommend reading the MVC fundamentals guide for a more complete explanation.
Basically you're right, you could do all the notification-related things yourself (i.e. with NotificationCenter) but since we're talking about UI-Stuff here I would greatly recommend you to use IBAction-Methods and IBOutlet-Properties in your code which you can easily connect to UI-Elements respectively their Callbacks in Interface Builder.
A very basic introduction to this topic can be found here:
iPhone SDK Interface Builder basic training
i hope that it is not too basic tough, and that I could lead you on the right track.
First of all delegate is NOT a Model.
Model is something passive that only holds the data (DB, plist, array, dictionary etc.).
While delegate is some set of functions that exist in order to react to some events.
Delegate is more likely to be a view controller in your case.
The view controller should react to user's action.
If the button tap should display some data from your model in some label then view controller should do all the work (receive user's action, take the necessary data from the model and display it on the view...).
I have two tabbar items(views) that use the same data, whats the best solution for getting the data?
Make two fetch request for the same
data in each view controller.
Make one fetch request in
appDelegate, and use
sharedApplication to get to the data
in appDelegate. I can use KVO and
notifications to notify the views if
the data has changed.
If i had to choose, i obviously would go for 2, but i want to make sure i am doing the right thing.
Can anyone tell me if this is the right approach?
I'm not sure why you'd be sticking data fetching-related stuff inside your app delegate, unless there's a good reason to do it there. (I can't really think of one). Having your view controllers observe the app delegate via KVO seems like a bad code smell to me.
I prefer to create data model classes (sometimes designed as Singletons) and use KVO or notifications with my view controllers. It makes for a cleaner design.
Here's a blog post by someone else on the subject.
I would recommend you use something like a singleton class. There is a very good example at bit-101 . The good thing about this example is that it extends easily to more complex cases, e.g. more tabs ...