What exactly does delegate do in xcode ios project?

Question

I have just been learning iPhone apps development but I have a hard time in understanding what delegate actually means? Can anyone tell me with example what it does and how important it is? Thanks for any helps!

Answer 1

It's a key concept to understand conceptually so getting your head around how to think about it ahead of the technical details is important. Simply put, a delegate is a callback.

Two main scenarios to use delegates:

1. A class or control wants to abstract out the details on how to do work (like retrieve data).
2. Allow others to hook code into a pipeline.

Examples: UITableView - a table view is just a control that knows how to render a list of cells. It handles all the heavy lifting of rendering, scrolling, etc... But, it has no idea how to load your data. So you implement a datasource delegate which has methods to get the cell data for a given row etc... That makes it easy on you. You just use the control and plug in the specifics for your data. The UITableView will do everything for you ... just answer a few specific questions for. A delegate answers those few specific questions.

A text control - you add a text control to your view and voila! you can type in it and alls good. But what if you want to do something when they start typing or when they're done typing? Well, the text control offers a delegate with methods that allow you to hook into the execution pipeline of the text control. It allows the text control to do everything for you and allows you to interject code where you need it. Many times, there's way to interject code to make a decision on whether something is allowed. The control will call back and ask, should I be able to do x? You can interject code and influence the behavior.

If you're creating a control or class, you can create your own protocol, datasource delegates etc... so your control can focus on doing what's advertised. For example, let's say you wanted to create a task control. You could:

First, create a contract. Hey, if you're going to provide data for my control, these are the questions I'm going to ask you. I'll take it from there... In this case, I'm going to ask you the number of tasks and I'm going to have you give me a task given the row number.

@protocol XXTaskBoardDelegate <NSObject>
-(NSInteger*)getTaskCount;
-(XXTask*)getTaskForRow:(NSInteger*)rowNumber;
@end

In the control or class, give the consumer a way to give us the delegate datasource class that will answer the questions the control will ask. At this point, the control is a pure control. It knows nothing about how you get your data. It's asking for an object (id) that implements a contract/protocol. id

@implementation XXTaskBoard
- (void)setDelegate:(id<XXTaskBoardDelegate>)newDelegate
{
    // the control stores the delegate so it can callback and ask you questions.
}

Then, for the delegate class, in the header declare you implement that formal protocol and in the implementation m file you provide the code.

@interface AppController : NSObject<XXTaskBoardDelegate> 
{
    //...
}

then, implement it in the implementation

@implementation AppController
- (NSInteger*)getTaskCount
{
    return [model queryTaskCount];
}

- (XXTask*)getTaskForRow:(NSInteger*)rowNumber
{
    return [[model tasks] getItem:(NSInteger*)rowNumber];
}

Answer 2

A delegate is an object that another class can pass messages to. In practice delegate classes have to conform to a delegate protocol.

For instance we will take a subclass of a table view controller. This is a delegate for your table view. First you define that it is a table view delegate by doing this:

MyTableViewController : UITableViewController <UITableViewDelegate>

This says that class MyTableViewController is a subclass of UITableViewController and CONFORMS to the UITableViewDelegate protocol.

Setting [tableView setDelegate:self] (or defining it as such in IB) then passes the self object to the tableview in order for the tableview to send messages to it.

The main message it sends is the didSelectRowAtIndexPath message which tells your class that the user has pressed a table view cell.

So the object that takes the click event (the table view) passes on the message that the cell has been clicked to the delegate object (which in this case is your MyTableViewController object).

Delegate protocols exist so that you can make sure that the delegate object has the necessary methods to deal with your messages. Methods in a delegate protocol can be @optional or enforced. Any methods that are optional don't have to be defined. In your MyTableViewController class the method didSelectRowAtIndexPath is optional - you don't have to have it. If the table view doesn't find the method it just won't call it.

However the cellForRowAtIndexPath is necessary and without it your app won't compile.

I hope this helps and is straightforwards for you. If you need any more info let me know.

Answer 3

Delegates are just way of getting callbacks from something. You pass a delegate (a pointer to an object that conforms to a protocol) to something and when it has new data for you or when an event occurs that something make a method call on the delegate.

For example, when events occur, like your app is put into the background or the app is about to terminate the UIApplication object will call your application delegate to let it know. When a CLLocationManager has a new GPS position is will call your delegate to pass it the new position. UITableViews call their delegates to get UITableViewCells to display in the table. There are many uses of delegates in iOS.