Spencer MacDonald

Helper Objects are used throughout Cocoa and CocoaTouch, and usually take the form of a delegate or dataSource. They are commonly used to add functionality to an existing class without having to subclass it.

In software engineering, the delegation pattern is a technique where an object outwardly expresses certain behaviour but in reality delegates responsibility for implementing that behavior to an associated object in an Inversion of Responsibility. The delegation pattern is the fundamental abstraction that underpins composition (also referred to as aggregation), mixins and aspects.

http://en.wikipedia.org/wiki/Delegation_pattern

The most common use of helper objects in iPhone development is when using UITableViews. When you instantiate a UITableViewController this class is automatically assigned to be the delegate and dataSource of the table view it holds.


self.tableView.delegate=self;
self.tableView.dataSource=self;


The UITableView then calls the appropriate delegate method when it needs information, such as:


- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath


This ask the delegate, what is the height for the row at a given index path, so in your UITableViewController (the delegate) you would write the following, if you wanted the row to be 44 pixels high (the default).


- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath{

return 44;
}


Writing your own delegate.

Most delegates are declared as a protocol, so the compiler knows that the delegate implements the required methods.

Example:

You have created a subclass of UITextView (e.g. OBCTextView), that you use in your application to enter both Tweets and SMS. You want to use the same class for both, BUT tweets have a character limit of 140 and SMS have a character limit of 160 (we will pretend that we are still in the 90s).

So we need to create a delegate that asks for the character limit.

- (NSInteger) charachterLimitForTextView: (OBCTextView *)textView;


As mentioned before this should be declared as a protocol

@protocol OBCTextViewDelegate

@end


The OBCTextView would call this method like so:

In our SMS view controller we would declare that we implement the delegate protocol.

@end


And then in the implementation file you would implement the following method

This method is required for OBCTextView to work so we should also add the key word @required to the protocol


@protocol OBCTextViewDelegate

@end


Optional methods

In your delegate, you may also want to implement optional methods. So for this example we will ask for the text color. You can specify that a method is optional using @optional.

@protocol OBCTextViewDelegate

@required
- (NSInteger)charachterLimitForTextView:(OBCTextView *)textView;

@optional
- (UIColor *)textViewColorForTextView:(OBCTextView *)textView;

@end

As it is optional the delegate class (helper object), does not have to implement it. If it does not implement it, and we attempt to call it, the application will crash. So how do we know if we should call the method or not ? The answer is respondsToSelector.

respondsToSelector allows you to ask an object at run time, if it implements a given selector. So for our example:

if your class wants to return a textview color, then you simply implement the method.

So that finishes off this post on helper objects, and how you can create your own. As always this example was a simple one to make it easy to follow. You could probably implement the above using setter methods.