Delegation

On your way to developing apps you will encounter many situations when you would want to respond to certain events in your app but you would find it difficult to implement such actions in code. 

For example, you are downloading a file from the internet and you want to constantly update the progress on the screen for your user. Now to actually download stuff from the internet you would use built in classes like NSURLRequest which have been built by Apple. Now what Apple does in order for you to actually accomplish the task of showing progress is that, it tells the class NSURLRequest to send messages while it is downloading, to a class of your choice. So the NSURLRequest class is delegating tasks that it should be doing to a class of your choice. This class of your choice is known as delegate.

I guess the concept should be very clear by now, some class sends messages to some other class so that the other class may complete tasks that the first class should have done.

Let us take another example where this is used, the TableView. The TableView is used in many iOS apps including Mail and iPod. The table view has cells, and each of those cells contain data. Many a times when you tap on a cell it does some action, for ex in the iPod app when you tap on a song it starts playing it. This is done by delegation. 

So whenever a cell is tapped on a table view a message is sent to a class defined by you, let us call that class yourClass. Now what is meant by sending a message is that a predefined method specified and decided by Apple engineers is called in yourClass. In the case we are discussing, this method is tableView didSelectRowAtIndexPath is called when a cell is tapped.

You have to implement this method in yourClass and in that you can include code to take whatever action you want in response to the cell-tapping. In case of the iPod app this method will start playing the selected song.

For detailed instructions on how to include a tableView in your app refer to this post.

If you have still not got a clear idea about delegation I am very sure when you actually start delving deeper and deeper into iOS development your concepts will become clearer.

UITableView

The UITableView is a very common design pattern used in iOS apps. They help in presenting a lot of data in limited screen real estate.

Setting up a UITableView requires a delegate and a data source to be implemented. In case you're not familiar with the delegation and data source concepts, think of it like this, the delegate is a class which tells the OS, how to present the table view, for ex its size, the modes in which the table view should be like editing, normal etc. The data source, as the name suggests provides data to the table view to present it on the screen.

This was about the concept of delegation and data source, to actually implement it, you should declare one of your classes (view controller, preferably with a nib file as well) as the UITableViewDelegate and the UITableViewDataSource. This is done by adding <UITableViewDelegate>, <UITableViewDataSource> after the class declaration in the header (.h) file, like this :

@interface MainViewMulti : UIViewController <UITableViewDelegate,UITableViewDataSource> {
//class members
}

To actually add the table view you can either do it through code or interface builder. Interface builder is preferred if you have relatively simple tasks. So click on the nib file linked with your view controller, it will open interface builder. Now in the library search for tableview and drag it to your nib.

After doing that right click on the table view you just dragged, there will be delegate and datasource outlets, drag it to the files owner. This tells the table view that the class you've declared is its delegate and data source.

Now that you have declared your class as the delegate and the data source it is time to implement mandatory methods which will provide data and set up the the tableviewcells.

The apple documentation says that the following methods are mandatory


- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section


- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath

The first method tells the device, how many rows do you want, that value is specified by the return value of that method.

The second method tells the device what contents to put in each row of the table view. This method is called for each row.

Right now you have the basic framework required for setting up a table view. You can use this as a template for all of you tableviews.

Now to actually put data in your table view let us declare an array in the header file, and populate it with nsstrings. Let us name the array tableDataSource, to populate it insert the following code in your viewDidLoad method


tableDataSource = [[NSArray alloc] initWithObjects:@"Monday", @"Tuesday", @"Wednesday", @"Thursday", @"Friday", @"Saturday", @"Sunday",nil];

In case you aren't familiar with arrays, this is one of the convenience methods to directly assign an array with objects.

Now the table view will contain as many rows as the number of objects in the array, so to tell this to the os add the following line to your -  numberOfRowsInSection method
return tableDataSource.count

The only thing left now is to populate each row of the tableview with the items in the array, to do this add the following code to your cellForRowAtIndexPath method

static NSString *SimpleTableIdentifier = @"SimpleTableIdentifier";
UITableViewCell *cell = [tableView
dequeueReusableCellWithIdentifier:SimpleTableIdentifier];
if (cell == nil) {
cell = [[[UITableViewCell alloc]initWithStyle:UITableViewCellStyleDefault
reuseIdentifier:SimpleTableIdentifier] autorelease];
}

NSUInteger row = [indexPath row];
cell.textLabel.text = [tableDataSource objectAtIndex:row];
return cell;

Now save everything (including the nib files) build your project and you should see the table view with the items you set in the simulator.

Next we will discuss how to put the table view in edit mode and how to add and delete items from the tableview.