ne of the neat features available in iPhone OS 3.0 is the GameKit framework. The GameKit framework contains APIs to allow communications over a Bluetooth network. Using these APIs, you can create peer-to-peer games and applications with ease. Unlike other mobile platforms, using Bluetooth as a communication channel in iPhone is way easier than expected. Hence, in this article, I will show you how to build a simple application that allows two iPhone or iPod Touch devices to communicate with each other.

Author’s Note: To test the concepts covered in this article, you would need two iPhones (3G or 3GS), or iPod Touches (second generation or later) running iPhone OS 3.0 or later.

Creating the Project

Using Xcode, create a new View-based Application project and name it as Bluetooth.

All the various APIs for accessing the Bluetooth is located in the GameKit framework. Hence, you need to add this framework to your project. Add a new Framework to the project by right-clicking on the Frameworks group in Xcode and selecting Add, Existing Frameworks. Select GameKit.framework (see Figure 1).

Figure 1. GameKit: Add the GameKit framework to the project in Xcode.

In the BluetoothViewController.h file, declare the following object, outlets, and actions:

#import #import @interface BluetoothViewController : UIViewController {    GKSession *currentSession;    IBOutlet UITextField *txtMessage;    IBOutlet UIButton *connect;    IBOutlet UIButton *disconnect;}@property (nonatomic, retain) GKSession *currentSession;@property (nonatomic, retain) UITextField *txtMessage;@property (nonatomic, retain) UIButton *connect;@property (nonatomic, retain) UIButton *disconnect;-(IBAction) btnSend:(id) sender;-(IBAction) btnConnect:(id) sender;-(IBAction) btnDisconnect:(id) sender;@end

The GKSession object is used to represent a session between two connected Bluetooth devices. You will make use of it to send and receive data between the two devices.

In the BluetoothViewController.m file, add in the following statements in bold:

#import "BluetoothViewController.h"#import @implementation [email protected] currentSession;@synthesize txtMessage;@synthesize connect;@synthesize disconnect;

Double-click on BluetoothViewController.xib to edit it in Interface Builder. Add the following views to the View window (see also Figure 2):

• Text Field
• Round Rect Button

Figure 2. Nice view: Populate the View window with the various views.

Perform the following actions:

• Control-click on the File’s Owner item and drag and drop it over the Text Field view. Select txtMessage.
• Control-click on the File’s Owner item and drag and drop it over the Connect button. Select connect.
• Control-click on the File’s Owner item and drag and drop it over the Disconnect button. Select disconnect.
• Control-click on the Send button and drag and drop it over the File’s Owner item. Select btnSend:.
• Control-click on the Connect button and drag and drop it over the File’s Owner item. Select btnConnect:.
• Control-click on the Disconnect button and drag and drop it over the File’s Owner item. Select btnDisconnect:.

Right-click on the File’s Owner item to verify that all the connections are made correctly (see Figure 3).

Figure 3. Check it: Verify the connections made for the various outlets and actions.

Back in Xcode, in the BluetoothViewController.m file, add in the following statements in bold:

- (void)viewDidLoad {        [connect setHidden:NO];    [disconnect setHidden:YES];    [super viewDidLoad];}- (void)dealloc {    [txtMessage release];    [currentSession release];    [super dealloc];}

Searching for Peer Devices

Now that all the plumbings for the project have been done, you can now focus on the APIs for accessing other Bluetooth devices.

In the BluetoothViewController.h file, declare a GKPeerPickerController object:

#import "BluetoothViewController.h"#import @implementation [email protected] currentSession;@synthesize txtMessage;@synthesize connect;@synthesize disconnect;GKPeerPickerController *picker;

The GKPeerPickerController class provides a standard UI to let your application discover and connect to another Bluetooth device. This is the easiest way to connect to another Bluetooth device.

To discover and connect to another Bluetooth device, implement the btnConnect: method as follows:

-(IBAction) btnConnect:(id) sender {    picker = [[GKPeerPickerController alloc] init];    picker.delegate = self;    picker.connectionTypesMask = GKPeerPickerConnectionTypeNearby;          [connect setHidden:YES];    [disconnect setHidden:NO];        [picker show];    }

The connectionTypesMask property indicates the types of connections that the user can choose from. There are two types available: GKPeerPickerConnectionTypeNearby and GKPeerPickerConnectionTypeOnline. For Bluetooth communication, use the GKPeerPickerConnectionTypeNearby constant. The GKPeerPickerConnectionTypeOnline constant indicates an Internet-based connection.

When remote Bluetooth devices are detected and the user has selected and connected to one of them, the peerPickerController:didConnectPeer:toSession: method will be called. Hence, implement this method as follows:

- (void)peerPickerController:(GKPeerPickerController *)picker               didConnectPeer:(NSString *)peerID                    toSession:(GKSession *) session {    self.currentSession = session;    session.delegate = self;    [session setDataReceiveHandler:self withContext:nil];picker.delegate = nil;    [picker dismiss];    [picker autorelease];}

When the user has connected to the peer Bluetooth device, you save the GKSession object to the currentSession property. This will allow you to use the GKSession object to communicate with the remote device.

If the user cancels the Bluetooth Picker, the peerPickerControllerDidCancel: method will be called. Define this method as follows:

- (void)peerPickerControllerDidCancel:(GKPeerPickerController *)picker{    picker.delegate = nil;    [picker autorelease];        [connect setHidden:NO];    [disconnect setHidden:YES];}

To disconnect from a connected device, use the disconnectFromAllPeers method from the GKSession object. Define the btnDisconnect: method as follows:

-(IBAction) btnDisconnect:(id) sender {    [self.currentSession disconnectFromAllPeers];    [self.currentSession release];    currentSession = nil;        [connect setHidden:NO];    [disconnect setHidden:YES];}

When a device is connected or disconnected, the session:peer:didChangeState: method will be called. Implement the method as follows:

- (void)session:(GKSession *)session            peer:(NSString *)peerID  didChangeState:(GKPeerConnectionState)state {    switch (state)    {        case GKPeerStateConnected:            NSLog(@"connected");            break;        case GKPeerStateDisconnected:            NSLog(@"disconnected");            [self.currentSession release];            currentSession = nil;                        [connect setHidden:NO];            [disconnect setHidden:YES];            break;    }}

Handling this event will allow you to know when a connection is established, or ended. For example, when the connection is established, you might want to immediately start sending data over to the other device.

Figure 4. On the lookout: The GKPeerPickerController looking for other devices.

Sending Data

To send data to the connected Bluetooth device, use the sendDataToAllPeers: method of the GKSession object. The data that you send is transmitted via an NSData object; hence you are free to define your own application protocol to send any types of data (e.g. binary data such as images). Define the mySendDataToPeers: method as follows:

- (void) mySendDataToPeers:(NSData *) data{    if (currentSession)         [self.currentSession sendDataToAllPeers:data                                    withDataMode:GKSendDataReliable                                           error:nil];    }

Define the btnSend: method as follows so that the text entered by the user will be sent to the remote device:

-(IBAction) btnSend:(id) sender{    //---convert an NSString object to NSData---    NSData* data;    NSString *str = [NSString stringWithString:txtMessage.text];    data = [str dataUsingEncoding: NSASCIIStringEncoding];            [self mySendDataToPeers:data];        }

Receiving Data

Figure 5. Name it: Display the names of devices found.

When data is received from the other device, the receiveData:fromPeer:inSession:context: method will be called. Implement this method as follows:

- (void) receiveData:(NSData *)data             fromPeer:(NSString *)peer            inSession:(GKSession *)session              context:(void *)context {    //---convert the NSData to NSString---    NSString* str;    str = [[NSString alloc] initWithData:data encoding:NSASCIIStringEncoding];        UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Data received"                                                     message:str                                                    delegate:self                                           cancelButtonTitle:@"OK"                                           otherButtonTitles:nil];    [alert show];    [alert release];    }

Here, the received data is in the NSData format. To display it using the UIAlertView class, you need to convert it to an NSString object.

Testing the Application

Figure 6. Connections: A remote device is prompting to connect to you.

That’s it! You are now ready to test the application. Press Command-R in Xcode to deploy the application onto two iPhones / iPod Touches. For this article, I assume you have two devices -- either iPhones or iPod Touches. In order to run this application, they both need to run at least iPhone OS 3.0.

Once the application is deployed to the two devices, launch the application on both devices. On each device, tap the Connect button. The GKPeerPickerController will display the standard UI to discover other devices (see Figure 4).

After a while, both application should be able to find each other (see Figure 5). When you tap on the name of the found device, the application will attempt to connect to it.

Figure 7. The All-Clear: Receiving data sent by the remote device.

Author's Note: If you test the application on the iPhone Simulator, you should be able to detect the two devices you have. However, that’s all it can do -- you won’t be able to connect to either device.

When another device tries to connect to you, you will see the popup as shown in Figure 6. Tap on Accept to connect and Decline to decline the connection.

If you are connected, you can now enter some text and start sending to the other device. Data received from another device will be shown in an alert view (see Figure 7).

Summary

In this article, you have seen how easy it is to connect two iPhones / iPod Touches using Bluetooth. Using the concepts shown in this article, you can build networked games and other interesting applications easily. In the next article, I will show you how you can make use of the Bluetooth connection to transmit voice data. Till then, have fun!