With the introduction of NFC on Windows Phone 8, the scenarios of device interaction become much more reachable to consumers and developers. By simply by tapping one WP8 device with another, we can perform a variety of tasks, like sharing a contact, photo, video, or app, pairing with Bluetooth, etc.

The Proximity API in WP8 provides two main classes for NFC related features: ProximityDevice and PeerFinder. ProximityDevice enables functionality like publish/subscribe for NFC messages to and from tags or another devices, while PeerFinder focuses on pairing two devices with a socket connection using NFC tapping or browsing without NFC involvement.

Launching an application from another device is one of the most important features that NFC provides. In most cases, launching the same application from another device is desired to enable further interactions on the same app context between two devices, such as multi-player games, messaging apps, media sharing, etc. Both ProximityDevice and PeerFinder can perform this task.

By using ProximityDevice, developers need to make a URI association for the app and then publish the custom URI to another device. This approach enables the app to interact with other applications as well. However, sometimes app developers do not want to disclose the interface to others due to confidentiality. In such cases, the URI association might not be the best option.

On the other hand, many developers have encountered issues where if both apps are running in the foreground on separate devices and either Fast App Resume (FAR) or BackgroundExecution is defined in the manifest, then passing the URI through NFC will not trigger the MapUri function to launch the desired page in the application. In such cases, the PeerFinder approach to launch the same app on another device becomes more reliable option for developers.

So, how do you launch the same app on another device using PeerFinder? The solution is pretty simple. Developers just need to include the following code in their application. As it requires NFC to make tapping gesture work, the application needs to have ID_CAP_PROXIMITY and ID_REQ_NFC defined in the manifest.

...
PeerFinder.TriggeredConnectionStateChanged += PeerFinder_TriggeredConnectionStateChanged;
PeerFinder.Start();
...
void PeerFinder_TriggeredConnectionStateChanged(object sender, TriggeredConnectionStateChangedEventArgs args)
{
//to-do
}

After PeerFinder is started on device A, then when another device (“B”) enters into proximity range, device B will behave according to the following rules:

• Case 1: If device A has the app but device B doesn’t, device B will get a prompt to search for the app in the store as in the first illustration.
• Case 2: If both device A and B have the app installed and the app is not yet launched on B, then B will get a prompt to launch the app as in the second illustration.
• Case 3: If both device A and B have the app installed and both have launched the same app, then TriggeredConnectionStateChanged will get notified and further actions can be performed.

If the app needs a socket connection for further action between two devices, then it can check the TriggeredConnectState property in the  TriggeredConnectionStateChangedEventArgs passed in with the  TriggeredConnectionStateChanged event. Once the state is changed to TriggeredConnectState.completed, then the Socket property under TriggeredConnectionStateChangedEventArgs will be ready to use.

Some developers want to distinguish between normal application launching and launching by PeerFinder. This can be achieved by using the URI mapper and checking the URI passed. In the PeerFinder case, the URI passed to the MapUri function will look like this:

/MainPage.xaml?ms_nfp_launchargs=Windows.Networking.Proximity.PeerFinder:StreamSocket


Launching the same app on another device using PeerFinder does not require URI association, which implies that no other application can pretend to be the application by associating the same URI theme. This ensures that only the expected app is launched. It also resolves the issue mentioned above in the FAR or BackgroundExecution case. However, if developers want to share their application experiences with other applications, then using ProximityDevice with URI association is the proper option to choose.