Integrating your app with the Windows Phone 8 Voice Commands

by Ishai Hachlili 29. October 2012 12:19

One of the new APIs in Windows Phone 8 is the Voice Commands API. This API allows you to integrate your own app with the main voice command functionality so when the phone’s start menu is held and the voice prompt comes up your app can be launched from it.

How it works

To add support for voice commands in your app you need to add a Voice Command Definition file to your project and register that file with the OS the first time your app launches. Once you do that, your app can be launched with the commands you defined.

When a command is launched, the target page associated with that command will be opened directly (just like the search extensions work in WP7).

To use this feature you need to add the following capabilities to your app (in the WMAppManifest.xml file)
(like all speech recognition solutions, the actual processing is done on the server side, the phone just streams the audio to the server and gets the text result back. that’s why networking must be enabled  for the app)

The Voice Command Definition File

The Voice Command Definition file, or VCD file, defines the commands your app will support.
You can add a new VCD file from the Add New Item menu.

Here’s the VCD file I’m using in this sample:

<?xml version="1.0" encoding="utf-8"?>
<VoiceCommands xmlns="">
<CommandSet xml:lang="en-us" Name="USEnglish">
<CommandPrefix> Play The Hunt </CommandPrefix>
<Example> Find Nearby Hunts </Example>
<Command Name="findHunts">
<Example> Find New Hunts </Example>
<ListenFor> Find {huntTypes} [Hunts]</ListenFor>
<Feedback> Searching For Hunts </Feedback>
<Navigate Target="/findhunts.xaml"/>

<PhraseList Label="huntTypes">
<Item> New </Item>
<Item> Nearby </Item>
<Item> My </Item>


In this VCD I’m defining a single command set for English. You can define additional command sets for other languages as well.
Let’s dig into this simple definition

Each command set can have a prefix.  When a user holds the start button and wants to launch a command in your app they will start the command with your prefix.
The prefix for the command set is optional. If it’s omitted your app name will be used as the prefix.
You might want to use a prefix if your app name is too long or hard to pronounce. You can also use it if you have multiple command sets for different languages.

Tip: If your app name is hard to pronounce, try using a prefix that phonetically corresponds to the app’s name.

The example text will show up in that initial dialog (not always, from my tests it seems to show up after the user already used your voice commands)

The voice command prompt


The next part of the XML file is a Command. You can have multiple commands, in this sample I only have one called findHunts.

The example text here will be shown when the command wasn’t found (the user has to tap the Tap Here link to see it)

Voice Command not foundsuppored commands for the app

Now we get to the most important parts of the command, the ListenFor and Navigate fields.

The ListenFor is the text you’re expecting for the command.

As you can see I’m using brackets to signify special meaning. The square brackets mean these words are optional (so in my example the user can say ‘Find New Hunts’ or just ‘Find New’).

The curly brackets are a phrase list. In this case the list is included in the XML file. You can also modify these lists at run time so if for example you want to use a list of products, you can get them from a web service and add them to the phrase list.

The Feedback is the text that will be displayed and spoken (using TTS) when this command is recognized.

The voice command feedback

The Navigate field defines the page you want opened with the recognized parameters

Register the VCD file

You’re supposed to register the VCD file once, on your app’s first launch.
Here’s a piece of code to do that:

private async void RegisterVoiceCommands()
await VoiceCommandService.InstallCommandSetsFromFileAsync(
new Uri("ms-appx:///VoiceCommands.xml", UriKind.RelativeOrAbsolute));

You can also check the InstalledCommandSets to see if you commands were already registered.

Handle the command in the target page

When a voice command for your apps is recognized, the target page will be launched and the URI for that page will include the recognized parameters in it.

The sample from Microsoft uses OnNavigatedTo to extract the query parameters from the NavigationContext.
If the page was opened from a voice command, a parameter named voiceCommandName will be in the query string. Testing for it’s existence is a good way to check if you got voice recognition results.

Here are the query string parameters you get when the user says “Play The Hunt, Find New Hunts”

[0]: {[voiceCommandName, findHunts]}
[1]: {[reco, open Play The Hunt Find New Hunts]}
[2]: {[huntTypes, New]}


If you have more than one command with the same target page, you can check voiceCommandName to figure out which command the user launched.

The reco parameter contains the whole recognized text. But it can only contain text that was an expected command. You can’t just get the text spoken by the user.
This is a big limitation, it means you have to launch the app with a command first and use TTS and Speech Recognition to carry on the conversation with your user. (I would prefer being able to say AppName, Remind me to call Someone tomorrow at 10 AM, instead it will have to be AppName, Add Call Reminder, or Remind Me To Call but the name and time will have to be asked for)
This is the same way hands free texting works in WP7, which works great but feels to verbose sometimes.

The huntTypes parameter contains the recognized word from my phrase list. I haven’t tried multiple phrase lists in one command, if that works, you’ll probably get more than one here. This means you don’t have to check the reco string at all. you can just use the phrase list to figure out what you want to do.

At this point, you can execute whatever code you need based on this command.

I’m using Caliburn Microfor my projects, so I won’t be overloading OnNavigatedTo in real projects. Instead, I’ll just add these three query string parameters as properties to my view model and Caliburn Micro will automagically parse the query string.
If you’re not using Caliburn Micro, you’re writing more code than you have to…

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Speech Recognition | Windows Phone 8

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About Ishai

Ishai is the founder of Smart Trivia, Inc. where he builds trivia games for smartphones using MvvmCross and Xamarin. He's also running the Silicon Valley Mobile/Cross Platform .Net Meetup and helps other companies build mobile apps using Xamarin as a consultant

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