Creating your own cards

You just learned how you can integrate the functionality of other applications with yours by using cards. However, a great way to get your application noticed is to export your application's cards, so other applications can import them. Creating cards is similar to creating a small, focused application. One of the key differences, however, is that unlike an application, a card can have multiple instances running at the same time. The BlackBerry 10 OS creates a new instance for each client application. In many cases, the BlackBerry 10 OS will pool your card (pooling is discussed a bit later). This section can help you understand how you can use cards to expose your application's functionality to other applications.

A card shares an execution context across multiple instances of itself. For example, all instances of a card run in the same workspace directory and permissions. This feature of cards is important when you consider how you use your resources. If you read and write files, make sure you coordinate and access across different instances of your card. Any cards your application exports are packaged in the same .bar file as the application.

Register a card target

Since a card is used through an invocation request, the first step in exporting a card is to register an invocation target for it. Registering card targets is exactly the same as registering application targets except for a minor difference. The <invoke-target-type> tag must specify one of the card styles supported by the BlackBerry 10 OS. The valid values for <invoke-target-type> are card.previewer, card.composer, and card.picker. Registering in this way informs the BlackBerry 10 OS that you are exporting a card and describes the style of the card, so the BlackBerry 10 OS can handle it appropriately.

Just like applications, cards are packaged and identified as invocation targets in the bar-descriptor.xml file. Here's an example that shows you how to declare cards:

<invoke-target id="com.acme.myapp">
  <invoke-target-type>card.previewer</invoke-target-type>
  <filter>
    <action>bb.action.VIEW</action>
    <action>bb.action.SHARE</action>
    <mime-type>image/png</mime-type>
    <mime-type>image/jpeg</mime-type>
    <property var="uris" value="file://"/>
  </filter>
</invoke-target>

Invocation requests for cards

Since your cards are bundled in the same executable file as your application, when you start your app, you should check whether the executable file needs to be the application or one of the exported cards. Inspecting the application start-up mode will reveal whether the process started when an application starts (for example, a user tapping on the application icon to start it) or as an invocation, and whether the invocation is for the full application or one of the exported cards. Here's how you can perform a check up during the start-up mode:

int main(int argc, char *argv[])
{
    ... // Other functions in main

    // An application may be invoked while it is running, 
    //so it should listen for invocations
    InvokeManager invokeManager;
    
    // If any Q_ASSERT statement(s) indicate that the slot failed to connect to 
    // the signal, make sure you know exactly why this has happened. This is not
    // normal, and will cause your app to stop working!!
    bool connectResult;
    
    // Since the variable is not used in the app, this is added to avoid a 
    // compiler warning.
    Q_UNUSED(connectResult);
    
    connectResult = QObject::connect(invokeManager, 
    SIGNAL(invoked(const bb::system::InvokeRequest&),
    &myApp, SLOT(onInvoke(const bb::system::InvokeRequest&));    
    
    // This is only available in Debug builds.
    Q_ASSERT(connectResult);

    ... // Other functions in the main

    switch(invokeManager.startupMode()) {
    case ApplicationStartupMode::LaunchApplication:
    // If the application was launched from the home screen, it can initialize
        break;
    case ApplicationStartupMode::InvokeCard:
    // If the application is invoked, 
    //it must wait until it receives an invoked(..) signal
    // so that it can determine the UI that it needs to initialize
        break;
    default:
    // Who am I and how did I get here?
        break;
    }

    // Drop the application into the exec loop 
    //so that it can start receiving messages
    return app.exec();
}

After you determine that the startup is configured for a card invocation, you need to wait to receive the invoked signal so that you can determine the card you want to start, by checking the target value in the invocation as follows:

void MyAppOrCard::invoked(const bb::system::InvokeRequest&  request)
{

    // Initiate the appropriate target based on the invoke.target-key
    if(request.target() == "com.acme.application") {
        // Full app is already running and should now handle this invoke
    } else if(request.target() == "com.acme.message.previewer") {
        // card should init as the specified target
    } else if(request.target() == "com.acme.message.composer") {
        // card should init as the specified target
    } else…
} else {
    // Error: Who am I and how did I get here? 
    // Did I declare my target names and check against the right strings?
}

Card targets receive invocation requests in the same way as application targets do. For more information, see Receiving invocation.

Requesting your card to be closed

After a card finishes its task, it can request to be closed. When this request is processed, the card is transitioned off the screen and its parent application is notified. While the card closes itself, it can also send a response message to the parent application. Here's how a card requests to be closed:
// Assemble response message
CardDoneMessage message;
message.setData(tr("Card: I am done. yay!"));
message.setDataType("text/plain");
message.setReason(tr("Success!"));

// Send message
invokeManager->sendCardDone(message);

Prepare your card for pooling

You can create your cards in such a way that they can be pooled when a user closes the card. Pooling helps to keep frequently used cards ready for reuse, which reduces loading and transition time when cards are stacked. For example, if the user is viewing one email message after another, instead of exporting a new card for every email, the BlackBerry 10 OS may pool the existing email previewer card and use it again.

When you're creating a card, you must consider several things before you handle pooling events, such as which resources your cards should clean up or keep when they're pooled. First, consider adding support for purging the state of the card. While pooled cards are not terminated, they may be suspended to keep them from processing while pooled. Once your card is pooled it should release resources such as files or database connections until it is resumed again. When a card receives a cardPooled notification it should clear its current state and listen for future invocations. In addition, cards must be written to support multiple instances which run simultaneously within the same sandbox.

Second, if an application with cards defines several card targets, then when it's retrieved from the pool, the card may be invoked to service any of the declared targets. In other words, if a .bar file bundles a composer, previewer, and picker under a single entry point, then when it's retrieved from the pool, the application or card may be invoked as any of these targets.

Third, cards generally operate in full screen mode. However, some applications may show the status bar and will require the card to be resized if it covers the status bar. Therefore, applications with cards should always include a cardResizeRequested(..) signal.

BlackBerry 10 OS manages the pooling of cards. The pooling is also dependent on varying factors of the device state. This means that in certain situations, a card may not be pooled. In such cases, the card receives the exit message that indicates to the card that it should terminate. See application in the API reference for more information.

Prepare your cards for resizing and rotation

Cards can be rotated and resized. Rotation occurs when a user rotates the BlackBerry device. When the device is rotated, a cardResize request is sent to the card and indicates the new dimensions of the card. How much a card is resized depends on whether the parent application requires the status bar to be shown, in which case the entire screen card is resized to allow the status bar to be displayed. Here's how a card handles the resizing request and resizes its orientation:

// Connect to the card resize request signal.

// If any Q_ASSERT statement(s) indicate that the slot failed to connect to 
// the signal, make sure you know exactly why this has happened. This is not
// normal, and will cause your app to stop working!!
bool connectResult;

// Since the variable is not used in the app, this is added to avoid a 
// compiler warning.
Q_UNUSED(connectResult);

connectResult = QObject::connect(invokeManager, 
SIGNAL(cardResize(const bb::system::CardResizeMessage&)), 
card, 
SLOT(cardResize(const bb::system::CardResizeMessage&)));

// This is only available in Debug builds.
Q_ASSERT(connectResult);  
...
  
void cardResizeRequested(const bb::system::CardResizeMessage& resizeMessage)
{
    // Resize card based on the following information
    // Width available to the card.
    resizeMessage.width();  
    // Height available to the card.         
    resizeMessage.height();
    // The edge of the device that points upwards.         
    resizeMessage.upEdge();  
    // Orientation of the device (portrait or landscape).      
    resizeMessage.orientation()  
    … 
    // Respond after resizing is done.
    invokeManager->cardResized(resizeMessage);
}

A card’s orientation is aligned with the orientation of the parent application. If the parent application's orientation is fixed, any child cards in the stack also have a fixed orientation.

When you create cards, you must add support for both landscape and portrait orientation so that the card follows the orientation of your application.

Card security

Cards have their own identity and sandbox environment that is shared with all instances of the card. Like applications, cards are processed in their own context and with their own permissions. It is important to pay attention to the capabilities and the data that you want to expose using your card.

Last modified: 2014-01-23

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