Creating your own cards

A great way to get your app noticed is to export your app's cards, so other apps can import them. Creating cards is similar to creating a small, focused app. One of the key differences, however, is that unlike an app, a card can have multiple instances running at the same time. The BlackBerry 10 OS creates a new instance for each client app. 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 app's functionality to other apps.

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 use the same 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 access across different instances of your card. Any cards that your app exports are packaged in the same .bar file as the app.

Registering a card target

Because 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 the same as declaring your app as a target 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 apps, cards are packaged and identified as invocation targets in the config.xml file. Here's an example that shows you how to declare a card:

<invoke-target id="com.acme.myapp">
    <property var="uris" value="file://"/>

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 app is notified. While the card closes itself, it can also send a response message to the parent application.

Preparing your card for pooling

You can create your cards so 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 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. When 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() signal, it should clear its current state and listen for future invocations. In addition, cards must be written to support multiple instances which run simultaneously in the same sandbox.

Second, if an app 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 app or card may be invoked as any of these targets.

Preparing your cards for resizing and rotation

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

Card security

Cards have their own identity and sandbox environment that is shared with all instances of the card. Like apps, 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.

To learn more about APIs that support card invocation and read code samples, see card.

Last modified: 2014-12-04

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