Publish and Subscribe QML Plugin

Overview

The Publish and Subscribe QML API enables QML applications to access values stored in the Value Space from QML. The publish/subscribe features therefore allow an easy to use form of IPC.

Publisher

The ValueSpacePublisher element allows us to publish key-value pairs of data to a known path. The path has the form of a directory structure path with the keys acting as files at the end of the path. The process is that the developer declares a ValueSpacePublisher element and then with a given path defines a list of keys. For example

 ValueSpacePublisher {
     id: battery
     path: "/power/battery"
     keys: ["charge", "charging"]
 }

To publish a value the key need only be set to the value. Here we see that the key is expressed as the last name in the path with dot notation followed by the key name.

     battery.charge = 50
     battery.charging = true

Subscriber

The ValueSpaceSubscriber element also defines the path to the key/value. For each key a different ValueSpaceSubscriber needs to be declared. In the above example using the keys 'charge' and 'charging' we will need subscribers for each one

 ValueSpaceSubscriber {
     id: batteryCharge
     path: "/power/battery/charge"
 }
 ValueSpaceSubscriber {
     id: batteryCharging
     path: "/power/battery/charging"
 }

Now the values being published can be read and used

 State {
     name: "low"
     when: batteryCharge.value < 25 && !batteryCharging.value
     PropertyChanges {
         target: visualCharge
         color: "red"
     }
 }

Examples

• Accessing Publish and Subscribe from QML

QML Elements

QML ValueSpacePublisher Element	The ValueSpacePublisher element represents a path in the value space where keys can be published.
QML ValueSpaceSubscriber Element	The QValueSpaceSubscriber class allows applications to read and subscribe to Value Space paths.