PropertyTreeChangeEvent Questions

Module Development
1

I'm trying to subscribe to property changes in my component model delegate, and usage of PropertyTreeChangeEvent is tripping me up.

My goal: subscribe to an array of objects at path this.props.tags.

1: Why do I have to call resolveReference()? How is that different than startup?

override fun onStartup() {
    log.infof("Starting up delegate for '%s'!", component.componentAddressPath)
    tags = component.createPropertyReference("this.props.tags", onValueChange, Origin.ANY)
    tags.resolveReference()
    tags.startup()
}

2: What is the distinction between these two?

val onValueChange: (changeEvent: PropertyTreeChangeEvent) -> Unit = { changeEvent ->
    changeEvent.readValue()       // Maybe an array of a QualifiedValues? Why?
    changeEvent.readCausalValue() // This one seems to make more sense.
}
2

Answered my own question 2:

changeEvent.readValue()       // Returns the value of the subscribed property. i.e. always "this.props.tags"
changeEvent.readCasualValue() // Returns the value of the property at the path that changed. i.e. possibly "this.props.tags[0].tagPath"
2 Likes
3

You don't want to use createPropertyReference; that's more "internal"/"under the hood" machinery. If you want to subscribe, you should be...subscribing :slight_smile:

First, obtain the backend PropertyTree for your property type:
props = component.getPropertyTreeOf(PropertyType.props);

Then you can issue a one-time read:
props.read("path").orElse(null)

Or create a subscription:
props.subscribe("path", Origin.ANY, someLambda)

You should maintain that list of Subscriptions returned by subscribe, and in your model's onShutdown make sure each one unsubscribes.

2 Likes
4

Awesome, thanks Paul.
I thought the whole thing seemed more convoluted than necessary :laughing: