On Change

This is a handler that watches for parameter changes and emits an event when the value of a parameter changes. There are two implicit variables that become available:

• $newValue
• $oldValue

template Abc {
    required int value; 
    onChange:value => Debug.Log($newValue); 
}

The comparison will use .Equals if IEquatable<T> is implemented, otherwise it will use == for primitives and classes, and will not work for structs without and IEquatable<T> implementation.

The event will fire after parameters were set but before the render block runs.

You can also use onChange as a template argument, it works slightly differently in that case but the concept is the same.

There are two differences:

• it fires after the template finished its execution for the frame
• it can observe public state fields and parameters.

template Something {
    
    render {
         SomeTemplate(onChange:someValue => Debug.Log($newValue + " " + $oldValue)
    }
    
}