How to replace deprectaed notifyProperty?

Question

Currently I have a getter xyz that is computed. To schedule a new computation I call notifyProperty(this, #xyz);.

In the the latest version of observe, notifyProperty is deprecated. How can I replace it? The documentation suggests to use this.notifyPropertyChange(#xyz, oldValue, newValue);. The problem is, that I don't have the oldValue (and not directly the newValue) as the getter is computed.

Answer 1

The suggestion from the devs is to keep the oldValue around in a private variable for reference. As for the newValue you can actually just pass the getter and it will compute it with the new values.

You will be looking at a class similar to this:

class MyElement extends Observable {
  @observable var foo, bar;
  var _oldValue;
  @reflectable get xyz => foo + bar;

  MyElement() {
    // we use the xyz getter to compute its own new value
    // notifyPropertyChange returns the new value for convenience :)
    notifyXyz() { _oldValue = notifyPropertyChange(#xyz, _oldValue, xyz); }
    onPropertyChange(this, #foo, notifyXyz);
    onPropertyChange(this, #bar, notifyXyz);
  }
}