Function onWatch

For the given object, if a watch is added to the given path the callback will be invoked. When all the watches are removed, the handle returned by the callback will be invoked.

A common use case for this is to notify changes to a property only if someone is watching it, this avoids unnecessary work from the watch if no one is watching the property.

class MyClass extends observable() {
    _foo = "foo";
    _bar = "bar";

    get foo() {
        return `${this._foo} ${this._bar}`;
    }

    get bar() {
        return this._bar;
    }
    set bar(value: string) {
        this._bar = value;
    }
}

const myClass = new MyClass();

onWatch(myClass, "foo", () =>
    watch(myClass, "bar", () => notifyChange(myClass, "foo"), {
        sync: true,
    })
);
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