owl/doc/observer.md

🦉 Observer 🦉

Owl need to be able to react to state changes. For example, whenever the state of a component is changed, we need to rerender it. To help with that, we have an Observer class. Its job is to observe some object state, and react to any change. To do that, it recursively replace all keys of the observed state by getters and setters.

For example, this code will display update in the console:

const observer = new owl.Observer();
observer.notifyCB = () => console.log("update");
observer.observe(obj);

const obj = { a: { b: 1 } };
obj.a.b = 2;

Technical Limitations

Since the observer uses getters and setters, it is actually unable to react to changes in two situations:

• adding a key to an object:

  const observer = new owl.Observer();
  const obj = { a: 1 };
  observer.observe(obj);
  obj.b = 2; // will do nothing
  

  In that case, we need a way to tell the observer that something happened. This can be done by using the set method:

  observer.set(obj, "b", 2);
  

• modifying an array by setting a new value at a given index:

  const observer = new owl.Observer();
  const obj = { todos: [{ id: 1, text: "todo" }] };
  observer.observe(obj);
  obj[0] = { id: 2, text: "othertodo" }; // will do nothing, and obj[0] is not observed
  

  In that case, the solution is the same, we can simply use the set method:

  observer.set(obj, 0, { id: 2, text: "othertodo" });