为什么用“何时”反应有效，而用“观察”或“拦截”反应无效

Question

Mobx-utils/fromPromise.state is not Observable！

const { observable, when, reaction, intercept, observe, isObservable } = mobx;
const { fromPromise, whenWithTimeout } = mobxUtils;
const promise = fetch('https://jsonplaceholder.typicode.com/todos')
  .then(res => res.json())
const result = fromPromise(promise);
console.assert(isObservable(result.state), 'state is NOT an isObservable');
/* WORKS!
when(
  () => result.state !== "pending",
  () => {
    console.log("Got ", result.value)
  }
);
*/
// NOT WORK, Why ?
observe(result, 'state', change => (console.log('observe', change)), true)
intercept(result, 'state', change => (console.log('intercept', change)));
reaction(
  () => result.state,
  state => console.log('reaction', state)
);

<script src="https://unpkg.com/mobx@3/lib/mobx.umd.js"></script>
<script src="https://unpkg.com/mobx-utils/mobx-utils.umd.js"></script>

Answer 1

result.state是不可观察的，但是如果你看一下它的实现，你会发现当你这样做时result.state它访问可观察的result._state。

private _state: IObservableValue<PromiseState> = observable(PENDING as any);
....
get state(): PromiseState {
    return this._state.get();
}

reaction和when工作，因为它们会对您阅读state时发生的_state访问做出反应。这在文档中有明确的解释。

observe和intercept不起作用，因为result不可观察。他们希望他们的第一个参数是可观察的。因此，即使更新的代码也无法正常工作

observe(result, '_state', change => (console.log('observe', change)), true)
intercept(result, '_state', change => (console.log('intercept', change)));

要修复它，请将result._state作为第一个参数。

observe(result._state, change => (console.log('observe', change)), true)
intercept(result._state, change => (console.log('intercept', change)));