can-value/doc/from.md

Observable values from other CanJS observables.

@function can-value.from from
@parent can-value/methods

@description Get an observable for getting (but not setting) a property on an object.

@signature `value.from( object, keyPath )`

  In the example below, a `keyObservable` is created that is one-way bound to the
  value at `outer.inner.key`. When `outer.inner.key` changes,
  `keyObservable.value` is updated, but changes to `keyObservable.value` do not
  update `outer.inner.key`.

  ```js
  import { ObservableObject, value} from "can";

  const outer = new ObservableObject({
    inner: {
      key: "hello"
    }
  });

  const keyObservable = value.from(outer, "inner.key");
  console.log(keyObservable.value); //-> "hello"

  try {
    keyObservable.value = "aloha";
  } catch(error) {
    // Error thrown because the value isn't settable
    console.log(error.message); //-> "Cannot set property value
    // of #<Observation> which has only a getter"
  }
  ```
  @codepen

  @param {Object} object The object from which to read.

  @param {String} keyPath A String of dot-separated keys, representing a path of properties.

  @return {Object} An observable compatible with [can-reflect.getValue]
  but not [can-reflect.setValue]; it also has a `value` property that
  can be used to get the value.