How to "react" on data changes with RxJS?

Question

RxJS beginner here: I have problems with saving and tracking data changes using RxJS. Say I structure my app in small views/widgets and every view/widget has its own state and should do things on data changes. How do I do that?

More concrete example. Let's say I have a widget called Widget and Widget has a title and button. The state should contain the title and the information if the button was already clicked. From reading the docs of RxJS it seems this would be a good starting point:

var widgetState = new Rx.Subject().startWith({
  wasClicked: false,
  title: 'foo'
});

Now I want to be notified if some data changes:

var widgetStateChanges = widgetState.subscribe(function(data) {
  console.log('data: ', data);
  // what do i do with the data here?
  // i would like to merge the new data into the old state
});

widgetStateChanges.onNext({ title: 'bar' });

I listen to the changes, but I don't know how to save them. I would also like to do special things, if a certain data change happens. Something like this.

widgetStateChanges.filter(function(e) {
  return e.wasClicked;
}).do(function(e) {
  console.log('Do something because was clicked now.');
});

However I can't filter a subscription (widgetStateChanges), only a subject (widgetState).

Answer 1

Use a BehaviorSubject to track observable state:

var widgetState = new Rx.BehaviorSubject({ wasClicked: false, title: 'foo' });

// change state, probably in response to UI events
// Note we always set the full state, not just the "delta"
widgetState.onNext({ wasClicked: true, title: 'foo2' });

// example listening to title input field and updating state
// assumes rxjs-jquery
$("#title").onAsObservable("change").subscribe (function (ev) {
    var oldState = widgetState.value;
    var newTitle = $("#title").val();
    // do not mutate the oldState object, instead clone it and change the title
    var newState = $.extend({}, oldState, { title: newTitle });

    // send the update
    widgetState.onNext(newState);
});

// listen to new state values, probably to update your HTML?
widgetState.subscribe(function (newState) { ... });

// listen only when wasClicked is true
widgetState
    .filter(function (s) { return s.wasClicked; })
    .subscribe(function (s) { ... });