switchfirst.js

  var SwitchFirstObservable = (function (__super__) {
    inherits(SwitchFirstObservable, __super__);
    function SwitchFirstObservable(source) {
      this.source = source;
      __super__.call(this);
    }

    SwitchFirstObservable.prototype.subscribeCore = function (o) {
      var m = new SingleAssignmentDisposable(),
        g = new CompositeDisposable(),
        state = {
          hasCurrent: false,
          isStopped: false,
          o: o,
          g: g
        };

      g.add(m);
      m.setDisposable(this.source.subscribe(new SwitchFirstObserver(state)));
      return g;
    };

    return SwitchFirstObservable;
  }(ObservableBase));

  var SwitchFirstObserver = (function(__super__) {
    inherits(SwitchFirstObserver, __super__);
    function SwitchFirstObserver(state) {
      this._s = state;
      __super__.call(this);
    }

    SwitchFirstObserver.prototype.next = function (x) {
      if (!this._s.hasCurrent) {
        this._s.hasCurrent = true;
        isPromise(x) && (x = observableFromPromise(x));
        var inner = new SingleAssignmentDisposable();
        this._s.g.add(inner);
        inner.setDisposable(x.subscribe(new InnerObserver(this._s, inner)));
      }
    };

    SwitchFirstObserver.prototype.error = function (e) {
      this._s.o.onError(e);
    };

    SwitchFirstObserver.prototype.completed = function () {
      this._s.isStopped = true;
      !this._s.hasCurrent && this._s.g.length === 1 && this._s.o.onCompleted();
    };

    inherits(InnerObserver, __super__);
    function InnerObserver(state, inner) {
      this._s = state;
      this._i = inner;
      __super__.call(this);
    }

    InnerObserver.prototype.next = function (x) { this._s.o.onNext(x); };
    InnerObserver.prototype.error = function (e) { this._s.o.onError(e); };
    InnerObserver.prototype.completed = function () {
      this._s.g.remove(this._i);
      this._s.hasCurrent = false;
      this._s.isStopped && this._s.g.length === 1 && this._s.o.onCompleted();
    };

    return SwitchFirstObserver;
  }(AbstractObserver));

  /**
   * Performs a exclusive waiting for the first to finish before subscribing to another observable.
   * Observables that come in between subscriptions will be dropped on the floor.
   * @returns {Observable} A exclusive observable with only the results that happen when subscribed.
   */
  observableProto.switchFirst = function () {
    return new SwitchFirstObservable(this);
  };