// 預設空的 observer 

const emptyObserver = {

  next: () => {},

  error: (err) => { throw err; },

  complete: () => {}

}

​

class Observer {

  constructor(destinationOrNext, error, complete) {

    switch (arguments.length) {

      case 0:

        // 空的 observer

        this.destination = this.safeObserver(emptyObserver);

        break;

      case 1:

        if (!destinationOrNext) {

          // 空的 observer

          this.destination = this.safeObserver(emptyObserver);

          break;

        }

        if (typeof destinationOrNext === 'object') {

          // 傳入了 observer 物件

          this.destination = this.safeObserver(destinationOrNext);

          break;

        }

      default:

        // 如果上面都不是，代表應該是傳入了一到三個 function

        this.destination = this.safeObserver(destinationOrNext, error, complete);

        break;

    }

  }

  safeObserver(observerOrNext, error, complete) {

    let next;

​

    if (typeof (observerOrNext) === 'function') {

      // observerOrNext 是 next function

      next = observerOrNext;

    } else if (observerOrNext) {

      // observerOrNext 是 observer 物件

      next = observerOrNext.next || () => {};

      error = observerOrNext.error || function(err) { 

        throw err 

      };

      complete = observerOrNext.complete || () => {};

    }

    // 最後回傳我們預期的 observer 物件

    return {

      next: next,

      error: error,

      complete: complete

    };

  }

  next(value) {

    if (!this.isStopped && this.next) {

      // 先判斷是否停止過

      try {

        this.destination.next(value); // 傳送值

      } catch (err) {

        this.unsubscribe();

        throw err;

      }

    }

  }

  error(err) {

    if (!this.isStopped && this.error) {

      // 先判斷是否停止過

      try {

        this.destination.error(err); // 傳送錯誤

      } catch (anotherError) {

        this.unsubscribe();

        throw anotherError;

      }

      this.unsubscribe();

    }

  }

​

  complete() {

    if (!this.isStopped && this.complete) {

      // 先判斷是否停止過

      try {

        this.destination.complete(); // 發送停止訊息

      } catch (err) {

        this.unsubscribe();

        throw err;

      }

      this.unsubscribe(); // 發送停止訊息後退訂

    }

  }

  unsubscribe() {

    this.isStopped = true;

  }

}

​

​

function create(subscriber) {

    var observable = {

        subscribe: function(observer) {

            subscriber(observer)

        }       

    };

    return observable;

}

​

var observable = create(function(observer) {

  observer.next(1);

  observer.next(2);

  observer.next(3);

  observer.complete(); // error: complete is not a function 

})

​

var observer = {

  next: function(value) {

    console.log(value)

  },

  complete: function() {

    console.log('complete!')

  }

}

​

observable.subscribe(observer)