在React / Flux中处理计时器

时间：2014-12-22 14:15:55

标签： javascript reactjs reactjs-flux

我正在开发一个应用程序，我希望计时器从60秒倒计时到0，然后更改一些内容，之后计时器将在60再次重新启动。

我已经在React和Flux中实现了这一点，但由于我是新手，我仍然遇到了一些问题。

我现在想为计时器添加一个开始/停止按钮。我不知道在哪里放置/处理定时器状态。

我有一个Timer.jsx组件，如下所示：

var React = require('react');
var AppStore = require('../stores/app-store.js');
var AppActions = require('../actions/app-actions.js');

function getTimeLeft() {
  return {
    timeLeft: AppStore.getTimeLeft()
  }
}

var Timer = React.createClass({
  _tick: function() {
    this.setState({ timeLeft: this.state.timeLeft - 1 });
    if (this.state.timeLeft < 0) {
      AppActions.changePattern();
      clearInterval(this.interval);
    }
  },
  _onChange: function() {
    this.setState(getTimeLeft());
    this.interval = setInterval(this._tick, 1000);
  },
  getInitialState: function() {
    return getTimeLeft();
  },
  componentWillMount: function() {
    AppStore.addChangeListener(this._onChange);
  },
  componentWillUnmount: function() {
    clearInterval(this.interval);
  },
  componentDidMount: function() {
    this.interval = setInterval(this._tick, 1000);
  },
  render: function() {
    return (
      <small>
        ({ this.state.timeLeft })
      </small>
    )
  }
});

module.exports = Timer;

它从商店检索倒计时持续时间，我只需要：

var _timeLeft = 60;

现在，当我想实现一个开始/停止按钮时，我觉得我也应该通过Flux Actions实现这个，对吗？所以我想在我的商店里有这样的东西：

dispatcherIndex: AppDispatcher.register(function(payload) {
  var action = payload.action;

  switch(action.actionType) {
    case AppConstants.START_TIMER:
      // do something
      break;
    case AppConstants.STOP_TIMER:
      // do something
      break;
    case AppConstants.CHANGE_PATTERN:
      _setPattern();
      break;
  }

  AppStore.emitChange();

  return true;
})

但是，由于我的Timer组件当前处理setInterval，我不知道如何让我的START / STOP_TIMER事件正常工作。我应该将setInterval内容从Timer组件移动到Store并以某种方式将其传递给我的组件吗？

可以找到完整代码here。

3 个答案:

答案 0 :(得分：20)

我最终下载了您的代码并实现了您想要的启动/停止/重置功能。我认为这可能是解释事情的最佳方式 - 显示您可以运行的代码以及一些评论。

我实际上最终得到了两个实现。我将其称为实施A和实施B.

我认为展示这两种实现会很有趣。希望它不会引起太多混乱。

对于记录，实现A是更好的版本。

以下是两种实现的简要说明：

实施A

此版本会跟踪App组件级别的状态。通过将props传递给Timer组件来管理定时器。计时器组件确实记录了它自己的剩余时间状态。

实施B

此版本使用TimerStore和TimerAction模块跟踪Timer组件级别的定时器状态，以管理组件的状态和事件。

实现B的一大（也可能是致命的）缺点是你只能有一个Timer组件。这是因为TimerStore和TimerAction模块基本上是单例。

<强> |

<强> |

实施A

<强> |

<强> |

此版本会跟踪App组件级别的状态。这里的大多数评论都在这个版本的代码中。

通过将props传递给计时器来管理计时器。

此实施的代码更改列表：

  • APP-constants.js
  • APP-actions.js
  • APP-store.js
  • App.jsx
  • Timer.jsx

应用-constants.js

这里我只添加了一个用于重置计时器的常量。

module.exports = {
  START_TIMER: 'START_TIMER',
  STOP_TIMER: 'STOP_TIMER',
  RESET_TIMER: 'RESET_TIMER',
  CHANGE_PATTERN: 'CHANGE_PATTERN'
};

应用-actions.js

我刚刚添加了一个处理重置计时器操作的调度方法。

var AppConstants = require('../constants/app-constants.js');
var AppDispatcher = require('../dispatchers/app-dispatcher.js');

var AppActions = {
  changePattern: function() {
    AppDispatcher.handleViewAction({
      actionType: AppConstants.CHANGE_PATTERN
    })
  },
  resetTimer: function() {
    AppDispatcher.handleViewAction({
      actionType: AppConstants.RESET_TIMER
    })
  },
  startTimer: function() {
    AppDispatcher.handleViewAction({
      actionType: AppConstants.START_TIMER
    })
  },
  stopTimer: function() {
    AppDispatcher.handleViewAction({
      actionType: AppConstants.STOP_TIMER
    })
  }
};

module.exports = AppActions;

应用-store.js

这是事情发生变化的地方。我在内部添加了详细的注释，我做了更改。

var AppDispatcher = require('../dispatchers/app-dispatcher.js');
var AppConstants = require('../constants/app-constants.js');
var EventEmitter = require('events').EventEmitter;
var merge = require('react/lib/Object.assign');


// I added a TimerStatus model (probably could go in its own file)
// to manage whether the timer is "start/stop/reset".
//
// The reason for this is that reset state was tricky to handle since the Timer
// component no longer has access to the "AppStore". I'll explain the reasoning for
// that later.
//
// To solve that problem, I added a `reset` method to ensure the state
// didn't continuously loop "reset". This is probably not very "Flux".
//
// Maybe a more "Flux" alternative is to use a separate TimerStore and
// TimerAction? 
//
// You definitely don't want to put them in AppStore and AppAction
// to make your timer component more reusable.
//
var TimerStatus = function(status) {
  this.status = status;
};

TimerStatus.prototype.isStart = function() {
  return this.status === 'start';
};

TimerStatus.prototype.isStop = function() {
  return this.status === 'stop';
};

TimerStatus.prototype.isReset = function() {
  return this.status === 'reset';
};

TimerStatus.prototype.reset = function() {
  if (this.isReset()) {
    this.status = 'start';
  }
};


var CHANGE_EVENT = "change";

var shapes = ['C', 'A', 'G', 'E', 'D'];
var rootNotes = ['A', 'A#', 'B', 'C', 'C#', 'D', 'D#', 'E', 'F', 'F#', 'G', 'G#'];

var boxShapes = require('../data/boxShapes.json');


// Added a variable to keep track of timer state. Note that this state is
// managed by the *App Component*.
var _timerStatus = new TimerStatus('start');


var _pattern = _setPattern();

function _setPattern() {
  var rootNote = _getRootNote();
  var shape = _getShape();
  var boxShape = _getBoxForShape(shape);

  _pattern = {
    rootNote: rootNote,
    shape: shape,
    boxShape: boxShape
  };

  return _pattern;
}

function _getRootNote() {
  return rootNotes[Math.floor(Math.random() * rootNotes.length)];
}

function _getShape() {
  return shapes[Math.floor(Math.random() * shapes.length)];
}

function _getBoxForShape(shape) {
  return boxShapes[shape];
}


// Simple function that creates a new instance of TimerStatus set to "reset"
function _resetTimer() {
  _timerStatus = new TimerStatus('reset');
}

// Simple function that creates a new instance of TimerStatus set to "stop"
function _stopTimer() {
  _timerStatus = new TimerStatus('stop');
}

// Simple function that creates a new instance of TimerStatus set to "start"
function _startTimer() {
  _timerStatus = new TimerStatus('start');
}


var AppStore = merge(EventEmitter.prototype, {
  emitChange: function() {
    this.emit(CHANGE_EVENT);
  },

  addChangeListener: function(callback) {
    this.on(CHANGE_EVENT, callback);
  },

  removeChangeListener: function(callback) {
    this.removeListener(CHANGE_EVENT, callback);
  },


  // Added this function to get timer status from App Store
  getTimerStatus: function() {
    return _timerStatus;
  },


  getPattern: function() {
    return _pattern;
  },

  dispatcherIndex: AppDispatcher.register(function(payload) {
    var action = payload.action;

    switch(action.actionType) {
      case AppConstants.RESET_TIMER:
        // Handle reset action
        _resetTimer();
        break;
      case AppConstants.START_TIMER:
        // Handle start action
        _startTimer();
        break;
      case AppConstants.STOP_TIMER:
        // Handle stop action
        _stopTimer();
        break;
      case AppConstants.CHANGE_PATTERN:
        _setPattern();
        break;
    }

    AppStore.emitChange();

    return true;
  })
});

module.exports = AppStore;

<强> App.jsx

App.jsx有很多变化，特别是我们已经从定时器组件将状态移动到App组件。代码中的详细注释。

var React = require('react');

var Headline = require('./components/Headline.jsx');
var Scale = require('./components/Scale.jsx');
var RootNote = require('./components/RootNote.jsx');
var Shape = require('./components/Shape.jsx');
var Timer = require('./components/Timer.jsx');


// Removed AppActions and AppStore from Timer component and moved
// to App component. This is done to to make the Timer component more
// reusable.
var AppActions = require('./actions/app-actions.js');
var AppStore = require('./stores/app-store.js');


// Use the AppStore to get the timerStatus state
function getAppState() {
  return {
    timerStatus: AppStore.getTimerStatus()
  }
}

var App = React.createClass({
  getInitialState: function() {
    return getAppState();
  },


  // Listen for change events in AppStore
  componentDidMount: function() {
    AppStore.addChangeListener(this.handleChange);
  },


  // Stop listening for change events in AppStore
  componentWillUnmount: function() {
    AppStore.removeChangeListener(this.handleChange);
  },


  // Timer component has status, defaultTimeout attributes.
  // Timer component has an onTimeout event (used for changing pattern)
  // Add three basic buttons for Start/Stop/Reset
  render: function() {
    return (
      <div>
        <header>
          <Headline />
          <Scale />
        </header>
        <section>
          <RootNote />
          <Shape />
          <Timer status={this.state.timerStatus} defaultTimeout="15" onTimeout={this.handleTimeout} />
          <button onClick={this.handleClickStart}>Start</button>
          <button onClick={this.handleClickStop}>Stop</button>
          <button onClick={this.handleClickReset}>Reset</button>
        </section>
      </div>
    );
  },


  // Handle change event from AppStore
  handleChange: function() {
    this.setState(getAppState());
  },


  // Handle timeout event from Timer component
  // This is the signal to change the pattern.
  handleTimeout: function() {
    AppActions.changePattern();
  },


  // Dispatch respective start/stop/reset actions
  handleClickStart: function() {
    AppActions.startTimer();
  },
  handleClickStop: function() {
    AppActions.stopTimer();
  },
  handleClickReset: function() {
    AppActions.resetTimer();
  }
});

module.exports = App;

<强> Timer.jsx

Timer也有很多更改，因为我删除了AppStore和AppActions依赖项，以使Timer组件更具可重用性。详细的评论在代码中。

var React = require('react');


// Add a default timeout if defaultTimeout attribute is not specified.
var DEFAULT_TIMEOUT = 60;

var Timer = React.createClass({

  // Normally, shouldn't use props to set state, however it is OK when we
  // are not trying to synchronize state/props. Here we just want to provide an option to specify
  // a default timeout.
  //
  // See http://facebook.github.io/react/tips/props-in-getInitialState-as-anti-pattern.html)
  getInitialState: function() {
    this.defaultTimeout = this.props.defaultTimeout || DEFAULT_TIMEOUT;
    return {
      timeLeft: this.defaultTimeout
    };
  },


  // Changed this to `clearTimeout` instead of `clearInterval` since I used `setTimeout`
  // in my implementation
  componentWillUnmount: function() {
    clearTimeout(this.interval);
  },

  // If component updates (should occur when setState triggered on Timer component
  // and when App component is updated/re-rendered)
  //
  // When the App component updates we handle two cases:
  // - Timer start status when Timer is stopped
  // - Timer reset status. In this case, we execute the reset method of the TimerStatus
  //   object to set the internal status to "start". This is to avoid an infinite loop
  //   on the reset case in componentDidUpdate. Kind of a hack...
  componentDidUpdate: function() {
    if (this.props.status.isStart() && this.interval === undefined) {
      this._tick();
    } else if (this.props.status.isReset()) {
      this.props.status.reset();
      this.setState({timeLeft: this.defaultTimeout});
    }
  },

  // On mount start ticking
  componentDidMount: function() {
    this._tick();
  },


  // Tick event uses setTimeout. I find it easier to manage than setInterval.
  // We just keep calling setTimeout over and over unless the timer status is
  // "stop".
  //
  // Note that the Timer states is handled here without a store. You could probably
  // say this against the rules of "Flux". But for this component, it just seems unnecessary
  // to create separate TimerStore and TimerAction modules.
  _tick: function() {
    var self = this;
    this.interval = setTimeout(function() {
      if (self.props.status.isStop()) {
        self.interval = undefined;
        return;
      }
      self.setState({timeLeft: self.state.timeLeft - 1});
      if (self.state.timeLeft <= 0) {
        self.setState({timeLeft: self.defaultTimeout});
        self.handleTimeout();
      }
      self._tick();
    }, 1000);
  },

  // If timeout event handler passed to Timer component,
  // then trigger callback.
  handleTimeout: function() {
    if (this.props.onTimeout) {
      this.props.onTimeout();
    }
  }
  render: function() {
    return (
      <small className="timer">
        ({ this.state.timeLeft })
      </small>
    )
  },
});

module.exports = Timer;

<强> |

<强> |

实施B

<强> |

<强> |

代码更改列表：

  • APP-constants.js
  • timer-actions.js （新）
  • timer-store.js （新）
  • APP-store.js
  • App.jsx
  • Timer.jsx

应用-constants.js

这些应该放在名为timer-constants.js的文件中，因为它们处理Timer组件。

module.exports = {
  START_TIMER: 'START_TIMER',
  STOP_TIMER: 'STOP_TIMER',
  RESET_TIMER: 'RESET_TIMER',
  TIMEOUT: 'TIMEOUT',
  TICK: 'TICK'
};

<强>定时器actions.js

此模块不言自明。我添加了三个事件 - 超时，勾选和重置。有关详细信息，请参阅代码。

var AppConstants = require('../constants/app-constants.js');
var AppDispatcher = require('../dispatchers/app-dispatcher.js');

module.exports = {

  // This event signals when the timer expires.
  // We can use this to change the pattern.
  timeout: function() {
    AppDispatcher.handleViewAction({
      actionType: AppConstants.TIMEOUT
    })
  },

  // This event decrements the time left
  tick: function() {
    AppDispatcher.handleViewAction({
      actionType: AppConstants.TICK
    })
  },

  // This event sets the timer state to "start"
  start: function() {
    AppDispatcher.handleViewAction({
      actionType: AppConstants.START_TIMER
    })
  },

  // This event sets the timer state to "stop"
  stop: function() {
    AppDispatcher.handleViewAction({
      actionType: AppConstants.STOP_TIMER
    })
  },

  // This event resets the time left and sets the state to "start"
  reset: function() {
    AppDispatcher.handleViewAction({
      actionType: AppConstants.RESET_TIMER
    })
  },
};

<强>定时器store.js

我从AppStore中分离出了计时器内容。这是为了使Timer组件更具可重用性。

定时器存储跟踪以下状态：

  • 计时器状态 - 可以＆＃34;开始＆＃34;或＆＃34;停止＆＃34;
  • 剩余时间 - 计时器剩余时间

Timer存储处理以下事件：

  • 计时器启动事件将计时器状态设置为启动。
  • 计时器停止事件将计时器状态设置为停止。
  • tick事件减少1
    • 的剩余时间
  • 计时器重置事件将剩余时间设置为默认值并将计时器状态设置为启动

以下是代码：

var AppDispatcher = require('../dispatchers/app-dispatcher.js');
var AppConstants = require('../constants/app-constants.js');
var EventEmitter = require('events').EventEmitter;
var merge = require('react/lib/Object.assign');

var CHANGE_EVENT = "change";
var TIMEOUT_SECONDS = 15;

var _timerStatus = 'start';
var _timeLeft = TIMEOUT_SECONDS;

function _resetTimer() {
  _timerStatus = 'start';
  _timeLeft = TIMEOUT_SECONDS;
}

function _stopTimer() {
  _timerStatus = 'stop';
}

function _startTimer() {
  _timerStatus = 'start';
}

function _decrementTimer() {
  _timeLeft -= 1;
}

var TimerStore = merge(EventEmitter.prototype, {
  emitChange: function() {
    this.emit(CHANGE_EVENT);
  },

  addChangeListener: function(callback) {
    this.on(CHANGE_EVENT, callback);
  },

  removeChangeListener: function(callback) {
    this.removeListener(CHANGE_EVENT, callback);
  },

  getTimeLeft: function() {
    return _timeLeft;
  },

  getStatus: function() {
    return _timerStatus;
  },

  dispatcherIndex: AppDispatcher.register(function(payload) {
    var action = payload.action;

    switch(action.actionType) {
      case AppConstants.START_TIMER:
        _startTimer();
        break;
      case AppConstants.STOP_TIMER:
        _stopTimer();
        break;
      case AppConstants.RESET_TIMER:
        _resetTimer();
        break;
      case AppConstants.TIMEOUT:
        _resetTimer();
        break;
      case AppConstants.TICK:
        _decrementTimer();
        break;
    }

    TimerStore.emitChange();

    return true;
  })
});

module.exports = TimerStore;

应用-store.js

这可以命名为pattern-store.js，但您需要对其进行一些更改才能重复使用。具体来说，我直接听取Timer的TIMEOUT动作/事件来触发模式更改。如果您想重用模式更改，您可能不想要这种依赖。例如，如果您想通过单击按钮或其他内容来更改模式。

除此之外，我刚从AppStore删除了所有与计时器相关的功能。

var AppDispatcher = require('../dispatchers/app-dispatcher.js');
var AppConstants = require('../constants/app-constants.js');
var EventEmitter = require('events').EventEmitter;
var merge = require('react/lib/Object.assign');

var CHANGE_EVENT = "change";

var shapes = ['C', 'A', 'G', 'E', 'D'];
var rootNotes = ['A', 'A#', 'B', 'C', 'C#', 'D', 'D#', 'E', 'F', 'F#', 'G', 'G#'];

var boxShapes = require('../data/boxShapes.json');

var _pattern = _setPattern();

function _setPattern() {
  var rootNote = _getRootNote();
  var shape = _getShape();
  var boxShape = _getBoxForShape(shape);

  _pattern = {
    rootNote: rootNote,
    shape: shape,
    boxShape: boxShape
  };

  return _pattern;
}

function _getRootNote() {
  return rootNotes[Math.floor(Math.random() * rootNotes.length)];
}

function _getShape() {
  return shapes[Math.floor(Math.random() * shapes.length)];
}

function _getBoxForShape(shape) {
  return boxShapes[shape];
}

var AppStore = merge(EventEmitter.prototype, {
  emitChange: function() {
    this.emit(CHANGE_EVENT);
  },

  addChangeListener: function(callback) {
    this.on(CHANGE_EVENT, callback);
  },

  removeChangeListener: function(callback) {
    this.removeListener(CHANGE_EVENT, callback);
  },

  getPattern: function() {
    return _pattern;
  },

  dispatcherIndex: AppDispatcher.register(function(payload) {
    var action = payload.action;

    switch(action.actionType) {
      case AppConstants.TIMEOUT:
        _setPattern();
        break;
    }

    AppStore.emitChange();

    return true;
  })
});

module.exports = AppStore;

<强> App.jsx

这里我只添加了一些启动/停止/重置按钮。单击时，将调度TimerAction。所以如果你点击了＆＃34;停止＆＃34;按钮，我们称之为TimerAction.stop() 

var React = require('react');

var Headline = require('./components/Headline.jsx');
var Scale = require('./components/Scale.jsx');
var RootNote = require('./components/RootNote.jsx');
var Shape = require('./components/Shape.jsx');
var Timer = require('./components/Timer.jsx');
var TimerActions = require('./actions/timer-actions.js');


var App = React.createClass({
  render: function() {
    return (
      <div>
        <header>
          <Headline />
          <Scale />
        </header>
        <section>
          <RootNote />
          <Shape />
          <Timer />
          <button onClick={this.handleClickStart}>Start</button>
          <button onClick={this.handleClickStop}>Stop</button>
          <button onClick={this.handleClickReset}>Reset</button>
        </section>
      </div>
    );
  },
  handleClickStart: function() {
    TimerActions.start();
  },
  handleClickStop: function() {
    TimerActions.stop();
  },
  handleClickReset: function() {
    TimerActions.reset();
  }
});

module.exports = App;

<强> Timer.jsx

主要变化之一是我们使用TimerAction和TimerStore而不是最初使用的AppAction和AppStore。原因是尝试使Timer组件更具可重用性。

Timer具有以下状态：

  • 状态计时器状态可以是＆＃34; start＆＃34;或＆＃34;停止＆＃34;
  • timeLeft 计时器上剩余的时间

请注意，我使用的是setTimeout而不是setInterval。我发现setTimeout更容易管理。

大部分逻辑都在_tick方法中。基本上我们一直打电话给setTimeout，只要状态是＆＃34;开始＆＃34;。

当计时器到达零时，我们发出timeout事件的信号。 TimerStore和AppStore正在监听此事件。

  1. TimerStore只会重置定时器。与重置事件相同。
  2. AppStore将更改模式。

    3. 如果计时器没有达到零，我们通过发信号＆＃34; tick＆＃34;来减去一秒钟。事件

    最后，我们需要处理定时器停止然后再启动的情况。这可以通过componentDidUpdate钩子来处理。当组件的状态发生变化或者父组件被重新渲染时，会调用此挂钩。

    在componentDidUpdate方法中，我们确保启动＆＃34;勾选＆＃34;只有当状态是＆＃34;开始＆＃34;并且超时标识符未定义。我们不希望运行多个setTimeout。

    var React = require('react');

var TimerActions = require('../actions/timer-actions.js');
var TimerStore = require('../stores/timer-store.js');

function getTimerState() {
  return {
    status: TimerStore.getStatus(),
    timeLeft: TimerStore.getTimeLeft()
  }
}

var Timer = React.createClass({
  _tick: function() {
    var self = this;
    this.interval = setTimeout(function() {
      if (self.state.status === 'stop') {
        self.interval = undefined;
        return;
      }

      if (self.state.timeLeft <= 0) {
        TimerActions.timeout();
      } else {
        TimerActions.tick();
      }
      self._tick();
    }, 1000);
  },
  getInitialState: function() {
    return getTimerState();
  },
  componentDidMount: function() {
    TimerStore.addChangeListener(this.handleChange);
    this._tick();
  },
  componentWillUnmount: function() {
    clearTimeout(this.interval);
    TimerStore.removeChangeListener(this.handleChange);
  },
  handleChange: function() {
    this.setState(getTimerState());
  },
  componentDidUpdate: function() {
    if (this.state.status === 'start' && this.interval === undefined) {
      this._tick();
    }
  },
  render: function() {
    return (
      <small className="timer">
        ({ this.state.timeLeft })
      </small>
    )
  }
});

module.exports = Timer;

答案 1 :(得分：7)

不要在组件中存储状态

使用flux的主要原因之一是集中应用程序状态。为此，您应完全避免使用组件的setState功能。此外，在组件保存其自身状态的范围内，它应仅适用于非常短暂性质的状态数据（例如，您可以在组件上本地设置状态，以指示鼠标是否悬停）。

使用Action Creators进行异步操作

在Flux中，商店意味着同步。 （请注意，这在Flux实现中有点争议，但我绝对建议您使存储同步。一旦在Stores中允许异步操作，它就会破坏单向数据流并损害应用程序推理。）。相反，异步操作应该存在于 Action Creator 中。在你的代码中我没有提到Action Creator，所以我怀疑这可能是你混乱的根源。不过，您的实际计时器应该存在于Action Creator中。如果您的组件需要影响计时器，它可以调用Action Creator上的方法，Action Creator可以创建/管理计时器，计时器可以调度将由商店处理的事件。

更新：请注意，在2014年的react-conf Flux面板上，一位开发大型Flux应用程序的开发人员表示，对于该特定应用程序，他们确实允许在商店中进行异步数据获取操作（GET但不是PUT或POST）。

Facebook's Flux Flow Chart

答案 2 :(得分：2)

我会从商店中删除计时器，而现在，只需管理那里的模式。您的计时器组件需要进行一些小的更改：

var Timer = React.createClass({
  _tick: function() {
    if (this.state.timeLeft < 0) {
      AppActions.changePattern();
      clearInterval(this.interval);
    } else {
      this.setState({ timeLeft: this.state.timeLeft - 1 });
    }
  },
  _onChange: function() {
    // do what you want with the pattern here
    // or listen to the AppStore in another component
    // if you need this somewhere else
    var pattern = AppStore.getPattern();
  },
  getInitialState: function() {
    return { timeLeft: 60 };
  },
  componentWillUnmount: function() {
    clearInterval(this.interval);
  },
  componentDidMount: function() {
    this.interval = setInterval(this._tick, 1000);
    AppStore.addChangeListener(this._onChange);
  },
  render: function() {
    return (
      <small>
        ({ this.state.timeLeft })
      </small>
    )
  }
});
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