docs(cn): translate docs/handling-events (reactjs#84)

Author: Ryanhui

content/docs/handling-events.md

@@ -1,43 +1,43 @@
 ---
 id: handling-events
-title: Handling Events
+title: 事件处理
 permalink: docs/handling-events.html
 prev: state-and-lifecycle.html
 next: conditional-rendering.html
 redirect_from:
   - "docs/events-ko-KR.html"
 ---
 
-Handling events with React elements is very similar to handling events on DOM elements. There are some syntactic differences:
+React 元素的事件处理和 DOM 元素的很相似，但是有一点语法上的不同:
 
-* React events are named using camelCase, rather than lowercase.
-* With JSX you pass a function as the event handler, rather than a string.
+* React 事件的命名采用小驼峰式（camelCase），而不是纯小写。
+* 使用 JSX 语法时你需要传入一个函数作为事件处理函数，而不是一个字符串。
 
-For example, the HTML:
+例如，传统的 HTML：
 
 ```html
 <button onclick="activateLasers()">
   Activate Lasers
 </button>
 ```
 
-is slightly different in React:
+在 React 中略微不同：
 
 ```js{1}
 <button onClick={activateLasers}>
   Activate Lasers
 </button>
 ```
 
-Another difference is that you cannot return `false` to prevent default behavior in React. You must call `preventDefault` explicitly. For example, with plain HTML, to prevent the default link behavior of opening a new page, you can write:
+在 React 中另一个不同点是你不能通过返回 `false` 的方式阻止默认行为。你必须显式的使用 `preventDefault` 。例如，传统的 HTML 中阻止链接默认打开一个新页面，你可以这样写：
 
 ```html
 <a href="#" onclick="console.log('The link was clicked.'); return false">
   Click me
 </a>
 ```
 
-In React, this could instead be:
+在 React 中，可能是这样的：
 
 ```js{2-5,8}
 function ActionLink() {
@@ -54,19 +54,19 @@ function ActionLink() {
 }
 ```
 
-Here, `e` is a synthetic event. React defines these synthetic events according to the [W3C spec](https://www.w3.org/TR/DOM-Level-3-Events/), so you don't need to worry about cross-browser compatibility. See the [`SyntheticEvent`](/docs/events.html) reference guide to learn more.
+在这里，`e` 是一个合成事件。React 根据 [W3C 规范](https://www.w3.org/TR/DOM-Level-3-Events/)来定义这些合成事件，所以你不需要担心跨浏览器的兼容性问题。如果想了解更多，请查看 [`SyntheticEvent`](/docs/events.html) 参考指南。
 
-When using React you should generally not need to call `addEventListener` to add listeners to a DOM element after it is created. Instead, just provide a listener when the element is initially rendered.
+使用 React 时，你一般不需要使用 `addEventListener` 为已创建的 DOM 元素添加监听器。React恰恰与之相反，你仅需要在该元素初始渲染的时候添加一个监听器。
 
-When you define a component using an [ES6 class](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Classes), a common pattern is for an event handler to be a method on the class. For example, this `Toggle` component renders a button that lets the user toggle between "ON" and "OFF" states:
+当你使用 [ES6 class](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Classes) 语法定义一个组件的时候，通常的做法是将事件处理函数声明为 class 中的方法。例如，下面的 `Toggle` 组件会渲染一个让用户切换开关状态的按钮：
 
 ```js{6,7,10-14,18}
 class Toggle extends React.Component {
   constructor(props) {
     super(props);
     this.state = {isToggleOn: true};
 
-    // This binding is necessary to make `this` work in the callback
+    // 为了在回调中使用 `this`，这个绑定是必不可少的
     this.handleClick = this.handleClick.bind(this);
   }
 
@@ -91,18 +91,18 @@ ReactDOM.render(
 );
 ```
 
-[**Try it on CodePen**](https://codepen.io/gaearon/pen/xEmzGg?editors=0010)
+[**在 CodePen 上尝试**](https://codepen.io/gaearon/pen/xEmzGg?editors=0010)
 
-You have to be careful about the meaning of `this` in JSX callbacks. In JavaScript, class methods are not [bound](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_objects/Function/bind) by default. If you forget to bind `this.handleClick` and pass it to `onClick`, `this` will be `undefined` when the function is actually called.
+你必须谨慎对待 JSX 回调函数中的 `this`，在 JavaScript 中，class 的方法默认不会[绑定](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_objects/Function/bind) `this`。如果你忘记绑定 `this.handleClick` 并把它传入了 `onClick`，当你调用这个函数的时候 `this` 的值为 `undefined`。
 
-This is not React-specific behavior; it is a part of [how functions work in JavaScript](https://www.smashingmagazine.com/2014/01/understanding-javascript-function-prototype-bind/). Generally, if you refer to a method without `()` after it, such as `onClick={this.handleClick}`, you should bind that method.
+这并不是 React 特有的行为；这其实与 [JavaScript 函数工作原理](https://www.smashingmagazine.com/2014/01/understanding-javascript-function-prototype-bind/)有关。通常情况下，如果你没有在方法后面添加 `()`，例如 `onClick={this.handleClick}`，你应该为这个方法绑定 `this`。
 
-If calling `bind` annoys you, there are two ways you can get around this. If you are using the experimental [public class fields syntax](https://babeljs.io/docs/plugins/transform-class-properties/), you can use class fields to correctly bind callbacks:
+如果觉得使用 `bind` 很麻烦，这里有两种方式可以解决。如果你正在使用实验性的 [public class fields 语法](https://babeljs.io/docs/plugins/transform-class-properties/)，你可以使用 class fileds 正确的绑定回调函数：
 
 ```js{2-6}
 class LoggingButton extends React.Component {
-  // This syntax ensures `this` is bound within handleClick.
-  // Warning: this is *experimental* syntax.
+  // 这种语法确保 `this` 绑定在 `handleClick` 内。
+  // 注意: 这是 *实验性* 语法。
   handleClick = () => {
     console.log('this is:', this);
   }
@@ -117,9 +117,9 @@ class LoggingButton extends React.Component {
 }
 ```
 
-This syntax is enabled by default in [Create React App](https://github.com/facebookincubator/create-react-app).
+[Create React App](https://github.com/facebookincubator/create-react-app) 默认启用此语法。
 
-If you aren't using class fields syntax, you can use an [arrow function](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Functions/Arrow_functions) in the callback:
+如果你没有使用 class fileds 语法，你可以在回调中使用[箭头函数](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Functions/Arrow_functions)：
 
 ```js{7-9}
 class LoggingButton extends React.Component {
@@ -128,7 +128,7 @@ class LoggingButton extends React.Component {
   }
 
   render() {
-    // This syntax ensures `this` is bound within handleClick
+    // 这种语法确保 `this` 绑定在 `handleClick` 内。
     return (
       <button onClick={(e) => this.handleClick(e)}>
         Click me
@@ -138,17 +138,18 @@ class LoggingButton extends React.Component {
 }
 ```
 
-The problem with this syntax is that a different callback is created each time the `LoggingButton` renders. In most cases, this is fine. However, if this callback is passed as a prop to lower components, those components might do an extra re-rendering. We generally recommend binding in the constructor or using the class fields syntax, to avoid this sort of performance problem.
+此语法问题在于每次渲染 `LoggingButton` 时都会创建不同的回调函数。在大多数情况下，这没什么问题，但如果该回调函数作为 prop 传入子组件时，这些组件可能会进行额外的重新渲染。我们通常建议在构造器中绑定或使用 class fileds 语法来避免这类性能问题。
 
-## Passing Arguments to Event Handlers {#passing-arguments-to-event-handlers}
+## 向事件处理程序传递参数 {#passing-arguments-to-event-handlers}
 
-Inside a loop it is common to want to pass an extra parameter to an event handler. For example, if `id` is the row ID, either of the following would work:
+在循环中，通常我们会为事件处理函数传递额外的参数。例如，若 `id` 是你要删除那一行的 ID，以下两种方式都可以向事件处理函数传递参数：
 
 ```js
 <button onClick={(e) => this.deleteRow(id, e)}>Delete Row</button>
 <button onClick={this.deleteRow.bind(this, id)}>Delete Row</button>
 ```
 
-The above two lines are equivalent, and use [arrow functions](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions) and [`Function.prototype.bind`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_objects/Function/bind) respectively.
+上述两种方式是等价的，分别通过[箭头函数](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions)和 [`Function.prototype.bind`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_objects/Function/bind) 来实现。
+
+在这两种情况下，React 的事件对象 `e` 会被作为第二个参数传递。如果通过箭头函数的方式，事件对象必须显式的进行传递，而通过 `bind` 的方式，事件对象以及更多的参数将会被隐式的进行传递。
 
-In both cases, the `e` argument representing the React event will be passed as a second argument after the ID. With an arrow function, we have to pass it explicitly, but with `bind` any further arguments are automatically forwarded.