v-on 监听 DOM 事件
event
例子
Vue 中使用 v-on 来监听 DOM 事件，这里用 v-on 监听了 button 上的 click 事件，并接收一个 add 函数作为响应函数，响应函数写在 methods 里面。add 函数第一个参数是 event，event 是原生的 DOM 事件，event 指的是刚刚点击的事件，输出 “true” 可以看出 event 就是刚刚点击的事件。（JS中的 event 的相关笔记：https://www.cnblogs.com/xiaoxuStudy/p/13131725.html ）
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实现每点击按钮一次页面上的数值加 2。
注意：‘this’ 在方法里指向当前 Vue 实例
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可以通过 Vue 的 $event 访问当前点击的 DOM 事件，可以通过 event 实现阻止默认的冒泡行为(event.stopPropagation())。
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事件修饰符
在组件开发过程中，为了保证 methods 方法只有纯粹的数据逻辑（和DOM解耦，易于单元测试），不去处理 DOM 相关的操作。Vue 中为 v-on 这个指令提供了事件修饰符，修饰符都是由点开头的，并且表示为指令的后缀。
（ Vue 官网中事件修饰符的部分：https://cn.vuejs.org/v2/guide/events.html#%E4%BA%8B%E4%BB%B6%E4%BF%AE%E9%A5%B0%E7%AC%A6 ）
事件修饰符 .stop
作用：取消事件冒泡。
( 关于事件冒泡的笔记：https://www.cnblogs.com/xiaoxuStudy/p/13126722.html#one )
例子
点击“点击”按钮，事件冒泡的顺序应该是 button -> div -> div -> template。
如果没有使用.stop修饰符，点击按钮的之后冒泡到 class=“inner” 的 div 时，触发了在该 div 上注册的事件处理程序，执行 clickInner 方法。继续冒泡，冒泡到 class=“outer” 的 div 时，触发了在该 div 上注册的事件处理程序，执行 clickOuter 方法，继续冒泡…
如果在 class=“inner” 上的 div使用了.stop修饰符，冒泡到 class=“inner” 的 div 之后就停止冒泡，不会触发在 class=“inner” 的 div 上注册的事件处理程序。
没有使用 .stop 修饰符：
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使用了 .stop 修饰符：
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v-on 监听自定义事件
（ Vue 官网中自定义事件的部分： https://cn.vuejs.org/v2/guide/components-custom-events.html#ad ）
Q：Vue 是单向数据流的，通过 Prop 从父组件向子组件传递值。那么，子组件怎么把信息传递给父组件？
A: 在 Vue 中，子组件可以通过调用内建的 $emit 方法并传入事件名称来触发一个事件，第一个参数为事件名，并同时在父组件上通过 v-on 来监听这个事件获取子组件传递的信息。
双向绑定
例子
父组件 App.vue 子组件 EventParentChild.vue 说明：子组件在 button 上监听 click 事件，button 被点击后，通过 $emit 方法传递一个消息给父组件，消息的名称叫做 ‘update:msg’ ，消息的正文是 ‘hi’ 这个字符串。父组件监听子组件传递来的消息，父组件通过 v-on (缩写为@) 来监听 “update:msg” 事件。在页面上点击按钮后，父组件监听到 “update:msg” 事件，此时会把子组件传递过来的消息同步更新到 msg 上。父组件的 msg 更新后，又会通过 Prop 将更新后的 msg 传给子组件。这就实现了一个双向绑定。
页面表现：
在 Vue 中，为了方便我们使用上例中双向绑定的模式，Vue 提供了 .sync 修饰符。使用 .sync 时需要注意：事件名的形式必须是 “ update : 父组件中的变量 ” 这样子的。
在上例的基础上，修改父组件的第 4 行代码，能够实现相同的页面效果。
等同于
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