虽然事件常常被用于处理用户输入和其它异步发生的行为，但事件本身是以同步方式发生的。
While events are often used to handle user input and other actions that happen asynchronously, the events themselves happen in synchronous fashion.
模板语句有副作用。这正是用户输入更新应用状态的方式。否则，响应事件就没有什么意义了。
A template statement has a side effect. It's how we update application state from user input. There would be no point to responding to an event otherwise.
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