This is a simple facility for periodical execution of a function. This essentially encapsulates the native clearInterval/setInterval mechanism found in native Window objects.
This is especially useful if you use one to interact with the user at given intervals (e.g. use a prompt or confirm call): this will avoid multiple message boxes all waiting to be actioned.
这个对象就是可以周期性的执行某个方法，但是在它内部维持了一个状态，可以防止由于某些原因一次调用没执行，然后下一次调用又来了，这样会造成连续执行两次方法。上面的第二断英文就是这个意思。
帮助文档上说这个对象只提供了一个方法stop，但是在我看的源码里还提供了一个事件onTimerEvent，应该可以在某个时候触发这个事件。但帮助文档上没有给出示例。
这个对象源码比较简单，这里直接贴出来了，就不再注释了：
var PeriodicalExecuter = Class.create({
initialize: function(callback, frequency) {
this.callback = callback;
this.frequency = frequency;
this.currentlyExecuting = false;
this.registerCallback();
},
registerCallback: function() {
this.timer = setInterval(this.onTimerEvent.bind(this), this.frequency * 1000);
},
execute: function() {
this.callback(this);
},
stop: function() {
if (!this.timer) return;
clearInterval(this.timer);
this.timer = null;
},
onTimerEvent: function() {
if (!this.currentlyExecuting) {
try {
this.currentlyExecuting = true;
this.execute();
} catch(e) {
/* empty catch for clients that don't support try/finally */
}
finally {
this.currentlyExecuting = false;
}
}
}
});
看一下示例：
new PeriodicalExecuter(function(pe) {
if (!confirm('Want me to annoy you again later?'))
pe.stop(); },
5);
// Note that there won't be a stack of such messages if the user takes too long
// answering to the question...