Prototype PeriodicalExecuter对象 学习

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...