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在指定的延迟时间之后调用一个函数或执行一个代码片段.
语法
var timeoutID = window.setTimeout(func, delay, [param1, param2, ...]); var timeoutID = window.setTimeout(code, delay);
说明:
timeoutID
是该延时操作的数字ID, 此ID随后可以用来作为window.clearTimeout方法的参数.func
是你想要在delay
毫秒之后执行的函数.code
在第二种语法,是指你想要在delay
毫秒之后执行的代码字符串 (使用该语法是不推荐的, 不推荐的原因和eval()一样)delay
是延迟的毫秒数 (一秒等于1000毫秒),函数的调用会在该延迟之后发生。如果省略该参数,delay取默认值0。实际的延迟时间可能会比 delay 值长,原因请查看下面的备注。
需要注意的是,IE9 及更早的 IE 浏览器不支持第一种语法中向延迟函数传递额外参数的功能。如果你想要在IE中达到同样的功能,你必须使用一种兼容代码 (查看callback arguments 一段).
例子
下文的例子在网页中设置了两个简单的按钮,以触发 setTimeout 和 clearTimeout 方法:按下第一个按钮会在 2s 后显示一个警告对话框,并将此次 setTimeout 的延时 ID 保存起来。按下第二个按钮可以取消这次延时调用行为。
HTML 内容
<p>Live Example</p>
<button onclick="delayedAlert();">Show an alert box after two seconds</button>
<p></p>
<button onclick="clearAlert();">Cancel alert before it happens</button>
JavaScript 内容
var timeoutID;
function delayedAlert() {
timeoutID = window.setTimeout(slowAlert, 2000);
}
function slowAlert() {
alert("That was really slow!");
}
function clearAlert() {
window.clearTimeout(timeoutID);
}
也可参考 clearTimeout()
示例.
回调参数
如果你需要向你的回调函数内传递一个参数, 而且还需要兼容IE9及以前的版本, 由于IE不支持传递额外的参数 (setTimeout()
或者 setInterval()
都不可以) ,但你可以引入下面的兼容代码.该代码能让IE也支持符合HTML5标准的定时器函数.
/*\ |*| |*| IE-specific polyfill which enables the passage of arbitrary arguments to the |*| callback functions of javascript timers (HTML5 standard syntax). |*| |*| https://developer.mozilla.org/en-US/docs/DOM/window.setInterval |*| |*| Syntax: |*| var timeoutID = window.setTimeout(func, delay, [param1, param2, ...]); |*| var timeoutID = window.setTimeout(code, delay); |*| var intervalID = window.setInterval(func, delay[, param1, param2, ...]); |*| var intervalID = window.setInterval(code, delay); |*| \*/ if (document.all && !window.setTimeout.isPolyfill) { var __nativeST__ = window.setTimeout; window.setTimeout = function (vCallback, nDelay /*, argumentToPass1, argumentToPass2, etc. */) { var aArgs = Array.prototype.slice.call(arguments, 2); return __nativeST__(vCallback instanceof Function ? function () { vCallback.apply(null, aArgs); } : vCallback, nDelay); }; window.setTimeout.isPolyfill = true; } if (document.all && !window.setInterval.isPolyfill) { var __nativeSI__ = window.setInterval; window.setInterval = function (vCallback, nDelay /*, argumentToPass1, argumentToPass2, etc. */) { var aArgs = Array.prototype.slice.call(arguments, 2); return __nativeSI__(vCallback instanceof Function ? function () { vCallback.apply(null, aArgs); } : vCallback, nDelay); }; window.setInterval.isPolyfill = true; }
IE Only Fix
如果你需要单独的针对IE9及之前浏览器的 hack 写法,你可以使用 JavaScript 条件注释:
/*@cc_on
// conditional IE < 9 only fix
@if (@_jscript_version <= 9)
(function(f){
window.setTimeout =f(window.setTimeout);
window.setInterval =f(window.setInterval);
})(function(f){return function(c,t){var a=[].slice.call(arguments,2);return f(function(){c.apply(this,a)},t)}});
@end
@*/
或者使用更加清晰的 IE HTML 条件注释:
<!--[if lte IE 9]><script>
(function(f){
window.setTimeout =f(window.setTimeout);
window.setInterval =f(window.setInterval);
})(function(f){return function(c,t){
var a=[].slice.call(arguments,2);
return f(function(){c.apply(this,a)},t)}
});
</script><![endif]-->
另一种方法是使用匿名函数包裹你的回调函数,这种方式要消耗更多资源:
var intervalID = setTimeout(function() { myFunc("one", "two", "three"); }, 1000);
此外,也可使用 function's bind:
setTimeout(function(arg1){}.bind(undefined, 10), 1000);
关于"this
"的问题
当你向 setTimeout()
(或者其他函数也行)传递一个函数时,该函数中的this会指向一个错误的值
.这个问题在 JavaScript reference 中进行了详细解释.
解释
由setTimeout()调用的代码运行在与所在函数
完全分离的执行环境上. 这会导致,这些代码中包含的 this
关键字会指向 window
(或全局
)对象,这和所期望的this的值是不一样的.查看下面的例子
:
myArray = ["zero", "one", "two"]; myArray.myMethod = function (sProperty) { alert(arguments.length > 0 ? this[sProperty] : this); }; myArray.myMethod(); // prints "zero,one,two" myArray.myMethod(1); // prints "one" setTimeout(myArray.myMethod, 1000); // prints "[object Window]" after 1 second setTimeout(myArray.myMethod, 1500, "1"); // prints "undefined" after 1,5 seconds // let's try to pass the 'this' object setTimeout.call(myArray, myArray.myMethod, 2000); // error: "NS_ERROR_XPC_BAD_OP_ON_WN_PROTO: Illegal operation on WrappedNative prototype object" setTimeout.call(myArray, myArray.myMethod, 2500, 2); // same error
正如你所看到的一样,我们没有任何方法将this对象传递给回调函数
.
解决方案
一个可用的解决 "this
" 问题的方法是使用两个非原生的setTimeout()
和 setInterval()
全局函数代替原生的.该非原生的函数通过使用Function.prototype.call
方法激活了正确的作用域.下面的代码显示了应该如何替换:
// Enable the passage of the 'this' object through the JavaScript timers var __nativeST__ = window.setTimeout, __nativeSI__ = window.setInterval; window.setTimeout = function (vCallback, nDelay /*, argumentToPass1, argumentToPass2, etc. */) { var oThis = this, aArgs = Array.prototype.slice.call(arguments, 2); return __nativeST__(vCallback instanceof Function ? function () { vCallback.apply(oThis, aArgs); } : vCallback, nDelay); }; window.setInterval = function (vCallback, nDelay /*, argumentToPass1, argumentToPass2, etc. */) { var oThis = this, aArgs = Array.prototype.slice.call(arguments, 2); return __nativeSI__(vCallback instanceof Function ? function () { vCallback.apply(oThis, aArgs); } : vCallback, nDelay); };
新特性检测:
myArray = ["zero", "one", "two"]; myArray.myMethod = function (sProperty) { alert(arguments.length > 0 ? this[sProperty] : this); }; setTimeout(alert, 1500, "Hello world!"); // the standard use of setTimeout and setInterval is preserved, but... setTimeout.call(myArray, myArray.myMethod, 2000); // prints "zero,one,two" after 2 seconds setTimeout.call(myArray, myArray.myMethod, 2500, 2); // prints "two" after 2,5 seconds
针对这个问题并没有原生的解决方案。
Function.prototype.bind()
方法,该方法允许显式地指定函数调用时 this 所指向的值 。该方法可以帮助你解决 this 指向不确定的问题。使用bind的例子:
myArray = ["zero", "one", "two"];
myBoundMethod = (function (sProperty) {
console.log(arguments.length > 0 ? this[sProperty] : this);
}).bind(myArray);
myBoundMethod(); // prints "zero,one,two" because 'this' is bound to myArray in the function
myBoundMethod(1); // prints "one"
setTimeout(myBoundMethod, 1000); // still prints "zero,one,two" after 1 second because of the binding
setTimeout(myBoundMethod, 1500, "1"); // prints "one" after 1.5 seconds
备注
你可以使用 window.clearTimeout()
来取消延迟操作。如果你希望你的代码被重复的调用 (比如每 N 毫秒一次),考虑使用 window.setInterval()
。
记住这一点:只有当调用setTimeout()的线程停止后,函数或代码段才能继续执行。
传递字符串字面量
向setTimeout()传递一个字符串而不是函数会遭受到与使用
eval一样的风险.
// 正确 window.setTimeout(function() { alert("Hello World!"); }, 500); // 不正确 window.setTimeout("alert(\"Hello World!\");", 500);
字符串会在全局作用域内被解释执行,所以当setTimeout()
函数执行完毕后,字符串中的变量不可用.
最小延迟和超时嵌套
Historically browsers implement setTimeout()
"clamping": successive setTimeout()
calls with delay
smaller than the "minimum delay" limit are forced to the use at least the minimum delay. The minimum delay, DOM_MIN_TIMEOUT_VALUE
, is 4 ms (stored in a preference in Firefox: dom.min_timeout_value
), with a DOM_CLAMP_TIMEOUT_NESTING_LEVEL
of 5ms.
In fact, 4ms is specified by the HTML5 spec and is consistent across browsers released in 2010 and onward. Prior to (Firefox 5.0 / Thunderbird 5.0 / SeaMonkey 2.2), the minimum timeout value for nested timeouts was 10 ms.
In addition to "clamping", the timeout can also fire later when the page (or the OS/browser itself) is busy with other tasks.
To implement a 0 ms timeout in a modern browser you can use window.postMessage()
as described here.
Inactive tabs
In (Firefox 5.0 / Thunderbird 5.0 / SeaMonkey 2.2) and Chrome 11, timeouts are clamped to firing no more often than once per second (1000ms) in inactive tabs; see bug 633421 for more information about this in Mozilla or crbug.com/66078 for details about this in Chrome.
浏览器兼容性
Feature | Chrome | Firefox (Gecko) | Internet Explorer | Opera | Safari |
---|---|---|---|---|---|
Basic support | 1.0 | 1.0 (1.7 or earlier) | 4.0 | 4.0 | 1.0 |
Supports parameters for callback*1 | (Yes) | (Yes) | 10.0 | (Yes) | ? |
Feature | Android | Chrome for Android | Firefox Mobile (Gecko) | IE Mobile | Opera Mobile | Safari Mobile |
---|---|---|---|---|---|---|
Basic support | 1.0 | 1.0 | 1.0 (1) | 6.0 | 6.0 | 1.0 |
Supports parameters for callback*1 | ? | ? | ? | ? | ? | ? |
*1 Whether it supports the optional parameters when in its first form or not.
规范
Part of DOM level 0, as specified in HTML5.