【 tulaoshi.com - Web开发 】
前段时间被IE和JavaScript脚本引擎的Memory Leak问题弄得郁闷坏了,不过幸好现在总算是柳暗花明了,并且找到了一些IE中使用脚本避免ML问题的方法。继续研究JavaScript的编写,有发现一些不算ML问题,但是可以节约IE内存使用的方法,在此和大家讨论讨论。
我们在JavaScript中编写代码,对于定义函数的语句:
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function foo()
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{
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// TODO: . . .
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return x;
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}
可以说是在熟悉不过了。当然除了这种定义函数的方法,我们还有另外几种方法也能定义函数:
(本文来源于图老师网站,更多请访问http://www.tulaoshi.com/webkaifa/)
var foo = function()
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{
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// TODO: . . .
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return x;
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}
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var foo = new Function('{/*todo*/return x;}');
后两种方法定义的JavaScript函数,在调用起来和第一种没有任何效果上的区别。
不过由于JavaScript是解释性语言,当我们定义一个函数的时候,解析引擎生成一个Function对象实例,然后把函数内容保存下来。所以每执行一次函数定义语句,就会生成一个函数。而不像编译语言,一个函数编译一次后就被任何语句调用。啊?难道JavaScript不能调用定义好的函数?不是这个意思了,当我们在制作JavaScript控件时,如果动态输出DHTML来作为控件的内容,就容易出现这样的问题。比如我们在一个HTML对象生成过程中,使用了inline方式定义的函数,那么这个元素生成几次,那个函数也就要同时生成几次。
(本文来源于图老师网站,更多请访问http://www.tulaoshi.com/webkaifa/)
function TestObject.prototype.Render(doc, id)
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{
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var span = doc.createElement('SPAN');
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span.Object = this;
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this.m_Element = span;
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if ( id == "NamedMethod" )
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{
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span.onclick = asdf;
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}
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else
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{
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span.onclick = function()
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{
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var asdf01 = ['a', 's', 'd', 'f'];
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var asdf02 = ['a', 's', 'd', 'f'];
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var asdf03 = ['a', 's', 'd', 'f'];
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var asdf04 = ['a', 's', 'd', 'f'];
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var asdf05 = ['a', 's', 'd', 'f'];
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var asdf06 = ['a', 's', 'd', 'f'];
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var asdf07 = ['a', 's', 'd', 'f'];
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var asdf08 = ['a', 's', 'd', 'f'];
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var asdf09 = ['a', 's', 'd', 'f'];
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var asdf10 = ['a', 's', 'd', 'f'];
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var asdf11 = ['a', 's', 'd', 'f'];
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var asdf12 = ['a', 's', 'd', 'f'];
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};
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}
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span.Name = this.m_Description;
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span.innerText = this.m_Name;
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span.style.display = 'block';
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return span;
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}
函数span.onclick = function()中的内容是用来占位置的,这样inline方式定义函数,每次Render()都就会生成一个新的函数对象。使用inline方式有什么不好呢?当对象实例多了的时候,会很明显的浪费内存空间呀,试验数据如下:
| Normal Method | Inline Method | Initialized 27.4 M27.4 M Rendered 33.4 M35.2 M
// IE消耗的内存数量(PM+VM)
单看绝对内存消耗差别不大,可是如果看相对内存消耗:(35.2-33.4)/(33.4-27.4) =
30% !!!,还是很可观的了,而且如果方法本省越大,inline时冗余数据就越多。
附测试代码:
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html
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head
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titleJScript Function Spending/title
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meta name="author" content="birdshome@博客园" /
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/head
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body onunload="ReleaseElements()"
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button id="NamedMethod" onclick="GenerateObjects(this)"
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Append Normal Elements/button
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button id="AnonymousMethod" onclick="GenerateObjects(this)"
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Append Inline Elements/button
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div id="container"
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/div
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script language="Javascript"
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function GenerateObjects(elmt)
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{
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var room = document.getElementById('container');
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for ( var i=0 ; i 1000 ; ++i )
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{
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var obj = new TestObject('__Object__' + i);
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room.appendChild(obj.Render(document, elmt.id));
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}
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}
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function TestObject(name)
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{
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this.m_Name = name;
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this.m_Description = '';
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this.m_Element = null;
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this.toString = function()
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{
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return '[class TestObject]';
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}
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}
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function TestObject.prototype.Render(doc, id)
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{
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var span = doc.createElement('SPAN');
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span.Object = this;
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this.m_Element = span;
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if ( id == "NamedMethod" )
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{
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span.onclick = asdf;
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}
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else
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{
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span.onclick = function()
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{
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var asdf01 = ['a', 's', 'd', 'f'];
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var asdf02 = ['a', 's', 'd', 'f'];
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var asdf03 = ['a', 's', 'd', 'f'];
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var asdf04 = ['a', 's', 'd', 'f'];
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var asdf05 = ['a', 's', 'd', 'f'];
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var asdf06 = ['a', 's', 'd', 'f'];
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var asdf07 = ['a', 's', 'd', 'f'];
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var asdf08 = ['a', 's', 'd', 'f'];
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var asdf09 = ['a', 's', 'd', 'f'];
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var asdf10 = ['a', 's', 'd', 'f'];
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var asdf11 = ['a', 's', 'd', 'f'];
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var asdf12 = ['a', 's', 'd', 'f'];
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};
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}
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span.Name = this.m_Description;
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span.innerText = this.m_Name;
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span.style.display = 'block';
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return span;
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}
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function asdf()
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{
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var asdf01 = ['a', 's', 'd', 'f'];
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var asdf02 = ['a', 's', 'd', 'f'];
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var asdf03 = ['a', 's', 'd', 'f'];
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var asdf04 = ['a', 's', 'd', 'f'];
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var asdf05 = ['a', 's', 'd', 'f'];
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var asdf06 = ['a', 's', 'd', 'f'];
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var asdf07 = ['a', 's', 'd', 'f'];
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var asdf08 = ['a', 's', 'd', 'f'];
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var asdf09 = ['a', 's', 'd', 'f'];
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var asdf10 = ['a', 's', 'd', 'f'];
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var asdf11 = ['a', 's', 'd', 'f'];
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var asdf12 = ['a', 's', 'd', 'f'];
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}
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/script
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script language="javascript"
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function ReleaseElements()
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{
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var room = document.getElementById('container');
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var spans = room.all.tags('SPAN');
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for ( var i=0 ; i spans.length ; ++i )
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{
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spans[i].Object = '';
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}
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}
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/script
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/body
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/html