什么叫回调函数?

本文详细介绍了回调函数的概念及其在计算机编程中的应用。通过生动的例子和专业的解释,帮助读者理解回调函数的工作原理,并提供了C语言示例代码加以说明。

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回调函数是应用程序提供给Windows系统DLL或其它DLL调用的函数,一般用于截获消息、获取系统信息或处理异步事件。应用程序把回调函数的地址指针告诉DLL,而DLL在适当的时候会调用该函数。回调函数必须遵守事先规定好的参数格式和传递方式,否则DLL一调用它就会引起程序或系统的崩溃。通常情况下,回调函数采用标准WindowsAPI的调用方式,即__stdcall,当然,DLL编制者可以自己定义调用方式,但客户程序也必须遵守相同的规定。在__stdcall方式下,函数的参数按从右到左的顺序压入堆栈,除了明确指明是指针或引用外,参数都按值传递,函数返回之前自己负责把参数从堆栈中弹出。



生动的描述
你到一个商店买东西,刚好你要的东西没有货,于是你在店员那里留下了你的电话,过了几天店里有货了,店员就打了你的电话,然后你接到电话后就到店里去取了货。在这个例子里,你的电话号码就叫回调函数,你把电话留给店员就叫登记回调函数,店里后来有货了叫做触发了回调关联的事件,店员给你打电话叫做调用回调函数,你到店里去取货叫做响应回调事件。(by常溪玲

专业的解释
In computer programming, a callback is a reference to executable code, or a piece of executable code, that is passed as an argument to other code. This allows a lower-level software layer to call a subroutine (or function) defined in a higher-level layer.

数学的定义
是设计模式里面template method pattern当template method的数量无限趋近于一的时候的极限。(by vczh )

通俗易懂的说法
想象你的朋友将要离开你家,走的时候,你告诉对方说“你到家后,给我打个电话,这样我才知道你安全到达了。” 这就是字面上的call back

举例说明
<span style="font-family:Courier New"><span class="co2" style="color:rgb(51,153,51)">#include <stdio.h></span>
<span class="co2" style="color:rgb(51,153,51)">#include <stdlib.h></span>
 
<span class="coMULTI" style="color:rgb(128,128,128); font-style:italic">/* The calling function takes a single callback as a parameter. */</span>
<span class="kw4" style="color:rgb(153,51,51)">void</span> PrintTwoNumbers<span class="br0" style="color:rgb(0,153,0)">(</span><span class="kw4" style="color:rgb(153,51,51)">int</span> <span class="br0" style="color:rgb(0,153,0)">(</span><span class="sy0" style="color:rgb(51,153,51)">*</span>numberSource<span class="br0" style="color:rgb(0,153,0)">)</span><span class="br0" style="color:rgb(0,153,0)">(</span><span class="kw4" style="color:rgb(153,51,51)">void</span><span class="br0" style="color:rgb(0,153,0)">)</span><span class="br0" style="color:rgb(0,153,0)">)</span> <span class="br0" style="color:rgb(0,153,0)">{</span>
    <span class="kw3" style="color:rgb(0,0,102)">printf</span><span class="br0" style="color:rgb(0,153,0)">(</span><span class="st0" style="color:rgb(255,0,0)">"%d and %d<span class="es1" style="color:rgb(0,0,153); font-weight:bold">\n</span>"</span><span class="sy0" style="color:rgb(51,153,51)">,</span> numberSource<span class="br0" style="color:rgb(0,153,0)">(</span><span class="br0" style="color:rgb(0,153,0)">)</span><span class="sy0" style="color:rgb(51,153,51)">,</span> numberSource<span class="br0" style="color:rgb(0,153,0)">(</span><span class="br0" style="color:rgb(0,153,0)">)</span><span class="br0" style="color:rgb(0,153,0)">)</span><span class="sy0" style="color:rgb(51,153,51)">;</span>
<span class="br0" style="color:rgb(0,153,0)">}</span>
 
<span class="coMULTI" style="color:rgb(128,128,128); font-style:italic">/* A possible callback */</span>
<span class="kw4" style="color:rgb(153,51,51)">int</span> overNineThousand<span class="br0" style="color:rgb(0,153,0)">(</span><span class="kw4" style="color:rgb(153,51,51)">void</span><span class="br0" style="color:rgb(0,153,0)">)</span> <span class="br0" style="color:rgb(0,153,0)">{</span>
    <span class="kw1" style="color:rgb(177,177,0)">return</span> <span class="br0" style="color:rgb(0,153,0)">(</span><span class="kw3" style="color:rgb(0,0,102)">rand</span><span class="br0" style="color:rgb(0,153,0)">(</span><span class="br0" style="color:rgb(0,153,0)">)</span> <span class="sy0" style="color:rgb(51,153,51)">%</span> <span class="nu0" style="color:rgb(0,0,221)">1000</span><span class="br0" style="color:rgb(0,153,0)">)</span> <span class="sy0" style="color:rgb(51,153,51)">+</span> <span class="nu0" style="color:rgb(0,0,221)">9001</span><span class="sy0" style="color:rgb(51,153,51)">;</span>
<span class="br0" style="color:rgb(0,153,0)">}</span>
 
<span class="coMULTI" style="color:rgb(128,128,128); font-style:italic">/* Another possible callback. */</span>
<span class="kw4" style="color:rgb(153,51,51)">int</span> meaningOfLife<span class="br0" style="color:rgb(0,153,0)">(</span><span class="kw4" style="color:rgb(153,51,51)">void</span><span class="br0" style="color:rgb(0,153,0)">)</span> <span class="br0" style="color:rgb(0,153,0)">{</span>
    <span class="kw1" style="color:rgb(177,177,0)">return</span> <span class="nu0" style="color:rgb(0,0,221)">42</span><span class="sy0" style="color:rgb(51,153,51)">;</span>
<span class="br0" style="color:rgb(0,153,0)">}</span>
 
<span class="coMULTI" style="color:rgb(128,128,128); font-style:italic">/* Here we call PrintTwoNumbers() with three different callbacks. */</span>
<span class="kw4" style="color:rgb(153,51,51)">int</span> main<span class="br0" style="color:rgb(0,153,0)">(</span><span class="kw4" style="color:rgb(153,51,51)">void</span><span class="br0" style="color:rgb(0,153,0)">)</span> <span class="br0" style="color:rgb(0,153,0)">{</span>
    PrintTwoNumbers<span class="br0" style="color:rgb(0,153,0)">(</span><span class="sy0" style="color:rgb(51,153,51)">&</span><span class="kw3" style="color:rgb(0,0,102)">rand</span><span class="br0" style="color:rgb(0,153,0)">)</span><span class="sy0" style="color:rgb(51,153,51)">;</span>
    PrintTwoNumbers<span class="br0" style="color:rgb(0,153,0)">(</span><span class="sy0" style="color:rgb(51,153,51)">&</span>overNineThousand<span class="br0" style="color:rgb(0,153,0)">)</span><span class="sy0" style="color:rgb(51,153,51)">;</span>
    PrintTwoNumbers<span class="br0" style="color:rgb(0,153,0)">(</span><span class="sy0" style="color:rgb(51,153,51)">&</span>meaningOfLife<span class="br0" style="color:rgb(0,153,0)">)</span><span class="sy0" style="color:rgb(51,153,51)">;</span>
    <span class="kw1" style="color:rgb(177,177,0)">return</span> <span class="nu0" style="color:rgb(0,0,221)">0</span><span class="sy0" style="color:rgb(51,153,51)">;</span>
<span class="br0" style="color:rgb(0,153,0)">}</span></span>

回调的作用
回调可用于通知机制,例如,有时要在程序中设置一个计时器,每到一定时间,程序会得到相应的通知,但通知机制的实现者对我们的程序一无所知。而此时,就需有一个特定原型的函数指针,用这个指针来进行回调,来通知我们的程序事件已经发生。

看完后,你觉得还有可补充的内容,欢迎给我留言:)

参考资料





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