解析WordPress中函数钩子hook的作用及基本用法_php技巧
WordPress 的插件机制实际上只的就是这个 Hook 了,它中文被翻译成钩子,允许你参与 WordPress 核心的运行,是一个非常棒的东西,下面我们来详细了解一下它。
钩子分类
钩子分为两种,一种叫做动作(action),还有一种叫做过滤器(filter)。这两种钩子实现原理基本一样,后边会讲到,使用上的区别在于过滤器有返回值,而动作没有。
动作的理念是让你在一种情况或者一个特别的位置执行一些函数,比如发送一封邮件等;过滤器则是让你修改 WordPress 核心需要用到的一个值,然后 WordPress 再用这些值做一些事情,比如函数的返回值等。
动作钩子
wp_head 是一个很常用的动作钩子,在开发主题过程中,开发者都会在 head 标签里加上 wp_head() 函数,事实上,正是这个函数调用了 wp_head 钩子。
如果插件开发者想要在 head 标签里添加一句话,就可以使用 wp_head 钩子,下边是个简单的例子。
//在 head 标签添加一些内容 function Bing_add_head_tag(){ echo '添加内容'; } add_action( 'wp_head', 'Bing_add_head_tag' );
添加代码后,查看前台的网页源代码,就能在 head 标签里看到我们添加的内容了。
上边是一个简单的例子,只是打印了一句话。利用这个钩子,我们还可以做一个遇到 404 页面给管理员发送一封邮件的插件,下边简单的写了一个。
//遇到 404 页面给管理员发送邮件 function Bing_404_page_mail(){ if( !is_404() ) return;//如果不是 404 页面就退出函数 $to = get_option( 'admin_email' );//获取管理员地址 $subject = '遇到 404 页面啦!';//邮件标题 $message = '404 页面的地址:http://' . $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI'];//邮件内容 wp_mail( $to, $subject, $message );//发送邮件 } add_action( 'wp_head', 'Bing_404_page_mail' );
过滤器钩子
根据我的亲身经历,可能过滤器钩子会难理解些,尤其对不熟悉 PHP 的人来说。
过滤器钩子可以让你改一些东西的值,过滤器回调函数会接受一个参数,这就是当前的值。还记得那个用来调用文章内容的 the_content() 函数吧,这个函数就提供了一个 the_content 过滤器。
给 the_content 钩子添加一个函数,这个函数需要接收一个参数,这个参数就是当前的值。
//文章内容全部链接新窗口打开 function Bing_autoblank( $content ){//$content 变量就是文章内容,因为其它过滤器也要过滤,所以这个内容可能是经过其它函数过滤的 $content = str_replace( '<a', '<a target="_blank"', $content );//添加 target="_blank" return $content;//必须要把过滤后的内容返回回去,否则值就丢了 } add_filter( 'the_content', 'Bing_autoblank' );
钩子原理
事实上,在调用 add_action() 和 add_filter() 的时候,只是向 $wp_filter 全局变量添加了一个数组元素,还要说的就是动作和过滤器是公用的一个全局变量,也就是说,过滤器和动作是不能重名的。
当调用 do_action() 的时候,会查找 $wp_filter 全局变量中都有哪些添加到此动作的函数,循环执行即可。
apply_filters() 比 do_action() 多了一个步奏,就是每次调用函数时还要接收这个函数的返回值,最后把经过多次过滤的值返回出去,供使用。
获取当前钩子列表
WordPress 的动作和过滤器是插件机制的核心部分,允许你在特定的地方主动添加自己需要执行的动作,一般使用 add_action() 和 add_filter() 函数挂载函数。
这些钩子被储存在 $wp_filter 全局变量里,所以,要获取钩子列表,可以直接获取 $wp_filter 全局变量。
<pre class="brush:php;toolbar:false"><?php var_dump( $GLOBALS['wp_filter'] ); ?>
上边的代码会打印出钩子列表。

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