PHP的学习-PHP的闭包

PHP的学习--PHP的闭包

php的闭包（Closure）也就是匿名函数，是PHP5.3引入的。

闭包的语法很简单，需要注意的关键字就只有use，use是连接闭包和外界变量。

<span style="color: #800080;">$a</span> = <span style="color: #0000ff;">function</span>() <span style="color: #0000ff;">use</span>(<span style="color: #800080;">$b</span>) {}

简单例子如下：

<span style="color: #0000ff;">function</span> <span style="color: #0000ff;">callback</span>(<span style="color: #800080;">$fun</span><span style="color: #000000;">) {</span><span style="color: #800080;">$fun</span><span style="color: #000000;">();}</span><span style="color: #800080;">$msg</span> = "Hello, everyone"<span style="color: #000000;">;</span><span style="color: #800080;">$fun</span> = <span style="color: #0000ff;">function</span> () <span style="color: #0000ff;">use</span>(<span style="color: #800080;">$msg</span><span style="color: #000000;">) {</span><span style="color: #0000ff;">print</span> "This is a closure use string value, msg is: <span style="color: #800080;">$msg</span>. <br>/n"<span style="color: #000000;">;};</span><span style="color: #800080;">$msg</span> = "Hello, everybody"<span style="color: #000000;">;</span><span style="color: #0000ff;">callback</span>(<span style="color: #800080;">$fun</span>);

结果是：This is a closure use string value, msg is: Hello, everyone.
/n

在PHP新开放的闭包语法中， 我们用use来使用闭包外部定义的变量的。这里我们使用了外部变量$msg，定义完之后，又对其值进行了改变，闭包被执行后输出的是原始值。以传值方式传递的基础类型参数，闭包use的值在闭包创建是就确定了。

小应用如下：

<span style="color: #008000;">/*</span><span style="color: #008000;">*  * 一个利用闭包的计数器产生器  * 这里其实借鉴的是python中介绍闭包时的例子...  * 我们可以这样考虑:  *      1. counter函数每次调用, 创建一个局部变量$counter, 初始化为1.  *      2. 然后创建一个闭包, 闭包产生了对局部变量$counter的引用.  *      3. 函数counter返回创建的闭包, 并销毁局部变量, 但此时有闭包对$counter的引用,   *          它并不会被回收, 因此, 我们可以这样理解, 被函数counter返回的闭包, 携带了一个游离态的  *          变量.  *      4. 由于每次调用counter都会创建独立的$counter和闭包, 因此返回的闭包相互之间是独立的.  *      5. 执行被返回的闭包, 对其携带的游离态变量自增并返回, 得到的就是一个计数器.  * 结论: 此函数可以用来生成相互独立的计数器.  </span><span style="color: #008000;">*/</span>  <span style="color: #0000ff;">function</span><span style="color: #000000;"> counter() {      </span><span style="color: #800080;">$counter</span> = 1<span style="color: #000000;">;      </span><span style="color: #0000ff;">return</span> <span style="color: #0000ff;">function</span>() <span style="color: #0000ff;">use</span>(&<span style="color: #800080;">$counter</span>) {<span style="color: #0000ff;">return</span> <span style="color: #800080;">$counter</span> ++<span style="color: #000000;">;};  }  </span><span style="color: #800080;">$counter1</span> =<span style="color: #000000;"> counter();  </span><span style="color: #800080;">$counter2</span> =<span style="color: #000000;"> counter();  </span><span style="color: #0000ff;">echo</span> "counter1: " . <span style="color: #800080;">$counter1</span>() . "<br>/n"<span style="color: #000000;">;  </span><span style="color: #0000ff;">echo</span> "counter1: " . <span style="color: #800080;">$counter1</span>() . "<br>/n"<span style="color: #000000;">;  </span><span style="color: #0000ff;">echo</span> "counter1: " . <span style="color: #800080;">$counter1</span>() . "<br>/n"<span style="color: #000000;">;  </span><span style="color: #0000ff;">echo</span> "counter1: " . <span style="color: #800080;">$counter1</span>() . "<br>/n"<span style="color: #000000;">;  </span><span style="color: #0000ff;">echo</span> "counter2: " . <span style="color: #800080;">$counter2</span>() . "<br>/n"<span style="color: #000000;">;  </span><span style="color: #0000ff;">echo</span> "counter2: " . <span style="color: #800080;">$counter2</span>() . "<br>/n"<span style="color: #000000;">;  </span><span style="color: #0000ff;">echo</span> "counter2: " . <span style="color: #800080;">$counter2</span>() . "<br>/n"<span style="color: #000000;">;  </span><span style="color: #0000ff;">echo</span> "counter2: " . <span style="color: #800080;">$counter2</span>() . "<br>/n"<span style="color: #000000;">;  </span>?>

闭包的作用

1. 减少foreach的循环的代码

比如手册http://php.net/manual/en/functions.anonymous.php 中的例子Cart

<span style="color: #000000;">php</span><span style="color: #008000;">//</span><span style="color: #008000;"> 一个基本的购物车，包括一些已经添加的商品和每种商品的数量。// 其中有一个方法用来计算购物车中所有商品的总价格。该方法使用了一个closure作为回调函数。</span><span style="color: #0000ff;">class</span><span style="color: #000000;"> Cart{    </span><span style="color: #0000ff;">const</span> PRICE_BUTTER  = 1.00<span style="color: #000000;">;    </span><span style="color: #0000ff;">const</span> PRICE_MILK    = 3.00<span style="color: #000000;">;    </span><span style="color: #0000ff;">const</span> PRICE_EGGS    = 6.95<span style="color: #000000;">;     </span><span style="color: #0000ff;">protected</span>   <span style="color: #800080;">$products</span> = <span style="color: #0000ff;">array</span><span style="color: #000000;">();         </span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">function</span> add(<span style="color: #800080;">$product</span>, <span style="color: #800080;">$quantity</span><span style="color: #000000;">)    {        </span><span style="color: #800080;">$this</span>->products[<span style="color: #800080;">$product</span>] = <span style="color: #800080;">$quantity</span><span style="color: #000000;">;    }         </span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">function</span> getQuantity(<span style="color: #800080;">$product</span><span style="color: #000000;">)    {        </span><span style="color: #0000ff;">return</span> <span style="color: #0000ff;">isset</span>(<span style="color: #800080;">$this</span>->products[<span style="color: #800080;">$product</span>]) ? <span style="color: #800080;">$this</span>->products[<span style="color: #800080;">$product</span>] :               <span style="color: #0000ff;">FALSE</span><span style="color: #000000;">;    }         </span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">function</span> getTotal(<span style="color: #800080;">$tax</span><span style="color: #000000;">)    {        </span><span style="color: #800080;">$total</span> = 0.00<span style="color: #000000;">;                 </span><span style="color: #800080;">$callback</span> =            <span style="color: #0000ff;">function</span> (<span style="color: #800080;">$quantity</span>, <span style="color: #800080;">$product</span>) <span style="color: #0000ff;">use</span> (<span style="color: #800080;">$tax</span>, &<span style="color: #800080;">$total</span><span style="color: #000000;">)            {                </span><span style="color: #800080;">$pricePerItem</span> = <span style="color: #008080;">constant</span>(<span style="color: #ff00ff;">__CLASS__</span> . "::PRICE_" .                    <span style="color: #008080;">strtoupper</span>(<span style="color: #800080;">$product</span><span style="color: #000000;">));                </span><span style="color: #800080;">$total</span> += (<span style="color: #800080;">$pricePerItem</span> * <span style="color: #800080;">$quantity</span>) * (<span style="color: #800080;">$tax</span> + 1.0<span style="color: #000000;">);            };        </span><span style="color: #008000;">//</span><span style="color: #008000;">使用用户自定义函数对数组中的每个元素做回调处理</span>        <span style="color: #008080;">array_walk</span>(<span style="color: #800080;">$this</span>->products, <span style="color: #800080;">$callback</span><span style="color: #000000;">);        </span><span style="color: #0000ff;">return</span> <span style="color: #008080;">round</span>(<span style="color: #800080;">$total</span>, 2<span style="color: #000000;">);;    }} </span><span style="color: #800080;">$my_cart</span> = <span style="color: #0000ff;">new</span><span style="color: #000000;"> Cart; </span><span style="color: #008000;">//</span><span style="color: #008000;"> 往购物车里添加条目</span><span style="color: #800080;">$my_cart</span>->add('butter', 1<span style="color: #000000;">);</span><span style="color: #800080;">$my_cart</span>->add('milk', 3<span style="color: #000000;">);</span><span style="color: #800080;">$my_cart</span>->add('eggs', 6<span style="color: #000000;">); </span><span style="color: #008000;">//</span><span style="color: #008000;"> 打出出总价格，其中有 5% 的销售税.</span><span style="color: #0000ff;">print</span> <span style="color: #800080;">$my_cart</span>->getTotal(0.05) . "\n"<span style="color: #000000;">;</span><span style="color: #008000;">//</span><span style="color: #008000;"> The result is 54.29</span>?>

这里如果我们改造getTotal函数必然要使用到foreach。

2. 减少函数的参数

<span style="color: #0000ff;">function</span> html(<span style="color: #800080;">$code</span> , <span style="color: #800080;">$id</span>="", <span style="color: #800080;">$class</span>=""<span style="color: #000000;">){</span><span style="color: #0000ff;">if</span> (<span style="color: #800080;">$id</span> !== "") <span style="color: #800080;">$id</span> = " id = \"<span style="color: #800080;">$id</span>\""<span style="color: #000000;"> ;</span><span style="color: #800080;">$class</span> = (<span style="color: #800080;">$class</span> !== "")? " class =\"<span style="color: #800080;">$class</span>\">":">"<span style="color: #000000;">;</span><span style="color: #800080;">$open</span> = "$code$id$class"<span style="color: #000000;">;</span><span style="color: #800080;">$close</span> = "<span style="color: #800080;">$code</span>>"<span style="color: #000000;">;</span><span style="color: #0000ff;">return</span> <span style="color: #0000ff;">function</span> (<span style="color: #800080;">$inner</span> = "") <span style="color: #0000ff;">use</span> (<span style="color: #800080;">$open</span>, <span style="color: #800080;">$close</span><span style="color: #000000;">){</span><span style="color: #0000ff;">return</span> "<span style="color: #800080;">$open$inner$close</span>"<span style="color: #000000;">;    };}</span>

如果是使用平时的方法，我们会把inner放到html函数参数中，这样不管是代码阅读还是使用都不如使用闭包。

3. 解除递归函数

<span style="color: #000000;">php</span><span style="color: #800080;">$fib</span> = <span style="color: #0000ff;">function</span>(<span style="color: #800080;">$n</span>) <span style="color: #0000ff;">use</span>(&<span style="color: #800080;">$fib</span><span style="color: #000000;">) {    </span><span style="color: #0000ff;">if</span>(<span style="color: #800080;">$n</span> == 0 || <span style="color: #800080;">$n</span> == 1) <span style="color: #0000ff;">return</span> 1<span style="color: #000000;">;    </span><span style="color: #0000ff;">return</span> <span style="color: #800080;">$fib</span>(<span style="color: #800080;">$n</span> - 1) + <span style="color: #800080;">$fib</span>(<span style="color: #800080;">$n</span> - 2<span style="color: #000000;">);}; </span><span style="color: #0000ff;">echo</span> <span style="color: #800080;">$fib</span>(2) . "\n"; <span style="color: #008000;">//</span><span style="color: #008000;"> 2</span><span style="color: #800080;">$lie</span> = <span style="color: #800080;">$fib</span><span style="color: #000000;">;</span><span style="color: #800080;">$fib</span> = <span style="color: #0000ff;">function</span>(){<span style="color: #0000ff;">die</span>('error');};<span style="color: #008000;">//</span><span style="color: #008000;">rewrite $fib variable </span><span style="color: #0000ff;">echo</span> <span style="color: #800080;">$lie</span>(5); <span style="color: #008000;">//</span><span style="color: #008000;"> error   because $fib is referenced by closure</span>

注意上题中的use使用了&，这里不使用&会出现错误fib(n-1)是找不到function的（前面没有定义fib的类型）

所以想使用闭包解除循环函数的时候就需要使用

<span style="color: #000000;">php</span><span style="color: #800080;">$recursive</span> = <span style="color: #0000ff;">function</span> () <span style="color: #0000ff;">use</span> (&<span style="color: #800080;">$recursive</span><span style="color: #000000;">){</span><span style="color: #008000;">//</span><span style="color: #008000;"> The function is now available as $recursive</span>}

这样的形式。

4. 延迟绑定

如果你需要延迟绑定use里面的变量，你就需要使用引用，否则在定义的时候就会做一份拷贝放到use中

<span style="color: #000000;">php</span><span style="color: #800080;">$result</span> = 0<span style="color: #000000;">; </span><span style="color: #800080;">$one</span> = <span style="color: #0000ff;">function</span><span style="color: #000000;">(){    </span><span style="color: #008080;">var_dump</span>(<span style="color: #800080;">$result</span><span style="color: #000000;">);}; </span><span style="color: #800080;">$two</span> = <span style="color: #0000ff;">function</span>() <span style="color: #0000ff;">use</span> (<span style="color: #800080;">$result</span><span style="color: #000000;">){    </span><span style="color: #008080;">var_dump</span>(<span style="color: #800080;">$result</span><span style="color: #000000;">);}; </span><span style="color: #800080;">$three</span> = <span style="color: #0000ff;">function</span>() <span style="color: #0000ff;">use</span> (&<span style="color: #800080;">$result</span><span style="color: #000000;">){    </span><span style="color: #008080;">var_dump</span>(<span style="color: #800080;">$result</span><span style="color: #000000;">);}; </span><span style="color: #800080;">$result</span>++<span style="color: #000000;">; </span><span style="color: #800080;">$one</span>();    <span style="color: #008000;">//</span><span style="color: #008000;"> outputs NULL: $result is not in scope</span><span style="color: #800080;">$two</span>();    <span style="color: #008000;">//</span><span style="color: #008000;"> outputs int(0): $result was copied</span><span style="color: #800080;">$three</span>();    <span style="color: #008000;">//</span><span style="color: #008000;"> outputs int(1)</span>

使用引用和不使用引用就代表了是调用时赋值，还是申明时候赋值