PHP 实现了一种代码复用的方法，称为 trait，复用trait

　　自 PHP 5.4.0 起，PHP 实现了一种代码复用的方法，称为 trait。

　　Trait 是为类似 PHP 的单继承语言而准备的一种代码复用机制。Trait 为了减少单继承语言的限制，使开发人员能够自由地在不同层次结构内独立的类中复用 method。

　　Trait 是 PHP 多重继承的一种解决方案。例如，需要同时继承两个 Abstract Class，这将会是件很麻烦的事情，Trait 就是为了解决这个问题。

　　它为传统继承增加了水平特性的组合。

例子1: 使用trait关键字定义trait

<span>trait first_trait{
    </span><span>public</span> <span>function</span><span> hello(){
        </span><span>return</span> 'hello'<span>;
    }
}</span>

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例子2: 在Class里使用trait，要使用use关键字，使用多个trait时用英文逗号隔开

<span>trait first_trait{
    </span><span>public</span> <span>function</span><span> hello(){
        </span><span>return</span> 'hello'<span>;
    }
}

trait second_trait{
    </span><span>public</span> <span>function</span><span> world(){
        </span><span>return</span> 'world'<span>;
    }
}

</span><span>class</span><span> first_class{
    </span><span>use</span> first_trait,<span>second_trait;
}
</span><span>$obj</span>=<span>new</span><span> first_class();
</span><span>echo</span> <span>$obj</span>-><span>hello();
</span><span>echo</span> <span>$obj</span>->world();

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例子3: 优先级

　　从基类继承的成员会被 trait 插入的成员所覆盖。优先顺序是来自当前类的成员覆盖了 trait 的方法，而 trait 则覆盖了被继承的方法。

　　例子：从基类继承的成员会被 trait 插入的成员所覆盖

<span>class</span><span> Base {
    </span><span>public</span> <span>function</span><span> sayHello() {
        </span><span>echo</span> 'Hello '<span>;
    }
}

trait SayWorld {
    </span><span>public</span> <span>function</span><span> sayHello() {
        parent</span>::<span>sayHello();
        </span><span>echo</span> 'World!'<span>;
    }
}

</span><span>class</span> MyHelloWorld <span>extends</span><span> Base {
    </span><span>use</span><span> SayWorld;
}

</span><span>$o</span> = <span>new</span><span> MyHelloWorld();
</span><span>$o</span>-><span>sayHello();
</span><span>//</span><span>输出的结果</span>
Hello World!

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　　例子：当前类的成员覆盖了 trait 的方法

<span>trait HelloWorld {
    </span><span>public</span> <span>function</span><span> sayHello() {
        </span><span>echo</span> 'Hello World!'<span>;
    }
}

</span><span>class</span><span> TheWorldIsNotEnough {
    </span><span>use</span><span> HelloWorld;
    </span><span>public</span> <span>function</span><span> sayHello() {
        </span><span>echo</span> 'Hello Universe!'<span>;
    }
}

</span><span>$o</span> = <span>new</span><span> TheWorldIsNotEnough();
</span><span>$o</span>-><span>sayHello();
</span><span>//</span><span>输出的结果</span>
Hello Universe!

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例子4: trait之间的嵌套

<span>trait first_trait{
    </span><span>public</span> <span>function</span><span> hello(){
        </span><span>echo</span> 'hello'<span>;
    }
}

trait second_trait{
    </span><span>//</span><span>trait之间的嵌套</span>
    <span>use</span><span> first_trait;
    </span><span>public</span> <span>function</span><span> world(){
        </span><span>echo</span> 'world'<span>;
    }
}

</span><span>class</span><span> first_class{
    </span><span>use</span><span> second_trait;
}
</span><span>$obj</span>=<span>new</span><span> first_class();
</span><span>echo</span> <span>$obj</span>-><span>hello();
</span><span>echo</span> <span>$obj</span>->world();

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例子5: 可以在trait中声明抽象方法，使用它的Class或trait必须实现抽象方法

<span>trait first_trait{
    </span><span>public</span> <span>function</span><span> hello(){
        </span><span>echo</span> 'hello'<span>;
    }
    </span><span>//</span><span>抽象方法</span>
    <span>public</span> <span>abstract</span> <span>function</span><span> test();
}

trait second_trait{
    </span><span>//</span><span>trait之间的嵌套</span>
    <span>use</span><span> first_trait;
    </span><span>public</span> <span>function</span><span> world(){
        </span><span>echo</span> 'world'<span>;
    }

    </span><span>//</span><span>实现first_trait 中的test方法</span>
    <span>public</span> <span>function</span><span> test(){
        </span><span>echo</span> '!'<span>;
    }
}

</span><span>class</span><span> first_class{
    </span><span>use</span><span> second_trait;
}
</span><span>$obj</span>=<span>new</span><span> first_class();
</span><span>echo</span> <span>$obj</span>-><span>hello();
</span><span>echo</span> <span>$obj</span>-><span>world();
</span><span>echo</span> <span>$obj</span>-><span>test();
</span><span>//</span><span>会输出</span>
helloworld！

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例子6: 冲突的解决

如果两个 trait 都插入了一个同名的方法，如果没有明确解决冲突将会产生一个致命错误。

为了解决多个 trait 在同一个类中的命名冲突，需要使用 insteadof 操作符来明确指定使用冲突方法中的哪一个。

以上方式仅允许排除掉其它方法，as 操作符可以将其中一个冲突的方法以另一个名称来引入，相当于方法的别名。

<span>trait A {
    </span><span>public</span> <span>function</span><span> smallTalk() {
        </span><span>echo</span> 'a'<span>;
    }
    </span><span>public</span> <span>function</span><span> bigTalk() {
        </span><span>echo</span> 'A'<span>;
    }
}

trait B {
    </span><span>public</span> <span>function</span><span> smallTalk() {
        </span><span>echo</span> 'b'<span>;
    }
    </span><span>public</span> <span>function</span><span> bigTalk() {
        </span><span>echo</span> 'B'<span>;
    }
}

</span><span>class</span><span> Talker {
    </span><span>use</span> A,<span> B {
        B</span>::smallTalk insteadof A; <span>//</span><span>trait B 的smallTalk方法会代替 trait A 的smallTalk方法</span>
        A::bigTalk insteadof B;  <span>//</span><span>trait A 的bigTalk方法会代替 trait B 的bigTalk方法</span>
<span>    }
}

</span><span>class</span><span> Aliased_Talker {
    </span><span>use</span> A,<span> B {
        B</span>::smallTalk insteadof A;<span>//</span><span>trait B 的smallTalk方法会代替 trait A 的smallTalk方法</span>
        A::bigTalk insteadof B;<span>//</span><span>trait A 的bigTalk方法会代替 trait B 的bigTalk方法</span>
        B::bigTalk <span>as</span> talk; <span>//</span><span>使用 as 操作符来定义了 talk方法 来作为 B 的 bigTalk方法 的别名</span>
<span>    }
}

</span><span>$obj</span>=<span>new</span><span> Talker();
</span><span>$obj</span>-><span>smallTalk();
</span><span>$obj</span>-><span>bigTalk();
</span><span>//</span><span>结果会输出 bA</span>
<span>$obj2</span>=<span>new</span><span> Aliased_Talker();
</span><span>$obj2</span>->talk();<span>//</span><span>会输出B</span>

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例子7: 修改方法的访问控制

<span>trait HelloWorld {
    </span><span>public</span> <span>function</span><span> sayHello() {
        </span><span>echo</span> 'Hello World!'<span>;
    }
}

</span><span>//</span><span> 修改 sayHello 的访问控制</span>
<span>class</span><span> MyClass1 {
    </span><span>use</span> HelloWorld { sayHello <span>as</span> <span>protected</span><span>; }
}

</span><span>//</span><span> 给方法一个改变了访问控制的别名
// 原版 sayHello 的访问控制则没有发生变化</span>
<span>class</span><span> MyClass2 {
    </span><span>use</span> HelloWorld { sayHello <span>as</span> <span>private</span><span> myPrivateHello; }
}</span>

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例子8: Trait 同样可以定义属性

<span>trait PropertiesTrait {
    </span><span>public</span> <span>$x</span> = 1<span>;
}

</span><span>class</span><span> PropertiesExample {
    </span><span>use</span><span> PropertiesTrait;
}

</span><span>$example</span> = <span>new</span><span> PropertiesExample;
</span><span>$example</span>->x;

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如果 trait 定义了一个属性，那类将不能定义同样名称的属性，否则会产生一个错误。如果该属性在类中的定义与在 trait 中的定义兼容（同样的可见性和初始值）则错误的级别是 E_STRICT，否则是一个致命错误。

<span>trait PropertiesTrait {
    </span><span>public</span> <span>$same</span> = <span>true</span><span>;
    </span><span>public</span> <span>$different</span> = <span>false</span><span>;
}

</span><span>class</span><span> PropertiesExample {
    </span><span>use</span><span> PropertiesTrait;
    </span><span>public</span> <span>$same</span> = <span>true</span>; <span>//</span><span> Strict Standards</span>
    <span>public</span> <span>$different</span> = <span>true</span>; <span>//</span><span> 致命错误</span>
}

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