PHP中Trait详解

php从以前到现在一直都是单继承的语言，无法同时从两个基类中继承属性和方法，为了解决这个问题，php出了Trait这个特性

用法：通过在类中使用use 关键字，声明要组合的Trait名称，具体的Trait的声明使用Trait关键词，Trait不能实例化

如下代码实例:

<?php

trait Dog{

    public $name="dog";

    public function bark(){

        echo "This is dog";

    }

}

class Animal{

    public function eat(){

        echo "This is animal eat";

    }

}

class Cat extends Animal{

    use Dog;

    public function drive(){

        echo "This is cat drive";

    }

}

$cat = new Cat();

$cat->drive();

echo "<br/>";

$cat->eat();

echo "<br/>";

$cat->bark();

?>

将会如下输出：

This is cat drive

This is animal eat

This is dog

再测试Trait、基类和本类对同名属性或方法的处理，如下代码

<?php

trait Dog{

    public $name="dog";

    public function drive(){

        echo "This is dog drive";

    }

    public function eat(){

        echo "This is dog eat";

    }

}

class Animal{

    public function drive(){

        echo "This is animal drive";

    }

    public function eat(){

        echo "This is animal eat";

    }

}

class Cat extends Animal{

    use Dog;

    public function drive(){

        echo "This is cat drive";

    }

}

$cat = new Cat();

$cat->drive();

echo "<br/>";

$cat->eat();

?>

如下显示：

This is cat drive

This is dog eat

所以：Trait中的方法会覆盖 基类中的同名方法，而本类会覆盖Trait中同名方法

注意点：当trait定义了属性后，类就不能定义同样名称的属性，否则会产生 fatal error，除非是设置成相同可见度、相同默认值。不过在php7之前，即使这样设置，还是会产生E_STRICT 的提醒

一个类可以组合多个Trait，通过逗号相隔，如下

use trait1,trait2

当不同的trait中，却有着同名的方法或属性，会产生冲突，可以使用insteadof或 as进行解决，insteadof 是进行替代，而as是给它取别名

如下实例：

<?php

trait trait1{

    public function eat(){

        echo "This is trait1 eat";

    }

    public function drive(){

        echo "This is trait1 drive";

    }

}

trait trait2{

    public function eat(){

        echo "This is trait2 eat";

    }

    public function drive(){

        echo "This is trait2 drive";

    }

}

class cat{

    use trait1,trait2{

        trait1::eat insteadof trait2;

        trait1::drive insteadof trait2;

    }

}

class dog{

    use trait1,trait2{

        trait1::eat insteadof trait2;

        trait1::drive insteadof trait2;

        trait2::eat as eaten;

        trait2::drive as driven;

    }

}

$cat = new cat();

$cat->eat();

echo "<br/>";

$cat->drive();

echo "<br/>";

echo "<br/>";

echo "<br/>";

$dog = new dog();

$dog->eat();

echo "<br/>";

$dog->drive();

echo "<br/>";

$dog->eaten();

echo "<br/>";

$dog->driven();

?>

输出如下：

This is trait1 eat

This is trait1 drive

This is trait1 eat

This is trait1 drive

This is trait2 eat

This is trait2 drive

as 还可以修改方法的访问控制

<?php

trait Animal{

    public function eat(){

        echo "This is Animal eat";

    }

}

class Dog{

    use Animal{

        eat as protected;

    }

}

class Cat{

    use Animal{

        Animal::eat as private eaten;

    }

}

$dog = new Dog();

$dog->eat();//报错，因为已经把eat改成了保护

$cat = new Cat();

$cat->eat();//正常运行，不会修改原先的访问控制

$cat->eaten();//报错，已经改成了私有的访问控制

?>

Trait也可以互相组合，还可以使用抽象方法，静态属性，静态方法等，实例如下

<?php

trait Cat{

    public function eat(){

        echo "This is Cat eat";

    }

}

trait Dog{

    use Cat;

    public function drive(){

        echo "This is Dog drive";

    }

    abstract public function getName();

    public function test(){

        static $num=0;

        $num++;

        echo $num;

    }

    public static function say(){

        echo "This is Dog say";

    }

}

class animal{

    use Dog;

    public function getName(){

        echo "This is animal name";

    }

}

$animal = new animal();

$animal->getName();

echo "<br/>";

$animal->eat();

echo "<br/>";

$animal->drive();

echo "<br/>";

$animal::say();

echo "<br/>";

$animal->test();

echo "<br/>";

$animal->test();

?>

输出如下：

This is animal name

This is Cat eat

This is Dog drive

This is Dog say

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