PHP 的 Trait 详解

Trait 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();//输出：This is cat drive
echo "
";
$cat->eat();//输出：This is animal eat
echo "
";
$cat->bark();//输出：This is dog

再测试Trait、基类和本类对同名属性或方法的处理

如下代码实例:

<?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(); //This is cat drive
echo "
";
$cat->eat(); //注意这里输出的是：This is dog eat

所以：Trait 中的方法或属性会覆盖 基类中的同名的方法或属性，而本类会覆盖Trait中同名的属性或方法

一个类可以组合多个Trait，通过逗号相隔

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();//输出：This is trait1 eat
echo "
";
$cat->drive();//输出：This is trait1 drive
echo "
";
echo "
";
echo "
";
$dog = new dog();
$dog->eat();//输出：This is trait1 eat
echo "
";
$dog->drive();//输出：This is trait1 drive
echo "
";
$dog->eaten();//输出：This is trait2 eat
echo "
";
$dog->driven();//输出：This is trait2 driven

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，使用抽象方法，静态属性，静态方法等

<?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();//输出：This is animal name
echo "
";
$animal->eat();//输出：This is Cat eat
echo "
";
$animal->drive();//输出：This is Dog drive
echo "
";
$animal::say();//输出：This is Dog say
echo "
";
$animal->test();//输出：1
echo "
";
$animal->test();//输出：2

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