php中的traits详解

PHP是单继承的语言,在PHP 5.4 Traits出现之前,PHP的类无法同时从两个基类继承属性或方法。php的Traits和Go语言的组合功能类似,通过在类中使用use关键字声明要组合的Trait名称,而具体某个Trait的声明使用trait关键词,Trait不能直接实例化。具体用法请看下面的代码:

<?php

trait Drive{
    public $carName = 'trait';

    public function driving()
    {
        echo "driving {$this->carName}\n";
    }
}

class Person{
    public function eat()
    {
        echo "eat\n";
    }
}

class Student extends Person{
    use Drive;

    public function study(){
        echo "study\n";
    }
}

$student = new Student();
$student->study();
$student->eat();
$student->driving();

输出结果如下:

study
eat
driving trait
上面的例子中,Student类通过继承Person,有了eat方法,通过组合Drive,有了driving方法和属性carName。

如果Trait、基类和本类中都存在某个同名的属性或者方法,最终会保留哪一个呢?通过下面的代码测试一下:

<?php

trait Drive{
    public function hello(){
        echo "hello drive\n";
    }

    public function driving(){
        echo "driving from drive\n";
    }
}

class Person{
    public function hello(){
        echo "hello person\n";
    }

    public function driving(){
        echo "driving from person\n";
    }
}

class Student extends Person{
    use Drive;

    public function hello(){
        echo "hello student\n";
    }
}

$student = new Student();
$student->hello();
$student->driving();

输出结果如下:

hello student
driving from drive
因此得出结论:当方法或属性同名时,当前类中的方法会覆盖 trait的 方法,而 trait 的方法又覆盖了基类中的方法。
如果要组合多个Trait,通过逗号分隔 Trait名称:


use Trait1, Trait2;
如果多个Trait中包含同名方法或者属性时,会怎样呢?答案是当组合的多个Trait包含同名属性或者方法时,需要明确声明解决冲突,否则会产生一个致命错误。
<?php

trait Trait1{
    public function hello()
    {
        echo "Trait1::hello\n";
    }

    public
    function hi()
    {
        echo "Trait1::hi\n";
    }
}

trait Trait2{
    public function hello()
    {
        echo "Trait2::hello\n";
    }

    public function hi()
    {
        echo "Trait2::hi\n";
    }
}

class Class1{
    use Trait1, Trait2;
}

输出结果如下:

PHP Fatal error:  Trait method hello has not been applied, 
because there are collisions with other trait methods on 
Class1 in ~/php54/trait_3.php on line 20

使用insteadof和as操作符来解决冲突,insteadof是使用某个方法替代另一个,而as是给方法取一个别名,具体用法请看代码:

<?php

trait Trait1
{
    public function hello()
    {
        echo "Trait1::hello\n";
    }

    public function hi()
    {
        echo "Trait1::hi\n";
    }
}

trait Trait2
{
    public function hello()
    {
        echo "Trait2::hello\n";
    }

    public function hi()
    {
        echo "Trait2::hi\n";
    }
}

class Class1
{
    use Trait1, Trait2 {
        Trait2::hello insteadof Trait1;
        Trait1::hi insteadof Trait2;
    }
}

class Class2
{
    use Trait1, Trait2 {
        Trait2::hello insteadof Trait1;
        Trait1::hi insteadof Trait2;
        Trait2::hi as hei;
        Trait1::hello as hehe;
    }
}

$Obj1 = new Class1();
$Obj1->hello();
$Obj1->hi();
echo "\n";
$Obj2 = new Class2();
$Obj2->hello();
$Obj2->hi();
$Obj2->hei();
$Obj2->hehe();

输出结果如下:

Trait2::hello
Trait1::hi

Trait2::hello
Trait1::hi
Trait2::hi
Trait1::hello
as关键词还有另外一个用途,那就是修改方法的访问控制:
Trait 也能组合Trait,Trait中支持抽象方法、静态属性及静态方法,测试代码如下:
<?php trait Hello
{
    public function sayHello()
    {
        echo "Hello\n";
    }
}

trait World
{
    use Hello;

    public function sayWorld()
    {
        echo "World\n";
    }

    abstract public function getWorld();

    public function inc()
    {
        static $c = 0;
        $c = $c + 1;
        echo "$c\n";
    }

    public static function doSomething()
    {
        echo "Doing something\n";
    }
}

class HelloWorld
{
    use World;

    public function getWorld()
    {
        return 'get World';
    }
}

$Obj = new HelloWorld();
$Obj->sayHello();
$Obj->sayWorld();
echo $Obj->getWorld() . "\n";
HelloWorld::doSomething();
$Obj->inc();
$Obj->inc();

输出结果如下:

Hello
World
get World
Doing something12




文章来源: php中的traits详解

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