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PHP中trait使用方法

说通俗点,PHP中使用trait关键字是为了解决一个类既想集成基类的属性和方法,又想拥有别的基类的方法,而trait一般情况下是和use搭配使用的。...

说通俗点,PHP中使用trait关键字是为了解决一个类既想集成基类的属性和方法,又想拥有别的基类的方法,而trait一般情况下是和use搭配使用的。

<?php
  trait Drive {
    public $carName = ‘trait‘;
    public function driving() {
      echo "driving {$this->carName}
";
    }
  }
  class Person {
    public function eat() {
      echo "eat
";
    }
  }
  class Student extends Person {
    use Drive;
    public function study() {
      echo "study
";
    }
  }
  $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?

<?php 
  trait Drive {
    public function hello() {
      echo "hello drive
";
    }
    public function driving() {
      echo "driving from drive
";
    }
  }
  class Person {
    public function hello() {
      echo "hello person
";
    }
    public function driving() {
      echo "driving from person
";
    }
  }
  class Student extends Person {
    use Drive;
    public function hello() {
      echo "hello student
";
    }
  }
  $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
";
  }
  public function hi() {
    echo "Trait1::hi
";
  }
}
trait Trait2 {
  public function hello() {
    echo "Trait2::hello
";
  }
  public function hi() {
    echo "Trait2::hi
";
  }
}
class Class1 {
  use Trait1, Trait2;
}
?>

输出结果如下:

复制代码 代码如下:

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
";
  }
  public function hi() {
    echo "Trait1::hi
";
  }
}
trait Trait2 {
  public function hello() {
    echo "Trait2::hello
";
  }
  public function hi() {
    echo "Trait2::hi
";
  }
}
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 "
";
$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
";
  }
}
trait World {
  use Hello;
  public function sayWorld() {
    echo "World
";
  }
  abstract public function getWorld();
  public function inc() {
    static $c = 0;
    $c = $c + 1;
    echo "$c
";
  }
  public static function doSomething() {
    echo "Doing something
";
  }
}
class HelloWorld {
  use World;
  public function getWorld() {
    return ‘get World‘;
  }
}
$Obj = new HelloWorld();
$Obj->sayHello();
$Obj->sayWorld();
echo $Obj->getWorld() . "
";
HelloWorld::doSomething();
$Obj->inc();
$Obj->inc();
?>

输出结果如下:

Hello
World
get World
Doing something
1
2