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PHP中的Traits详解

原文链接: http://tabalt.net/blog/php-traits/

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:

<?php  trait Hello {   public function hello() {    echo "hello,trait/n";   }  }  class Class1 {   use Hello {    hello as protected;   }  }  class Class2 {   use Hello {    Hello::hello as private hi;   }  }  $Obj1 = new Class1();  $Obj1->hello(); # 报致命错误,因为hello方法被修改成受保护的  $Obj2 = new Class2();  $Obj2->hello(); # 原来的hello方法仍然是公共的  $Obj2->hi();  # 报致命错误,因为别名hi方法被修改成私有的  

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 something 1 2 

原文链接: http://tabalt.net/blog/php-traits/

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