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/