PHP 5.4+ Traits 特性

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。
<?phptrait 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:
<?phptrait 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:
<?phptrait 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
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