Usage and examples of Trait in PHP

PHP is a single-inheritance language. Before the emergence of PHP 5.4 Traits, PHP classes could not inherit properties or methods from two base classes at the same time. In order to solve this problem, PHP introduced Traits. This feature. (The combination function of Traits and Go language is somewhat similar)

Usage: Use the use keyword in the class to declare the Trait name to be combined, and the declaration of a specific Trait uses the trait keyword, and the Trait cannot be directly instantiated. change.

<?php
trait Drive {
    public $carName = &#39;BMW&#39;;
    public function driving() {
        echo "driving {$this->carName}\n";
    }
}
 
class Person {
    public function age() {
        echo "i am 18 years old\n";
    }
}
 
class Student extends Person {
    use Drive;
    public function study() {
        echo "Learn to drive \n";
    }
}
 
$student = new Student();
$student->study();  //输出：Learn to drive 
$student->age();    //输出：i am 18 years old
$student->driving();//输出：driving BMW

Conclusion:

The Student class inherits Person and has the age method.

By combining Drive, it has the driving method and attribute carName.

If there is a property or method with the same name in Trait, base class and this class, which one will be retained in the end? Test it with the following code:

<?php
 
trait Drive {
    public function hello() {
        echo "hello 周伯通\n";
    }
    public function driving() {
        echo "周伯通不会开车\n";
    }
}
 
class Person {
    public function hello() {
        echo "hello 大家好\n";
    }
    public function driving() {
        echo "大家都会开车\n";
    }
}
 
class Student extends Person {
    use Drive;//trait 的方法覆盖了基类Person中的方法，所以Person中的hello 和driving被覆盖
    public function hello() {
        echo "hello 新学员\n";//当方法或属性同名时，当前类中的方法会覆盖 trait的 方法,所以此处hello会覆盖trait中的
        hello
    }
}
 
$student = new Student();
$student->hello();    //输出：hello 新学员
$student->driving();  //输出：周伯通不会开车

Conclusion: When a method or attribute has the same name, the method in the current class will override the trait's method, and the trait's method will override the method in the base class.

If you want to combine multiple Traits, separate the Trait names by commas:

use Trait1, Trait2;

What will happen if multiple Traits contain methods or properties with the same name? Woolen cloth? The answer is that when multiple combined Traits contain properties or methods with the same name, they need to be explicitly declared to resolve conflicts, otherwise a fatal error will occur.

<?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：Fatal error:  Trait method hello has not been applied, because there are collisions with other trait
 methods on Class1 in

Use insteadof and as operators to resolve conflicts. Insteadof uses a method to replace another, while as gives an alias to the method. Please see the code for specific usage:

<?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();

Output

Trait2::hello
Trait1::hi 
 
Trait2::hello
Trait1::hi 
Trait2::hi
Trait1::hello

<?php
trait Hello {
    public function hello() {
        echo "hello,我是周伯通\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(); # 输出: hello,我是周伯通,因为原来的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

Uncaught Error: Call to protected method Class1::hello() from context '' in D:\web\mytest\p.php:18

Trait can also be combined with Trait. Trait supports abstract methods, static properties and static methods. The test code is as follows:

<?php
trait Hello {
    public function sayHello() {
        echo "Hello 我是周伯通\n";
    }
}
 
trait World {
    use Hello;
    public function sayWorld() {
        echo "hello 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 &#39;do you get World ?&#39;;
    }
}
 
$Obj = new HelloWorld();
$Obj->sayHello();
$Obj->sayWorld();
echo $Obj->getWorld() . "\n";
HelloWorld::doSomething();
$Obj->inc();
$Obj->inc();

Output

Hello 我是周伯通
hello world
do you get World ?
Doing something12

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