How to implement new traits of code reuse in PHP

I came into contact with traits while reading the yii2 source code, so I studied it and wrote a blog to record it.

Since PHP 5.4.0, PHP has implemented a method of code reuse called traits.

Traits is a code reuse mechanism prepared for single inheritance languages ​​like PHP. Traits are designed to reduce the constraints of single-inheritance languages ​​and allow developers to freely reuse method sets in independent classes within different hierarchies. The semantics of traits and class composition define a way to reduce complexity and avoid the typical problems associated with traditional multiple inheritance and mixins.

Trait is similar to a class, but is only designed to combine functionality in a fine-grained and consistent way. Trait cannot be instantiated by itself. It adds a combination of horizontal features to traditional inheritance; that is, members of application classes do not need to be inherited.

Trait example

The code is as follows:

<?php
trait ezcReflectionReturnInfo {
    function getReturnType() { /*1*/ }
    function getReturnDescription() { /*2*/ }
}
class ezcReflectionMethod extends ReflectionMethod {
    use ezcReflectionReturnInfo;
    /* ... */
}
class ezcReflectionFunction extends ReflectionFunction {
    use ezcReflectionReturnInfo;
    /* ... */
}
?>

Priority

Members inherited from the base class are inserted by trait covered by members. The order of precedence is that members from the current class override the trait's methods, and the trait overrides the inherited methods.

Priority order example

The code is as follows:

<?php
class Base {
    public function sayHello() {
        echo &#39;Hello &#39;;
    }
}
trait SayWorld {
    public function sayHello() {
        parent::sayHello();
        echo &#39;World!&#39;;
    }
}
class MyHelloWorld extends Base {
    use SayWorld;
}
$o = new MyHelloWorld();
$o->sayHello();
?>

The above routine will output: Hello World!

SayWorld where members inherited from the base class are inserted Covered by the sayHello method in Trait. Its behavior is consistent with the methods defined in the MyHelloWorld class. The order of precedence is that methods in the current class override trait methods, which in turn override methods in the base class.

Another example of priority order

The code is as follows:

<?php
trait HelloWorld {
    public function sayHello() {
        echo &#39;Hello World!&#39;;
    }
}
class TheWorldIsNotEnough {
    use HelloWorld;
    public function sayHello() {
        echo &#39;Hello Universe!&#39;;
    }
}
$o = new TheWorldIsNotEnough();
$o->sayHello();
?>

The above routine will output: Hello Universe!

Multiple traits

are separated by commas and multiple traits are listed in the use statement. They can all be inserted into a class.

Examples of usage of multiple traits

The code is as follows:

<?php
trait Hello {
    public function sayHello() {
        echo &#39;Hello &#39;;
    }
}
trait World {
    public function sayWorld() {
        echo &#39;World&#39;;
    }
}
class MyHelloWorld {
    use Hello, World;
    public function sayExclamationMark() {
        echo &#39;!&#39;;
    }
}
$o = new MyHelloWorld();
$o->sayHello();
$o->sayWorld();
$o->sayExclamationMark();
?>

The above routine will output: Hello World!

Conflict resolution

If two traits insert a method with the same name, a fatal error will occur if the conflict is not resolved explicitly.

In order to resolve the naming conflict of multiple traits in the same class, you need to use the insteadof operator to explicitly specify which of the conflicting methods to use.

The above method only allows to exclude other methods. The as operator can introduce one of the conflicting methods under another name.

Example of conflict resolution

The code is as follows:

<?php
trait A {
    public function smallTalk() {
        echo &#39;a&#39;;
    }
    public function bigTalk() {
        echo &#39;A&#39;;
    }
}
trait B {
    public function smallTalk() {
        echo &#39;b&#39;;
    }
    public function bigTalk() {
        echo &#39;B&#39;;
    }
}
class Talker {
    use A, B {
        B::smallTalk insteadof A;
        A::bigTalk insteadof B;
    }
}
class Aliased_Talker {
    use A, B {
        B::smallTalk insteadof A;
        A::bigTalk insteadof B;
        B::bigTalk as talk;
    }
}
?>

In this example Talker uses traits A and B. Since A and B have conflicting methods, they define using smallTalk from trait B and bigTalk from trait A.

Aliased_Talker Use the as operator to define talk as an alias of B's ​​bigTalk.

Modify the access control of the method

Using the as syntax can also be used to adjust the access control of the method.

An example of modifying the access control of a method

The code is as follows:

<?php
trait HelloWorld {
    public function sayHello() {
        echo &#39;Hello World!&#39;;
    }
}
// 修改 sayHello 的访问控制
class MyClass1 {
    use HelloWorld { sayHello as protected; }
}
// 给方法一个改变了访问控制的别名
// 原版 sayHello 的访问控制则没有发生变化
class MyClass2 {
    use HelloWorld { sayHello as private myPrivateHello; }
}
?>

Composed from trait to trait

Just like Just as classes can use traits, other traits can also use traits. By using one or more traits when a trait is defined, it can combine some or all members of other traits.

Example of composing trait from trait

The code is as follows:

<?php
trait Hello {
    public function sayHello() {
        echo &#39;Hello &#39;;
    }
}
trait World {
    public function sayWorld() {
        echo &#39;World!&#39;;
    }
}
trait HelloWorld {
    use Hello, World;
}
class MyHelloWorld {
    use HelloWorld;
}
$o = new MyHelloWorld();
$o->sayHello();
$o->sayWorld();
?>

The above routine will output: Hello World!

Abstract of Trait Members

In order to enforce requirements on the classes used, traits support the use of abstract methods.

Indicates an example of mandatory requirements through abstract methods

The code is as follows:

<?php
trait Hello {
    public function sayHelloWorld() {
        echo &#39;Hello&#39;.$this->getWorld();
    }
    abstract public function getWorld();
}
class MyHelloWorld {
    private $world;
    use Hello;
    public function getWorld() {
        return $this->world;
    }
    public function setWorld($val) {
        $this->world = $val;
    }
}
?>

Static member of Trait

Traits can be defined by static members and static methods.

Example of static variables

The code is as follows:

<?php
trait Counter {
    public function inc() {
        static $c = 0;
        $c = $c + 1;
        echo "$c\n";
    }
}
class C1 {
    use Counter;
}
class C2 {
    use Counter;
}
$o = new C1(); $o->inc(); // echo 1
$p = new C2(); $p->inc(); // echo 1
?>

Example of static method

The code is as follows:

<?php
trait StaticExample {
    public static function doSomething() {
        return &#39;Doing something&#39;;
    }
}
class Example {
    use StaticExample;
}
Example::doSomething();
?>

Examples of static variables and static methods

The code is as follows:

<?php
trait Counter {
    public static $c = 0;
    public static function inc() {
        self::$c = self::$c + 1;
        echo self::$c . "\n";
    }
}
class C1 {
    use Counter;
}
class C2 {
    use Counter;
}
C1::inc(); // echo 1
C2::inc(); // echo 1
?>

Attributes
Trait can also define attributes.

Example of defining attributes

The code is as follows:

<?php
trait PropertiesTrait {
    public $x = 1;
}
class PropertiesExample {
    use PropertiesTrait;
}
$example = new PropertiesExample;
$example->x;
?>

If a trait defines an attribute, that class will not be able to define an attribute with the same name, otherwise a mistake. If the property's definition in the class is compatible with its definition in the trait (same visibility and initial value) then the error level is E_STRICT, otherwise it is a fatal error.

Conflict examples

The code is as follows:

<?php
trait PropertiesTrait {
    public $same = true;
    public $different = false;
}
class PropertiesExample {
    use PropertiesTrait;
    public $same = true; // Strict Standards
    public $different = true; // 致命错误
}
?>

Differences in Use

Examples of different uses

The code is as follows:

<?php
namespace Foo\Bar;
use Foo\Test;  // means \Foo\Test - the initial \ is optional
?>
<?php
namespace Foo\Bar;
class SomeClass {
    use Foo\Test;   // means \Foo\Bar\Foo\Test
}
?>

The first use is use Foo\Test for namespace, and what is found is \Foo\Test. The second use is to use a trait. Found \Foo\Bar\Foo\Test.

__CLASS__ and __TRAIT__
__CLASS__ Returns the class name of the use trait, __TRAIT__ returns the trait name

Example The code is as follows

The code is as follows:

<?php
trait TestTrait {
    public function testMethod() {
        echo "Class: " . __CLASS__ . PHP_EOL;
        echo "Trait: " . __TRAIT__ . PHP_EOL;
    }
}
class BaseClass {
    use TestTrait;
}
class TestClass extends BaseClass {
}
$t = new TestClass();
$t->testMethod();
//Class: BaseClass
//Trait: TestTrait

Trait singleton

The example is as follows

The code is as follows:

<?php
trait singleton {    
    /**
     * private construct, generally defined by using class
     */
    //private function __construct() {}
    public static function getInstance() {
        static $_instance = NULL;
        $class = __CLASS__;
        return $_instance ?: $_instance = new $class;
    }
    public function __clone() {
        trigger_error(&#39;Cloning &#39;.__CLASS__.&#39; is not allowed.&#39;,E_USER_ERROR);
    }
    public function __wakeup() {
        trigger_error(&#39;Unserializing &#39;.__CLASS__.&#39; is not allowed.&#39;,E_USER_ERROR);
    }
}
/**
* Example Usage
*/
class foo {
    use singleton;
    private function __construct() {
        $this->name = &#39;foo&#39;;
    }
}
class bar {
    use singleton;
    private function __construct() {
        $this->name = &#39;bar&#39;;
    }
}
$foo = foo::getInstance();
echo $foo->name;
$bar = bar::getInstance();
echo $bar->name;

Call trait method

Although it is not obvious, if the trait method can be defined as a static method in a normal class, it can be called

实例如下

代码如下:

<?php 
trait Foo { 
    function bar() { 
        return &#39;baz&#39;; 
    } 
} 
echo Foo::bar(),"\\n"; 
?>

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