php-doc-en/language/oop5/traits.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- $Revision$ -->
 <sect1 xml:id="language.oop5.traits" xmlns="http://docbook.org/ns/docbook">
  <title>Traits</title>
  <para>
   As of PHP 5.4.0, PHP implements a method of code reuse called Traits.
  </para>
  <para>
   Traits are a mechanism for code reuse in single inheritance languages such as
   PHP. A Trait is intended to reduce some limitations of single inheritance by
   enabling a developer to reuse sets of methods freely in several independent
   classes living in different class hierarchies. The semantics of the combination
   of Traits and classes is defined in a way which reduces complexity, and avoids
   the typical problems associated with multiple inheritance and Mixins.
  </para>
  <para>
   A Trait is similar to a class, but only intended to group functionality in a
   fine-grained and consistent way. It is not possible to instantiate a Trait on
   its own. It is an addition to traditional inheritance and enables horizontal
   composition of behavior; that is, the application of class members without
   requiring inheritance.
  </para>
  
  <example xml:id="language.oop5.traits.basicexample">
    <title>Trait example</title>
    <programlisting role="php">
<![CDATA[
<?php
trait ezcReflectionReturnInfo {
    function getReturnType() { /*1*/ }
    function getReturnDescription() { /*2*/ }
}

class ezcReflectionMethod extends ReflectionMethod {
    use ezcReflectionReturnInfo;
    /* ... */
}

class ezcReflectionFunction extends ReflectionFunction {
    use ezcReflectionReturnInfo;
    /* ... */
}
?>
]]>
    </programlisting>
   </example>
  
  <sect2 xml:id="language.oop5.traits.precedence">
   <title>Precedence</title>
   <para>
    An inherited member from a base class is overridden by a member inserted
    by a Trait. The precedence order is that members from the current class
    override Trait methods, which in turn override inherited methods.
   </para>
   <example xml:id="language.oop5.traits.precedence.examples.ex1">
    <title>Precedence Order Example</title>
    <para>
     An inherited method from a base class is overridden by the
     method inserted into MyHelloWorld from the SayWorld Trait. The behavior is
     the same for methods defined in the MyHelloWorld class. The precedence order
     is that methods from the current class override Trait methods, which in
     turn override methods from the base class.
    </para>
    <programlisting role="php">
<![CDATA[
<?php
class Base {
    public function sayHello() {
        echo 'Hello ';
    }
}

trait SayWorld {
    public function sayHello() {
        parent::sayHello();
        echo 'World!';
    }
}

class MyHelloWorld extends Base {
    use SayWorld;
}

$o = new MyHelloWorld();
$o->sayHello();
?>
]]>
    </programlisting>
    &example.outputs;
    <screen>
<![CDATA[
Hello World!
]]>
    </screen>
   </example>
   <example xml:id="language.oop5.traits.precedence.examples.ex2">
    <title>Alternate Precedence Order Example</title>
    <programlisting role="php">
<![CDATA[
<?php
trait HelloWorld {
    public function sayHello() {
        echo 'Hello World!';
    }
}

class TheWorldIsNotEnough {
    use HelloWorld;
    public function sayHello() {
        echo 'Hello Universe!';
    }
}

$o = new TheWorldIsNotEnough();
$o->sayHello();
?>
]]>
    </programlisting>
    &example.outputs;
    <screen>
<![CDATA[
Hello Universe!
]]>
    </screen>
   </example>
  </sect2>
  
  <sect2 xml:id="language.oop5.traits.multiple">
   <title>Multiple Traits</title>
   <para>
    Multiple Traits can be inserted into a class by listing them in the use
    statement, separated by commas.
   </para>
   <example xml:id="language.oop5.traits.multiple.ex1">
    <title>Multiple Traits Usage</title>
    <programlisting role="php">
<![CDATA[
<?php
trait Hello {
    public function sayHello() {
        echo 'Hello ';
    }
}

trait World {
    public function sayWorld() {
        echo 'World';
    }
}

class MyHelloWorld {
    use Hello, World;
    public function sayExclamationMark() {
        echo '!';
    }
}

$o = new MyHelloWorld();
$o->sayHello();
$o->sayWorld();
$o->sayExclamationMark();
?>
]]>
    </programlisting>
    &example.outputs;
    <screen>
<![CDATA[
Hello World!
]]>
    </screen>
   </example>
  </sect2>
  
  <sect2 xml:id="language.oop5.traits.conflict">
   <title>Conflict Resolution</title>
   <para>
    If two Traits insert a method with the same name, a fatal error is produced,
    if the conflict is not explicitly resolved.
   </para>
   <para>
    To resolve naming conflicts between Traits used in the same class,
    the <literal>insteadof</literal> operator needs to be used to chose exactly
    one of the conflicting methods.
   </para>
   <para>
    Since this only allows one to exclude methods, the <literal>as</literal>
    operator can be used to allow the inclusion of one of the conflicting
    methods under another name. 
   </para>
   <example xml:id="language.oop5.traits.conflict.ex1">
    <title>Conflict Resolution</title>
    <para>
      In this example, Talker uses the traits A and B.
      Since A and B have conflicting methods, it defines to use
      the variant of smallTalk from trait B, and the variant of bigTalk from
      trait A.
    </para>
    <para>
      The Aliased_Talker makes use of the <literal>as</literal> operator
      to be able to use B's bigTalk implementation under an additional alias
      <literal>talk</literal>.
    </para> 
    <programlisting role="php">
<![CDATA[
<?php
trait A {
    public function smallTalk() {
        echo 'a';
    }
    public function bigTalk() {
        echo 'A';
    }
}

trait B {
    public function smallTalk() {
        echo 'b';
    }
    public function bigTalk() {
        echo 'B';
    }
}

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;
    }
}
?>
]]>
    </programlisting>
   </example>
  </sect2>
  
  <sect2 xml:id="language.oop5.traits.visibility">
   <title>Changing Method Visibility</title>
   <para>
    Using the <literal>as</literal> syntax, one can also adjust the visibility
    of the method in the exhibiting class.
   </para>
   <example xml:id="language.oop5.traits.visibility.ex1">
    <title>Changing Method Visibility</title>
    <programlisting role="php">
<![CDATA[
<?php
trait HelloWorld {
    public function sayHello() {
        echo 'Hello World!';
    }
}

// Change visibility of sayHello
class MyClass1 {
    use HelloWorld { sayHello as protected; }
}

// Alias method with changed visibility
// sayHello visibility not changed
class MyClass2 {
    use HelloWorld { sayHello as private myPrivateHello; }
}
?>
]]>
    </programlisting>
   </example>
  </sect2>
  
  <sect2 xml:id="language.oop5.traits.composition">
   <title>Traits Composed from Traits</title>
   <para>
    Just as classes can make use of traits, so can other traits. By using one
    or more traits in a trait definition, it can be composed partially or
    entirely of the members defined in those other traits.
   </para>
   <example xml:id="language.oop5.traits.composition.ex1">
    <title>Traits Composed from Traits</title>
    <programlisting role="php">
<![CDATA[
<?php
trait Hello {
    public function sayHello() {
        echo 'Hello ';
    }
}

trait World {
    public function sayWorld() {
        echo 'World!';
    }
}

trait HelloWorld {
    use Hello, World;
}

class MyHelloWorld {
    use HelloWorld;
}

$o = new MyHelloWorld();
$o->sayHello();
$o->sayWorld();
?>
]]>
    </programlisting>
    &example.outputs;
    <screen>
<![CDATA[
Hello World!
]]>
    </screen>
   </example>
  </sect2>
  
  <sect2 xml:id="language.oop5.traits.abstract">
   <title>Abstract Trait Members</title>
   <para>
    Traits support the use of abstract methods in order to impose requirements
    upon the exhibiting class.
   </para>
   <example xml:id="language.oop5.traits.abstract.ex1">
    <title>Express Requirements by Abstract Methods</title>
    <programlisting role="php">
<![CDATA[
<?php
trait Hello {
    public function sayHelloWorld() {
        echo 'Hello'.$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;
    }
}
?>
]]>
    </programlisting>
   </example>
  </sect2>
  
  <sect2 xml:id="language.oop5.traits.static">
   <title>Static Trait Members</title>
   <para>
    Traits can define both static members and static methods.
   </para>
   <example xml:id="language.oop5.traits.static.ex1">
    <title>Static Variables</title>
    <programlisting role="php">
<![CDATA[
<?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
?>
]]>
    </programlisting>
   </example>
   <example xml:id="language.oop5.traits.static.ex2">
    <title>Static Methods</title>
    <programlisting role="php">
<![CDATA[
<?php
trait StaticExample {
    public static function doSomething() {
        return 'Doing something';
    }
}

class Example {
    use StaticExample;
}

Example::doSomething();
?>
]]>
    </programlisting>
   </example>
  </sect2>
  
  <sect2 xml:id="language.oop5.traits.properties">
   <title>Properties</title>
   <para>
    Traits can also define properties.
   </para>
   <example xml:id="language.oop5.traits.properties.example">
    <title>Defining Properties</title>
    <programlisting role="php">
<![CDATA[
<?php
trait PropertiesTrait {
    public $x = 1;
}

class PropertiesExample {
    use PropertiesTrait;
}

$example = new PropertiesExample;
$example->x;
?>
]]>
    </programlisting>
   </example>
   <para>
    If a trait defines a property then a class can not define a property with
    the same name, otherwise an error is issued. It is an
    <constant>E_STRICT</constant> if the class definition is compatible (same
    visibility and initial value) or fatal error otherwise.
   </para>
   <example xml:id="language.oop5.traits.properties.conflicts">
    <title>Conflict Resolution</title>
    <programlisting role="php">
<![CDATA[
<?php
trait PropertiesTrait {
    public $same = true;
    public $different = false;
}

class PropertiesExample {
    use PropertiesTrait;
    public $same = true; // Strict Standards
    public $different = true; // Fatal error
}
?>
]]>
    </programlisting>
   </example>
  </sect2>

 </sect1>
 
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