PHP: Hypertext Preprocessor

Trait

PHP 实现了一种代码复用的方法，称为 trait。

Trait 是为类似 PHP 的单继承语言而准备的一种代码复用机制。Trait 为了减少单继承语言的限制，使开发人员能够自由地在不同层次结构内独立的类中复用 method。Trait 和 Class 组合的语义定义了一种减少复杂性的方式，避免传统多继承和 Mixin 类相关典型问题。

Trait 和 Class 相似，但仅仅旨在用细粒度和一致的方式来组合功能。无法通过 trait 自身来实例化。它为传统继承增加了水平特性的组合；也就是说，应用的几个 Class 之间不需要继承。

示例 #1 Trait 示例

<?php
trait ezcReflectionReturnInfo {
    function getReturnType() { /*1*/ }
    function getReturnDescription() { /*2*/ }
}

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

class ezcReflectionFunction extends ReflectionFunction {
    use ezcReflectionReturnInfo;
    /* ... */
}
?>

优先级

从基类继承的成员会被 trait 插入的成员所覆盖。优先顺序是来自当前类的成员覆盖了 trait 的方法，而 trait 则覆盖了被继承的方法。

示例 #2 优先顺序示例

从基类继承的成员被插入的 SayWorld Trait 中的 MyHelloWorld 方法所覆盖。其行为 MyHelloWorld 类中定义的方法一致。优先顺序是当前类中的方法会覆盖 trait 方法，而 trait 方法又覆盖了基类中的方法。

<?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();
?>

以上示例会输出：

Hello World!

示例 #3 另一个优先级顺序的例子

<?php
trait HelloWorld {
    public function sayHello() {
        echo 'Hello World!';
    }
}

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

$o = new TheWorldIsNotEnough();
$o->sayHello();
?>

以上示例会输出：

Hello Universe!

多个 trait

通过逗号分隔，在 use 声明列出多个 trait，可以都插入到一个类中。

示例 #4 多个 trait 的用法

<?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();
?>

以上示例会输出：

Hello World!

冲突的解决

如果两个 trait 都插入了一个同名的方法，如果没有明确解决冲突将会产生一个致命错误。

为了解决多个 trait 在同一个类中的命名冲突，需要使用 insteadof 操作符来明确指定使用冲突方法中的哪一个。

以上方式仅允许排除掉其它方法，as 操作符可以为某个方法引入别名。注意，as 操作符不会对方法进行重命名，也不会影响其方法。

示例 #5 冲突的解决

在本例中 Talker 使用了 trait A 和 B。由于 A 和 B 有冲突的方法，其定义了使用 trait B 中的 smallTalk 以及 trait A 中的 bigTalk。

Aliased_Talker 使用了 as 操作符来定义了 talk 来作为 B 的 bigTalk 的别名。

<?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;
    }
}
?>

修改方法的访问控制

使用 as 语法还可以用来调整方法的访问控制。

示例 #6 修改方法的访问控制

<?php
trait HelloWorld {
    public function sayHello() {
        echo 'Hello World!';
    }
}

// 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; }
}
?>

从 trait 来组成 trait

正如 class 能够使用 trait 一样，其它 trait 也能够使用 trait。在 trait 定义时通过使用一个或多个 trait，能够组合其它 trait 中的部分或全部成员。

示例 #7 从 trait 来组成 trait

<?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();
?>

以上示例会输出：

Hello World!

Trait 的抽象成员

为了对使用的类施加强制要求，trait 支持抽象方法的使用。支持 public 、protected 和 private 方法。PHP 8.0.0 之前，仅支持 public 和 protected 抽象方法。

警告

自 PHP 8.0.0 起，具体方法的签名必须遵循签名兼容性规则。以前，其签名可能有所不同。

示例 #8 表示通过抽象方法来进行强制要求

<?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;
    }
}
?>

Trait 的静态成员

Traits 可以定义静态变量、静态方法和静态属性。

注意:
自 PHP 8.1.0 起，弃用直接在 trait 上调用静态方法或者访问静态属性。静态方法和属性应该仅在使用了 trait 的 class 中访问。

示例 #9 静态变量

<?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
?>

示例 #10 静态方法

<?php
trait StaticExample {
    public static function doSomething() {
        return 'Doing something';
    }
}

class Example {
    use StaticExample;
}

Example::doSomething();
?>

示例 #11 静态属性

<?php
trait StaticExample {
    public static $static = 'foo';
}

class Example {
    use StaticExample;
}

echo Example::$static;
?>

属性

Trait 同样可以定义属性。

示例 #12 定义属性

<?php
trait PropertiesTrait {
    public $x = 1;
}

class PropertiesExample {
    use PropertiesTrait;
}

$example = new PropertiesExample;
$example->x;
?>

Trait 定义了一个属性后，类就不能定义同样名称的属性，否则会产生 fatal error。有种情况例外：属性是兼容的（同样的访问可见度、类型、readonly 修饰符和初始默认值）。

示例 #13 解决冲突

<?php
trait PropertiesTrait {
    public $same = true;
    public $different1 = false;
    public bool $different2;
    public bool $different3;
}

class PropertiesExample {
    use PropertiesTrait;
    public $same = true;
    public $different1 = true; // Fatal error
    public string $different2; // Fatal error
    readonly protected bool $different3; // Fatal error
}
?>

常量

自 PHP 8.2.0 起，trait 也可以定义常量。

示例 #14 定义常量

<?php
trait ConstantsTrait {
    public const FLAG_MUTABLE = 1;
    final public const FLAG_IMMUTABLE = 5;
}

class ConstantsExample {
    use ConstantsTrait;
}

$example = new ConstantsExample;
echo $example::FLAG_MUTABLE; // 1
?>

如果 trait 定义了常量，然后类不能定义相同名称的常量，除非两者兼容（相同的可见性、初始化值和 final），否则会发出 fatal error。

示例 #15 解决冲突

<?php
trait ConstantsTrait {
    public const FLAG_MUTABLE = 1;
    final public const FLAG_IMMUTABLE = 5;
}

class ConstantsExample {
    use ConstantsTrait;
    public const FLAG_IMMUTABLE = 5; // Fatal error
}
?>

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用户贡献的备注 25 notes

down

656

Safak Ozpinar / safakozpinar at gmail ¶

12 years ago

Unlike inheritance; if a trait has static properties, each class using that trait has independent instances of those properties.

Example using parent class:
<?php
class TestClass {
    public static $_bar;
}
class Foo1 extends TestClass { }
class Foo2 extends TestClass { }
Foo1::$_bar = 'Hello';
Foo2::$_bar = 'World';
echo Foo1::$_bar . ' ' . Foo2::$_bar; // Prints: World World
?>

Example using trait:
<?php
trait TestTrait {
    public static $_bar;
}
class Foo1 {
    use TestTrait;
}
class Foo2 {
    use TestTrait;
}
Foo1::$_bar = 'Hello';
Foo2::$_bar = 'World';
echo Foo1::$_bar . ' ' . Foo2::$_bar; // Prints: Hello World
?>

down

454

greywire at gmail dot com ¶

12 years ago

The best way to understand what traits are and how to use them is to look at them for what they essentially are:  language assisted copy and paste.

If you can copy and paste the code from one class to another (and we've all done this, even though we try not to because its code duplication) then you have a candidate for a trait.

down

248

Stefan W ¶

11 years ago

Note that the "use" operator for traits (inside a class) and the "use" operator for namespaces (outside the class) resolve names differently. "use" for namespaces always sees its arguments as absolute (starting at the global namespace):

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

On the other hand, "use" for traits respects the current namespace:

<?php
namespace Foo\Bar;
class SomeClass {
    use Foo\Test;   // means \Foo\Bar\Foo\Test
}
?>

Together with "use" for closures, there are now three different "use" operators. They all mean different things and behave differently.

down

JustAddingSomeAdditionalUseCase ¶

1 year ago

I have not seen this specific use case:

"Wanting to preserve action of parent class method, the trait one calling ::parent & also the child class mehod action".

// Child class.
use SuperTrait {
  initialize as initializeOr;
}
public function initialize(array &$element) {
  ...
  $this->initializeOr($element);
}
// Trait.
public function initialize(array &$element) {
  ...
  parent::initialize($element);
}
// Parent class.
public function initialize(array &$element) {
  ...
}

down

105

t8 at AT pobox dot com ¶

12 years ago

Another difference with traits vs inheritance is that methods defined in traits can access methods and properties of the class they're used in, including private ones.

For example:
<?php
trait MyTrait
{
  protected function accessVar()
  {
    return $this->var;
  }

}

class TraitUser
{
  use MyTrait;

  private $var = 'var';

  public function getVar()
  {
    return $this->accessVar();
  }
}

$t = new TraitUser();
echo $t->getVar(); // -> 'var'                                                                                                                                                                                                                          

?>

down

chris dot rutledge at gmail dot com ¶

12 years ago

It may be worth noting here that the magic constant __CLASS__ becomes even more magical - __CLASS__ will return the name of the class in which the trait is being used.

for example

<?php
trait sayWhere {
    public function whereAmI() {
        echo __CLASS__;
    }
}

class Hello {
    use sayWHere;
}

class World {
    use sayWHere;
}

$a = new Hello;
$a->whereAmI(); //Hello

$b = new World;
$b->whereAmI(); //World
?>

The magic constant __TRAIT__ will giev you the name of the trait

down

qeremy (!) gmail ¶

9 years ago

Keep in mind; "final" keyword is useless in traits when directly using them, unlike extending classes / abstract classes.

<?php
trait Foo {
    final public function hello($s) { print "$s, hello!"; }
}
class Bar {
    use Foo;
    // Overwrite, no error
    final public function hello($s) { print "hello, $s!"; }
}

abstract class Foo {
    final public function hello($s) { print "$s, hello!"; }
}
class Bar extends Foo {
    // Fatal error: Cannot override final method Foo::hello() in ..
    final public function hello($s) { print "hello, $s!"; }
}
?>

But this way will finalize trait methods as expected;

<?php
trait FooTrait {
    final public function hello($s) { print "$s, hello!"; }
}
abstract class Foo {
    use FooTrait;
}
class Bar extends Foo {
    // Fatal error: Cannot override final method Foo::hello() in ..
    final public function hello($s) { print "hello, $s!"; }
}
?>

down

yeu_ym at yahoo dot com ¶

5 years ago

Here is an example how to work with visiblity and conflicts.

<?php

trait A
{
    private function smallTalk()
    {
        echo 'a';
    }

    private function bigTalk()
    {
        echo 'A';
    }
}

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

    private function bigTalk()
    {
        echo 'B';
    }
}

trait C
{
    public function smallTalk()
    {
        echo 'c';
    }

    public function bigTalk()
    {
        echo 'C';
    }
}

class Talker
{
    use A, B, C {
        //visibility for methods that will be involved in conflict resolution
        B::smallTalk as public;
        A::bigTalk as public;

        //conflict resolution
        B::smallTalk insteadof A, C;
        A::bigTalk insteadof B, C;

        //aliases with visibility change
        B::bigTalk as public Btalk;
        A::smallTalk as public asmalltalk;
        
        //aliases only, methods already defined as public
        C::bigTalk as Ctalk;
        C::smallTalk as cmallstalk;
    }

}

(new Talker)->bigTalk();//A
(new Talker)->Btalk();//B
(new Talker)->Ctalk();//C

(new Talker)->asmalltalk();//a
(new Talker)->smallTalk();//b
(new Talker)->cmallstalk();//c

down

canufrank ¶

8 years ago

A number of the notes make incorrect assertions about trait behaviour because they do not extend the class.

So, while "Unlike inheritance; if a trait has static properties, each class using that trait has independent instances of those properties.

Example using parent class:
<?php
class TestClass {
    public static $_bar;
}
class Foo1 extends TestClass { }
class Foo2 extends TestClass { }
Foo1::$_bar = 'Hello';
Foo2::$_bar = 'World';
echo Foo1::$_bar . ' ' . Foo2::$_bar; // Prints: World World
?>

Example using trait:
<?php
trait TestTrait {
    public static $_bar;
}
class Foo1 {
    use TestTrait;
}
class Foo2 {
    use TestTrait;
}
Foo1::$_bar = 'Hello';
Foo2::$_bar = 'World';
echo Foo1::$_bar . ' ' . Foo2::$_bar; // Prints: Hello World
?>"

shows a correct example, simply adding
<?php
require_once('above');
class Foo3 extends Foo2 {
}
Foo3::$_bar = 'news';
echo Foo1::$_bar . ' ' . Foo2::$_bar . ' ' . Foo3::$_bar; 

// Prints: Hello news news

I think the best conceptual model of an incorporated trait is an advanced insertion of text, or as someone put it "language assisted copy and paste." If Foo1 and Foo2 were defined with $_bar, you would not expect them to share the instance. Similarly, you would expect Foo3 to share with Foo2, and it does.

Viewing this way explains away a lot of  the 'quirks' that are observed above with final, or subsequently declared private vars,

down

rawsrc ¶

6 years ago

About the (Safak Ozpinar / safakozpinar at gmail)'s great note, you can still have the same behavior than inheritance using trait with this approach : 
<?php

trait TestTrait {
    public static $_bar;
}

class FooBar {
    use TestTrait;
}

class Foo1 extends FooBar {

}
class Foo2 extends FooBar {

}
Foo1::$_bar = 'Hello';
Foo2::$_bar = 'World';
echo Foo1::$_bar . ' ' . Foo2::$_bar; // Prints: World World

down

balbuf ¶

8 years ago

(It's already been said, but for the sake of searching on the word "relative"...)

The "use" keyword to import a trait into a class will resolve relative to the current namespace and therefore should include a leading slash to represent a full path, whereas "use" at the namespace level is always absolute.

down

marko at newvibrations dot net ¶

8 years ago

As already noted, static properties and methods in trait could be accessed directly using trait. Since trait is language assisted c/p, you should be aware that static property from trait will be initialized to the value trait property had in the time of class declaration. 

Example:

<?php

trait Beer {
    protected static $type = 'Light';
    public static function printed(){
        echo static::$type.PHP_EOL;
    }
    public static function setType($type){
        static::$type = $type;
    }
}

class Ale {
    use Beer;
}

Beer::setType("Dark");

class Lager {
    use Beer;
}

Beer::setType("Amber");

header("Content-type: text/plain");

Beer::printed();  // Prints: Amber
Ale::printed();   // Prints: Light
Lager::printed(); // Prints: Dark

?>

down

qschuler at neosyne dot com ¶

10 years ago

Note that you can omit a method's inclusion by excluding it from one trait in favor of the other and doing the exact same thing in the reverse way.

<?php

trait A {
    public function sayHello()
    {
        echo 'Hello from A';
    }

    public function sayWorld()
    {
        echo 'World from A';
    }
}

trait B {
    public function sayHello()
    {
        echo 'Hello from B';
    }

    public function sayWorld()
    {
        echo 'World from B';
    }
}

class Talker {
    use A, B {
        A::sayHello insteadof B;
        A::sayWorld insteadof B;
        B::sayWorld insteadof A;
    }
}

$talker = new Talker();
$talker->sayHello();
$talker->sayWorld();

?>

The method sayHello is imported, but the method sayWorld is simply excluded.

down

Edward ¶

12 years ago

The difference between Traits and multiple inheritance is in the inheritance part.   A trait is not inherited from, but rather included or mixed-in, thus becoming part of "this class".   Traits also provide a more controlled means of resolving conflicts that inevitably arise when using multiple inheritance in the few languages that support them (C++).  Most modern languages are going the approach of a "traits" or "mixin" style system as opposed to multiple-inheritance, largely due to the ability to control ambiguities if a method is declared in multiple "mixed-in" classes.

Also, one can not "inherit" static member functions in multiple-inheritance.

down

ryan at derokorian dot com ¶

12 years ago

Simple singleton trait.

<?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('Cloning '.__CLASS__.' is not allowed.',E_USER_ERROR);
    }
    
    public function __wakeup() {
        trigger_error('Unserializing '.__CLASS__.' is not allowed.',E_USER_ERROR);
    }
}

/**
 * Example Usage
 */

class foo {
    use singleton;
    
    private function __construct() {
        $this->name = 'foo';
    }
}

class bar {
    use singleton;
    
    private function __construct() {
        $this->name = 'bar';
    }
}

$foo = foo::getInstance();
echo $foo->name;

$bar = bar::getInstance();
echo $bar->name;

down

katrinaelaine6 at gmail dot com ¶

7 years ago

Adding to "atorich at gmail dot com":

The behavior of the magic constant __CLASS__ when used in traits is as expected if you understand traits and late static binding (http://php.net/manual/en/language.oop5.late-static-bindings.php).

<?php

$format = 'Class: %-13s | get_class(): %-13s | get_called_class(): %-13s%s';

trait TestTrait {
    public function testMethod() {
        global $format;
        printf($format, __CLASS__, get_class(), get_called_class(), PHP_EOL);
    }
    
    public static function testStatic() {
        global $format;
        printf($format, __CLASS__, get_class(), get_called_class(), PHP_EOL);
    }
}

trait DuplicateTrait {
    public function duplMethod() {
        global $format;
        printf($format, __CLASS__, get_class(), get_called_class(), PHP_EOL);
    }
    
    public static function duplStatic() {
        global $format;
        printf($format, __CLASS__, get_class(), get_called_class(), PHP_EOL);
    }
}

abstract class AbstractClass {
    
    use DuplicateTrait;
    
    public function absMethod() {
        global $format;
        printf($format, __CLASS__, get_class(), get_called_class(), PHP_EOL);
    }
    
    public static function absStatic() {
        global $format;
        printf($format, __CLASS__, get_class(), get_called_class(), PHP_EOL);
    }
}

class BaseClass extends AbstractClass {
    use TestTrait;
}

class TestClass extends BaseClass { }

$t = new TestClass();

$t->testMethod();
TestClass::testStatic();

$t->absMethod();
TestClass::absStatic();

$t->duplMethod();
TestClass::duplStatic();

?>

Will output:

Class: BaseClass     | get_class(): BaseClass     | get_called_class(): TestClass    
Class: BaseClass     | get_class(): BaseClass     | get_called_class(): TestClass    
Class: AbstractClass | get_class(): AbstractClass | get_called_class(): TestClass    
Class: AbstractClass | get_class(): AbstractClass | get_called_class(): TestClass    
Class: AbstractClass | get_class(): AbstractClass | get_called_class(): TestClass    
Class: AbstractClass | get_class(): AbstractClass | get_called_class(): TestClass

Since Traits are considered literal "copying/pasting" of code, it's clear how the methods defined in DuplicateTrait give the same results as the methods defined in AbstractClass.

down

D. Marti ¶

12 years ago

Traits are useful for strategies, when you want the same data to be handled (filtered, sorted, etc) differently.

For example, you have a list of products that you want to filter out based on some criteria (brands, specs, whatever), or sorted by different means (price, label, whatever). You can create a sorting trait that contains different functions for different sorting types (numeric, string, date, etc). You can then use this trait not only in your product class (as given in the example), but also in other classes that need similar strategies (to apply a numeric sort to some data, etc).

<?php
trait SortStrategy {
    private $sort_field = null;
    private function string_asc($item1, $item2) {
        return strnatcmp($item1[$this->sort_field], $item2[$this->sort_field]);
    }
    private function string_desc($item1, $item2) {
        return strnatcmp($item2[$this->sort_field], $item1[$this->sort_field]);
    }
    private function num_asc($item1, $item2) {
        if ($item1[$this->sort_field] == $item2[$this->sort_field]) return 0;
        return ($item1[$this->sort_field] < $item2[$this->sort_field] ? -1 : 1 );
    }
    private function num_desc($item1, $item2) {
        if ($item1[$this->sort_field] == $item2[$this->sort_field]) return 0;
        return ($item1[$this->sort_field] > $item2[$this->sort_field] ? -1 : 1 );
    }
    private function date_asc($item1, $item2) {
        $date1 = intval(str_replace('-', '', $item1[$this->sort_field]));
        $date2 = intval(str_replace('-', '', $item2[$this->sort_field]));
        if ($date1 == $date2) return 0;
        return ($date1 < $date2 ? -1 : 1 );
    }
    private function date_desc($item1, $item2) {
        $date1 = intval(str_replace('-', '', $item1[$this->sort_field]));
        $date2 = intval(str_replace('-', '', $item2[$this->sort_field]));
        if ($date1 == $date2) return 0;
        return ($date1 > $date2 ? -1 : 1 );
    }
}

class Product {
    public $data = array();
    
    use SortStrategy;
    
    public function get() {
        // do something to get the data, for this ex. I just included an array
        $this->data = array(
            101222 => array('label' => 'Awesome product', 'price' => 10.50, 'date_added' => '2012-02-01'),
            101232 => array('label' => 'Not so awesome product', 'price' => 5.20, 'date_added' => '2012-03-20'),
            101241 => array('label' => 'Pretty neat product', 'price' => 9.65, 'date_added' => '2012-04-15'),
            101256 => array('label' => 'Freakishly cool product', 'price' => 12.55, 'date_added' => '2012-01-11'),
            101219 => array('label' => 'Meh product', 'price' => 3.69, 'date_added' => '2012-06-11'),
        );
    }
    
    public function sort_by($by = 'price', $type = 'asc') {
        if (!preg_match('/^(asc|desc)$/', $type)) $type = 'asc';
        switch ($by) {
            case 'name':
                $this->sort_field = 'label';
                uasort($this->data, array('Product', 'string_'.$type));
            break;
            case 'date':
                $this->sort_field = 'date_added';
                uasort($this->data, array('Product', 'date_'.$type));
            break;
            default:
                $this->sort_field = 'price';
                uasort($this->data, array('Product', 'num_'.$type));
        }
    }
}

$product = new Product();
$product->get();
$product->sort_by('name');
echo '<pre>'.print_r($product->data, true).'</pre>';
?>

down

bscheshirwork at gmail dot com ¶

7 years ago

https://3v4l.org/mFuQE

1. no deprecate if same-class-named method get from trait
2. replace same-named method ba to aa in C

trait ATrait {
    public function a(){
        return 'Aa';
    }
}

trait BTrait {
    public function a(){
        return 'Ba';
    }
}

class C {
    use ATrait{
        a as aa;
    }
    use BTrait{
        a as ba;
    }
    
    public function a() {
        return static::aa() . static::ba();
    }
}

$o = new C;
echo $o->a(), "\n";

class D {
    use ATrait{
        ATrait::a as aa;
    }
    use BTrait{
        BTrait::a as ba;
    }
    
    public function a() {
        return static::aa() . static::ba();
    }
}

$o = new D;
echo $o->a(), "\n";

class E {
    use ATrait{
        ATrait::a as aa;
        ATrait::a insteadof BTrait;
    }
    use BTrait{
        BTrait::a as ba;
    }
    
    public function e() {
        return static::aa() . static::ba();
    }
}

$o = new E;
echo $o->e(), "\n";

class F {
    use ATrait{
        a as aa;
    }
    use BTrait{
        a as ba;
    }
    
    public function f() {
        return static::aa() . static::ba();
    }
}

$o = new F;
echo $o->f(), "\n";

AaAa 
AaBa 

Deprecated: Methods with the same name as their class will not be constructors in a future version of PHP; E has a deprecated constructor in /in/mFuQE on line 48 
AaBa 

Fatal error: Trait method a has not been applied, because there are collisions with other trait methods on F in /in/mFuQE on line 65

down

Kristof ¶

10 years ago

don't forget you can create complex (embedded) traits as well

<?php
trait Name {
  // ...
}
trait Address {
  // ...
}
trait Telephone {
  // ...
}
trait Contact {
  use Name, Address, Telephone;
}
class Customer {
  use Contact;
}
class Invoce {
  use Contact;
}
?>

down

Carlos Alberto Bertholdo Carucce ¶

8 years ago

If you want to resolve name conflicts and also change the visibility of a trait method, you'll need to declare both in the same line:

trait testTrait{
    
    public function test(){
        echo 'trait test';
    }
    
}

class myClass{
    
    use testTrait {
        testTrait::test as private testTraitF;
    }
    
    public function test(){
        echo 'class test';
        echo '<br/>';
        $this->testTraitF();
    }
    
}

$obj = new myClass(); 
$obj->test(); //prints both 'trait test' and 'class test'
$obj->testTraitF(); //The method is not accessible (Fatal error: Call to private method myClass::testTraitF() )

down

Oddant ¶

11 years ago

I think it's obvious to notice that using 'use' followed by the traits name must be seen as just copying/pasting lines of code into the place where they are used.

down

guidobelluomo at gmail dot com ¶

4 years ago

If you override a method which was defined by a trait, calling the parent method will also call the trait's override. Therefore if you need to derive from a class which has a trait, you can extend the class without losing the trait's functionality:

<?php

trait ExampleTrait
{
    public function output()
    {
        parent::output();
        echo "bar<br>";
    }
}

class Foo
{
    public function output()
    {
        echo "foo<br>";
    }
}

class FooBar extends Foo
{
    use ExampleTrait;
}

class FooBarBaz extends FooBar
{
    use ExampleTrait;
    public function output()
    {
        parent::output();
        echo "baz";
    }
}

(new FooBarBaz())->output();
?>

Output:
foo
bar
baz

down

84td84 at gmail dot com ¶

9 years ago

A note to 'Beispiel #9 Statische Variablen'. A trait can also have a static property:

trait Counter {
    static $trvar=1;

    public static function stfunc() {
        echo "Hello world!"
    }
}

class C1 {
    use Counter;
}

print "\nTRVAR: " . C1::$trvar . "\n";   //prints 1

$obj = new C1();
C1::stfunc();   //prints  Hello world!
$obj->stfunc();   //prints Hello world!

A static property (trvar) can only be accessed using the classname (C1).
But a static function (stfunc) can be accessed using the classname or the instance ($obj).

down

cody at codysnider dot com ¶

7 years ago

/*
DocBlocks pertaining to the class or trait will NOT be carried over when applying the trait.

Results trying a couple variations on classes with and without DocBlocks that use a trait with a DocBlock
*/

<?php

/**
 * @Entity
 */
trait Foo
{
    protected $foo;
}

/**
 * @HasLifecycleCallbacks
 */
class Bar
{
    use \Foo;
    
    protected $bar;
}

class MoreBar
{
    use \Foo;
    
    protected $moreBar;
}

$w = new \ReflectionClass('\Bar');
echo $w->getName() . ":\r\n";
echo $w->getDocComment() . "\r\n\r\n";

$x = new \ReflectionClass('\MoreBar');
echo $x->getName() . ":\r\n";
echo $x->getDocComment() . "\r\n\r\n";

$barObj = new \Bar();
$y = new \ReflectionClass($barObj);
echo $y->getName() . ":\r\n";
echo $y->getDocComment() . "\r\n\r\n";

foreach($y->getTraits() as $traitObj) {
    echo $y->getName() . " ";
    echo $traitObj->getName() . ":\r\n";
    echo $traitObj->getDocComment() . "\r\n";
}

$moreBarObj = new \MoreBar();
$z = new \ReflectionClass($moreBarObj);
echo $z->getName() . " ";
echo $z->getDocComment() . "\r\n\r\n";

foreach($z->getTraits() as $traitObj) {
    echo $z->getName() . " ";
    echo $traitObj->getName() . ":\r\n";
    echo $traitObj->getDocComment() . "\r\n";
}

down

artur at webprojektant dot pl ¶

12 years ago

Trait can not have the same name as class because it will show: Fatal error: Cannot redeclare class