Learning PHP--New Traits

在阅读yii2源码的时候接触到了trait，就学习了一下，写下博客记录一下。

自 PHP 5.4.0 起，PHP 实现了代码复用的一个方法，称为 traits。

Traits 是一种为类似 PHP 的单继承语言而准备的代码复用机制。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方法集。Traits 和类组合的语义是定义了一种方式来减少复杂性，避免传统多继承和混入类（Mixin）相关的典型问题。

Trait 和一个类相似，但仅仅旨在用细粒度和一致的方式来组合功能。Trait 不能通过它自身来实例化。它为传统继承增加了水平特性的组合；也就是说，应用类的成员不需要继承。

Trait 示例

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

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

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

优先级

从基类继承的成员被 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。

优先顺序示例

<?<span>php
</span><span>class</span><span> Base {
    </span><span>public</span> <span>function</span><span> sayHello() {
        </span><span>echo</span> 'Hello '<span>;
    }
}

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

</span><span>class</span> MyHelloWorld <span>extends</span><span> Base {
    </span><span>use</span><span> SayWorld;
}

</span><span>$o</span> = <span>new</span><span> MyHelloWorld();
</span><span>$o</span>-><span>sayHello();
</span>?>

以上例程会输出：Hello World!

从基类继承的成员被插入的 SayWorld Trait 中的 sayHello 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。

另一个优先级顺序的例子

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

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

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

以上例程会输出：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。

多个 trait 的用法的例子

<?<span>php
trait Hello {
    </span><span>public</span> <span>function</span><span> sayHello() {
        </span><span>echo</span> 'Hello '<span>;
    }
}

trait World {
    </span><span>public</span> <span>function</span><span> sayWorld() {
        </span><span>echo</span> 'World'<span>;
    }
}

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

</span><span>$o</span> = <span>new</span><span> MyHelloWorld();
</span><span>$o</span>-><span>sayHello();
</span><span>$o</span>-><span>sayWorld();
</span><span>$o</span>-><span>sayExclamationMark();
</span>?>

以上例程会输出：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 操作符可以将其中一个冲突的方法以另一个名称来引入。

冲突解决的例子

<?<span>php
trait A {
    </span><span>public</span> <span>function</span><span> smallTalk() {
        </span><span>echo</span> 'a'<span>;
    }
    </span><span>public</span> <span>function</span><span> bigTalk() {
        </span><span>echo</span> 'A'<span>;
    }
}

trait B {
    </span><span>public</span> <span>function</span><span> smallTalk() {
        </span><span>echo</span> 'b'<span>;
    }
    </span><span>public</span> <span>function</span><span> bigTalk() {
        </span><span>echo</span> 'B'<span>;
    }
}

</span><span>class</span><span> Talker {
    </span><span>use</span> A,<span> B {
        B</span>::<span>smallTalk insteadof A;
        A</span>::<span>bigTalk insteadof B;
    }
}

</span><span>class</span><span> Aliased_Talker {
    </span><span>use</span> A,<span> B {
        B</span>::<span>smallTalk insteadof A;
        A</span>::<span>bigTalk insteadof B;
        B</span>::bigTalk <span>as</span><span> talk;
    }
}
</span>?>

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。

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。

修改方法的访问控制的例子

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

</span><span>//</span><span> 修改 sayHello 的访问控制</span>
<span>class</span><span> MyClass1 {
    </span><span>use</span> HelloWorld { sayHello <span>as</span> <span>protected</span><span>; }
}

</span><span>//</span><span> 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</span>
<span>class</span><span> MyClass2 {
    </span><span>use</span> HelloWorld { sayHello <span>as</span> <span>private</span><span> myPrivateHello; }
}
</span>?>

从 trait 来组成 trait

正如类能够使用 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，它能够组合其它 trait 中的部分或全部成员。

从 trait 来组成 trait的例子

<?<span>php
trait Hello {
    </span><span>public</span> <span>function</span><span> sayHello() {
        </span><span>echo</span> 'Hello '<span>;
    }
}

trait World {
    </span><span>public</span> <span>function</span><span> sayWorld() {
        </span><span>echo</span> 'World!'<span>;
    }
}

trait HelloWorld {
    </span><span>use</span> Hello,<span> World;
}

</span><span>class</span><span> MyHelloWorld {
    </span><span>use</span><span> HelloWorld;
}

</span><span>$o</span> = <span>new</span><span> MyHelloWorld();
</span><span>$o</span>-><span>sayHello();
</span><span>$o</span>-><span>sayWorld();
</span>?>

以上例程会输出：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。

表示通过抽象方法来进行强制要求的例子

<?<span>php
trait Hello {
    </span><span>public</span> <span>function</span><span> sayHelloWorld() {
        </span><span>echo</span> 'Hello'.<span>$this</span>-><span>getWorld();
    }
    </span><span>abstract</span> <span>public</span> <span>function</span><span> getWorld();
}

</span><span>class</span><span> MyHelloWorld {
    </span><span>private</span> <span>$world</span><span>;
    </span><span>use</span><span> Hello;
    </span><span>public</span> <span>function</span><span> getWorld() {
        </span><span>return</span> <span>$this</span>-><span>world;
    }
    </span><span>public</span> <span>function</span> setWorld(<span>$val</span><span>) {
        </span><span>$this</span>->world = <span>$val</span><span>;
    }
}
</span>?>

Trait 的静态成员

Traits 可以被静态成员静态方法定义。

静态变量的例子

<?<span>php
trait Counter {
    </span><span>public</span> <span>function</span><span> inc() {
        </span><span>static</span> <span>$c</span> = 0<span>;
        </span><span>$c</span> = <span>$c</span> + 1<span>;
        </span><span>echo</span> "<span>$c</span>\n"<span>;
    }
}

</span><span>class</span><span> C1 {
    </span><span>use</span><span> Counter;
}

</span><span>class</span><span> C2 {
    </span><span>use</span><span> Counter;
}

</span><span>$o</span> = <span>new</span> C1(); <span>$o</span>->inc(); <span>//</span><span> echo 1</span>
<span>$p</span> = <span>new</span> C2(); <span>$p</span>->inc(); <span>//</span><span> echo 1</span>
?>

静态方法的例子

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

</span><span>class</span><span> Example {
    </span><span>use</span><span> StaticExample;
}

Example</span>::<span>doSomething();
</span>?>

静态变量和静态方法的例子

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

</span><span>class</span><span> C1 {
    </span><span>use</span><span> Counter;
}

</span><span>class</span><span> C2 {
    </span><span>use</span><span> Counter;
}

C1</span>::inc(); <span>//</span><span> echo 1</span>
C2::inc(); <span>//</span><span> echo 1</span>
?>

属性

Trait 同样可以定义属性。

定义属性的例子

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

</span><span>class</span><span> PropertiesExample {
    </span><span>use</span><span> PropertiesTrait;
}

</span><span>$example</span> = <span>new</span><span> PropertiesExample;
</span><span>$example</span>-><span>x;
</span>?>

如果 trait 定义了一个属性，那类将不能定义同样名称的属性，否则会产生一个错误。如果该属性在类中的定义与在 trait 中的定义兼容（同样的可见性和初始值）则错误的级别是 E_STRICT，否则是一个致命错误。

冲突的例子

<?<span>php
trait PropertiesTrait {
    </span><span>public</span> <span>$same</span> = <span>true</span><span>;
    </span><span>public</span> <span>$different</span> = <span>false</span><span>;
}

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

Use的不同

不同use的例子

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

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

第一个use是用于 namespace 的 use Foo\Test，找到的是 \Foo\Test，第二个 use 是使用一个trait，找到的是\Foo\Bar\Foo\Test。

__CLASS__和__TRAIT__

__CLASS__ 返回 use trait 的 class name，__TRAIT__返回 trait name

示例如下

<?<span>php
trait TestTrait {
    </span><span>public</span> <span>function</span><span> testMethod() {
        </span><span>echo</span> "Class: " . <span>__CLASS__</span> . <span>PHP_EOL</span><span>;
        </span><span>echo</span> "Trait: " . __TRAIT__ . <span>PHP_EOL</span><span>;
    }
}

</span><span>class</span><span> BaseClass {
    </span><span>use</span><span> TestTrait;
}

</span><span>class</span> TestClass <span>extends</span><span> BaseClass {

}

</span><span>$t</span> = <span>new</span><span> TestClass();
</span><span>$t</span>-><span>testMethod();

</span><span>//</span><span>Class: BaseClass
//Trait: TestTrait</span>

Trait单例

实例如下

<?<span>php

trait singleton {    
    </span><span>/*</span><span>*
     * private construct, generally defined by using class
     </span><span>*/</span>
    <span>//</span><span>private function __construct() {}</span>
    
    <span>public</span> <span>static</span> <span>function</span><span> getInstance() {
        </span><span>static</span> <span>$_instance</span> = <span>NULL</span><span>;
        </span><span>$class</span> = <span>__CLASS__</span><span>;
        </span><span>return</span> <span>$_instance</span> ?: <span>$_instance</span> = <span>new</span> <span>$class</span><span>;
    }
    
    </span><span>public</span> <span>function</span><span> __clone() {
        </span><span>trigger_error</span>('Cloning '.<span>__CLASS__</span>.' is not allowed.',<span>E_USER_ERROR</span><span>);
    }
    
    </span><span>public</span> <span>function</span><span> __wakeup() {
        </span><span>trigger_error</span>('Unserializing '.<span>__CLASS__</span>.' is not allowed.',<span>E_USER_ERROR</span><span>);
    }
}

</span><span>/*</span><span>*
* Example Usage
</span><span>*/</span>

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

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

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

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

调用trait方法

虽然不很明显，但是如果Trait的方法可以被定义为在普通类的静态方法，就可以被调用

实例如下

<?<span>php 
trait Foo { 
    </span><span>function</span><span> bar() { 
        </span><span>return</span> 'baz'<span>; 
    } 
} 

</span><span>echo</span> Foo::bar(),"\\n"<span>; 
</span>?>

以上就介绍了PHP的学习--Traits新特性，包括了方面的内容，希望对PHP教程有兴趣的朋友有所帮助。