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
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<?<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>$c</span>
"<span>;
}
}
</span><span>class</span><span> C1 {
</span><span>use</span><span> Counter;
}
</span><span>class</span><span> C2 {
</span><span>use</span><span> Counter;
}
</span><span>$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>$c</span> . "
"<span>;
}
}
</span><span>class</span><span> C1 {
</span><span>use</span><span> Counter;
}
</span><span>class</span><span> C2 {
</span><span>use</span><span> Counter;
}
C1</span>::inc(); <span>//</span><span> echo 1</span>
C2::inc(); <span>//</span><span> echo 1</span>
?>
属性
Trait 同样可以定义属性。
定义属性的例子
<?<span>php
trait PropertiesTrait {
</span><span>public</span> <span>$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 FooBar;
</span><span>use</span> FooTest; <span>//</span><span> means FooTest - the initial is optional</span>
?>
<?<span>php
namespace FooBar;
</span><span>class</span><span> SomeClass {
</span><span>use</span> FooTest; <span>//</span><span> means FooBarFooTest</span>
<span>}
</span>?>
第一个use是用于 namespace 的 use FooTest，找到的是 FooTest，第二个 use 是使用一个trait，找到的是FooBarFooTest。
__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。
