Detailed explanation and application of Trait in PHP

Starting from PHP version 5.4.0, PHP provides a new concept of code reuse, which is Trait. Trait literally means "characteristics" and "features". We can understand that using the Trait keyword can add new characteristics to classes in PHP.

Everyone who is familiar with object-oriented knows that there are two methods of code reuse commonly used in software development: inheritance and polymorphism. In PHP, only single inheritance can be achieved. Traits avoid this. The following is a comparative explanation through a simple example.

1. Inheritance VS Polymorphism VS Trait

Now there are two classes

Publish.php

and

Answer.php

. To add LOG function to it, record the actions inside the class. There are several options:

Inheritance

Polymorphism

Trait

1.1. Inheritance

such as Picture:



The code structure is as follows:

// Log.php
<?php
Class Log
{
    public function startLog()
    {
        // echo ...
    }
    public function endLog()
    {
        // echo ...
    }
}
// Publish.php
<?php
Class Publish extends Log
{
}
// Answer.php
<?php
Class Answer extends Log
{
}

You can see that inheritance does meet the requirements. But this violates the object-oriented principle. The relationship between operations such as Publish and Answer and Log is not the relationship between the subclass and the parent class. So it is not recommended to use it this way.

1.2. Polymorphism

As shown in the figure:



Implementation code:

// Log.php
<?php
Interface Log
{
    public function startLog();
    public function endLog();
}
// Publish.php
<?php
Class Publish implements Log
{
    public function startLog()
    {
        // TODO: Implement startLog() method.
    }
    public function endLog()
    {
        // TODO: Implement endLog() method.
    }
}
// Answer.php
<?php
Class Answer implements Log
{
    public function startLog()
    {
        // TODO: Implement startLog() method.
    }
    public function endLog()
    {
        // TODO: Implement endLog() method.
    }
}

The logging operations should be the same, so the logging implementation in the Publish and Answer actions is also the same. Obviously, this violates the DRY (Don’t Repeat Yourself) principle. Therefore, it is not recommended to implement it this way.

1.3. Trait

As shown:



The implementation code is as follows:

// Log.php
<?php
trait Log{
    public function startLog() {
        // echo ..
    }
    public function endLog() {
        // echo ..
    }
}
// Publish.php
<?php
class Publish {
    use Log;
}
$publish = new Publish();
$publish->startLog();
$publish->endLog();
// Answer.php
<?php
class Answer {
    use Log;
}
$answer = new Answer();
$answer->startLog();
$answer->endLog();

As you can see, we have achieved code reuse without increasing the complexity of the code.

1.4. Conclusion

Although the inheritance method can also solve the problem, its idea violates the object-oriented principle and seems very crude; the polymorphic method is also feasible, However, it does not comply with the DRY principle in software development and increases maintenance costs. The Trait method avoids the above shortcomings and achieves code reuse relatively elegantly.

2. The scope of Trait

After understanding the benefits of Trait, we also need to understand the rules in its implementation. Let’s talk about scope first. This is easier to prove. The implementation code is as follows:

<?php
class Publish {
    use Log;
    public function doPublish() {
        $this->publicF();
        $this->protectF();
        $this->privateF();
    }
}
$publish  = new Publish();
$publish->doPublish();
执行上述代码输出结果如下:
public function
protected function
private function

It can be found that the scope of Trait is visible inside the Trait class that references it. It can be understood that the use keyword copies the implementation code of the Trait into the class that references the Trait.

3. Priority of attributes in Trait

When it comes to priority, there must be a reference object for comparison. The reference object here is the class that references the Trait. and its parent class.

Use the following code to prove the priority of attributes in Trait applications:

<?php
trait Log
{
    public function publicF()
    {
        echo __METHOD__ . &#39; public function&#39; . PHP_EOL;
    }
    protected function protectF()
    {
        echo __METHOD__ . &#39; protected function&#39; . PHP_EOL;
    }
}
class Question
{
    public function publicF()
    {
        echo __METHOD__ . &#39; public function&#39; . PHP_EOL;
    }
    protected function protectF()
    {
        echo __METHOD__ . &#39; protected function&#39; . PHP_EOL;
    }
}
class Publish extends Question
{
    use Log;
    public function publicF()
    {
        echo __METHOD__ . &#39; public function&#39; . PHP_EOL;
    }
    public function doPublish()
    {
        $this->publicF();
        $this->protectF();
    }
}
$publish = new Publish();
$publish->doPublish();
上述代码的输出结果如下:
Publish::publicF public function
Log::protectF protected function

Through the above example, we can summarize the priorities in Trait applications as follows:

Members from the current class override the trait method

trait overrides the inherited method

The class member priority is:

当前类>Trait>父类

4. Insteadof and As keywords

In a class, you can reference multiple Traits, as follows:

<?php
trait Log
{
    public function startLog()
    {
        echo __METHOD__ . &#39; public function&#39; . PHP_EOL;
    }
    protected function endLog()
    {
        echo __METHOD__ . &#39; protected function&#39; . PHP_EOL;
    }
}
trait Check
{
    public function parameterCheck($parameters) {
        // do sth
    }
}
class Publish extends Question
{
    use Log,Check;
    public function doPublish($para) {
        $this->startLog();
        $this->parameterCheck($para);
        $this->endLog();
    }
}

Through the above method, we can reference multiple Traits in a class. When referencing multiple Traits, it is easy to cause problems. The most common problem is what to do if there are properties or methods with the same name in two Traits? At this time, you need to use

Insteadof

and

as

are the two keywords. Please see the following implementation code:

<?php
trait Log
{
    public function parameterCheck($parameters)
    {
        echo __METHOD__ . &#39; parameter check&#39; . $parameters . PHP_EOL;
    }
    public function startLog()
    {
        echo __METHOD__ . &#39; public function&#39; . PHP_EOL;
    }
}
trait Check
{
    public function parameterCheck($parameters)
    {
        echo __METHOD__ . &#39; parameter check&#39; . $parameters . PHP_EOL;
    }
    public function startLog()
    {
        echo __METHOD__ . &#39; public function&#39; . PHP_EOL;
    }
}
class Publish
{
    use Check, Log {
        Check::parameterCheck insteadof Log;
        Log::startLog insteadof Check;
        Check::startLog as csl;
    }
    public function doPublish()
    {
        $this->startLog();
        $this->parameterCheck(&#39;params&#39;);
        $this->csl();
    }
}
$publish = new Publish();
$publish->doPublish();

Execute the above code and the output result is as follows:

Log::startLog public function
Check::parameterCheck parameter checkparams
Check::startLog public function

Just like the literal meaning, the

insteadof

keyword replaces the latter with the former, and the

as

keyword gives an alias to the replaced method.

When referencing Trait, the use keyword is used, and the use keyword is also used to refer to the namespace. The difference between the two is that when referencing Trait, it is used inside the class.

The above is the detailed explanation and application of Trait in PHP. For more related content, please pay attention to the PHP Chinese website (www.php.cn)!