What is php chain writing?

php chain writing method is "$tree->setAge()->setID()->setName();", where in the php class this pointer points to the class/object itself, it Like a rope connecting the class/object to the attributes and functions of the class.

## Operating environment for this article: Windows 7 system, PHP version 7.1 , DELL G3 computer

The chain writing method exists in many object-oriented programming languages. My simple understanding is that it is a linked list, and it is indeed very familiar with the use of linked lists. The this pointer is the basis of chained operations.

This pointer has similar functions in different programming languages, but the specific details are different.
But there is a basis for pointing to the object/function/class itself. Based on this basis, chain writing can be implemented. Take php as an example.
In the PHP class, the this pointer ($this) points to the class/object itself. It is like a rope connecting the class/object with the attributes and functions of the class. And you can use it to call the corresponding properties and functions (methods). As shown below:

Demo of php program:

<?php
/**
 * Created by PhpStorm.
 * User: dell
 * Date: 2017/3/31
 * Time: 21:42
 */
/*
*注释的规范写法：@param  [参数类型]  参数名   参数解释 
 * @return 返回值  [类型]   返回值名   返回值说明
*/



class Tree{
    private  $id ;
    private  $name;
    private  $age;
    /*
 *
 * @param  null
 * @return  null
 *
 */
    public function  _construct()
    {
        //构造函数
    }
    /*
 *
 * @param  [array] $arr 参数1
 * @return  null
 *
 */
    public function  init( $arr = array() )
    {
        //初始化函数
        $arrs = array();
        //设置默认参数
        $arrs = [
            &#39;id&#39;=> 1,
            &#39;name&#39;=> &#39;user&#39;,
            &#39;age&#39; => 18
        ];
        if( !(is_array($arr)) )
        {
        //如果传进来的数据不是数组则默认是id
            $this->id = intval( $arr );
        }
        $arrs = array_merge( $arrs , $arr );//合并数组
        //给类的属性赋值
        $this->id = $arrs[&#39;id&#39;];
        $this->name = $arrs[&#39;name&#39;];
        $this->age = $arrs[&#39;age&#39;];
    }
    /*
 *
 * @param  [int] $data 参数1
 * @return  $this
 *
 */
    public function  setID(  $data = 0 )
    {
        //判断是不是数字
        $this->id =is_numeric( $data )? $data : intval($data)  ;//判断是否是数字，是赋值，否则转为数字在赋值
        return $this;//返回this指针
    }
    /*
 *
 * @param  [string] $data 参数1
 * @return  $this
 *
 */
    public function  setName(  $data = &#39;user&#39; )
    {
        //判断是不是数字
        $this->name =is_string( $data )? $data : strval($data)  ;//判断是否是字符串，是赋值，否则转为字符串在赋值
        return $this;//返回this指针
    }
    /*
 *
 * @param  [int] data 参数1
 * @return  $this
 *
 */
    public function  setAge(  $data = 18 )
    {
        //判断是不是数字
        $this->age =is_numeric( $data )? $data : intval($data)  ;//判断是否是数字，是赋值，否则转为数字在赋值
        return $this;//返回this指针
    }
};

$tree = new Tree();
$array = array();
$array[&#39;id&#39;] = 10;
$array[&#39;name&#39;] = &#39;admin&#39;;
$array[&#39;age&#39;] = 20;
$tree->init($array);
var_dump($tree);//打印结果
$tree->setAge()->setID()->setName();//链式写法
var_dump($tree);//在打印看链式写法是否有效
?>

Output result:

This one above In the demo, a tree is built using the PHP class, with ID, name and age. Initialize properties through the initialization function init. Then assign values ​​to attributes through chain writing, and finally print the result to verify whether it is successful.

The premise that functions/methods can implement chain writing is that the return value is this pointer (return $this in php, return this in js, and similar in other languages). Only in this way can the this pointer be returned after calling this function. Logically speaking, what is returned at this time is the class itself, and then continue to call the function and return the this pointer, and so on.
Of course, each programming language has its differences. For example, c is ->, php->, and js or jq is...; the calling form is different but the principle is the same.

One point, there is a this pointer, return this pointer, and then you can implement chain writing and can be used cyclically ->fun1(return this)->fun2(return this)->fun3 (return this)…… .

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