Laravel 动态属性的实现

什么是动态属性

假设我们有一个 User 模型：

class User extends Model
{
    /**
     * 获取与用户关联的电话号码记录。
     */
    public function phone()
    {
        return $this->hasOne('App\Phone');
    }

    public function getFullNameAttribute()
    {
        return "{$this->first_name} {$this->last_name}";
    }
}

$user = User::find(1);
// 这种调用不存在的属性就是 laravel 中的动态属性
$user->full_name;
$user->phone

动态属性的实现

1. 万恶之源 __get()

PHP 的魔术方法 __get()，当读取不可访问属性的值时，__get()会被调用。我们在在使用动态属性时
都会触发这个魔术方法。然后我顺利的在 Eloquent\Model.php 中找到了这个这个方法：

/**
* Dynamically retrieve attributes on the model.
*
* @param  string  $key
* @return mixed
*/
public function __get($key)
{
    return $this->getAttribute($key);
}

2. getAttribute

继续追踪：

/**
     * Get an attribute from the model.
     *
     * @param  string  $key
     * @return mixed
     */
    public function getAttribute($key)
    {
        if (! $key) {
            return;
        }

        // If the attribute exists in the attribute array or has a "get" mutator we will
        // get the attribute's value. Otherwise, we will proceed as if the developers
        // are asking for a relationship's value. This covers both types of values.
        if (array_key_exists($key, $this->attributes) ||
            $this->hasGetMutator($key)) {
            return $this->getAttributeValue($key);
        }

        // Here we will determine if the model base class itself contains this given key
        // since we don't want to treat any of those methods as relationships because
        // they are all intended as helper methods and none of these are relations.
        if (method_exists(self::class, $key)) {
            return;
        }

        return $this->getRelationValue($key);
    }

第二个 if 处理了model有这个attribute和model有对应访问器的情况。

3. getRelationValue

继续追踪, 关联关系的动态属性用法：

/**
    * Get a relationship.
    *
    * @param  string  $key
    * @return mixed
    */
public function getRelationValue($key)
{
    // If the key already exists in the relationships array, it just means the
    // relationship has already been loaded, so we'll just return it out of
    // here because there is no need to query within the relations twice.
    if ($this->relationLoaded($key)) {
        return $this->relations[$key];
    }

    // If the "attribute" exists as a method on the model, we will just assume
    // it is a relationship and will load and return results from the query
    // and hydrate the relationship's value on the "relationships" array.
    if (method_exists($this, $key)) {
        return $this->getRelationshipFromMethod($key);
    }
}

这里可以看到首先会读一个关联关系的缓存，若没有缓存才会判断是否存在和所调用属性同名的方法，
如果存在则调用 getRelationshipFromMethod($key) 方法。

4. getRelationshipFromMethod($method)

protected function getRelationshipFromMethod($method)
    {
        $relations = $this->$method();

        if (! $relations instanceof Relation) {
            throw new LogicException('Relationship method must return an object of type '
                .'Illuminate\Database\Eloquent\Relations\Relation');
        }

        $this->setRelation($method, $results = $relations->getResults());

        return $results;
    }

setRelation 的意思是将没有加载的 relation 进行加载，那么下次需要时就可以在getRelationValue($key)的第一个 if 中即返回需要的结果。

此方法的返回值返回的是 Collection 类型，也就是说动态属性访问关联模型返回的是Collection类型，而如果我们直接调用方法返回的则是 Relation 类型。

本作品采用《CC 协议》，转载必须注明作者和本文链接