douzhi3667 2018-04-29 08:08
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PHP Laravel:理解这个闭包

I have this snippet from the Laravel documentation:

DB::table('users')
            ->whereExists(function ($query) {
                $query->select(DB::raw(1))
                      ->from('orders')
                      ->whereRaw('orders.user_id = users.id');
            })
            ->get();

I need to understand two things.

  1. Where does the $query parameter to the closure come from? I suspect that there is something happening under the hood that I don't understand. The function takes 1 parameter, the $query, but where does it come from, how does this function know what is in this parameter, what is passed into the function?
  2. It appears that this closure doesn't return a value, there is no return statement. So how does the whereExists method know the return value of the closure?
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1条回答 默认 最新

  • doushi3715 2018-04-29 08:22
    关注

    Refer to the source: https://github.com/laravel/framework/blob/987a21f39f203c76665f6014cbef10451689fbdd/src/Illuminate/Database/Query/Builder.php#L1333

    As you can see the closure is treated like a callback.

    So whereExists passes $query to it. $query is an instance of self (Builder) class, so the code in the closure is just updating the object.

    /**
     * Add an exists clause to the query.
     *
     * @param  \Closure $callback
     * @param  string   $boolean
     * @param  bool     $not
     * @return $this
     */
    public function whereExists(Closure $callback, $boolean = 'and', $not = false)
    {
        $query = $this->forSubQuery();
        // Similar to the sub-select clause, we will create a new query instance so
        // the developer may cleanly specify the entire exists query and we will
        // compile the whole thing in the grammar and insert it into the SQL.
        call_user_func($callback, $query);
        return $this->addWhereExistsQuery($query, $boolean, $not);
    }
    
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