Gridview replaceTags optimized to render content only if necessary using callbacks

[kartik-v]

This enhancement will update the GridView::replaceTags property behavior

replaceTags array, tags to replace in the rendered layout. Enter this as an associative array of the format $key => $callback, where:

• $key: string, defines the flag.

• $callback: string|array, the callback function name that will return the value to be replaced. This can be a
  global function name or a callback setting in an array format as understood by PHP's call_user_func_array method. For example:

  For example:

function renderTag1() { // global function
    $string = ''; // do your stuff to render;
    return $string;
};
echo GridView::widget([
    'replaceTags' => [
        '{tag1}' => 'renderTag1'
    ]
    // other gridview settings
]);

Alternatively you can return a function name from your class or object as an array format. For example:

class YourClass {
  public function renderToken2() {       // object function
     $string = ''; // do your stuff to render;
     return $string;
  }

  public static function renderToken3() { // static function
     $string = ''; // do your stuff to render;
     return $string;
  }

  public function render() {
     return GridView::widget([
         'replaceTags' => [
             '{token2}' => [$this, 'renderToken2'],
             '{token3}' => [YourClass::class, 'renderToken3']
         ]
         // other gridview settings
    ]);
  }
}

[kartik-v]

API Documentation is updated to reflect this

[urielgolab]

I'm not confident enough to make changes to file \yii2-grid\src*GridViewTrait.php* at line 1644, function kartik\grid\GridView::replaceLayoutPart, but I think it should treat callback as an array, and send it the parameters to that function.