> For the complete documentation index, see [llms.txt](https://docs.apryse.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.apryse.com/reusable-content/guide/guide-gs-flutter-addapi2.md).

# guide/gs/flutter/addAPI2

***

### Adding the \`getPageCropBox\` function

#### 2. Define the new function in Dart

The `constants.dart` file contains constants grouped by their purpose. The names of functions, parameters, buttons, toolbars, and other relevant constants are stored here. Add a constant to the `Functions` class:

```dart
class Functions {
    static const getPageCropBox = "getPageCropBox";
}
```

The `document_view.dart` file handles function calls for the widget, while `pdftron_flutter.dart` handles function calls for the plugin. If you want to implement for both the widget and plugin then add functions to both files, otherwise add your function to whichever you prefer.

Note that since we use JSON to transfer the information between Flutter and the native code, one extra step of decoding is required here.

In `document_view.dart`, add the `getPageCropBox` method to the `DocumentViewController` class:

```dart
Future<PTRect> getPageCropBox(int pageNumber) {
    return _channel.invokeMethod(Functions.getPageCropBox, <String, dynamic>{'pageNumber': pageNumber}).then((value) => jsonDecode(value));
}
```

In `pdftron_flutter.dart`, add the `getPageCropBox` method to the `PdftronFlutter` class:

```dart
static Future<PTRect> getPageCropBox(int pageNumber) {
    return _channel.invokeMethod(Functions.getPageCropBox, <String, dynamic>{'pageNumber': pageNumber}).then((value) => jsonDecode(value));
}
```

The `options.dart` file contains constants and classes for convenience. In this exercise, we add a new class called `PTRect` to this file for easier access of information.

```dart
class PTRect {
  double x1, y1, x2, y2, width, height;
  PTRect(this.x1, this.y1, this.x2, this.y2, this.width, this.height);

  factory PTRect.fromJson(dynamic json) {
    return PTRect(getDouble(json['x1']), getDouble(json['y1']), getDouble(json['x2']), getDouble(json['y2']), getDouble(json['width']), getDouble(json['height']));
  }

  // a helper for JSON number decoding
  static getDouble(dynamic value) {
    if (value is int) {
      return value.toDouble();
    } else {
      return value;
    }
  }
}
```


---

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