颤振：EdgeInsetsGeometry？ 不能分配给 EdgeInsetsGeometry

Question

I am trying to make helper constants in my project so that I don't have to type the syntax again and again. This is the code.

import 'package:flutter/material.dart';

// Images

Map<String, String> images = {
  'APP_LOGO': 'lib/resources/globals/assets/images/app_logo.png'
};

// Colors

Map<String, Color> colors = {
  'PRIMARY_FONT_COLOR': const Color(0xFF000000),
  'SECONDARY_FONT_COLOR': const Color(0xFF4FAFF5),
};

// Paddings

Map<String, EdgeInsetsGeometry> paddings = {
  // ALL
  'P_1': const EdgeInsets.all(5.0),
  'P_2': const EdgeInsets.all(10.0),
  'P_3': const EdgeInsets.all(15.0),
  'P_4': const EdgeInsets.all(20.0),

  // HORIZONTAL
  'PX_1': const EdgeInsets.symmetric(vertical: 0, horizontal: 5.0),
  'PX_2': const EdgeInsets.symmetric(vertical: 0, horizontal: 10.0),
  'PX_3': const EdgeInsets.symmetric(vertical: 0, horizontal: 15.0),
  'PX_4': const EdgeInsets.symmetric(vertical: 0, horizontal: 20.0),

  // VERTICAL
  'PY_1': const EdgeInsets.symmetric(vertical: 5.0, horizontal: 0),
  'PY_2': const EdgeInsets.symmetric(vertical: 10.0, horizontal: 0),
  'PY_3': const EdgeInsets.symmetric(vertical: 15.0, horizontal: 0),
  'PY_4': const EdgeInsets.symmetric(vertical: 20.0, horizontal: 0),
};

// Text Alignment

Map<String, TextAlign> text_alignment = {
  'CENTER': TextAlign.center,
  'LEFT': TextAlign.left,
  'RIGHT': TextAlign.right,
  'JUSTIFY': TextAlign.justify,
};

Now when I try to use these constants inside a widget like following,

Image.asset(global_constants.images['APP_LOGO'].toString()),
Text(
    local_strings.headings['ABOUT_BENJI'].toString(),
    style: local_constants.text_styles['ABOUT_BENJI'],
    textAlign: global_constants.text_alignment['CENTER'],
 ),

It works as expected but I don't know what happens in case of the Padding Widget. Use the constant for padding, padding: global_constants.paddings['PX_5'] .

The error I get Error: The argument type 'EdgeInsetsGeometry?' can't be assigned to the parameter type 'EdgeInsetsGeometry' because 'EdgeInsetsGeometry?' is nullable and 'EdgeInsetsGeometry' isn't. Error: The argument type 'EdgeInsetsGeometry?' can't be assigned to the parameter type 'EdgeInsetsGeometry' because 'EdgeInsetsGeometry?' is nullable and 'EdgeInsetsGeometry' isn't.

Now I do understand the error but, how do I overcome this. I mean all the other constants are working except the padding widget. Any suggestions are welcome. Thanks

Answer 1

This happens because the [] operator returns a nullable type of your map values. This is by design so it can return null if the given key is not in the map.

To fix this, you need to add a ! null assertion operator and cast to a non nullable type, if you are sure that the variable is non null. So in your case,

padding: global_constants.paddings['PX_5']!

However, to be honest I recommend against your current design pattern - you are much better off using constants instead of maps, so

abstract class Paddings {
  static const P_1 = const EdgeInsets.all(5.0);
  static const P_2 = const EdgeInsets.all(10.0);
  static const P_3 = const EdgeInsets.all(15.0);
  static const P_4 = const EdgeInsets.all(20.0);
}

and so on. That way, you sidestep the nullability problem altogether.