Dart - initializing final class members depedent on each other-CodePudding

I like to use final members in my classes whenever it makes sense, but I ran into a problem trying to do something relatively simple:

import 'dart:ui';

class Foo {
  final PictureRecorder recorder;
  final Canvas canvas;
  final int margin;

  Foo({
    this.margin = 10,
  })  : recorder = PictureRecorder(),
        canvas = Canvas(recorder) { // this line errors out
  }
}

The error I'm getting from the compiler is:

The instance member 'recorder' can't be accessed in an initializer. Try replacing the reference to the instance member with a different expression

Is there another way of doing what I want without making the canvas field non-final?

I know I can make the class look similar "from the outside" using a non-final private field and a custom getter, but it feels like an unnecessary workaround.

CodePudding user response：

You can do this

class Foo {
  final PictureRecorder recorder;
  late final Canvas canvas;
  final int margin;

  Foo({
    this.margin = 10,
  }) : recorder = PictureRecorder() {
    canvas = Canvas(recorder);
  }
}

CodePudding user response：

Here's the workaround I have in mind, but I'd rather do without:

import 'dart:ui';

class Foo {
  final PictureRecorder recorder;
  Canvas? _canvas;
  final int margin;

  Canvas get canvas => _canvas!;

  PatternVisualizer({
    this.margin = 10,
  }) : recorder = PictureRecorder() {
    _canvas = Canvas(recorder);
  }
}

CodePudding user response：

Try using late keyword for the canvas.

late final Canvas canvas;

And the constructor should be updated like this :

Foo({
    this.margin = 10,
  }) : recorder = PictureRecorder() {
    canvas = Canvas(recorder);
  }

Check this post to get more info: https://dev.to/pktintali/late-variables-in-dart-dart-learning-series-1-2opf