Flutter error: Error: Can't access 'this' in a field initializer to read 'computeMaxScale'

Question

After running flutter upgrade I'm facing some problems with flutter

I have this class that stores minScale and maxScale.

In the constructor, I want to assert the interval between them. As those values can be a double or an instance of an enum, I have to compute them before the comparison.

Code of the constructor and asserts:

class ScaleBoundaries {
  final dynamic _minScale;
  final dynamic _maxScale;
  Size size;
  ImageInfo imageInfo;

  ScaleBoundaries(this._minScale, this._maxScale, { @required this.size, @required this.imageInfo}) :
        assert(_minScale is double || _minScale is PhotoViewScaleBoundary),
        assert(_maxScale is double || _maxScale is PhotoViewScaleBoundary),
        assert(computeMinScale() <= computeMaxScale());

This gives me the following error:

compiler message: file:///.../photo_view/lib/photo_view_scale_boundaries.dart:14:37: Error: Can't access 'this' in a field initializer to read 'computeMaxScale'.
compiler message:         assert(computeMinScale() <= computeMaxScale());
compiler message:                                     ^^^^^^^^^^^^^^^
compiler message: file:///.../photo_view/lib/photo_view_scale_boundaries.dart:14:16: Error: Can't access 'this' in a field initializer to read 'computeMinScale'.
compiler message:         assert(computeMinScale() <= computeMaxScale());
compiler message:                ^^^^^^^^^^^^^^^

That is my flutter doctor output:

Doctor summary (to see all details, run flutter doctor -v):
[✓] Flutter (Channel master, v0.7.1-pre.15, on Linux, locale en_US.UTF-8)
[!] Android toolchain - develop for Android devices (Android SDK 28.0.1)
    ✗ Android license status unknown.
[✓] Android Studio (version 3.1)
[✓] IntelliJ IDEA Community Edition (version 2018.1)
[!] VS Code (version 1.25.1)
[✓] Connected devices (1 available)

! Doctor found issues in 2 categories.

The only solution that I found out is removing the assert, but that is not okay. How can I address that?

For more information, see the whole file and project: photo_view_scale_boundaries.dart

Answer 1

You can do that assert within the body of your constructor

class Foo {
  Foo() {
    assert(computeMinScale() <= computeMaxScale());
  }    
}