CODEWITHSUNDEEP
flutterdart
loupdaniel
loupdaniel

Reputation: 125

Can't figure out this Error "The method 'map' can't be unconditionally invoked because the receiver can be 'null'"

I'm following this speed code tutorial and I'm facing some problems during somewhere on 5:08.

import 'package:flutter/material.dart';
import 'dart:math';

class WaveColorPainter extends CustomPainter {
  Paint? _paint;
  List<Color>? colors;
  @override
  void paint(Canvas canvas, Size size) {
    colors = List.from(Colors.accents);
    colors!.removeRange(6, 13); //null safety
    List<Color> gradColors = colors.map(
      (color) => color.withOpacity(
        Random().nextDouble().clamp(0.5, 0.9),
      ),
    );
    final Gradient gradient = LinearGradient(colors: gradColors);

    _paint = Paint()
      ..style = PaintingStyle.stroke
      ..strokeWidth = 1.5
      ..shader = gradient.createShader(
        Rect.fromLTWH(
          0,
          20,
          size.width,
          40,
        ),
      );

    canvas.translate(0, size.height / 2);
    canvas.scale(1, -1);

    for (int i = 0; i < size.width.toInt(); i++) {
      double x = i.toDouble();
      double r = 2 * sin(i) - 2 * cos(4 * i) + sin(2 * i - pi * 24);
      r = r * 5;
      canvas.drawLine(Offset(x, r), Offset(x, -r), _paint!);
    } // frequency bar
  }

  @override
  bool shouldRepaint(covariant CustomPainter oldDelegate) {
    return false;
  }
}

Up above is my code and I tried both ? and ! on every variables and functions but I can't figure out what the problem is.

And this is how my vscode looks right now:

Result

Upvotes: 1

Views: 61

Answers (2)

My Car
My Car

Reputation: 4566

You can use Yeasin Sheikh's answer, but here are two more ways:

List<Color> gradColors = colors == null ? [] : colors!.map((color) => color.withOpacity(Random().nextDouble().clamp(0.5, 0.9))).toList();

or

List<Color> gradColors = (colors ?? []).map((color) => color.withOpacity(Random().nextDouble().clamp(0.5, 0.9))).toList();

Upvotes: 1

Md. Yeasin Sheikh
Md. Yeasin Sheikh

Reputation: 63604

You are using nullable dataType List<Color>? colors;, So it is possible to get null.

You can pass empty list on null case

    List<Color> gradColors = colors
            ?.map(
              (color) => color.withOpacity(
                Random().nextDouble().clamp(0.5, 0.9),
              ),
            )
            .toList() ??
        [];

Upvotes: 3

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