Flutter Stepper控件报错：无法赋值给'Widget Function'参数的解决方法

你按照YouTube教程实现Stepper控件时，编写的controlsBuilder代码出现**无法赋值给参数类型'Widget Function'**错误，问题出在函数签名的类型匹配上。

原代码片段：

controlsBuilder: (context, {onStepContinue, onStepCancel}) {
  final isLastStep = currentStep == getSteps().length - 1;

  return Container(
    margin: EdgeInsets.only(top: 50),
    child: Row(
      children: [
        Expanded(
          child: ElevatedButton(
            child: Text(isLastStep ? "Confirm" : "NEXT"),
            onPressed: onStepContinue,
          ),
        ),
        const SizedBox(width: 12),
        if (currentStep != 0)
          Expanded(
            child: ElevatedButton(
              child: Text("BACK"),
              onPressed: onStepCancel,
            ),
          ),
      ],
    ),
  );
},

解决方法 #

Flutter的Stepper.controlsBuilder要求的函数签名中，命名参数onStepContinue和onStepCancel必须显式指定VoidCallback?类型，修改后的代码如下：

controlsBuilder: (BuildContext context, {VoidCallback? onStepContinue, VoidCallback? onStepCancel}) {
  final isLastStep = currentStep == getSteps().length - 1;

  return Container(
    margin: EdgeInsets.only(top: 50),
    child: Row(
      children: [
        Expanded(
          child: ElevatedButton(
            child: Text(isLastStep ? "Confirm" : "NEXT"),
            onPressed: onStepContinue,
          ),
        ),
        const SizedBox(width: 12),
        if (currentStep != 0)
          Expanded(
            child: ElevatedButton(
              child: Text("BACK"),
              onPressed: onStepCancel,
            ),
          ),
      ],
    ),
  );
},

原因说明 #

新版本Flutter对类型推断的要求更严格，未指定类型的命名参数会导致编译器无法匹配controlsBuilder要求的Widget Function(BuildContext, {VoidCallback?, VoidCallback?})类型，显式声明参数类型即可解决类型不匹配问题。

内容的提问来源于stack exchange，提问作者Mario Hooi