这个常量表达式的计算抛出一个表达式 [英] Evaluation of this constant expression throws an expression

问题描述

我的 dart 分析在代码中显示以下错误:

My dart analysis is showing the following error in the code :

error: Evaluation of this constant expression throws an exception. 
        (const_eval_throws_exception at [laundry] 
        lib\pick_drop_ui\pages\works.dart:97) error: Arguments of a constant creation must be constant expressions. 
        (const_with_non_constant_argument at [laundry] 
        lib\pick_drop_ui\pages\works.dart:98) error: Arguments of a constant creation must be constant expressions. 
        (const_with_non_constant_argument at [laundry] 
        lib\pick_drop_ui\pages\works.dart:104) error: Evaluation of this constant expression throws an exception. 
       (const_eval_throws_exception at [laundry] 
       lib\pick_drop_ui\pages\works.dart:104)

在以下代码中:

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

class work extends StatefulWidget {
  @override
  _workState createState() => _workState();
}

class _workState extends State<work> {

  var workdata;                        //Variable to get the snapshort of the works available in the firestore

  @override
  void initState() {
    // TODO: implement initState
    super.initState();
    print("Called work");
    setState(() {
      workdata = getData();
    });
  }


  getworkdetails(){
    if(workdata != null){
      return StreamBuilder(
        stream: workdata,
        builder: (context,snapshot){
          if(snapshot.data != null){
          return ListView.builder(
            shrinkWrap: true,
            itemCount: snapshot.data.documents.length,
            itemBuilder: (context,i){
              return workcards(snapshot.data.documents[i].data['Name of customer'],snapshot.data.documents[i].data['Address']);
            },
          );
          }else{
            return Text("Malfunction");
          }
        },
      );
    }else{
      print("getting workdata");
    }
  }




  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Jobs Assigned"),
      ),

      body: getworkdetails(),
    );
  }
}


getData() {
  return Firestore.instance.collection('Jobs').snapshots();
}


class workcards extends StatelessWidget{

  final  name;
  final  address;

  workcards(this.name,this.address);


  @override
  Widget build(BuildContext context) {
    // TODO: implement build
    return Card(
      color: Colors.blueGrey[50],
      child: InkWell(
        splashColor: Colors.blue[100].withAlpha(100),
        onTap: () {
        },
        child: Column(
          mainAxisSize: MainAxisSize.min,
          children: <Widget>[
            const ListTile(
              leading: Icon(Icons.view_module),
              // Error: invalid constant value
              title: Text(
                name,
                style: TextStyle(
                  fontWeight: FontWeight.w600,
                  letterSpacing: .5,
                ),
              ),
              // Error: invalid constant value
              subtitle: Text(address),
            ),
            ButtonBar(
              children: <Widget>[
                RaisedButton(
                  child: const Text('OPEN'),
                  onPressed: () {/* ... */},
                  focusElevation: 10,
                ),
                RaisedButton(
                  child: const Text('SHARE'),
                  onPressed: () {/* ... */},
                  focusElevation: 20,
                ),
              ],
            ),
          ],
        ),
      ),
    );
  }
}

推荐答案

去掉 ListTile 前的 const 关键字.

Remove the const keyword before the ListTile.

一般来说，为了有一个编译时常量的小部件，它的所有属性也应该是编译时常量.在您的情况下，这是不可能的，因为 title 属性只能通过评估 name 来实现，这仅在运行时才知道.

Generally, in order to have a widget that is compile-time constant, all of its properties should also be compile-time constants. In your case it isn't possible because the title property can only be achieved by evaluating name, which is only known at runtime.

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