参数类型“Future<>”不能分配给参数类型“”

Posted 2023-02-23

【中文标题】参数类型“Future<>”不能分配给参数类型“”

【英文标题】：The argument type 'Future<>' can't be assigned to the parameter type ''

【发布时间】：2021-01-05 23:46:21

【问题描述】：

我正在尝试构建一个 GridView 列表。当我创建一个简单的“玩家”列表时，这一切都有效，但现在我尝试使用 sqflite 持久化数据，但出现错误：

错误：无法将参数类型“Future”分配给参数类型“int”

这是我的列表构建器小部件：

class PlayerList extends StatelessWidget 
  @override
  Widget build(BuildContext context) 
    return Consumer<PlayerData>(
      builder: (context, playerData, child) 
        return GridView.builder(
            gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(
              crossAxisCount: 2,
              mainAxisSpacing: 20,
              crossAxisSpacing: 20,
            ),
            itemCount: playerData.playerCount,
            itemBuilder: (BuildContext context, int index) 
              final player = playerData.players[index];
              return RawMaterialButton(
                  fillColor: Color(0x991c5597),
                  elevation: 20,
                  shape: RoundedRectangleBorder(
                    borderRadius: BorderRadius.circular(10),
                  ),
                  child: Padding(
                    padding: const EdgeInsets.all(10.0),
                    child: Column(
                      crossAxisAlignment: CrossAxisAlignment.center,
                      mainAxisAlignment: MainAxisAlignment.spaceEvenly,
                      children: <Widget>[
                        CircleAvatar(
                          backgroundColor: Colors.tealAccent,
                          radius: 30,
                          child: Icon(
                            Icons.star,
                            color: Color(0xFF16377a),
                            size: 30,
                          ),
                        ),
                        Text(player.name, style: smallTitleText),
                      ],
                    ),
                  ),
                  onPressed: () 
                    playerData.makeActive(player.name);
                    playerData.printList();
                    Navigator.pushNamed(context, PlayScreen.id);
                  );
            );
      ,
    );
  

这就是我获取列表计数的方式：

Future<int> get playerCount async 
  final Database db = await getDBConnector();
  final List<Map<String, dynamic>> maps = await db.query('players');
  if (maps.length != null) 
    return maps.length;
   else  return 0;

【问题讨论】：

你到底在哪里得到错误？

我想添加日志，以便我们知道问题到底出在哪里。

【参考方案1】：

出现错误是因为 playerCount 没有返回 int，而是返回了 Future。首先，您必须将您的类更改为有状态小部件并加载数据，如下所示

class PlayerList extends StatefulWidget 
  @override
  _PlayerListState createState() => _PlayerListState();


class _PlayerListState extends State<PlayerList> 
  int count;

  @override
  void initState() 
    super.initState();
    loadPlayerCount();
  

  @override
  Widget build(BuildContext context) 
    return Consumer<PlayerData>(
      builder: (context, playerData, child) 
        return GridView.builder(
            gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(
              crossAxisCount: 2,
              mainAxisSpacing: 20,
              crossAxisSpacing: 20,
            ),
            itemCount: count??0,
            itemBuilder: (BuildContext context, int index) 
              final player = playerData.players[index];
              return RawMaterialButton(
                  fillColor: Color(0x991c5597),
                  elevation: 20,
                  shape: RoundedRectangleBorder(
                    borderRadius: BorderRadius.circular(10),
                  ),
                  child: Padding(
                    padding: const EdgeInsets.all(10.0),
                    child: Column(
                      crossAxisAlignment: CrossAxisAlignment.center,
                      mainAxisAlignment: MainAxisAlignment.spaceEvenly,
                      children: <Widget>[
                        CircleAvatar(
                          backgroundColor: Colors.tealAccent,
                          radius: 30,
                          child: Icon(
                            Icons.star,
                            color: Color(0xFF16377a),
                            size: 30,
                          ),
                        ),
                        Text(player.name, style: smallTitleText),
                      ],
                    ),
                  ),
                  onPressed: () 
                    playerData.makeActive(player.name);
                    playerData.printList();
                    Navigator.pushNamed(context, PlayScreen.id);
                  );
            );
      ,
    );
  

  void loadPlayerCount() async
    final Database db = await getDBConnector();
    final List<Map<String, dynamic>> maps = await db.query('players');
    if (maps.length != null) 
      setState(() 
        this.count = maps.length;
      );

     else 
      setState(() 
        this.count = 0;
      );
    
  

【参考方案2】：

用FutureBuilder包裹你的GridView

类似的东西

return FutureBuilder(
       future: playerData.playerCount,
       builder : (context,snapshot)
         if(!(snapshot.hasData))
          return Container();
         
         else
           return GridView.builder(
        gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(
          crossAxisCount: 2,
          mainAxisSpacing: 20,
          crossAxisSpacing: 20,
        ),
        itemCount: snapshot.data,
        itemBuilder: (BuildContext context, int index) 
          final player = playerData.players[index];
          return RawMaterialButton(
              fillColor: Color(0x991c5597),
              elevation: 20,
              shape: RoundedRectangleBorder(
                borderRadius: BorderRadius.circular(10),
              ),
              child: Padding(
                padding: const EdgeInsets.all(10.0),
                child: Column(
                  crossAxisAlignment: CrossAxisAlignment.center,
                  mainAxisAlignment: MainAxisAlignment.spaceEvenly,
                  children: <Widget>[
                    CircleAvatar(
                      backgroundColor: Colors.tealAccent,
                      radius: 30,
                      child: Icon(
                        Icons.star,
                        color: Color(0xFF16377a),
                        size: 30,
                      ),
                    ),
                    Text(player.name, style: smallTitleText),
                  ],
                ),
              ),
              onPressed: () 
                playerData.makeActive(player.name);
                playerData.printList();
                Navigator.pushNamed(context, PlayScreen.id);
              );
         );
         ,
       );
      
    
);