映射<动态，动态>即使存在数据也返回空值

Question

我有一个包含数据的映射。但当我执行这个map["content"]时，它告诉我null。有什么问题吗?

This is the codefinal messages = snap.data!.snapshot.value as Map<dynamic, dynamic>;This is what I'm doingmessages["idFrom"]

Answer 1

消息是Map<dynamic, dynamic>或Map<int, dynamic>从您的响应

Map<int, dynamic> messages = {
1673561668839422: {
'timeStamp': 1673561668840253,
'idTo': 'hMKNXQxOHTSi6WqbuHN5uabQwea2',
'idFrom': '7BEeF0rHihfIFRvv9ClqIdAzzPP2',
'type': 0,
'content': 'Hi'
},
1673562395023337: {
'timeStamp': 1673562395023396,
'idTo': 'hMKNXQxOHTSi6WqbuHN5uabQwea2',
'idFrom': '7BEeF0rHihfIFRvv9ClqIdAzzPP2',
'type': 0,
'content': 'How are you'
},
...
};

不能执行messages['idFrom']，因为idFrom是嵌套Map的键，即

{
'timeStamp': 1673561668840253,
'idTo': 'hMKNXQxOHTSi6WqbuHN5uabQwea2',
'idFrom': '7BEeF0rHihfIFRvv9ClqIdAzzPP2',
'type': 0,
'content': 'Hi'
}

如果你想访问idFrom，你可以访问messages[int]['idFrom']，其中int是Map消息的密钥之一。

交货. .print(messages[1673561668839422]['idFrom'])

7 beef0rhihfifrvv9clqidazzpp2