android - flutter 如何检查 Map 是否为空？

Question

map 定义如下:

Map<String,dynamic> allAvailableTime;

当用户初始化页面时 map 应该是空的:

我想在从 map 获取数据之前执行空值检查，但是这些不起作用...

检查 1:

allAvailableTime == null ?

错误:

type 'String' is not a subtype of type 'Widget'

检查 2:

allAvailableTime == {} ?

检查应该返回 True，但它返回 False ..

检查 3:

allAvailableTime.isEmpty ?

错误3:

The getter 'isEmpty' was called on null.
Receiver: null
Tried calling: isEmpty

Answer 1

对 null 检查。它应该可以为空

void main() {
 
 //1. Making Map Nullable 
 Map<String,dynamic>? allAvailableTime;
 print(allAvailableTime==null);
 //Output: true
  
  
 //2. If you dont want to make it nullable
 Map<String,dynamic> allAvailableTime1 = {};
 print(allAvailableTime1.isEmpty);
  //Output: true
}

没有 NullSafety

void main() {
 
 //1. Making Map Nullable 
 Map<String,dynamic> allAvailableTime;
 print(allAvailableTime==null);
 //Output: true
  
  
 //2. If you dont want to make it nullable
 Map<String,dynamic> allAvailableTime1 = {};
 print(allAvailableTime1.isEmpty);
  //Output: true
}