为什么我不能把一个数转换成双数？

Question

int weight = json['weight'];
double weight = json['weight'];

num weight = json['weight'];
num.toDouble();

Answer 1

当从Firestore解析100时（它实际上不支持“数字类型”，但将其转换），按照标准，它将被解析为int。
Dart不会自动“巧妙地”强制转换这些类型。事实上，您不能将int强制转换为double，这就是您面临的问题。如果可能的话，您的代码就可以正常工作了。

相反，您可以自己解析它：

double weight = json['weight'].toDouble();

同样有效的方法是将JSON解析为num，然后将其分配给double，后者将num强制转换为double。

double weight = json['weight'] as num;

double a = 100; // this will not compile

double b = 100 as num; // this will compile, but is still marked as an "unnecessary cast"

double a = 100 as double; // does not compile because int is not the super class of double

double b = (100 as num) as double; // compiles, you can also omit the double cast

double weight;

weight = 100; // cannot compile because 100 is considered an int
// is the same as
weight = 100 as double; // which cannot work as I explained above
// Dart adds those casts automatically