Flutter:将值分配给const color时出错 [英] Flutter: Error in assigning value to const color

问题描述

const color = Colors.red; // no error
const color100 = Colors.red[100]; // error

如果 Colors.red 是一个编译时间常数，为什么 Colors.red [100] 不是.

If Colors.red is a compile time constant, why Colors.red[100] isn't.

推荐答案

因为您只能将常量值分配给 const 变量.

Because you can only assign constant values to a const variable.

并且没有/不能将运算符 T1运算符[](T2 i)的返回类型声明为 const

And the return type of an operator T1 operator [](T2 i) isn't/can't be declared as const

所以你不能做 const a = b [c];

不/不能将函数 T1 x(T2 y)的返回类型声明为 const

Like the return type of a function T1 x(T2 y) isn't/can't be declared as const

所以你不能做 const z = x(y);

由于使用运算符或函数时，不会在调用时创建返回值，因此不会返回常量.那只能通过构造函数来实现.

Because when you use an operator or a function, you are not creating the return value at the moment of the call, so you are not returning a constant value. That can only be achieved with a constructor.

因此，如果声明 ClassA 的构造函数为 const

So you can do const a = ClassA(); if the constructor of ClassA is declared const

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