色调不是Flutter中的恒定值 [英] Color shades are not constant values in Flutter

问题描述

例如，当您使用Colors.blue时，它将返回一个恒定的Color对象，但是如果您选择使用阴影(即Colors.blue [300])，则该对象不是恒定的.例如，当您具有采用可选Color参数的方法时，该方法的默认值必须为常数，这一点很重要.那么我们如何使颜色阴影保持不变?

When you use Colors.blue, for example this returns a constant Color object, but if you choose to use a shade instead, i.e. Colors.blue[300], then this object is NOT constant. This is important, for example, when you have a method that takes an optional Color parameter, whose default value must be constant. So how do we make a Color shade constant?

static const Color mainColor = Colors.blue \\All good!
static const Color shade = Colors.blue[400] \\ERROR: Const variables must be initialized with a constant value

推荐答案

那么我们如何使颜色阴影常数不变?

So how do we make a Color shade constant?

不能.若要选择特定的阴影，应使用[]运算符，就像调用方法一样，并且由于方法返回的值在运行时会有所不同，因此从方法调用返回的值不能用作常量. /p>

You can't. To choose a specific shade, you should use the [] operator, which is just like calling a method, and since the value a method returns varies on run-time, the value returned from a method call cannot be use as constant.

这很重要，例如，当您有一个采用可选Color参数的方法时，其默认值必须为常数.

This is important, for example, when you have a method that takes an optional Color parameter, whose default value must be constant.

如果您的情况如此简单，只需使用Colors.blue[400]的实际值，即Color(0xFF42A5F5).

If your situation is as simple as this one, just use the real value of Colors.blue[400], which is Color(0xFF42A5F5).

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