小编典典

颤振使用什么颜色系统,为什么我们使用“ const Color”而不是“ new Color”

flutter

今天,我看完以下代码片段,这些代码片段在Flutter中实现了渐变

return new Container(
  ...
  decoration: new BoxDecoration(
    gradient: new LinearGradient(
      colors: [
        const Color(0xFF3366FF), 
        const Color(0xFF00CCFF),
      ]
      begin: const FractionalOffset(0.0, 0.0),
      end: const FractionalOffset(1.0, 0.0),
      stops: [0.0, 1.0],
      tileMode: TileMode.clamp
    ),
  ),
),

它提出了两个问题:

1)这是什么颜色系统0xFF3366FF?它看起来有点类似于十六进制,但事实并非如此。

2)为什么我们使用constfor const Color()相反,new Color()我理解两者之间的不同,但是这里的const对我来说并不直观,我希望它能创建一个new Color()类实例,类似于我们的用法new Text("Some text")。如果需要使用const,为什么还不TileMode.clamp使用const?


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2020-08-13

共1个答案

小编典典

从Flutter来源

class Color {
  /// Construct a color from the lower 32 bits of an [int].
  ///
  /// The bits are interpreted as follows:
  ///
  /// * Bits 24-31 are the alpha value.
  /// * Bits 16-23 are the red value.
  /// * Bits 8-15 are the green value.
  /// * Bits 0-7 are the blue value.
  ///
  /// In other words, if AA is the alpha value in hex, RR the red value in hex,
  /// GG the green value in hex, and BB the blue value in hex, a color can be
  /// expressed as `const Color(0xAARRGGBB)`.
  ///
  /// For example, to get a fully opaque orange, you would use `const
  /// Color(0xFFFF9000)` (`FF` for the alpha, `FF` for the red, `90` for the
  /// green, and `00` for the blue).
  const Color(int value) : value = value & 0xFFFFFFFF;

const 实例被规范化。

如果您const Color(0xFF00CCFF)的代码中有多个实例,则只会创建一个实例。

const实例在编译时评估。在Dart VM中,这是加载代码的时间,但是在Flutter生产中,将使用AoT编译,因此const值会带来很小的性能优势。

2020-08-13