Flutter 16進制數字符串轉 int

Flutter 中 Color 接收 int 型十六進制數，若是顏色數據是從後臺獲取，則通常拿到字符串，此時須要轉爲 int

Flutter 中 Color 定義：

(new) Color(int value) → Color
dart.ui

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).
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第一種方法 16進制數字符串轉 int

int _hexToInt(String hex) {
  int val = 0;
  int len = hex.length;
  for (int i = 0; i < len; i++) {
    int hexDigit = hex.codeUnitAt(i);
    if (hexDigit >= 48 && hexDigit <= 57) {
      val += (hexDigit - 48) * (1 << (4 * (len - 1 - i)));
    } else if (hexDigit >= 65 && hexDigit <= 70) {
      // A..F
      val += (hexDigit - 55) * (1 << (4 * (len - 1 - i)));
    } else if (hexDigit >= 97 && hexDigit <= 102) {
      // a..f
      val += (hexDigit - 87) * (1 << (4 * (len - 1 - i)));
    } else {
      throw new FormatException("Invalid hexadecimal value");
    }
  }
  return val;
}
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第二種方法

若是後臺返回的字符串中包含 '#', '0x' 前綴, 或者包含透明度，可處理一下

class HexColor extends Color {
  static int _getColorFromHex(String hexColor) {
    hexColor = hexColor.toUpperCase().replaceAll("#", "");
    hexColor = hexColor.replaceAll('0X', '');
    if (hexColor.length == 6) {
     hexColor = "FF" + hexColor;
    }
    return int.parse(hexColor, radix: 16);
  }

  HexColor(final String hexColor) : super(_getColorFromHex(hexColor));
}
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