toString方法

String toString()
override

返回格式异常的描述。

描述总是包含消息。

如果存在且为字符串的来源，描述将包含（至少是部分）来源。如果还提供了偏移量，则包含在来源中的部分将包含该偏移量，并且偏移量会被标记。

如果来源是字符串并且它的偏移量之前有一个换行符，则只包含包含偏移量的行，它的行号也将作为描述的一部分。行列偏移量是基于1的。

实现

String toString() {
  String report = "FormatException";
  Object? message = this.message;
  if (message != null && "" != message) {
    report = "$report: $message";
  }
  int? offset = this.offset;
  Object? source = this.source;
  if (source is String) {
    if (offset != null && (offset < 0 || offset > source.length)) {
      offset = null;
    }
    // Source is string and offset is null or valid.
    if (offset == null) {
      if (source.length > 78) {
        source = source.substring(0, 75) + "...";
      }
      return "$report\n$source";
    }
    int lineNum = 1;
    int lineStart = 0;
    bool previousCharWasCR = false;
    for (int i = 0; i < offset; i++) {
      int char = source.codeUnitAt(i);
      if (char == 0x0a) {
        if (lineStart != i || !previousCharWasCR) {
          lineNum++;
        }
        lineStart = i + 1;
        previousCharWasCR = false;
      } else if (char == 0x0d) {
        lineNum++;
        lineStart = i + 1;
        previousCharWasCR = true;
      }
    }
    if (lineNum > 1) {
      report += " (at line $lineNum, character ${offset - lineStart + 1})\n";
    } else {
      report += " (at character ${offset + 1})\n";
    }
    int lineEnd = source.length;
    for (int i = offset; i < source.length; i++) {
      int char = source.codeUnitAt(i);
      if (char == 0x0a || char == 0x0d) {
        lineEnd = i;
        break;
      }
    }
    int length = lineEnd - lineStart;
    int start = lineStart;
    int end = lineEnd;
    String prefix = "";
    String postfix = "";
    if (length > 78) {
      // Can't show entire line. Try to anchor at the nearest end, if
      // one is within reach.
      int index = offset - lineStart;
      if (index < 75) {
        end = start + 75;
        postfix = "...";
      } else if (end - offset < 75) {
        start = end - 75;
        prefix = "...";
      } else {
        // Neither end is near, just pick an area around the offset.
        start = offset - 36;
        end = offset + 36;
        prefix = postfix = "...";
      }
    }
    String slice = source.substring(start, end);
    int markOffset = offset - start + prefix.length;
    return "$report$prefix$slice$postfix\n${" " * markOffset}^\n";
  } else {
    // The source is not a string.
    if (offset != null) {
      report += " (at offset $offset)";
    }
    return report;
  }
}