在UITextField的开头添加恒定的国家/地区代码

此答案假定一个起始国家代码字符串，该字符串的末尾包括连字符，例如：self.countryCode = @"+1-";。文本字段最初应包含“ + 1-”。

我已经使回答的方式比您的原始意图更全面，因为它可以处理您忽略的许多用例，例如，具有多个字符的复制和粘贴操作，不适当的连字符删除，不适当的连字符添加，中线插入等。仍然不是很完美，因为您的原始答案在某些方面是不确定的...例如，如果您指定用户只能输入数字，则代码可以更简洁。

全文包含的注释中逐行描述了以下实现。

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
    // Combine the new text with the old
    NSMutableString *combinedText = [[textField.text stringByReplacingCharactersInRange:range withString:[NSString stringWithFormat:@"%@", string]] mutableCopy];

    // If the user deletes part of the country code or tries
    // to edit it in any way, don't allow it
    if (combinedText.length < self.countryCode.length ||
        ![combinedText hasPrefix:self.countryCode]) {
        return NO;
    }

    //  Limit text field characters to 12
    if (combinedText.length > self.countryCode.length + 12) {
        return NO;
    }

    // If the user tries to add a hyphen where there's supposed
    // to be a hyphen, allow them to do so.
    if ([string isEqualToString:@"-"] &&
        (range.location == self.countryCode.length + 3 ||
        range.location == self.countryCode.length + 7)) {
        return  YES;
    }

    // Remove all the hyphens other than the one directly
    // following the country code        
    [combinedText replaceOccurrencesOfString:@"-" withString:@"" options:0 range:NSMakeRange(self.countryCode.length, [combinedText length] - self.countryCode.length)];

    // Auto-add the hyphens before the 4th and 7th digits
    if (combinedText.length > self.countryCode.length + 3)
        [combinedText insertString:@"-" atIndex:self.countryCode.length + 3];
    if (combinedText.length > self.countryCode.length + 7)
        [combinedText insertString:@"-" atIndex:self.countryCode.length + 7];

    // Store the original cursor position
    UITextPosition *pos = [textField selectedTextRange].start;

    // Count up the original number of hyphens
    NSUInteger originalNumberOfHyphens = [[textField.text componentsSeparatedByString:@"-"] count] - 1;
    // Count up the new number of hyphens
    NSUInteger newNumberOfHyphens = [[combinedText componentsSeparatedByString:@"-"] count] - 1;

    // Create a cursor offset to reflect the difference
    // in the number of hyphens
    float offset = newNumberOfHyphens - originalNumberOfHyphens;

    // Update the text field to contain the combined text
    textField.text = combinedText;

    // Update the cursor position appropriately
    if (string.length > 0) {
        UITextPosition* cursor = [textField positionFromPosition:[textField beginningOfDocument] offset:range.location + range.length + offset + string.length];
        textField.selectedTextRange = [textField textRangeFromPosition:cursor toPosition:cursor];
    } else {
        UITextPosition* cursor = [textField positionFromPosition:pos inDirection:UITextLayoutDirectionLeft offset:1-offset];
        textField.selectedTextRange = [textField textRangeFromPosition:cursor toPosition:cursor];
    }

    // No need to replace the string since it's already been done
    return NO;
}