-(BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string{
    // Check for non-numeric characters
    NSUInteger lengthOfString = string.length;
    for (NSInteger loopIndex = 0; loopIndex < lengthOfString; loopIndex++) {//只允许数字输入
        unichar character = [string characterAtIndex:loopIndex];
        if (character < 48) return NO; // 48 unichar for 0
        if (character > 57) return NO; // 57 unichar for 9
    }
    // Check for total length
    NSUInteger proposedNewLength = textField.text.length - range.length + string.length;
    if (proposedNewLength > 3) return NO;//限制长度
    return YES;
    }

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