UITextField 输入金额，小数点的控制输入

#pragma mark --- UITextFieldDelegate ---
- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
    // 判断是否有小数点
    if ([textField.text containsString:@"."]) {
        self.isHaveDian = YES;
    }else{
        self.isHaveDian = NO;
    }
    
    if (string.length > 0) {
        
        //当前输入的字符
        unichar single = [string characterAtIndex:0];
        
        // 不能输入.0-9以外的字符
        if (!((single >= '0' && single <= '9') || single == '.'))
        {
            return NO;
        }
        
        // 只能有一个小数点
        if (self.isHaveDian && single == '.') {
            return NO;
        }
        
        // 如果第一位是.则前面加上0.
        if ((textField.text.length == 0) && (single == '.')) {
            textField.text = @"0";
        }
        
        // 如果第一位是0则后面必须输入点，否则不能输入。
        if ([textField.text hasPrefix:@"0"]) {
            if (textField.text.length > 1) {
                NSString *secondStr = [textField.text substringWithRange:NSMakeRange(1, 1)];
                if (![secondStr isEqualToString:@"."]) {
                    return NO;
                }
            }else{
                if (![string isEqualToString:@"."]) {
                    return NO;
                }
            }
        }
        
        // 小数点后最多能输入两位
        if (self.isHaveDian) {
            NSRange ran = [textField.text rangeOfString:@"."];
            // 由于range.location是NSUInteger类型的，所以这里不能通过(range.location - ran.location)>2来判断
            if (range.location > ran.location) {
                if ([textField.text pathExtension].length > 5) {
                    return NO;
                }
            }
        }
    }
    return YES;
}