[__NSCFString appendString：]：无参数错误

Question

我试图在打印预览选项中显示字符串值。如果我没有在任何文本字段中输入任何文本，并且点击打印按钮，应用程序会崩溃。并且错误是[__NSCFString appendString：]：无argument'.Here是我的代码

-(NSMutableString*)pageStringToPrint { 

    NSMutableString * cnoteString = [[NSMutableString alloc]init]; 

    [cnoteString setString:@"Class: "]; 
    [cnoteString appendString:self.cnote.classname]; 
    [cnoteString appendString:@"\nTeacher: "]; 
    [cnoteString appendString:self.cnote.teacher]; 
    [cnoteString appendString:@"\nSource: "]; 
    [cnoteString appendString:self.cnote.source]; 
    [cnoteString appendString:@"\nTopic: "]; 
    [cnoteString appendString:self.cnote.topic]; 

    NSDateFormatter *dateFormat = [[NSDateFormatter alloc] init]; 
    [dateFormat setDateFormat:@"MM-dd-yyyy"]; 
    NSString *dateString = [dateFormat stringFromDate:self.cnote.date]; 
    [cnoteString appendString:@"\nDate: "]; 
    [cnoteString appendString: dateString]; 

    cnoteString = [NSMutableString stringWithFormat:@"<div id=\"mainContainer\"><div id=\"header\">%@</div>",[cnoteString kv_encodeHTMLCharacterEntities]]; 

这是我如何创建Self.cnoe.classname

- (void)viewDidLoad 
{ 
    [super viewDidLoad]; 
    // Do any additional setup after loading the view. 
    [self saveCNote ]; 
    self.classNameTextField.text = self.cnote.classname; 
    self.topicTextField.text = self.cnote.topic; 
    self.sourceTextField.text = self.cnote.source; 
    self.teacherTextField.text = self.cnote.teacher; 


    self.essentialQuestionTextView.placeholder = @"Essential Question"; 
    self.essentialQuestionTextView.layer.cornerRadius = 7.0f; 
    self.essentialQuestionTextView .layer.borderColor = [[UIColor grayColor] CGColor]; 
    self.essentialQuestionTextView.layer.borderWidth= 1.0f; 
    self.essentialQuestionTextView.delegate = self; 
    self.essentialQuestionTextView.text = self.cnote.essentialQuestion; 

    // set the date 
    self.dateTextField.delegate = self; 
    NSDateFormatter *dateFormat = [[NSDateFormatter alloc] init]; 
    [dateFormat setDateFormat:@"MM-dd-yyyy"]; 
    self.dateTextField.text = [dateFormat stringFromDate:self.cnote.date]; 
    } 

保存C注：

- (void)saveCNote { 

    self.cnote.classname = self.classNameTextField.text; 
    self.cnote.topic = self.topicTextField.text; 
    self.cnote.source = self.sourceTextField.text; 
    self.cnote.teacher = self.teacherTextField.text; 


    [self.cnote save]; 
} 

Answer 1

你应该为nil这样添加判断：

-(NSMutableString*)pageStringToPrint { 

    NSMutableString * cnoteString = [[NSMutableString alloc]init]; 

    [cnoteString setString:@"Class: "]; 
    [cnoteString appendString:self.note.classname ?: @""]; 
    [cnoteString appendString:@"\nTeacher: "]; 
    [cnoteString appendString:self.note.teacher ?: @""]; 
    [cnoteString appendString:@"\nSource: "]; 
    [cnoteString appendString:self.note.source ?: @""]; 
    [cnoteString appendString:@"\nTopic: "]; 
    [cnoteString appendString:self.note.topic ?: @""]; 

    NSDateFormatter *dateFormat = [[NSDateFormatter alloc] init]; 
    [dateFormat setDateFormat:@"MM-dd-yyyy"]; 
    NSString *dateString = [dateFormat stringFromDate:self.note.date] ?: @""; 
    [cnoteString appendString:@"\nDate: "]; 
    [cnoteString appendString: dateString]; 

    cnoteString = [NSMutableString stringWithFormat:@"<div id=\"mainContainer\"><div id=\"header\">%@</div>",[cnoteString kv_encodeHTMLCharacterEntities]];