[__NSCFString appendString:]: nil argument' 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在单击打印按钮时应用程序崩溃。错误是 [__NSCFString appendString:]: nil argument '.这是我的代码

-(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]];

这就是我创建 Self.cnoe.classname 的方式

- (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];
  }

保存注释：

- (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

你应该像这样为nil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]];