How to Unarchive data and print? App crashing - ios

Question

I have a save method that saves info entered from a ViewController. I put that object with the saved info into an array, and I want to archive that array to a file. The app works fine until unarchive and try to NSLog the info. The app then crashes. Can anyone help me figure out why?
- (IBAction)Save:(UIButton *)sender {
self.homeworkAssignment = [[Homework alloc] init];
self.homeworkAssignment.className = self.ClassNameField.text;
self.homeworkAssignment.assignmentTitle = self.AssignmentTitleField.text;
self.homeworkAssignment.assignmentDiscription = self.DiscriptionTextView.text;
self.homeworkAssignment.pickerDate = self.DatePicker.date;
NSMutableArray *MyHomeworkArray = [[NSMutableArray alloc] init];
[MyHomeworkArray addObject:self.homeworkAssignment];
//Create file
NSString *filePath = [self dataFilePath];
//Archive my object
[NSKeyedArchiver archiveRootObject:MyHomeworkArray toFile:filePath];
//Unarchive my object to check
Homework *archivedHomework = [NSKeyedUnarchiver unarchiveObjectWithFile:filePath];
NSLog(#"%# %#", archivedHomework.className, archivedHomework.assignmentTitle);
//^^^^^If I comment this line out, the app does not crash^^^^^^^^^
And this is the method that creates the file
- (NSString*)dataFilePath
{
NSString *docDir = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) objectAtIndex:0];
NSString *filePath = [docDir stringByAppendingPathComponent:#"MyHomework.data"];
NSFileHandle *file = [NSFileHandle fileHandleForWritingAtPath:filePath];
if (!file) {
NSLog(#"Attempting to create the file");
if (![[NSFileManager defaultManager] createFileAtPath:filePath contents:nil attributes:nil]) {
NSLog(#"Failed to create file");
}
else
file = [NSFileHandle fileHandleForWritingAtPath:filePath];
}
return filePath;
}
Here is the output error info:
2013-10-09 20:22:03.524 HW1ARC[902:11303] -[__NSArrayM assignmentTitle]: unrecognized selector sent to instance 0x71c72a0
2013-10-09 20:22:03.524 HW1ARC[902:11303] * Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSArrayM assignmentTitle]: unrecognized selector sent to instance 0x71c72a0'
* First throw call stack:
(0x1c96012 0x10d3e7e 0x1d214bd 0x1c85bbc 0x1c8594e 0x28df 0x10e7705 0x1b2c0 0x1b258 0xdc021 0xdc57f 0xdb6e8 0x2df1d3 0x1c5eafe 0x1c5ea3d 0x1c3c7c2 0x1c3bf44 0x1c3be1b 0x1bf07e3 0x1bf0668 0x17ffc 0x1dbd 0x1ce5 0x1)
libc++abi.dylib: terminate called throwing an exception
(lldb)
EDIT :
Homework.h
#interface Homework : NSObject
<
NSCoding
>
#property (nonatomic, strong) NSString *className;
#property (nonatomic, strong) NSString *assignmentTitle;
#property (nonatomic, strong) NSString *assignmentDiscription;
#property (nonatomic, strong) NSDate *pickerDate;
#property (nonatomic, strong) UISwitch *Switch;
#end
Homework.m
- (void)encodeWithCoder:(NSCoder *)aCoder
{
[aCoder encodeObject:self.className forKey:#"className"];
[aCoder encodeObject:self.assignmentTitle forKey:#"assignmentTitle"];
[aCoder encodeObject:self.assignmentDiscription forKey:#"assignmentDiscription"];
[aCoder encodeObject:self.pickerDate forKey:#"pickerDate"];
[aCoder encodeObject:self.Switch forKey:#"Switch"];
}
- (id)initWithCoder:(NSCoder *)aDecoder
{
self = [super init];
if (self) {
self.className = [aDecoder decodeObjectForKey:#"className"];
self.assignmentDiscription = [aDecoder decodeObjectForKey:#"assignmentDiscription"];
self.assignmentTitle = [aDecoder decodeObjectForKey:#"assignmentTitle"];
self.pickerDate = [aDecoder decodeObjectForKey:#"pickerDate"];
self.Switch = [aDecoder decodeObjectForKey:#"Switch"];
}
return self;
}
#end

Answer 1

Look at what you are doing. You create a Homework object, add it to an array, then archive the array.
But when you unarchive the data, you assign the unarchived object (which is an array) to a Homework variable. You then attempt to use the array like a Homework object. This is why the error is coming from __NSArrayM (the internal representation of a mutable array).
This code:
//Unarchive my object to check
Homework *archivedHomework = [NSKeyedUnarchiver unarchiveObjectWithFile:filePath];
NSLog(#"%# %#", archivedHomework.className, archivedHomework.assignmentTitle);
should be:
//Unarchive my object to check
NSMutableArray *homeworkArray = [NSKeyedUnarchiver unarchiveObjectWithFile:filePath];
Homework *archivedHomework = homeworkArray[0];
NSLog(#"%# %#", archivedHomework.className, archivedHomework.assignmentTitle);

Answer 2

Show the header of the Homework class, as well as it's implementations of initWithCoder and encodeWithCoder.
(In order for what you are trying to do to work, the Homework class must conform to NSCoding, which means it has to implement those 2 methods.)
Your method dataFilePath doesn't really make sense. It creates a path for a file in the documents directory, which is fine. It also creates an NSFileHandle as a local variable and then doesn't do anything with that file handle. There is no reason to create a file handle. It is not doing you any good, and might prevent your file from writing at all. Just build a path string and then use archiveRootObject: toFile: on the array, like you are doing.
The error you report, unrecognized selector assignmentTitle sent to an NSArray object sounds like you have a zombie (an object that is being deallocated and then has a message sent to it after it's memory is used for another object.)
Is your project ARC? Zombies are unusual with ARC, although still not impossible.