数据加载但无法保存iPhone应用程序

Question

I've been stuck on this for approximately two weeks. I hate posting things that have been asked a lot but I really have gone through them all.

I used Ray Wenderlich's tutorial for saving data in an iPhone app.

http://www.raywenderlich.com/tutorials

So that is the setup I have going on in my app. I'm saving very simple objects. My Card object consists of a name, type, and image. That's all. So the tutorial is quite close to mine. Which is making this more frustrating.

The thing is, I have some NSLog statements in there for loading. I have it displaying the folder it's using to load and what objects it does load. Right now it is displaying this.

Loading cards from /Users/zach/Library/Application Support/iPhone Simulator/7.0.3-64/Applications/E3DB01FD-A37E-4A69-840B-43830F2BDE2C/Library/Private Documents
2013-11-04 00:02:50.073 CardMinder[84170:a0b] ()

So it seems to be trying to load them, but there's nothing there to load. Here is my function to save data.

- (void)saveData {
    if (_data == nil) return;

    [self createDataPath];

    NSString *dataPath = [_docPath stringByAppendingPathComponent:kDataFile];
    NSMutableData *data = [[NSMutableData alloc] init];
    NSKeyedArchiver *archiver = [[NSKeyedArchiver alloc] initForWritingWithMutableData:data];
    [archiver encodeObject:_data forKey:kDataKey];
    [archiver finishEncoding];

    NSLog(@"%@",dataPath);
    NSLog(@"%@",data);

    [data writeToFile:dataPath atomically:YES];

}

Which is really just what's posted in that tutorial. I know if you feel generous enough to help me out i'll have to post some more code but I don't want to flood the post with useless stuff so just let me know and i'll get it out here.

I really appreciate anyone that can help, I have recently entered the desperation state and need help.

Thanks

UPDATE

    NSError *error;
    [data writeToFile:dataPath options:NSDataWritingAtomic error:&error];
    NSLog(@"error: %@", error.localizedFailureReason);

These are the methods for the CardData class. I'm doing the name, type, and a bool here.

- (void)encodeWithCoder:(NSCoder *)aCoder
{
    [aCoder encodeObject:_name forKey:kNameKey];
    [aCoder encodeObject:_cardType forKey:kTypeKey];
    [aCoder encodeBool:_checkedOut forKey:kOutKey];

}

- (id)initWithCoder:(NSCoder *)aDecoder
{
    NSString *name = [aDecoder decodeObjectForKey:kNameKey];
    NSString *cardType = [aDecoder decodeObjectForKey:kTypeKey];
    BOOL checkedOut = [aDecoder decodeBoolForKey:kOutKey];
    return [self initWithName:name cardType:cardType _Bool:checkedOut];
}

UPDATE 2

I just put some more NSLog statements in and I found out that when I press the "Save card" button in my app, it doesn't seem to execute the saveData function at all. I have log statements galore in that saveData function and when I click the saveCard button it doesn't show any of those logs. Why would that be happening?

This is my saveButton code.

- (IBAction)saveNewCard:(id)sender
{
    NSString *cardName = self.nameField.text;

    _cardDoc.data.name = cardName;


    CardDoc *newCard = [[CardDoc alloc] initWithName:cardName cardType:cardTypeString _Bool:NO image:chosenIcon];

    [_cardDoc saveData];
    NSLog(@"Card save button pressed!");

    CardViewController *cardViewController = (CardViewController *)[self.navigationController.viewControllers objectAtIndex:self.navigationController.viewControllers.count-2];

    [cardViewController.cards addObject:newCard];
    [self.navigationController popToRootViewControllerAnimated:YES];


}

Answer 1

A few things.

Post the results of your log statements so we know what you are seeing.

In order for your approach to work, your _data object needs to conform to the NSCoding protocol. That means you need to add the protocol declaration to your interface, and implement the methods encodeWithCoder and initWithCoder.

In those methods you need to save all the state data for your object / load the state back into your object.

Those methods are the most likely source of problems with your code. Post those methods if you need help with them, and walk though them in the debugger.

You might also look at the NSKeyedArchvier class method archivedDataWithRootObject. That method takes an object and encodes it into an NSData object in one step. The method archiveRootObject:toFile: take it a step further, and writes the data directly to a file for you.

NSKeyedUnarchiver has the corresponding methods unarchiveObjectWithData and unarchiveObjectWithFile to recreate your object from data/a file.

Answer 2

You should use writeToFile:options:error: instead of writeToFile:atomically: ; that will give you an error message that should prove helpful. (The equivalent to atomically:YES is the option constant NSDataWritingAtomic .) Make sure you're getting back a return value of YES ; if not, the error should be set.

If you're getting a value of NO but the error is not set, it means you're messaging nil . A quirk of Objective-C is that messaging nil is completely valid. If the method is defined to return something, you'll even get a result: 0 or equivalent ( NO , nil , etc.)

In this case, you're messaging _cardDoc . There's no return result to detect. This is a bit harder to defensively code around, but [_cardDoc saveData] is actually [nil saveData] . The debugger will just breeze past the line.

Generally, if something absolutely should not be nil , you can use NSAssert :

NSAssert(_cardData, @"_cardData should not be nil");
[_cardData saveData];

But use this sparingly; you'll probably come to usually appreciate this behaviour.