Question

Okay I have spent the last 2 days trying to sort this one out. I have a class thats controlling my property list, It is used as a singleton class so its only instantiated once and it has one other method in it that is used to save the values to the plist.

The plist works sweet no matter what you do up untill you remove the app from the multitasking bar (i.e. close the app completely) then all of the values in the plist are reset to null.. However the plist is not lost.

So I am thinking now maybe I am overwriting the values in my plist that I have made in the root document directory for read and write capability. I am hoping with my code example you guys might be able to spot my mistake… I’m not sure what else I need to add.. if you have any question that will help you help me answer this problem then please let me know.

any help would be hugely appreciated and my children’s children’s children will be in debuted to you.

#import "EnginePropertiesController.h"

static EnginePropertiesController *sharedMyManager = nil;

@implementation EnginePropertiesController

@synthesize pSignature;
@synthesize pVersion;
@synthesize rNumber;
@synthesize dVReturned;
@synthesize cacheValue;

@synthesize manu;
@synthesize mod;
@synthesize submod;


#pragma mark Singleton Methods
+ (id)sharedManager {
    @synchronized(self) {
        if (sharedMyManager == nil)
            sharedMyManager = [[self alloc] init];
    }
    return sharedMyManager;
}
- (id)init {
    if (self = [super init]) {

            // get paths from root direcory
            NSArray *paths = NSSearchPathForDirectoriesInDomains (NSDocumentDirectory, NSUserDomainMask, YES);
            // get documents path
            NSString *documentsPath = [paths objectAtIndex:0];
            // get the path to our Data/plist file
            NSString *plistPath = [documentsPath stringByAppendingPathComponent:@"EngineProperties.plist"];
            NSLog(@"myplist path read = %@", plistPath);

            // check to see if Data.plist exists in documents
            if (![[NSFileManager defaultManager] fileExistsAtPath:plistPath])
            {
                // if not in documents, get property list from main bundle
                plistPath = [[NSBundle mainBundle] pathForResource:@"EngineProperties" ofType:@"plist"];

            }

            // read property list into memory as an NSData object
            NSData *plistXML = [[NSFileManager defaultManager] contentsAtPath:plistPath];
            NSString *errorDesc = nil;
            NSPropertyListFormat format;
            // convert static property liost into dictionary object
            NSDictionary *tempRoot = (NSMutableDictionary *)[NSPropertyListSerialization propertyListFromData:plistXML mutabilityOption:NSPropertyListMutableContainersAndLeaves format:&format errorDescription:&errorDesc];
            manu = [cacheValue objectForKey:@"Manu"];
            mod = [cacheValue objectForKey:@"Mod"];
            submod = [cacheValue objectForKey:@"SubMod"];
            if (tempRoot && [tempRoot count]){
                // assign values
                self.pVersion = [tempRoot objectForKey:@"PSignature"];
                self.pVersion = [tempRoot objectForKey:@"PVersion"];
                self.rNumber = [tempRoot objectForKey:@"RNumber"];
                self.dVReturned = [tempRoot objectForKey:@"DVReturned"];
                cacheValue = [tempRoot objectForKey:@"Cache Value"];
        }
    }
    return self;
}

- (void) saveData:(NSString *)methodName pSignature:(NSString *)TemppSignature pVersion:(NSNumber *)TemppVersion rNumber:(NSNumber *)TemprNumber dVReturned:(NSNumber *)TempdvReturned cacheValue:(NSNumber *)cValue
{
    // get paths from root direcory
    NSArray *paths = NSSearchPathForDirectoriesInDomains (NSDocumentDirectory, NSUserDomainMask, YES);
    // get documents path
    NSString *documentsPath = [paths objectAtIndex:0];
    // get the path to our Data/plist file
    NSString *plistPath = [documentsPath stringByAppendingPathComponent:@"EngineProperties.plist"];

    // set the variables to the values in the text fields
    self.pSignature = TemppSignature;
    self.pVersion = TemppVersion;
    self.rNumber = TemprNumber;
    self.dVReturned = TempdVReturned;

    //This checks with methodName I would like to save the value from
    //The reason for this is there are 3 different cache values I get at seperate times to save This if statment just makes sure they are in the correct order.
    if ([methodName isEqualToString:@"Getmanu"]) {
        self.manu = cValue; 
    } else if ([methodName isEqualToString:@"GetMod"]) {
        self.mod = cValue;
    } else if ([methodName isEqualToString:@"GetSubMod"]) {
        self.submod = cValue;
    }        

    //Set up cacheValue Dictionary that will be added to the property list root dictionary
    self.cacheValue = [NSDictionary dictionaryWithObjectsAndKeys:
                       manu, @"Manu",
                       mod, @"Mod",
                       subMod, @"SubMod", nil];

    //Pass appropriate values into plist
    NSDictionary *plistDict = [NSDictionary dictionaryWithObjectsAndKeys:
                               protocolSignature, @"Protocol Signature",
                               pVersion, @"Protocol Version",
                               rNumber, @"Request Number",
                               dVReturned, @"Data Version returned",
                               cacheValue, @"Cache Value", nil];


    NSString *error = nil;
    // create NSData from dictionary
    NSData *plistData = [NSPropertyListSerialization dataFromPropertyList:plistDict format:NSPropertyListXMLFormat_v1_0 errorDescription:&error];

    // check is plistData exists
    if(plistData) {
        // write plistData to our Data.plist file
        [plistData writeToFile:plistPath atomically:YES];
    } else {
        NSLog(@"Error in saveData: %@", error);
    }
}


@end

Answer 1

The Problem: nil Values

Here’s a problem: pVersion is assigned twice, while pSignature remains nil.

self.pVersion = [tempRoot objectForKey:@"PSignature"];
self.pVersion = [tempRoot objectForKey:@"PVersion"];

This should probably be

self.pSignature = [tempRoot objectForKey:@"PSignature"];
self.pVersion = [tempRoot objectForKey:@"PVersion"];

This in turn leads to trouble at saving time:

NSDictionary *plistDict = [NSDictionary dictionaryWithObjectsAndKeys:
                           pSignature, @"PSignature",
                           pVersion, @"PVersion",
                           rNumber, @"RNumber",
                           dVReturned, @"DVReturned",
                           cacheValue, @"Cache Value", nil];

This constructor doesn’t know how many arguments it has, it just keeps going until it hits nil. If pSignature is nil, it will save an empty dictionary.

The Fix: Robust NSDictionary Creation

While the immediate solution is to fix the pSignature reading, it’s always fragile to construct dictionaries using dictionaryWithObjectsAndKeys:. It’s much better to store each object individually, using setObject:forKey. This throws an exception if the object is nil, so test before calling.

NSMutableDictionary *plistDict = [NSMutableDictionary dictionary];
if (pSignature) [plistDict setObject:pSignature forKey:@"PSignature"];
if (pVersion) [plistDict setObject:pVersion forKey:@"PVersion"];
// ... and so on.

The Alternative: NSCoding

Another option for storage is to serialize. See the Archives and Serializations Programming Guide. This has tradeoffs: while saving and loading is simpler when it works, the resulting files are less human-readable.

Debugging

Finally, three places to put log statements, breakpoints or dialog boxes that help in cases like this:

1. Log the dictionaries being loaded and the dictionaries being saved. If you open your app and trigger a save immediately, are the dictionaries identical?
2. Check the first argument sent to nil-terminated methods like arrayWithObjects: and dictionaryWithObjectsAndKeys:
3. On the simulator, log the path used for saving and open it in Finder. Is there a file there? If it’s a property list, you can open it in XCode and see if it has the content you expect.