Why NSMutableDictionary don't want write to file? - iphone

Question

- (void)viewDidLoad
{
[super viewDidLoad];
if ([[NSFileManager defaultManager] fileExistsAtPath:pathString])
{
infoDict = [[NSMutableDictionary alloc] initWithContentsOfFile:pathString];
}
else
{
infoDict = [[NSMutableDictionary alloc]initWithObjects:[NSArray arrayWithObjects:#"BeginFrame",#"EndFrame", nil] forKeys:[NSArray arrayWithObjects:[NSNumber numberWithBool:YES],[NSNumber numberWithBool:YES], nil]];
if ([infoDict writeToFile:pathString atomically:YES])
{
NSLog(#"Created");
}
else
{
NSLog(#"Is not created");
NSLog(#"Path %#",pathString);
}
}
This is my code. I check if file is created, if not - I create a NSMutableDictionary and I write it to file at path, but writeToFile method returns NO. Where is problem? If I create this file with NSFileManager it works, but doesn't when I want to write a dictionary.

Answer 1

writeToFile:atomically only works if the dictionary you call it on is a valid property list object (see docs).
For a NSDictionary to be a valid property list object, among other things, its keys must be strings, but in your example the keys are NSNumber instances.

Answer 2

You can not control the content you are going to write sometimes. For example, you can't avoid a null value when you are going to write a JSON object that is gotten from a server.
NSData is compatible with these "invalid" values, so converting NSArray or NSDictionary to NSData is an ideal way in these cases.
write:
NSData *data = [NSKeyedArchiver archivedDataWithRootObject:jsonObject];
[data writeToFile:path atomically:YES];
read:
NSData *data = [NSData dataWithContentsOfFile:path];
NSDictionary *jsonObject = [NSKeyedUnarchiver unarchiveObjectWithData:data];