How do I rewrite a plist if its data types are immutable?

I am getting comfortable with using plists for initializing my app. I now want to save app state back to the plist used to initialize the app and I find myself stuck. At application startup I ingest the plist into an NSDictionary which is immutable. I now want to update the NSDictionary by replacing old values with new values for existing keys and write to the plist via [NSDictionary writeToFile:atom开发者_C百科ically]. How do I get around the immutability of NSDictionary?

Thanks, Doug

UPDATE - Not quite there yet

I followed zneak's suggestion and ingested my settings file into an NSMutableDictionary. Works fine. Prior to writing the plist out I confirm that new values now replace old values. Cool. Good to go.

Problem: when I write the file thusly:

if ([self.settings writeToFile:plistPath atomically:YES] == NO) {

    NSLog(@"Unable to write plist");
}

The method happily completes properly - the conditional is YES rather then NO - but I see no file. Where has the file gone?

Shouldn't I see the new file in my directory tree?

Make it a NSMutableDictionary by calling -[myDict mutableCopy], then use this one for writing the file; or simply load the plist using [NSMutableDictionary dictionaryWithContentsOfFile:(NSString*)] to get a mutable instance to start with instead of an immutable one.

Use an NSMutableDictionary (you can use -[NSDictionary mutableCopy] to create it). You can then use writeToFile just as with an NSDictionary.