Why need I add self when assign attibute for one model? - objective-c

Question

I have one model.
I want to update one attribute each 3 seconds after init, and I need to change the attribute in one other view controller later.
the code is:
MODEL
#interface Ap : NSObject
#property (nonatomic, retain) NSString *address;
#property (nonatomic, retain) NSString *sessionId;
+ (id)sharedInstance;
#end
#implementation Ap
#synthesize sessionId, address;
-(id) init {
if (self = [super init]) {
self.address = nil;
self.sessionId = nil;
[NSTimer scheduledTimerWithTimeInterval:3 target:self selector:#selector(heartbeat) userInfo:nil repeats:YES];
}
return self;
}
-(void) updateSession{
sessionId = #"some string";
}
- (void) update{
self.sessionId = #"some value from network";
}
+ (Ap *)sharedInstance {
if (!sharedInstance) {
sharedInstance = [[super allocWithZone:NULL] init];
}
return sharedInstance;
}
#end
CONTROLLER
- (void) viewDidLoad {
[super viewDidLoad];
[[Ap sharedInstance] updateSession];
}
The error is: (lldb) bad access when model update
And I change the updateSession method to
-(void) updateSession{
self.sessionId = #"some string";
}
The error is gone, can anyone tell me why?

Answer 1

You should use self.ap = [[Ap alloc] init]; in your app delegate. Your model object might be getting released when you try to call update on that which might cause a crash.
Your code will look like this,
#interface SomeAppDelegate : UIResponder{
#property(nonatomic, retain) Ap *ap; //declare property here
#end
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
if(!self.ap){
self.ap = [[Ap alloc] init]; //use property here as self.ap which will retain ap. Otherwise it will not retain it.
}
}
Update:
Looks like you have removed the previous question and added a completely new question to the previous one. Anyways I will try to answer this one as well.
The error is gone, can anyone tell me why?
The reason is same as what I mentioned above. If you use self.sessionId = #"some string";, you are retaining the object. Because it is a property and a property will internally retain since you have declared that property as retain. But if you use sessionId = #"some string";, its scope is only inside that method since you are not using the property. You are directly setting the value without calling property and it will autoreleased after that method. And hence you will get a bad access.
I would suggest you to go through the apple documentation to understand more about properties and its working.