How properly assign singleton to a variable in iOS? - ios

Question

I have a project written in Objective-C. Inside it I use singleton.
Its declaration is:
+ (id)sharedInstance
{
static id sharedInstance = nil;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
sharedInstance = [[self alloc] init];
});
return sharedInstance;
}
I init it inside each class where I use it by this code:
singApp = [XXXsingApplication sharedInstance];
When I run Xcode Analyzer I get following warning:
Instance variable used while 'self' is not set to the result of
'[(super or self) init...]'
Why I get this warning?
I suppose I have some general misunderstanding of matter, because AFAIK it has only single instance and must not be init again to avoid creating several instances?
EDIT:
My init looks like:
-(id)init
{
self = [super init];
// init read-only values
_appSwVersion = APP_SOFT_VERSION;
_appSwDateTime = APP_SOFT_DATETIME;
_websDns = WEBS_DNS;
_testWebsUrl = TEST_WEBS_DNS_URL;
// init properties dictionary
_propListDict = [[NSMutableDictionary alloc] init];
#try {
if (self) {
// load settings
[self loadSettingsFromFile];
// init custom Bundle
[self setCustomBundle];
}
}
#catch (NSException *exception) {
NSLog(#"EXCEPTION\nName-> %#\nDescription-> %#", [exception name], [exception description]);
}
return self;
}

Answer 1

Exceptions are there to catch programming errors. You don't catch and handle exceptions in Objective-C. When an exception happens, you let it crash. This is Objective-C, not Java or C++. You don't handle exceptions, you fix the code.
You should check whether self == nil immediately after calling [super init]. Think about what you are doing: You are assigning to an instance variable which will lead to a crash if self == nil. Sure, there will be cases where people are 100% sure that self cannot be nil. But five lines later, you check whether self == nil. That combination is a clear indication of a bug. If you check, you must check before you assign to an instance variable. If you assign to an instance variable, any check is too late.

Answer 2

Your code should look like this.
-(id)init {
self = [super init];
#try {
if (self) {
// init read-only values
_appSwVersion = APP_SOFT_VERSION;
_appSwDateTime = APP_SOFT_DATETIME;
_websDns = WEBS_DNS;
_testWebsUrl = TEST_WEBS_DNS_URL;
// init properties dictionary
_propListDict = [[NSMutableDictionary alloc] init];
// load settings
[self loadSettingsFromFile];
// init custom Bundle
[self setCustomBundle];
}
} #catch (NSException *exception) {
NSLog(#"EXCEPTION\nName-> %#\nDescription-> %#", [exception name], [exception description]);
}
return self;
}