trying to understand the Singleton concept in objective-c with many variables - objective-c

Question

I'm trying to understand the Singleton concept in objective-c.
Most examples that I did found just refer to a single variable.
I'm a bit lost about how to adapt the examples to handle many variables as per an example the accelerometer values that return x, y, z.
Can you guide me a bit further ?

Answer 1

A Singleton refers to a special object that can only exist once inside the lifespan of your application. That object can have as many variables and properties as necessary.
// Singleton.h
#interface Singleton : NSObject
#property (readwrite) int propertyA;
#property (readwrite) int propertyB;
#property (readwrite) int propertyC;
+ (Singleton *)sharedInstance;
#end
The key to a Singleton is that it can only be created once. Usually in Objective-C we use the #synchronized() directive to make sure it only gets created once. We put this in a convenience class method called sharedInstance and return our Singleton. Since the Singleton is just an object it can easily have multiple properties, variables, and methods.
// Singleton.m
#import "Singleton.h"
#interface Singleton ()
{
int variableA;
int variableB;
int variableC;
}
#end
#implementation Singleton
static Singleton *sharedInstance = nil;
+ (Singleton *)sharedInstance
{
#synchronized(self) {
if (sharedInstance == nil) {
sharedInstance = [[Singleton alloc] init];
}
}
return sharedInstance;
}
+ (id)allocWithZone:(NSZone *)zone {
#synchronized(self) {
if (sharedInstance == nil) {
sharedInstance = [super allocWithZone:zone];
return sharedInstance;
}
}
return nil;
}
- (id)init {
self = [super init];
if (self) {
// Inits
}
return self;
}
#end
This is not the ONLY way to create a Singleton. Remember the important part is it can only be created once. So you can take advantage of newer Grand Central Dispatch calls when developing for OSX and iOS such as dispatch_once.
Talking to the Singleton
So lets say you have another object elsewhere talking to the Singleton. This can be done anywhere you #import "Singleton.h"
- (void)someMethod
{
// Setting properties
int valueA = 5;
[[Singleton sharedInstance] setPropertyA:valueA];
// Reading properties
int valueB = [[Singleton sharedInstance] propertyB];
}