Delegating the even callback in objective-C - objective-c

Question

I am trying to wrap my head around how does the an event callback delegation works. So far I have written following code which btw works just fine:
Bridge.h
#protocol BridgeDelegate <NSObject>
- (void) bridgeLock;
#end
#interface Bridge : NSObject
+(instancetype) sharedInstance;
#property (weak, nonatomic) id<BridgeDelegate> bridgeDelegate;
- (void) wipe;
#end
Bridge.m
#implementation Bridge
+(instancetype) sharedInstance {
static dispatch_once_t pred;
static id shared = nil;
dispatch_once(&pred, ^{
shared = [[super alloc] initUniqueInstance];
});
return shared;
}
-(instancetype) initUniqueInstance {
return [super init];
}
- (void) wipe
{
NSLog(#"lock in bridge called");
if(self.bridgeDelegate)
{
[self.bridgeDelegate bridgeLock];
}
}
#end
Plugin.h
#interface Plugin : NSObject<BridgeDelegate>
#property (strong, nonatomic) Bridge *bridge;
- (void) pluginInitialize;
#end
Plugin.m
#implementation Plugin
- (void) pluginInitialize
{
self.bridge = [Bridge sharedInstance];
self.bridge.bridgeDelegate = self;
}
- (void)bridgeLock
{
NSLog(#"lock in plugin called");
}
#end
When I call the following code in applicationDidBecomeActive
Bridge* bridge = [Bridge sharedInstance];
Plugin* plugin = [[Plugin alloc] init];
[plugin pluginInitialize];
[bridge wipe];
I get the following expected output:
lock in bridge called
lock in plugin called
Now my questions:
How exactly is the delegate work? In the sense, Plugin is only implementing the function bridgewipe(), right? Why and how bridgeLock is being called at first place?
Does this have anything to the fact that Bridge is a singleton. Had I made Bridge a non singleton class, will the end result be same.

Answer 1

1.How exactly is the delegate work? In the sense, Plugin is only implementing the function bridgewipe(), right? Why and how bridgeLock is being called at first place?
In the above pasted code "Plugin.m" is implementing - (void)bridgeLock
Does this have anything to the fact that Bridge is a singleton. Had I made Bridge a non singleton class, will the end result be same.
No

Answer 2

Bridge* bridge = [Bridge sharedInstance];
this bridge we call it B1;
B1 is a instance make by the method "sharedInstance";
then you call the following code :
Plugin* plugin = [[Plugin alloc] init];
[plugin pluginInitialize];
pluginInitialize method your code is
{
self.bridge = [Bridge sharedInstance];
self.bridge.bridgeDelegate = self;
}
when code executed, self.bridge is also a instance make by the method "sharedInstance"; it's equal to B1 with address and also make B1's delegate == self;
so when you call [bridge wipe];
It will nslog #"lock in bridge called";
Because self.bridgeDelegate is not nil, so delegate will call the bridgeLock method;
Then nslog #"lock in plugin called".
About your second question, when you make Bridge a non singleton class, I think the result will be different.

Answer 3

#hariszaman, explanation is right but I would like to expand more on this so it can help someone in the future. Basically this is what happening.
I am creating an instance of the Bridge class. This instance in the memory has a reference variable of type BridgeDelegate.
As I instancetiate Plugin, BridgeDelegate variable starts pointing to the the Plugin class instance.
Now when lock is called, it calls the bridgelock method of the class that is pointed by BridgeDelegate pointer which in this case is Plugin.
It doesn't matter if the Bridge class is not singleton because following line in pluginInitialize:
{
self.bridge = [Bridge sharedInstance];
self.bridge.bridgeDelegate = self;
}
will be changed to :
{
self.bridge = [[Bridge alloc] init];
self.bridge.bridgeDelegate = self;
}
and steps 1,2 and 3 will be repeated the same way.