iOS KVO explained or How Key Value Observations work

There are so many ways to transfer data between objects. delegate, blocks, notifications, etc. Understanding them and how to use them properly is one of the most critical aspects of developing apps. Which ones should I used and why is very important. With that being said, here is one more that I've used many times before but always seemed to avoid really understanding, until now. KVO.

Key-Value Observing (KVO) is another way to learn about other objects without them really offering it up easily. The basis of this technique is if you want to know when a value of a different object changes you can create a KVO to detect and inform you of when this actually happens. Many apple APIs use this like AVPlayer. Personally I am not a big fan but I can appreciate why it is used.

Lets get down to it. Here is the declaration;

 [object addObserver:(NSObject *)anObserver
         forKeyPath:(NSString *)keyPath
            options:(NSKeyValueObservingOptions)options
            context:(void *)context];

-object is the object you want to observe the change on
-anObserver should whom you want to be listening, usually self.
-keyPath is the name of the property you want to listen for. Example is when a label text changes you would put in @"text"
-options is for when it actually changes, what do you want back. There are 4 options of which you can choose multiple;    
    NSKeyValueObservingOptionNew = 0x01, when you want what the new value is
    NSKeyValueObservingOptionOld = 0x02, when you want what the old value was
    NSKeyValueObservingOptionInitial = 0x04, when you want to know before it happens
    NSKeyValueObservingOptionPrior  = 0x08 when you want to know after it happens (can't get the old one then)
-context is a unique identifier that you can reference when the call back method is actually called. I needs to be unique and nothing else. So people like to get creative with this sometimes. Most of the time I see something like this;

static void *AVPlayerDemoPlaybackViewControllerStatusObservationContext = &AVPlayerDemoPlaybackViewControllerStatusObservationContext;

This is just a pointer that references itself. It is small and unique so it fits perfectly. And when you want to find it, you can do so easily. 

Now that you are listening, now what? How do I actually do something? Well you need to run the actually method it plans to run when it needs to of course. It is;

- (void)observeValueForKeyPath:(NSString*) path

                      ofObject:(id)object

                        change:(NSDictionary*)change

                       context:(void*)context

-path is the string you want

-object is the actual object 

-change is a dictionary of the items you requested

-context is that unique context you created above

Since this method is called for all value changes you should isolate what you are looking for each time by referencing path, context and/or object. Once you have done that, grab the change dictionary and see what was actually changed. 

Note: Most of the time you want to compare the new and old one when you get them to make sure it was not actually just updated with the same thing twice.

   if (context == AVPlayerDemoPlaybackViewControllerStatusObservationContext){
        AVPlayerItemStatus status = [[change objectForKey:NSKeyValueChangeNewKey] integerValue];
        self.streamStatusType = StreamStatusTypeUnknownState;
        switch (status){
            case AVPlayerItemStatusUnknown:{
                self.streamStatusType = StreamStatusTypeUnknownState;
                break;
            }
            case AVPlayerItemStatusReadyToPlay:{
                self.streamStatusType = StreamStatusTypeReadyToPlay;
                break;
            }
            case AVPlayerItemStatusFailed: {
                self.streamStatusType = StreamStatusTypeFailed;
                break;
            }
        }
     }

And that is about it!

Bonus note: removing the observer can actually crash the app randomly is seems so it is good practice to add a @try @catch around it just in case.

        @try {

            [self.playerItem removeObserver:self forKeyPath:@"status"];

        }

        @catch (NSException *exception) {

            

        }

        @finally {

            

        }

References:
Apple KVO intro
Apple KVO example

Creating an object with multiple delegates in iOS 8

Sometimes one delegate is just not enough and notifications are just not reliable or their timing is to unpredictable. Here comes the solution, multiple delegates! Bonus, this should be multithreading safe.

The multiple delegates still will not give you an order on which receiver gets the information first but if you want that, you aren't programming correctly.

First you need to make a delegate protocol;

@protocol AudioPlayerDelegate <NSObject>

@optional

- (void)AudioPlayer:(AudioPlayer *)audioEngine onStateChange:(StreamStatusType)state;

@end

in the interface, create the delegate calls;

//Delegates

- (void)addDelegate:(id<AudioPlayerDelegate>)aDelegate;

- (void)removeDelegate:(id<AudioPlayerDelegate>)aDelegate;

Also in the interface create a private delegate as a hashtable;

@private

  NSHashTable *delegates;

in the implementation, setup the delegates in the init;

  delegates = [NSHashTable weakObjectsHashTable];

and create the methods;

#pragma mark - Delegates

- (void)addDelegate:(id<AudioPlayerDelegate>)aDelegate {

    [delegates addObject:aDelegate];

}

- (void)removeDelegate:(id<AudioPlayerDelegate>)aDelegate {

    [delegates removeObject:aDelegate];

}

when you want to use them, here is the best and safest way to do so;

    __weak AudioPlayer *weakSelf = self;

    id<AudioPlayerDelegate> delegate = nil;

    for (delegate in [delegates copy]) {

        if (delegate && [delegate respondsToSelector:@selector(AudioPlayer:onStateChange:)]) {

            dispatch_async(dispatch_get_main_queue(), ^{

                [delegate AudioPlayer:weakSelf onStateChange:weakSelf.streamStatusType];

            });

       }

    }

This might be overkill but it is always good to be thread safe. 

And that's it! 

How to make a 3D planet in xcode using SceneKit

For some reason I've always wanted to create a spinning 3D globe but never had the time or ability. I decided last weekend to see if I could find someone or some way to make one. I checked all the usual places but the few I found were either overly complicated, way out of date, or no longer worked. So I decided to put my hat in the ring. I thought this might also be a good way to play with scenekit since I have never touched it before.

Wow. It worked better than I could ever imagine. Here is how to make the whole world in a few steps and about 15 lines of code.

First create a new project using scene kit. I like objective-c but you can do whatever you want with the language. You should be able to run it right away and see an airplane that you can move around the screen in 3 dimensions. Don't worry about that.

Here is the quick code;

-(void)worldSetup{

    SCNView *scnView = (SCNView *)self.view;

    

    myScene = [[SCNScene alloc] init];

    

    SCNSphere *planetSphere = [SCNSphere sphereWithRadius:5.0];

    SCNNode *sphereNode = [SCNNode nodeWithGeometry:planetSphere];

    [myScene.rootNode addChildNode:sphereNode];

    

    SCNMaterial *corona = [SCNMaterial material];

    corona.diffuse.contents = [UIImage imageNamed:@"earth"];

    corona.specular.contents = [UIColor colorWithWhite:0.6 alpha:1.0];

    corona.shininess = 0.5;

    [planetSphere removeMaterialAtIndex:0];

    planetSphere.materials = @[corona];

    

    // create and add a light to the scene

    scnView.autoenablesDefaultLighting = YES;

    

    // create and add a camera to the scene

    scnView.allowsCameraControl = true;

    

    mySphere = planetSphere;

    

    scnView.scene = myScene;
}

earth

in celebration of seeing pluto,
I added some love

Here is the alternate slightly more custom code.

-(void)worldSetup{

    SCNView *scnView = (SCNView *)self.view;

    

    myScene = [[SCNScene alloc] init];

    

    SCNSphere *planetSphere = [SCNSphere sphereWithRadius:5.0];

    SCNNode *sphereNode = [SCNNode nodeWithGeometry:planetSphere];

    [myScene.rootNode addChildNode:sphereNode];

    

    SCNMaterial *corona = [SCNMaterial material];

    corona.diffuse.contents = [UIImage imageNamed:@"earth"];

    corona.specular.contents = [UIColor colorWithWhite:0.6 alpha:1.0];

    corona.shininess = 0.5;

    [planetSphere removeMaterialAtIndex:0];

    planetSphere.materials = @[corona];

    

    // create and add a light to the scene

    SCNNode *lightNode = [SCNNode node];

    lightNode.light = [SCNLight light];

    lightNode.light.type = SCNLightTypeAmbient;

    lightNode.light.color = [UIColor colorWithWhite:0.37 alpha:1.0];

    lightNode.position = SCNVector3Make(0, 50, 50);

    [myScene.rootNode addChildNode:lightNode];

    

    SCNNode *omniLightNode = [SCNNode node];

    omniLightNode.light = [SCNLight light];

    omniLightNode.light.type = SCNLightTypeOmni;

    omniLightNode.light.color = [UIColor colorWithWhite:.70 alpha:1.0];

    omniLightNode.position = SCNVector3Make(0, 40, 40);

    [myScene.rootNode addChildNode:omniLightNode];

    

    // create and add a camera to the scene

    scnView.allowsCameraControl = true;

    

    // create and add a camera to the scene

    SCNNode *cameraNode = [SCNNode node];

    cameraNode.camera = [SCNCamera camera];

    cameraNode.position = SCNVector3Make(0, 0, 20);

    [myScene.rootNode addChildNode:cameraNode];

    

    mySphere = planetSphere;

    

    scnView.scene = myScene;

}

You still need to get an 'earth' texture from the internet somewhere, should be easy to find, probably from NASA. And that is it!

Here is a link to the app in github. https://github.com/Darkin/3DPlanet

Helpful CocoaPods

The classic and helpful AFNetworking
Github - CocoaPod pod "AFNetworking", "~> 2.0"

The robust and powerful JSON data network restful RESTKIT
Github - CocoaPod pod 'RestKit', '~> 0.24.0'
                                # Testing and Search are optional components
                                pod 'RestKit/Testing', '~> 0.24.0'
                                pod 'RestKit/Search',  '~> 0.24.0'

The nice JSON to/from NSDictionary template MANTLE
Github - CocoaPod  pod 'Mantle', '~>2.0.2'

The pretty much required facebook 1 2 3
Github - CocoaPod 'FBSDKCoreKit', 'FBSDKLoginKit', 'FBSDKShareKit' .

The ever helpful image downloader and cacher SDWebImage
Github - CocoaPod pod 'SDWebImage', '~>3.7'