Essential Cocos2D (Sample)

Strategies for Accessing Other Nodes

Many times over, you may wonder how to get access to a particular node that lives somewhere else in the node hierarchy. This article answer this question.

Accessing the Currently Running Scene via Director

images_s/en_GB/3296/1/_/images/icons/emoticons/add.png Benefits

images_s/en_GB/3296/1/_/images/icons/emoticons/forbidden.png Drawbacks

  • readily available

  • few lines of code

  • running scene changes before scene deallocs

  • real risk of casting object to wrong class

You can access the running scene via CCDirector like so:

CCScene* runningScene = [CCDirector sharedDirector].runningScene;

Now if you need to access your specific subclass of CCScene, you'll have to cast it. Since this poses the risk that the runningScene may not actually be a subclass of MyScene, you should verify that:

CCScene* runningScene = [CCDirector sharedDirector].runningScene;
if ([runningScene isKindOfClass:[MyScene class]])
    MyScene* myScene = (MyScene*)runningScene;
    // use myScene here
    // error handling here, or ignore

If you execute CCDirector's replaceScene, pushScene or popScene methods, the runningScene property will point to the new scene. Any code that accesses the runningScene from this point forward may not receive the scene object you might expect. Therefore the isKindOfClass check in the above code sample.

Accessing a Specific Scene via Shared Instance (Singleton-like)

images_s/en_GB/3296/1/_/images/icons/emoticons/add.png Benefits

images_s/en_GB/3296/1/_/images/icons/emoticons/forbidden.png Drawbacks

  • shared scene available until it deallocs

  • no casting required

  • requires more setup code

  • can not replace scene with other scene of the same class

Instead of going through the director you can also turn your CCScene subclass into a temporary singleton. This has the advantage that access is still possible during a scene change, until the scene has been deallocated. It also removes the necessity of casting the returned scene.

It does require a bit more setup code. Here's an example implementation of a Scene singleton that's accessible for the lifetime of the scene. Contrary to a regular singleton, you can not initialize it by executing the sharedMyScene accessor - in this case the temporary singleton asserts. Here's the interface:

MyScene interface
@interface MyScene : CCScene
    // custom instance vars here...
+(MyScene*) sharedMyScene;
// custom methods here...

And the implementation:

MyScene implementation
@implementation MyScene
static MyScene* sharedInstance;
-(id) init
    self = [super init];
    if (self)
        NSAssert1(sharedInstance == nil, 
                  @"there's already an active instance of %@", 
                  NSStringFromClass([self class]));
        sharedInstance = self;
    return self;
-(void) dealloc
    // you could also nil the instance in cleanup to let it go earlier
    sharedInstance = nil;
+(MyScene*) sharedMyScene
              @"%@ temporary singleton is not yet initialized!", 
              NSStringFromClass([self class]));
    return sharedInstance;

The MyScene init method asserts that sharedInstance is nil. If it were not, we knew there would be another MyScene still active, and child nodes accessing the wrong instance could cause bugs that are hard to track down. That also means you can't replace MyScene with another instance of MyScene, you'll have to go through an intermediate (loading) scene. You can work around this by setting the sharedInstance to nil in the cleanup method, but then the sharedInstance would be nil the moment you call one of CCDirector's scene changing methods (replaceScene, pushScene, popScene, etc).

The sharedMyScene method asserts that sharedInstance is not nil. This is for cases where the code is used as if it were a true singleton. A true singleton would instantiate itself when its shared instance accessor is first executed. But we better not do this for shared access to a CCScene object because the sharedMyScene method can't ensure that the scene is properly run by CCDirector. It's better to leave this up to the user and let the CCScene behave normally concerning the Cocos2D node hierarchy.

Now you can access the running MyScene object from any other node:

// import the header
#import "MyScene.h"
// somewhere in a method:
MyScene* myScene = [MyScene sharedMyScene];

It is good practice to access sharedMyScene only from nodes which are children or grandchildren of MyScene, and are not used in any other scene as well. That way it is ensured that child nodes accessing the MyScene object can do so safely for their entire lifetime, since their lifetime begins later and ends earlier than that of the scene.

Accessing Sibling Nodes via Parent

images_s/en_GB/3296/1/_/images/icons/emoticons/add.png Benefits

images_s/en_GB/3296/1/_/images/icons/emoticons/forbidden.png Drawbacks

  • readily available

  • reasonably fast

  • requires casting

  • requires tagging

  • sibling may be nil

To gain access to nodes which you know have the same parent, you can do so by sending getChildByTag to the parent:

CCNode* otherNode = [self.parent getChildByTag:123];

Of course you need to properly tag the nodes first, and you may have to cast the returned node.

The alternative would be to subclass the parent node's class if not done already, and give the parent class properties with which you can access the commonly used child nodes.

@interface MyParent : CCNode
@property (assign) CCSprite* childSprite;
@property (assign) CCLabelBMFont* childLabel;
@implementation MyParent
@synthesize childSprite, childLabel;

You need to assign childSprite and childLabel once when you initialize respectively add them as child nodes. Then the child nodes can access each other with a little casting:

// in the child sprite class:
MyParent* myParent = (MyParent*)self.parent;
CCLabelBMFont* childLabel = myParent.childLabel;

A common issue is caused by a sibling like childSprite receiving a strong (retaining) reference to another sibling like childLabel, and vice versa. This can easily lead to a retain cycle if you're not careful, even under ARC!

It is always better to relay access to sibling and their child nodes to the parent node. The parent node can safely retain references to a child or grandchild node. All other strong references (child retains parent or grandparent, child retains sibling or sibling's child) are prone to causing a retain cycle.

Accessing Other Nodes via Parent and Grandparents

images_s/en_GB/3296/1/_/images/icons/emoticons/add.png Benefits

images_s/en_GB/3296/1/_/images/icons/emoticons/forbidden.png Drawbacks

  • readily available

  • ok for batched nodes

  • requires casting

  • difficult to change node hierarchy

  • potential to break badly (crash, bugs)

Now wait a minute … couldn't you access the scene simply by using self.parent with the appropriate number of parents?

Yes, indeed you could:

// Hi Grand-grand-grand-grandparent!
MyScene* myScene = (MyScene*)self.parent.parent.parent.parent.parent;
Don't try this at home!

The above code is not bad practice. It is terrible practice!

This code breaks as soon as you add or remove a node from the hierarchy, or move the node executing the above code to some other place in the node hierarchy. Anything that changes the number of parent calls necessary to reach the scene, or whatever parent node you want to get, is going to break the above code. At best, the returned reference is nil and nothing happens. At worst it's a totally different object and that will crash with either "does not respond to selector" or EXC_BAD_ACCESS.

Not only that, there can be subtle, hard to debug issues introduced as well. For example if you were to change a common property of the parent, such as position and scale, you may only notice on screen that there's something wrong with the position or scale of some nodes. Or possibly all nodes if suddenly you position or scale the entire layer or scene. These issues are not obviously attributed to the use of the multiple-parent-accessor style. It's side effects like these that will drive you crazy.

Finally, it's a total slap in the face for code reuse and maintenance. You can't use the node using the above code anywhere else unless the fifth parent is the parent it needs for whatever reason. And you're also going to hesitate making beneficial or necessary changes to your node hierarchy, because it'll break your code.

There is an acceptable use for self.parent.parent, and that is when you want to bypass a sprite-batched sprite's or particle-batched particle's parent node:

// ok if parent is CCSpriteBatchNode or CCParticleBatchNode
MyParent* myParent = (MyParent*)self.parent.parent;

It is acceptable because the relationship of the sprite/particle and batch node is hardly going to change.

Similarly, you may find multi-parent-accessor style convenient in very self contained nodes. But frankly one has to wonder how one got there with this multi-parent-accessor style? Maybe that node is so self-contained mainly because it was locked into this specific node hierarchy setup by using the multi-parent-accessor style. In any case, such self-contained nodes usually grow over time, they evolve and change. So even in that case it seems counterproductive to use the multi-parent-accessor style because it prevents them from changing and evolving.

Accessing a Grandparent's Properties via Shared Instance

images_s/en_GB/3296/1/_/images/icons/emoticons/add.png Benefits

images_s/en_GB/3296/1/_/images/icons/emoticons/forbidden.png Drawbacks

  • no casting required

  • requires extra code

  • should only be used for child nodes

  • tempting to get into bad habit of sharing many nodes

This of course raises the question: how the frack are child nodes supposed to gain access to grandparent or grand-grandparent properties? Specifically if they're not the scene.

Answer: the same way you can access the Scene as a temporary singleton. See the earlier paragraph on accessing a specific scene via shared instance. There's nothing that prevents you from changing any parent node to have a sharedParentNode or similar accessor method that returns the instance. The only thing you have to ensure by discipline, is not to use this accessor by any other node but children and grandchildren of that node.

I know what you're thinking. Don't give many or most of your scene's nodes a static accessor to conveniently access practically any node from anywhere!

Because that as well makes the node hierarchy very rigid and hard to change by assuming that a certain node must exist and be initialized. This makes re-use difficult because you created a strong dependency between one branch of the node hierarchy and a specific node that's outside that particular branch. Hence the advice that only child nodes should assume that one of their parents exists and is of a specific class. That way, the access remains self contained within that branch of the node hierarchy.

Accessing A Node That May be Anywhere

images_s/en_GB/3296/1/_/images/icons/emoticons/add.png Benefits

images_s/en_GB/3296/1/_/images/icons/emoticons/forbidden.png Drawbacks

  • read the text

  • read the text

Now comes the difficult question. If you shouldn't just make any node a temporary singleton, and multi-parent-accessor style is bad news, how could unrelated node A with 3 parents gain access to unrelated node B with 5 parents - without any of them knowing where they are?

Here's some questions for you if you run into this situation:

  • Why does node A need a reference to node B (or vice versa)?

  • Can the access be relayed / routed through other (controlling) nodes?

  • If they require direct access to each other, how come they're in so very different branches of the node hierarchy?

  • Do they each store data that maybe should be global in the scene or a singleton for game data?

Setting any lingering design issues aside - (this is your homework assignment and yes, there will be a test on monday!) - the most common misconception is that you need direct access to the other node.

Rigidly Independent Message Relay

Assume the case where you have a player node that lives in a gameplay layer, and a score label that lives in the UI layer. Now the player kills a bad guy and that gives him score. We register the kill in the player node, so how do we inform the score label?

The most obvious solution seems to be that the player needs a reference to the score label. But that creates a strong dependency to the score label, and how that score is displayed (or if at all). Now depending on how rigorous you want to model your "dependency borders", you could route the score update from the player via the gameplay layer to the scene which then informs the UI layer which in turn notifies the score label. That is the ultra-rigid variant of separating different code modules from one another. If you had to separate the ties between gameplay layer and UI layer, you'd only have to modify the scene's routing methods in order to cut out and ignore the messages going to either the UI layer or the gameplay layer. This makes your node hierarchy flexible, but it can also be tedious and in rare cases costly.

Reasonably Independent Message Relay

A more reasonable approach seems to be to allow the player direct access to the UI layer via a shared instance of the UI layer. The UI layer then informs the score label. So how is that better than accessing the score label itself?

First, it's more reasonable because you probably will always have the UI layer where the player lives. So it's reasonably safe to assume that it'll exist, and safe to require it. Not so much for the score label though. The score label might be changed from a numeric label to a score bar or something else entirely. Furthermore, wouldn't it be cool if there would be an animation playing where the score appears over the player's or enemy's head and floats towards the score label? In addition to updating the score label (or bar), the UI layer could also kick off such an extra animation. The player doesn't know about this. In fact, he doesn't need to. All he knows and should know is that he scored. Whether it was big time or not, that's up to the UI layer.

Event-Driven or Polling

It is this: in a branching, tree-like system of objects such as the Cocos2D node hierarchy, there's really no way around performing some message routing.

In general there are two strategies:

  1. event-driven (messaging)

  2. polling (frequent updates)

For example, in a multiplayer game the UI layer could be informed about every player joining or leaving (messaging). It could then store a reference to each player while the player remains active. In order to update the player's status (hitpoints, mana, etc) it could periodically check the player's properties related to hitpoints and mana (polling).

The alternative is for the UI layer to rely solely on messaging. So besides being informed when players join and leave, the UI layer would also receive messages passing in a specific player, and the type of change that occured. Or simply a general request to update all player UI values if there aren't that many.

When to Use Polling

Polling is really only preferable in two instances:

  1. very frequent changes are the norm and expected (ie several times per second)

  2. the observed object does not send the required messages (and the object can not be changed to do so)

The latter is mainly to pull out data from objects which were never designed to inform other about certain changes. Since you're writing most of the classes yourself and can change the sending node, the only real question is whether the properties you're interested in polling are expected to change frequently or not. Frequently as in probably every couple frames. In that case polling may be suitable. In all other cases event-driven is the way to go. And that means sending a message to another node whenever a change occurs. Messaging also has the added benefit that the target won't hold a strong reference to the sending node, therefore eliminating potential retain cycle issues.

Informing Other Nodes via Messaging and Notifications

There are generally two ways of informing another node that a change occured:

  1. send (and route) message to node

  2. send a NSNotification

Routing Messages

images_s/en_GB/3296/1/_/images/icons/emoticons/add.png Benefits

images_s/en_GB/3296/1/_/images/icons/emoticons/forbidden.png Drawbacks

  • best practice

  • leads to cleaner, modular code design

  • reasonable shortcuts possible
    (via shared instance accessors)

  • can be applied too rigidly

  • requires adding methods that only forward (route) a message

For number one, there's a difficult truth to accept: in hierarchical systems with many branches, direct access between individual nodes is often not possible, and where it may be possible it is not advisable. Hence the general recommendation to refactor your scene or the scene's first-level child nodes (often layers and nodes) to be message relaying controllers.

For example if your scene contains the gameplay and UI layers, you'll want the scene to give access to the gameplay and UI layer via properties:

GamePlayLayer* gameLayer = [GameScene sharedGameScene].gamePlayLayer;
UILayer* uiLayer = [GameScene sharedGameScene].uiLayer;

Now that you have access to individual layers, the player node for example could then send the score update message to the UI layer:

-(void) killedEnemy:(Enemy*)enemy
    UILayer* uiLayer = [GameScene sharedGameScene].uiLayer;
    [uiLayer updateScore:enemy.scoreWhenKilled 
                 painful:(self.currentWeapon == kWeaponMegaBlaster)];

The UI layer is fed with just the information it needs. Notice that although we could send both the player and enemy nodes to the UI layer, it doesn't really need to know about these objects. In fact the less the UI layer knows about game objects, the better. The UI layer can then do its job, which is not only updating the score label but possibly also playing audio and particle effects:

-(void) updateScore:(int)score atPosition:(CGPoint)killPos painful:(BOOL)painful
    [self.scoreLabel updateScore:score];
    if (painful)
        [self createBloodAndGoreEffectAtPosition:killPos];
        [self playPositionalAudio:@"Wuaaarghsplash.caf" atPosition:killPos];
// for conveniently updating the score and nothing else
-(void) updateScore:(int)score
    [self updateScore:score atPosition:CGPointZero painful:NO];

Everything that should happen on the UI side, like playing audio, particle effects and updating the HUD, is now handled on the UI side. Furthermore, no dependencies are introduced by unnecessarily passing in the player or enemy nodes, or passing in the weapon types. Finally, for the common case where only the score needs to be updated (ie item pickup etc) a convenience method allows to skip the two extra parameters that are only intended for kill scores.


images_s/en_GB/3296/1/_/images/icons/emoticons/add.png Benefits

images_s/en_GB/3296/1/_/images/icons/emoticons/forbidden.png Drawbacks

  • message can have multiple receivers

  • sender does not know about receivers

  • can help to reduce code dependencies

  • several times slower than ObjC message send

  • additional parameters require temporary NSDictionary

  • forgetting to unregister could leak or crash

  • globally unique identifier string required

Notifications are useful when the sender doesn't know anything about the receiver. Not even whether the sent message will be received at all, or by whom. Generally NSNotification are only used when the sender has no way of easily informing the receiver(s) via regular Objective-C messaging. You can learn more about notifications in Apple's Notification Programming Topics guide.

Sender and receiver only have to agree on a common notification name, and perhaps named parameters to pass along. The latter is particularly disadvantageous for fast paced games, since you have to create a temporary NSDictionary and populate it with the data every time you send a notification with additional parameters. That's an overhead you should try to avoid in notifications that are sent every frame or hundreds of times per second. I'll leave this out of this discussion, there are examples in Apple's notification guide.

The receiver needs to register and unregister itself at the appropriate time to receive a specific notification via the NSNotificationCenter:

-(void) onEnter
    [super onEnter];
    // register for player died notification
    NSNotificationCenter* notifyCenter = [NSNotificationCenter defaultCenter];
    [notifyCenter addObserver:self
                       object:nil]; // nil means that notification could be sent from any object
-(void) cleanup
    [super cleanup];
    // deregister as observer
    [[NSNotificationCenter defaultCenter] removeObserver:self];

In cases where you want to receive the message only when it is sent from a specific object, you'd have to pass the object in as the last parameter. Otherwise leave it nil to receive notifications with this name from all objects.

The kPlayerDiedNotification is just an NSString. The notification names should be defined in a global header file, and the notification string can be anything, as long as the string is unique:

static NSString* kPlayerDiedNotification = @"player died";

Defining the notification strings globally instead of writing the strings inline with the message parameters is good practice. This prevents any potential typo that could lead to difficult-to-debug issues like not sending or receiving (some) notifications.

To send a message you would use the postNotificationName method and optionally pass in an object that the receiver might need:

NSNotificationCenter* notifyCenter = [NSNotificationCenter defaultCenter];
[notifyCenter postNotificationName:kPlayerDiedNotification

The receiver then has to implement the selector with a specific signature: it returns void and receives an NSNotification* object:

-(void) playerDied:(NSNotification*)notification
    CCLOG(@"player died: %@ - received object: %@ - user info: %@", 
          notification, notification.object, notification.userInfo);

The received NSNotification gives you access to the passed in object, and the userInfo NSDictionary should you choose to use it.