Design Questions from a newbie

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Design Questions from a newbie

(15 posts) (5 voices)

Started 3 months ago by Tanner85
Latest reply from Tanner85

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Tanner85
Member

Hello everybody,
I started with Objective-C and Sparrow just 2 weeks ago.
I have everything set up and running, I've also managed to learn basics and tutorials here and there.

I'm trying to build a 2D horizontal scrolling platformer with "Screens" piled one after another (using BEParallax class as a starting point) and for now it works. I have a player sprite that stays at the screen's center and sometimes I spawn an zombie enemy that wants to reach the player.

Here comes my question: now that i've understood the basics from Sparrow, how can I manage the different character classes to communicate to each other for doing collision detection or simply increase life counter and so on?

I've seen people speaking of SINGLETONS and design patterns, then someone speaks about MESSAGING an event to other classes to achieve status changes...

So what is the best approach and how do I implement a good game logic?

Thanks in advance.

Posted 3 months ago #
nilaf
Moderator

Depends on what you want to share between classes

Events help to communicate between classes.Use this in case of reporting a collision etc. http://wiki.sparrow-framework.org/manual/event_handling

If you want share common flags such as pauseflag , level number etc use a singleton class.
http://forum.sparrow-framework.org/topic/accesing-variables

Posted 3 months ago #
alex_h
Moderator

I would recommend handling your collision detection from a central run loop (onEnterFrame event handler most likely) in the main class of your game. Use a repeat loop to check each relevant object for collision with your player.

Posted 3 months ago #
Jugibur
Moderator
Hi Tanner85,

The Singleton-Pattern is used if you want ensure that your class get instanced only once and have a global access. I.e. I'm using a texture manager as singleton like that:
SPTexture *texture = [[SXMediaManager shared] textureByKey:@"textureKey"];
One goal of using patterns is to get reusable, extendable and exchangeable objects.
With the event system you can avoid closely coupled objects, because you communicate indirect to the listeners of an event.

If some listener got an message you can get the dispatcher (target) like this:
- (void)onSomethingHappened:(SPEvent*)event { YourObject object = (YourObject*)event.target; [object someMethod]; }
Note: In this case you must import the "YourObject"-class.

If you want more separation you can use protocols, i.e.
- (void)onSomethingHappened:(SPEvent*)event { id object = (id<SomeProtocol>)event.target; [object someMethod]; // SomeProtocol knows this method }
Posted 3 months ago #
alex_h
Moderator

Incidentally, if you are really curious to learn about design patterns, I can highly recommend reading this book:

"Head First Design Patterns"
http://www.amazon.co.uk/Head-First-Design-Patterns-Freeman/dp/0596007124

It's focus is Java, but as usual the concepts concerned are equally relevant to Objective C or any other comparable language.

Posted 3 months ago #
Tanner85
Member

Hello guys, thanks for the replies.

Now I have some questions for you as I'm proceeding with development of my first horizontal scrolling game.

I have a stage which is a class "Game : SPStage";

This stage does have a sprite "mContents" containing everything on the screen. The player can go left or right and the stage has a cameraX and cameraSpeed properties, indicating what portion of the scrolling background is on the screen the camera direction.

I also have a spawnEnemy method that creates an enemy sprite that runs against the player.
Now, it's obvious that:
- if the player runs against the enemy (in pseudocode):
ENEMYSPRITE.X += SPEED + SPSTAGE.CAMERASPEED; //we are going to reach the enemy soon

- otherwise if the player is escaping from the enemy:
ENEMYSPRITE.X += SPEED - SPSTAGE.CAMERASPEED; //we are escaping far from enemy

Since the enemy is a SPSprite that updates its X coordinate in "OnEnterFrame" but CAMERASPEED is property of the Game class, what's the correct way to access that property from the EnemySprite class?

self.stage.cameraspeed does not work (since Game is a subclass of SPStage) so i'm using this cast

[(Game *)[self.parent parent] cameraSpeed];

where the first parent refers to the parent sprite "mContents", and the second parent obviously refers to the stage...

To let the cast, I'm including game.h in EnemySprite.h and vice versa, which to me sounds a bit weird..

Bet I'm confused.. help's really appreciated

Sorry for bad english.

Posted 3 months ago #
Jugibur
Moderator
Hi Tanner85,

you could get the property in a more direct way like this:
((Game*)[SPStage mainStage]).cameraSpeed;
What with this approach isn't ideal, is that the child can manipulate the properties of his parent. If this is a single-man-project this isn't a big problem, in fact, you can do what you want But to prevent the temptation it's better to have a read-only property .
If you want write a property you could send an event or call a method. In this case the parent can decide if writing to this property is allowed.

One more thing is to prevent dependencies, that means two classed doesn't known there real opponent. They communicating via methods they known from a special protocol where all methods listed that they need.
Posted 3 months ago #
Tanner85
Member

I think the communication of changing game states between classes is one of the hardest showstoppers for newbies...
Thanks for the reply, i'm investigating further to better address those problems.

P.s. what about "To let the cast, I'm including game.h in EnemySprite.h and vice versa, which to me sounds a bit weird.."?
Am I going to encounter problems?

Posted 3 months ago #

Jugibur
Moderator

Sorry to irritate you...

Because you ask about design patterns I wrote above that it's not the best way to give full access to each other (parent and child). So you can build your game like in your example but I want to give you a more elegant way.

I will give you an protocol example:

/// File: GameStageProtocol.h
///
@protocol GameStageProtocol <NSObject>
 
@required
 
@property (nonatomic, readonly) float cameraSpeed;
 
@end

/// File: Game.h
///
#import "GameStageProtocol.h"
 
@interface Game : SPStage <GameStageProtocol>
{
    float mCameraSpeed;
}
 
/// implements that protocol here
@property (nonatomic, readonly) float cameraSpeed;
 
@end

/// File: Game.m
///
@implementation Game
 
- (float)getCameraSpeed
{
    return mCameraSpeed;
}
 
@end

Note: Your Sprite doesn't have access to your accessor class "game" or whatever.
You just say them: There is a pointer to a Stage that will implement the GameStageProtocol. So you can be sure that there are the needed methods or properties, like "cameraSpeed".

/// File: YourSprite.h
///
#import "GameStageProtocol.h"
 
@interface YourSprite : SPSprite
{
    SPStage<GameStageProtocol>* mStage;
}
 
- (id)initWithStage:(SPStage<GameStageProtocol>*)stage;
 
@end

/// File: YourSprite.m
///
#import "YourSprite.h"
 
@implementation YourSprite
 
- (id)initWithStage:(SPStage<GameStageProtocol>*)stage
{
    if ((self = [super init]))
    {
        mStage = [stage retain];
    }
    return self;
}
 
- (void)dealloc
{
    [mStage release];
    [super dealloc];
}
 
@end

Now you have an capsuled easy access to the cameraSpeed property with this:

float theCameraSpeed = mStage.cameraSpeed;

Hope it's now clearer for you. If not, don't hesitate to ask.

Jugibur

Posted 3 months ago #

Tanner85
Member

thank you jugibur!
In fact, my implementation does have flaws and I started discovering the first bugs just today.

Every Sparrow project derived from Scaffold or others' demos do have a Game : SPStage class where the newbie like me usually adds a lot of properties and methods concerning game logic. Actors of this game logic resides in other subclasses of SPSprite, which in fact don't know anything about "Game" subclass and its variables, causing trouble when dealing with changing game states and value sharing (sharing camera speed for me).

I will look about protocols (which I don't know much about for now).

Posted 3 months ago #
Tanner85
Member

I tried the protocols approach then I refuged in the most elegant and sparrow-compliant way, EVENTS... I'm rewriting all the game logic using EVENTS to handle everything and it works like a charm.. On the fly question: does SPStage handle the OnEnterFrame event?

Posted 3 months ago #
Jugibur
Moderator
SPStage is derived from SPDisplayObjectContainer, so you can dispatch events and also receive them.
Just add this inside your successor of SPStage:
[self addEventListener:@selector(onEnterFrame:) atObject:self forType:SP_EVENT_TYPE_ENTER_FRAME]; - (void)onEnterFrame:(SPEnterFrameEvent*)event { ... }
Posted 3 months ago #
Tanner85
Member

Sorry if I ask questions-burst

Player Sprite does an animation that takes over control from the Player for a certain time. Then the control should return to him.
Which is the best for delayed invocation of a "release player" method?

1) delayInvocationAtTarget:self byTime:30.0] releasePlayer;

2) performSelector:@selector(releasePlayer) withObject:nil afterDelay:(NSTimeInterval)delay

I've seen both in tutorials and open source games, either are claimed to be not precise (but are often used anyway).

Hope this is of interest for everyone. Thanks in advance.

Posted 3 months ago #
borrrden
Moderator

I recommend number 1 for the following reason:

If you pause your game, you will probably want to stop the timer. With number 1 it is as simple as not advancing the juggler, but number 2 is more complicated. You will have to delete the invocation and create a new one with the remaining time after your pause is done.

Posted 3 months ago #
Tanner85
Member

Thanks for all your replies.

I have another question regarding SPMovieClip.
My player sprite has four animations to perform during gameplay.
Whats the correct way to handle the animation switching?

4 SPMovieClips are allocated and initialized in the init method.
Then I have 4 methods to play them but I don't know the way to switch between them, for now I use the .visible property but it does not seem much elegant.

I tried to assign to a blank SPMovieClip object the others actually containing animations: _currentClip = _playerWalkClip;

I need to know what happens when I add that _currentClip as a child to the sprite and if assigning other clips to that blank object will actually change the animation displayed on screen (which is not happening right now).

Thanks in advance.

Posted 3 months ago #

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