Timer in xCode

1. Declare NSTimer in class ".h" file

@property (nonatomic, assign) NSTimer *timer;

2. Synthesize in class ".m" file

@synthesize timer;

3. Now use this, where you want. But always invalidate timer before use. So that timer could start from scratch

[timer invalidate];

timer = nil;

//Start The Timer

timer = [NSTimer scheduledTimerWithTimeInterval:1 target:self selector:@selector(timerTick:) userInfo:nil repeats:YES];

//if repeats is set to NO, then timer will run only once. otherwise it will be called, until you stop the timer or stop the application.

4. To Stop Timer: Just use the following code

[timer invalidate];

timer = nil;

xCode Car Race with Box2d

Finally i have created Objective C based car race from the Javascript reference link
http://www.codekites.com/make-racing-car-box2d-javascript/ by - Silver Moon

My output will be like this

Right One is Accelerator and Left one is Break with reverser :)

So here are the steps. 
Steps:

1. You have to install cocos2d in Mac. You can take help from below link to install Box2d in xcode.
    
     http://www.raywenderlich.com/25736/how-to-make-a-simple-iphone-game-with-cocos2d-2-x-tutorial

2. Coding Part :

Create default box2d project

3. Go to HelloWorldLayer.h and add following variables

@property (nonatomic) float max_steer_angle;

@property (nonatomic) float steering_angle;

@property (nonatomic) float steer_speed;

@property (nonatomic) float defaultCarSpeed;

4. Now go to HelloWorldLayer.mm and delete code inside init method and put the code as below
Also Import 

#import "Car.h"

#import "ClickableSprite.h"

@interface HelloWorldLayer()

@property (nonatomic) Car *car;//Add this 

Now - 

-(id) init

{

if( (self=[super init])) {

// enable events

self.car = [[Car alloc] init];

        self.startTrack = NO;

self.isTouchEnabled = YES;

self.isAccelerometerEnabled = YES;

        max_steer_angle = M_PI/3;

        steer_speed = 1;

        steering_angle = 0;//-max_steer_angle;

        

CGSize s = [CCDirector sharedDirector].winSize;

[self createMenu];

// init physics

[self initPhysics];

        CGPoint p = CGPointMake(s.width/2-20, 230);

        [self addCarBody:p];

        

[self scheduleUpdate];

}

return self;

}

5. Now add following code in createMenu function 

//Accelerator

    ClickableSprite *accelerator = [ClickableSprite spriteWithFile:@"accelerator.png" rect:CGRectMake(0, 0, 73, 140)];
    accelerator.target = self;
    accelerator.selector = @selector(accelerateCar);
    accelerator.selectorEnd = @selector(accelerateCarStop);
    accelerator.clickableSize = CGSizeMake(73, 140);
    accelerator.touchPriority = 2;
    accelerator.position = ccp(size.width-200, 100);
    [self addChild:accelerator z:1];

    

    //Reverse

    ClickableSprite *reversor = [ClickableSprite spriteWithFile:@"accelerator.png" rect:CGRectMake(0, 0, 73, 140)];
    reversor.target = self;
    reversor.selector = @selector(reverseCar);
    reversor.selectorEnd = @selector(reverseCarStop);
    reversor.clickableSize = CGSizeMake(73, 140);
    reversor.touchPriority = 2;
    reversor.position = ccp(100, 90);
    [self addChild:reversor z:1];

6. Now add some following functions

-(void) accelerometer:(UIAccelerometer *)accelerometer didAccelerate:(UIAcceleration *)acceleration

{

    if (acceleration.y > 0) {

        [self moveLeft:acceleration.y];

    } else if (acceleration.y < 0) {

        [self moveRight:acceleration.y];

    }

}

-(void) accelerateCar

{

    self.car.gear = 1;

    [self.car start_engine];

}

-(void) accelerateCarStop

{

    self.car.gear = 0;

    [self.car stop_engine];

}

-(void) reverseCar

{

    self.car.gear = -1;

    [self.car start_engine];

}

-(void) reverseCarStop

{

    self.car.gear = 0;

    [self.car stop_engine];

}

7. Now change the gravity in initPhysics function

gravity.Set(0.0f, 0.0f);


8. Now add updateCar in draw function

-(void) draw
{

    [self updateCar];

}

9. add updateCar function

-(void) updateCar
{

    //NSLog(@" - > : %c", self.startTrack);

    if(self.startTrack == YES)
    {
        world->Step(1/60, 8, 3);

        

        world->ClearForces();

        

        //Driving

        b2Vec2 direction = self.car.left_wheel->GetTransform().q.GetYAxis();
        direction.operator*=(self.car.engine_speed);
        self.car.left_wheel->ApplyForce(direction, self.car.left_wheel->GetPosition());

        

        direction = self.car.right_wheel->GetTransform().q.GetYAxis();
        direction.operator*=(self.car.engine_speed);
        self.car.right_wheel->ApplyForce(direction, self.car.right_wheel->GetPosition());

        

        //Steering

        float angle = steering_angle - self.car.leftWheelJoint->GetJointAngle();

        self.car.leftWheelJoint->SetMotorSpeed(angle*steer_speed);

        

        angle = steering_angle - self.car.rightWheelJoint->GetJointAngle();

        self.car.rightWheelJoint->SetMotorSpeed(angle*steer_speed);

    }
}

10. Now add following functions

-(void) addCarBody:(CGPoint)p
{

    CCNode *parent = [self getChildByTag:kTagParentNode];

    //We have a 64x64 sprite sheet with 4 different 32x32 images.  The following code is
    //just randomly picking one of the images
    int idx = (CCRANDOM_0_1() > .5 ? 0:1);
    int idy = (CCRANDOM_0_1() > .5 ? 0:1);
    PhysicsSprite *sprite = [PhysicsSprite spriteWithTexture:spriteTexture_ rect:CGRectMake(32 * idx,32 * idy,32,32)];
    [parent addChild:sprite];

    sprite.position = ccp( p.x, p.y);

    // Define the dynamic body.
    //Set up a 1m squared box in the physics world
    b2BodyDef bodyDef;

    bodyDef.type = b2_dynamicBody;

    bodyDef.position.Set(p.x/PTM_RATIO, p.y/PTM_RATIO);
    bodyDef.linearDamping = 10.0;
    bodyDef.angularDamping = 10.0;
    bodyDef.gravityScale = 1.0;
    b2Body *body = world->CreateBody(&bodyDef);

    // Define another box shape for our dynamic body.

    b2PolygonShape dynamicBox;

    dynamicBox.SetAsBox(1.0f, 2.0f);//These are mid points for our 1m box

    // Define the dynamic body fixture.

    b2FixtureDef fixtureDef;
    fixtureDef.shape = &dynamicBox;
    fixtureDef.density = 1.0f;
    fixtureDef.friction = 0.0f;
    fixtureDef.restitution = 0.2f;
    body->CreateFixture(&fixtureDef);

    

    [sprite setPhysicsBody:body];

    

    //Left Wheel Start Here
    PhysicsSprite *left_wheel = [PhysicsSprite spriteWithTexture:spriteTexture_ rect:CGRectMake(32 * idx, 32 * idy, 32, 32)];
[parent addChild:left_wheel];
    p.x -= 30;
    p.y += 40;
    b2Body *leftwheelB = [self createBox:p wd:0.2f ht:0.35f];
    [left_wheel setPhysicsBody:leftwheelB];

    

    left_wheel.position = ccp(p.x, p.y);

    

    b2RevoluteJointDef joint_def_left;
    joint_def_left.Initialize(body, leftwheelB, leftwheelB->GetWorldCenter());
    joint_def_left.enableMotor = YES;
    joint_def_left.maxMotorTorque = 100000;

    

    joint_def_left.enableLimit = YES;
    joint_def_left.lowerAngle = -1*M_PI/3;
    joint_def_left.upperAngle = M_PI/3;

    

    self.car.leftWheelJoint = (b2RevoluteJoint*)world->CreateJoint(&joint_def_left);

    

    //Right Wheel Start Here

    PhysicsSprite *right_wheel = [PhysicsSprite spriteWithTexture:spriteTexture_ rect:CGRectMake(32 * idx, 32 * idy, 32, 32)];
    [parent addChild:right_wheel];

    p.x += 60;//35 before and 35 more so 70

    b2Body *rightwheelB = [self createBox:p wd:0.2f ht:0.35f];

    [right_wheel setPhysicsBody:rightwheelB];
    right_wheel.position = ccp(p.x, p.y);  

        

    b2RevoluteJointDef joint_def_right;
    joint_def_right.Initialize(body, rightwheelB, rightwheelB->GetWorldCenter());
    joint_def_right.enableMotor = YES;
    joint_def_right.maxMotorTorque = 100000;

    

    joint_def_right.enableLimit = YES;
    joint_def_right.lowerAngle = -1*M_PI/3;
    joint_def_right.upperAngle = M_PI/3;

    

    self.car.rightWheelJoint = (b2RevoluteJoint *)world->CreateJoint(&joint_def_right);

    

    //Left Bottom Wheel Start Here

    PhysicsSprite *left_rear_wheel = [PhysicsSprite spriteWithTexture:spriteTexture_ rect:CGRectMake(32 * idx, 32 * idy, 32, 32)];
[parent addChild:left_rear_wheel];
    p.x -= 60;
    p.y -= 80;
    b2Body *leftRearWheelB = [self createBox:p wd:0.2f ht:0.35f];
    [left_rear_wheel setPhysicsBody:leftRearWheelB];
    left_rear_wheel.position = ccp(p.x, p.y);

    

    b2PrismaticJointDef joint_def_left_rear;
    joint_def_left_rear.Initialize(body, leftRearWheelB, leftRearWheelB->GetWorldCenter(), b2Vec2(1,0));

    

    joint_def_left_rear.enableLimit = YES;
    joint_def_left_rear.lowerTranslation = 0;
    joint_def_left_rear.upperTranslation = 0;

    

    world->CreateJoint(&joint_def_left_rear);

    

    //Right Bottom Wheel Start Here

    PhysicsSprite *right_rear_wheel = [PhysicsSprite spriteWithTexture:spriteTexture_ rect:CGRectMake(32 * idx, 32 * idy, 32, 32)];
[parent addChild:right_rear_wheel];

    p.x += 60;//35 before and 35 more so 70

    //p.y += 130;// same as above 65 before and 65 now so 0

    b2Body *rightRearWheelB = [self createBox:p wd:0.2f ht:0.35f];
    [right_rear_wheel setPhysicsBody:rightRearWheelB];
    right_rear_wheel.position = ccp(p.x, p.y);

    

    b2PrismaticJointDef joint_def_right_rear;
    joint_def_right_rear.Initialize(body, rightRearWheelB, rightRearWheelB->GetWorldCenter(), b2Vec2(1,0));

    

    joint_def_right_rear.enableLimit = YES;
    joint_def_right_rear.lowerTranslation = 0;
    joint_def_right_rear.upperTranslation = 0;

    

    self.car.carBody = body;
    self.car.left_wheel = leftwheelB;
    self.car.right_wheel = rightwheelB;
    self.car.left_rear_wheel = leftRearWheelB;
    self.car.right_rear_wheel = rightRearWheelB;

    

    world->CreateJoint(&joint_def_right_rear);

      

    [self.car stop_engine];

    self.startTrack = YES;

}

-(b2Body *) createBox :(CGPoint)p wd:(float)wd ht:(float)ht
{
    b2BodyDef wheelDef;
    wheelDef.type = b2_dynamicBody;
    wheelDef.position.Set(p.x/PTM_RATIO, p.y/PTM_RATIO);
    b2Body *wheelbody = world->CreateBody(&wheelDef);

    // Define another box shape for our dynamic body.
    b2PolygonShape dynamicBoxWheel;

    dynamicBoxWheel.SetAsBox(wd, ht);//These are mid points for our 1m box

    // Define the dynamic body fixture.
    b2FixtureDef fixtureDefWheel;
    fixtureDefWheel.shape = &dynamicBoxWheel;
    fixtureDefWheel.density = 2.0f;
    fixtureDefWheel.friction = 0.3f;
    wheelbody->CreateFixture(&fixtureDefWheel);

    

    return wheelbody;
}

11. Now Move Right and Left

-(void) moveRight
{

    steering_angle = -max_steer_angle;

    steer_speed = 1;

}
-(void) moveLeft
{

    steering_angle = max_steer_angle;

    steer_speed = 1;

}

12. Now Add Car.h and Car.mm

Car.h Code: 

#import <Foundation/Foundation.h>

#import "PhysicsSprite.h"

@interface Car : PhysicsSprite

@property (nonatomic) float top_engine_speed;

@property (nonatomic) bool engineOn;

@property (nonatomic) float gear;

@property (nonatomic) float engine_speed;

@property (nonatomic, assign) b2Body *left_wheel;

@property (nonatomic, assign) b2Body *right_wheel;

@property (nonatomic, assign) b2Body *left_rear_wheel;

@property (nonatomic, assign) b2Body *right_rear_wheel;

@property (nonatomic) b2RevoluteJoint *leftWheelJoint;

@property (nonatomic) b2RevoluteJoint *rightWheelJoint;

@property (nonatomic, assign) b2Body *carBody;

-(id) init;

-(void) start_engine;

-(void) stop_engine;

@end

Car.mm Code: 

#import "Car.h"

@implementation Car

@synthesize top_engine_speed, engineOn, gear, engine_speed;

@synthesize left_wheel, right_wheel, left_rear_wheel, right_rear_wheel, carBody, leftWheelJoint, rightWheelJoint;

-(id) init

{

    top_engine_speed = 240.0;

    engineOn = NO;

    gear = 1.0;

    return self;

}

-(void) start_engine

{

    engineOn = YES;

    engine_speed = gear * top_engine_speed;

}

-(void) stop_engine

{

    engineOn = NO;

    engine_speed = 0;

}

@end

13. Now Add ClickableSprite.h and ClickableSprite.mm

ClickableSprite.h Code: 

#import "CCSprite.h"

@interface ClickableSprite : CCSprite <CCTargetedTouchDelegate>

@property (nonatomic, assign) id<NSObject> target;

@property (nonatomic) SEL selector;

@property (nonatomic) SEL selectorEnd;

@property (nonatomic) CGSize clickableSize;

@property (nonatomic) int touchPriority;

@end

ClickableSprite.mm Code:

#import "ClickableSprite.h"

@implementation ClickableSprite

- (BOOL)containsTouchLocation:(UITouch *)touch {

    CGPoint p = [self convertTouchToNodeSpaceAR:touch];

    CGSize size = self.contentSize;

    if(!CGSizeEqualToSize(self.clickableSize, CGSizeZero))

        size = self.clickableSize;

    CGRect r = CGRectMake(-size.width*0.5, -size.height*0.5, size.width, size.height);

    return CGRectContainsPoint(r, p);

}

- (BOOL)ccTouchBegan:(UITouch *)touch withEvent:(UIEvent *)event {

    NSLog(@"Tound Began");

    [self.target performSelector:self.selector withObject:self];

    return [self containsTouchLocation:touch];

}

- (void)ccTouchEnded:(UITouch *)touch withEvent:(UIEvent *)event {

    if(![self containsTouchLocation:touch])

        return;

    NSLog(@"Tound End");

    [self.target performSelector:self.selectorEnd withObject:self];

}

- (void)onEnter {

    [super onEnter];

    CCDirector *director = [CCDirector sharedDirector];

    [[director touchDispatcher] addTargetedDelegate:self priority:self.touchPriority swallowsTouches:YES];

}

- (void)onExit {

    CCDirector *director = [CCDirector sharedDirector];

    [[director touchDispatcher] removeDelegate:self];

    [super onExit];

}

@end

You can Contact me also to get these files. If any issue. I will love to help.