Question

please can anyone show me how to implement a countdown timer for starting a game in cocos2d for the iPhone.

What I mean is that on pressing “play” a new scene comes up with the numbers “3”, “2”, “1” displayed and then the word “GO!”.

Answer 1

From “cocos2d Best Practices”:

Try NOT to use Cocoa’s NSTimer. Instead use cocos2d’s own scheduler.

So this is example of using cocos2d’s scheduler to animate your label, even with some effect.

In @interface:

int timeToPlay;
CCLabelTTF * prepareLabel;
CCLabelTTF * timeoutLabel;
CCMenu *menu;

In init:

timeToPlay=4;

CGSize s = [CCDirector sharedDirector].winSize;
prepareLabel = [CCLabelTTF labelWithString:@"Prepare to play!" fontName:@"Marker Felt" fontSize:40];
prepareLabel.position = ccp(s.width/2.0f, 150);

timeoutLabel = [CCLabelTTF labelWithString:@"3" fontName:@"Marker Felt" fontSize:60];
timeoutLabel.position = ccp(s.width/2.0f, 90);

[self addChild:prepareLabel];
[self addChild:timeoutLabel];

timeoutLabel.visible=NO;
prepareLabel.visible=NO;

...
CCMenuItem *Play = [CCMenuItemFont itemFromString:@"PLAY" 
                                           target:self 
                                         selector:@selector(aboutToPlay:)];
...

aboutToPlay:

-(void) aboutToPlay: (id) sender {
    [self removeChild:menu cleanup:YES];
    timeoutLabel.visible=YES;
    prepareLabel.visible=YES;
    [self schedule: @selector(tick:) interval:1];
}

And tick:

-(void) tick: (ccTime) dt
{
    if(timeToPlay==1) [self play];
    else {
        timeToPlay--;
        NSString * countStr;

        if(timeToPlay==1)
        countStr = [NSString stringWithFormat:@"GO!"];
        else
        countStr = [NSString stringWithFormat:@"%d", timeToPlay-1];

        timeoutLabel.string = countStr;

        //and some cool animation effect
        CCLabelTTF* label = [CCLabelTTF labelWithString:countStr fontName:@"Marker Felt" fontSize:60];

        label.position = timeoutLabel.position;
        [self addChild: label z: 1001];
        id scoreAction = [CCSequence actions:
                            [CCSpawn actions:
                              [CCScaleBy actionWithDuration:0.4 scale:2.0],
                              [CCEaseIn actionWithAction:[CCFadeOut actionWithDuration:0.4] rate:2],
                              nil],
                            [CCCallBlock actionWithBlock:^{
                              [self removeChild:label cleanup:YES];
                            }],
                            nil];
        [label runAction:scoreAction];

    }

}

Play:

-(void) play {
    [[CCDirector sharedDirector] replaceScene:[CCTransitionSlideInL transitionWithDuration:0.4 scene:[GamePlay node]]];
}