Question

I have created an animated game app, In which I am using NSTimer to move an image after a particular interval of time.

timer = [NSTimer scheduledTimerWithTImerInterval:0.2 target:self selector:@selector(MoveVirus) userinfo:nil repeats:YES];

this timer calls up the function MoveVirus, MoverVirus moves virus (an Image on screen).

It works fine in the beginning but the speed of motion automatically increases.
The increased speed of motion destroys every further logic.

I don’t know what is the problem with it?

Please help to solve this problem.

Answer 1

NSTimer isn’t necessarily meant for this sort of use…From the docs on NSTimer:

A timer is not a real-time mechanism; it fires only when one of the
run loop modes to which the timer has been added is running and able
to check if the timer’s firing time has passed. Because of the various
input sources a typical run loop manages, the effective resolution of
the time interval for a timer is limited to on the order of 50-100
milliseconds. If a timer’s firing time occurs while the run loop is in
a mode that is not monitoring the timer or during a long callout, the
timer does not fire until the next time the run loop checks the timer.
Therefore, the actual time at which the timer fires potentially can be
a significant period of time after the scheduled firing time.

A better approach, if you are moving UIImages that are contained in a UIImageView would be to use the class animation methods on UIView. You can still get the same result of moving the image after a particular time if you use the animation method that contains a delay. The method is:

+ (void)animateWithDuration:(NSTimeInterval)duration delay:(NSTimeInterval)delay options:(UIViewAnimationOptions)options animations:(void (^)(void))animations completion:(void (^)(BOOL finished))completion

Using this method, you can specify the time of the animation (how long it takes to animate the move of your UIImages), how long to wait before starting this animation, a set on animation options, a block of animation code, and a block that executes when the animation is complete.