Iphone，objective-c如何为平台游戏制作一个Jump方法

I have this IBAction that is suppose to make a character onscreen jump, however when its called it just moves the character up once, then each call after that the character just moves down more and more. This function should just be called then the character would 'jump' up and then fall straight down off the screen since i havent put in any collision with the ground. ANy suggestions why this is happening? tim is the name of my UIImageView that holds the chracater, btw.I have this IBAction that is suppose to make a