利用arc4random()生成隨機背景色
int r = arc4random() % 255;
int g = arc4random() % 255;
int b = arc4random() % 255;
CGFloat rr = r / 255.0;
CGFloat rg = g / 255.0;
CGFloat rb = b / 255.0;
self.backgroundColor = [[UIColor alloc]initWithRed:rr green:rg blue:rb alpha:1];
