pragmatically scroll UIScrollView upon shake gesture

Question

HI,

I have a view that has three UIScrollViews on the screen. I want to randomly scroll the UIScrollViews to different positions whenever a user shakes the iPhone, but I am unable to get it done.

I have implemented the following in my ViewController class to detect and handle the shake gesture, the 3 UIScrollViews also belong to the same class. The shake gesture is detected, but the UIScrollViews do not change. What am I doing Wrong??

i have tried both motionBegan and motionEnded.

-(BOOL)canBecomeFirstResponder {
    return YES;
}

-(void)viewDidAppear:(BOOL)animated {
    [super viewDidAppear:animated];
    [self becomeFirstResponder];
}

- (void)viewWillDisappear:(BOOL)animated {
    [self resignFirstResponder];
    [super viewWillDisappear:animated];
}

- (void)motionEnded:(UIEventSubtype)motion withEvent:(UIEvent *)event
{
    if (motion == UIEventSubtypeMotionShake)
    {
  int randomTag = arc4random() % [dirContents count];

  CGRect nextImageView = [[scrollView1 viewWithTag:2] frame];
  [scrollView1 scrollRectToVisible:nextImageView animated:YES];

  randomTag = arc4random() % [dirContents count];
  nextImageView = [[scrollView2 viewWithTag:4] frame];
  [scrollView2 scrollRectToVisible:nextImageView animated:YES];

  randomTag = arc4random() % [dirContents count];
  nextImageView = [[scrollView3 viewWithTag:4] frame];
  [scrollView3 scrollRectToVisible:nextImageView animated:YES];
  NSLog(@"Shake Detected End");
    }
}

Thank You

Answer 1

Have you tried using SetContentOffset instead of scrollRectToVisible yet?

if the images in your tableView are of equal height the offset per "Element" is always the same.

[scrollView3 setContentOffset:yourRandomOffsetInPixels animated:YES];

maybe this works. also consider, that The Problem might be that your shake-detection Method runs on a separate Thread this would mean that you have to call your motionEnded Method on the mainthread like so:

[self performSelectorOnMainThread:@selector(motionEnded) withObject:nil waitUntilDone:NO];

Answer 2

Did you check your nextImageView variable to see if it was correct ?

Further more if you are trying will have the motion of a slot machine, I would recommend you to use UITableView instead of doing it by yourself with scrollView

Answer 3

Just one quick question. In your example code, you generate a random tag:

randomTag = arc4random() % [dirContents count];

but then you use a specific tag value (4 in this case)? I assume it still doesn't work when you use the randomTag value? and that you were just doing some testing?

 nextImageView = [[scrollView2 viewWithTag:4] frame];