animation - Layer masking based on positioning iOS -

i've never worked animations on ios , hoping pointed in right direction. have far animated bunch of pill shaped objects in fashion similar have below, have yet implement correct masking based on y positioning of shape.

the pill shapes uiviews. i'm looking way backgrounds change based on positioning; background not move, pills.

edit:

based on rob's answer below, tying animation device's movement so:

- (void)setanimationtodevicemotion {     motionmanager = [[cmmotionmanager alloc] init];     motionmanager.devicemotionupdateinterval = 0.1f;     nsoperationqueue *motionqueue = [[nsoperationqueue alloc] init];      [motionmanager startdevicemotionupdatesusingreferenceframe:cmattitudereferenceframexarbitraryzvertical toqueue:motionqueue withhandler:^(cmdevicemotion *motion, nserror *error) {         if (!error) {             double pitch = motion.attitude.pitch;             nslog(@"%f", pitch);             [self addmasktoview:self.imageview withadjustment:pitch];         }     }]; } 

assuming 8 pill shapes reveal single image underneath,

• create uibezierpath 8 pill shapes
• create cashapelayer layer uses uibezierpath (or more accurately, cgpath)
• apply cashapelayer mask layer uiimageview

if want animate on time, replace mask (e.g. in response accelerometer event, cadisplaylink or nstimer, or uipangesture), or update path of cashapelayer have applied mask uiimageview's layer.

thus, might have like:

- (void)addmasktoview:(uiview *)view withadjustment:(cgfloat)adjustment {     cashapelayer *mask = [cashapelayer layer];     mask.path = [[self pillspathwithadjustment:adjustment forview:view] cgpath];      self.imageview.layer.mask = mask; }  - (uibezierpath *)pillspathwithadjustment:(cgfloat)adjustment forview:(uiview *)view {     uibezierpath *path = [uibezierpath bezierpath];      (nsinteger = 0; < knumberofpills; i++)         [self addpillpathnumber:i forview:view topath:path adjustment:adjustment];      return path; }  - (void)addpillpathnumber:(nsinteger)index forview:(uiview *)view topath:(uibezierpath *)path adjustment:(cgfloat)adjustment {     cgfloat const pillspacing = 5.0f;     cgfloat const pillwidth = view.bounds.size.width / knumberofpills - pillspacing;     cgfloat const pillheight =  view.bounds.size.height * 0.4;     cgfloat const cornerrounding = 10.0f;      cgpoint point = cgpointmake(index * (pillwidth + pillspacing) + pillspacing / 2.0, sinf((float) index * m_pi / knumberofpills + adjustment * m_pi * 2.0) * 100.0);      // top      point.x += cornerrounding;     [path movetopoint:point];     point.x += pillwidth - cornerrounding * 2.0;     [path addlinetopoint:point];      // top right corner      [path addarcwithcenter:cgpointmake(point.x, point.y + cornerrounding) radius:cornerrounding startangle:3 * m_pi_2 endangle:2.0 * m_pi clockwise:yes];     point.x += cornerrounding;     point.y += cornerrounding;      // right      point.y += pillheight - cornerrounding * 2.0;     [path addlinetopoint:point];      // lower right corner      [path addarcwithcenter:cgpointmake(point.x - cornerrounding, point.y) radius:cornerrounding startangle:0 endangle:m_pi_2 clockwise:yes];      // bottom      point.y += cornerrounding;     point.x -= cornerrounding;     point.x -= pillwidth - cornerrounding * 2.0;     [path addlinetopoint:point];      // lower left corner      [path addarcwithcenter:cgpointmake(point.x, point.y - cornerrounding) radius:cornerrounding startangle:m_pi_2 endangle:m_pi clockwise:yes];      // left      point.y -= cornerrounding;     point.x -= cornerrounding;     point.y -= pillheight - cornerrounding * 2.0;     [path addlinetopoint:point];      // upper left corner      [path addarcwithcenter:cgpointmake(point.x + cornerrounding, point.y) radius:cornerrounding startangle:m_pi endangle:3.0 * m_pi_2 clockwise:yes];      [path closepath]; } 

by way of illustration, how i'd animate using cadisplaylink:

- (void)startdisplaylink {     self.displaylink = [cadisplaylink displaylinkwithtarget:self selector:@selector(handledisplaylink:)];     self.starttime = cacurrentmediatime();     [self.displaylink addtorunloop:[nsrunloop currentrunloop] formode:nsdefaultrunloopmode]; }  - (void)stopdisplaylink {     [self.displaylink invalidate];     self.displaylink = nil; }  - (void)handledisplaylink:(cadisplaylink *)displaylink {     cgfloat seconds;     cgfloat fractionsofsecond = modff(cacurrentmediatime() - self.starttime, &seconds);      [self addmasktoview:self.imageview withadjustment:fractionsofsecond]; }