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iOS7 Day-by-Day :: Day 0 :: UIKit Dynamics

Posted on 19 Sep 2013 Written by Sam Davies

This post is part of a daily series of posts introducing the most exciting new parts of iOS7 for developers – #iOS7DayByDay. To see the posts you’ve missed check out the introduction page, but have a read through the rest of this post first!

With the introduction of iOS7 Apple made it very clear that they are pushing the interaction between devices and the real world. One of the new APIs they introduced was UIKit Dynamics – a 2-dimensional physics engine which lies underneath the entirety of UIKit. In day 0 of this blog series we’re going to take a look at UIKit Dynamics and build a Newton’s cradle simulation.

The code for this blog post is available in the github repo which accompanies this series – at github.com/ShinobiControls/iOS7-day-by-day

The physical universe

In order to model the physics of real world we use UIDynamicBehavior subclasses to apply different behaviors to objects which adopt the UIDynamicItem protocol. Examples of behaviors include concepts such as gravity, collisions and springs. Although you can create your own objects which adopt the UIDynamicItem protocol, importantly UIView already does this. TheUIDynamicBehavior objects can be composited together to generate a behavior object which contains all the behavior for a given object or set of objects.

Once we have specified the behaviors for our dynamic objects we can provide them to a UIDynamicAnimator instance – the physics engine itself. This runs the calculations to determine how the different objects should interact given their behaviors. The follows shows a conceptual overview of the UIKit Dynamics world:


Building a pendulum

Remembering back to high school science – one of the simplest objects studied in Newtonian physics is a pendulum. Let’s create aUIView to represent the ball-bearing:

UIView *ballBearing = [[UIView alloc] initWithFrame:CGRectMake(0,0,40,40)];
ballBearing.backgroundColor = [UIColor lightGrayColor];
ballBearing.layer.cornerRadius = 10;
ballBearing.layer.borderColor = [UIColor grayColor].CGColor;
ballBearing.layer.borderWidth = 2;
ballBearing.center = CGPointMake(200, 300);
[self.view addSubview:ballBearing];

Now we can add some behaviors to this ball bearing. We’ll create a composite behavior to collect the behavior together:

UIDynamicBehavior *behavior = [[UIDynamicBehavior alloc] init];

Next we’ll start adding the behaviors we wish to model – first up gravity:

UIGravityBehavior *gravity = [[UIGravityBehavior alloc] initWithItems:@[ballBearing]];
gravity.magnitude = 10;
[behavior addChildBehavior:gravity];

UIGravityBehavior represents the gravitational attraction between an object and the Earth. It has properties which allow you to configure the vector of the gravitational force (i.e. both magnitude and direction). Here we are increasing the magnitude of the force, but keeping it directed in an increasing y direction.

The other behavior we need to apply to our ball bearing is an attachment behavior – which represents the string from which it hangs:

CGPoint anchor = ballBearing.center;
anchor.y -= 200;
UIAttachmentBehavior *attachment = [[UIAttachmentBehavior alloc] initWithItem:ballBearing attachedToAnchor:anchor];
[behavior addChildBehavior:attachment];

UIAttachmentBehavior instances attach dynamic objects either to an anchor point or to another object. They have properties which control the behavior of the attaching string – specifying its frequent, damping and length. The default values for this ensure a completely rigid attachment, which is what we want for a pendulum.

Now the behaviors are specified on the ball bearing we can create the physics engine to look after it all, which is defined as an ivar UIDynamicAnimator *_animator;:

_animator = [[UIDynamicAnimator alloc] initWithReferenceView:self.view];
[_animator addBehavior:behavior];

UIDynamicAnimator represents the physics engine which is required to model the dynamic system. Here we create it and specify which view it should use as its reference view (i.e. specifying the spatial universe) and add the composite behavior we’ve built.

With that we’ve actually created our first UIKit Dynamics system. However, if you run up the app, nothing will happen. This is because the system starts in and equilibrium state – we need to perturb the system to see some motion.

Gesture responsive behaviors

We need to add a gesture recognizer to the ball bearing to allow the user to play with the pendulum:

UIPanGestureRecognizer *gesture = [[UIPanGestureRecognizer alloc] initWithTarget:self action:@selector(handleBallBearingPan:)];
[ballBearing addGestureRecognizer:gesture];

In the target for the gesture recognizer we apply a constant force behavior to the ball bearing:

- (void)handleBallBearingPan:(UIPanGestureRecognizer *)recognizer
    // If we're starting the gesture then create a drag force
    if (recognizer.state == UIGestureRecognizerStateBegan) {
        if(_userDragBehavior) {
            [_animator removeBehavior:_userDragBehavior];
        _userDragBehavior = [[UIPushBehavior alloc] initWithItems:@[recognizer.view] mode:UIPushBehaviorModeContinuous];
        [_animator addBehavior:_userDragBehavior];

    // Set the force to be proportional to distance the gesture has moved
    _userDragBehavior.pushDirection = CGVectorMake([recognizer translationInView:self].x / 10.f, 0);
    // If we're finishing then cancel the behavior to 'let-go' of the ball
    if (recognizer.state == UIGestureRecognizerStateEnded) {
        [_animator removeBehavior:_userDragBehavior];
        _userDragBehavior = nil;

UIPushBehavior represents a simple linear force applied to objects. We use the callback to apply a force to the ball bearing, which displaces it. We have an ivar UIPushBehavior *_userDragBehavior which we create when a gesture start, remembering to add it to the dynamics animator. We set the size of the force to be proportional to the horizontal displacement. In order for the pendulum to swing we remove the push behavior when the gesture has ended.

Combining multiple pendulums

A Newton’s cradle is an arrangement of identical pendulums, such that the ball bearings are almost touching.

Newtons Cradle

(photo courtest of flickr/xraijs)

To recreate this using UIKit Dynamics we need to create multiple pendulums – following the same pattern for each of them as we did above. They should be spaced so that they aren’t quite touching (see the sample code for details).

We also need to add a new behavior which will describe how the ball bearings collide with each other. We now have an ivar to store the ball bearings NSArray *_ballBearings;:

UICollisionBehavior *collision = [[UICollisionBehavior alloc] initWithObjects:_ballBearings];
[behavior addChildBehavior:collision];

Here we’re using a collision behavior and a set of objects which are modeled in the system. Collision behaviors can also be used to model objects hitting boundaries such as view boundaries, or arbitrary bezier path boundaries.

If you run the app now and try to move one of the pendulums you’ll notice that the cradle doesn’t behave as you would expect it to. This is because the collisions are currently not elastic. We need to add a special type of dynamic behavior to specify various shared properties:

UIDynamicItemBehavior *itemBehavior = [[UIDynamicItemBehavior alloc] initWithItems:_ballBearings];
// Elasticity governs the efficiency of the collisions
itemBehavior.elasticity = 1.0;
itemBehavior.allowsRotation = NO;
itemBehavior.resistance = 0.5;
[behavior addChildBehavior:itemBehavior];

We use UIDynamicItemBehavior to specify the elasticity of the collisions, along with some other properties such as resistance (pretty much air resistance) and rotation. If we allow rotation we can specify the angular resistance. The dynamic item behavior also allows setting of linear and angular velocity which can be useful when matching velocities with gestures.

Running the app up now will show a Newton’s cradle which behaves exactly as you would expect it in the real world. Maybe as an extension you could investigate drawing the strings of the pendulums as well as the ball bearings.

Completed Cradle

The code which accompanies this post represents the completed Newton’s cradle project. It uses all the elements introduced, but just tidies them up a little into a demo app.


This introduction to UIKit Dynamics has barely scratched the surface – with these building blocks really complex physical systems can be modeled. This opens the door for creating apps which are heavily influenced by our inherent understanding of motion and object interactions from the real world.

Don’t forget that you can get the code for this project on github at github.com/ShinobiControls/iOS7-day-by-day. If you have any feedback/comments then feel free to use the comments box below, or hit me up on twitter – @iwantmyrealname.


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