Trying to get UIView to bounce using UIKit Dynamics

Question

I am trying to get a UIView that is in the shape of a circle to "bounce" using UIKit Dynamics. It is supposed to bounce when it reaches the bottom of the screen. The circle is a standard UIView that I create in the viewDidLoad method as follows:

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    self.orangeBall = [[UIView alloc] initWithFrame:CGRectMake(100.0, 100.0, 50.0, 50.0)];
    self.orangeBall.backgroundColor = [UIColor orangeColor];
    self.orangeBall.layer.cornerRadius = 25.0;
    self.orangeBall.layer.borderColor = [UIColor blackColor].CGColor;
    self.orangeBall.layer.borderWidth = 0.0;
    [self.view addSubview:self.orangeBall];

    // Initialize the property UIDynamicAnimator
    self.animator = [[UIDynamicAnimator alloc] initWithReferenceView:self.view];

    [self demoGravity];

}

I am trying to get the bounce effect done inside the demoGravity method as follows:

-(void)demoGravity{

UIGravityBehavior *gravityBehavior = [[UIGravityBehavior alloc] initWithItems:@[self.orangeBall]];

[self.animator addBehavior:gravityBehavior];

UICollisionBehavior *collisionBehavior = [[UICollisionBehavior alloc] initWithItems:@[self.orangeBall]];

[collisionBehavior addBoundaryWithIdentifier:@"myView"
                                   fromPoint:self.orangeBall.frame.origin
                                     toPoint:CGPointMake(self.orangeBall.frame.origin.x, self.orangeBall.frame.origin.y + self.view.frame.size.height)];

[self.animator addBehavior:collisionBehavior];
}

I want the "ball" to fall straight down on the screen, and once it reaches the bottom of the view, to stop, and "bounce". However, when I run my app, all I see is the circle at the top of the screen fall off the screen. What am I doing wrong?

I originally asked why the ball was not falling and bouncing. Not simply why it was motionless. I edited the code in the question, as well as the concluding problem to incorporate your suggestion, but unfortunately I still was experiencing trouble. You pointed to your sample code in your book, which I did, and saw the relevant block of code where you added the behaviour in your "doButton" method, but I unfortunately still haven't found a solution. — syedfa, Jun 18 '15 at 01:28

score 0 · Accepted Answer · edited Nov 12 '17 at 19:40

The reason why my ball was not bouncing was two fold:

It wasn't falling because I did not have a gravityBehaviour attached to my UIDynamicAnimator object:

UIGravityBehavior *gravityBehavior = [[UIGravityBehavior alloc] initWithItems:@[self.orangeBall]];

[self.animator addBehavior:gravityBehavior];

It wasn't bouncing because I needed to set the boolean property of CollisionBehaviour: "TranslatesReferenceBoundsIntoBoundary" to "YES".
```
[collisionBehavior setTranslatesReferenceBoundsIntoBoundary:YES];
```

A special thanks to Ray Wenderlich where the following article provided me with the solution, and the following paragraph is a direct quote from the article:

Rather than explicitly adding boundary co-ordinates, the above code sets the translatesReferenceBoundsIntoBoundary property to YES. This causes the boundary to use the bounds of the reference view supplied to the UIDynamicAnimator.

Trying to get UIView to bounce using UIKit Dynamics

1 Answers1