UIDynamics and Autolayout

Question

Recently I used UIDynamics to animate an image view into place. However, because its autolayout y-pos constraint was set to off-screen, when navigating away from the screen and then returning to it, my image view was being placed off-screen again. The animation took about 3 seconds, so after three seconds I just reset the constraint. That feels a little hacky.

So my question is this: what is the proper way to handle autolayout and UIDynamics at the same time?

Proper way to handle UIDynamics and AutoLayout? Keep them separate at all costs. You can add dynamics to a view and then INTERNALLY to that view lay it out with auto layout. But don't use them together. — Fogmeister, Mar 20 '14 at 17:16

score 5 · Accepted Answer · answered Mar 20 '14 at 17:17

5

This is not really a dynamics problem. Autolayout is incompatible with any view animation, or any manual setting of the frame: when layout comes along, it is the constraints that will be obeyed. It is up to you, if you move a view manually in any way, to update the constraints to match its new position/size/whatever.

Having said that: with UIKit Dynamics, when the animation ends, the animator will pause, and the animator's delegate is notified:

https://developer.apple.com/library/ios/documentation/uikit/reference/UIDynamicAnimatorDelegate_Protocol/Reference/Reference.html#//apple_ref/occ/intfm/UIDynamicAnimatorDelegate/dynamicAnimatorDidPause:

So that is the moment to update the constraints.

answered Mar 20 '14 at 17:17

matt

515,959
87
875
1,141

*"Autolayout is incompatible with any view animation"*...although, you can animate a view layout that does not incorporate `UIKitDynamics` by updating the autolayout constraints themselves. (Or am I doing something that will bite me later?) – Anthony C Mar 23 '14 at 23:19
@AnthonyC What you're doing is perfectly right. The problem is that it doesn't work in all situations. Even outside of UIKit Dynamics, the autolayout constraints on a view not match the aspects of that view that you want to animate. For example, applying a rotation transform to a constrained view is typically a disaster. See my little essay here: http://stackoverflow.com/a/14105757/341994 – matt Mar 24 '14 at 01:31

score 3 · Answer 2 · answered Aug 08 '14 at 11:24

You have a nice solution provided by Geppy Parziale in this tutorial.

Basically you can create an object that conforms to UIDynamicItem:

@interface DynamicHub : NSObject <UIDynamicItem>
    @property(nonatomic, readonly) CGRect bounds;
    @property(nonatomic, readwrite) CGPoint center;
    @property(nonatomic, readwrite) CGAffineTransform transform;
@end

That needs to init the bounds or it will crash:

- (id)init {
    self = [super init];
    if (self) {
        _bounds = CGRectMake(0, 0, 100, 100);
    }
    return self;
}

And then you use UIDynamics on that object and use the intermediate values to update your constraints:

DynamicHub *dynamicHub = [[DynamicHub alloc] init];

UISnapBehavior *snapBehavior = [[UISnapBehavior alloc] initWithItem:dynamicHub
                                           snapToPoint:CGPointMake(50.0, 150.0)];
[snapBehavior setDamping:.1];

snapBehavior.action = ^{
     self.firstConstraint.constant = [dynamicHub center].y;
     self.secondConstraint.constant = [dynamicHub center].x;
};

[self.animator addBehavior:snapBehavior];

Why not just use a UIView instead of a DynamicHub? – eddy Jul 30 '15 at 19:13 — eddy, Jul 30 '15 at 19:13

UIDynamics and Autolayout

2 Answers2