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d428565 Mar 12, 2017
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Shared Element Transitions

Top Level

The core idea behind the shared element transition API is that any screen can define some number of views that are SharedElementGroups and each SharedElementGroup can have some number of views that are SharedElements inside of it.

Each SharedElementGroup has an identifier (a string) which uniquely identifies it on the screen.

Each SharedElement has an identifier (a string) which uniquely identifies it inside of the SharedElementGroup

Also, SharedElements need not live inside of a SharedElementGroup, but then they can only be used in the to portion of the transition, and not the from part.

If a transitionGroup is specified as an option in the navigation call (ie, Navigator.push), then a shared element transition will be attempted with the provided id of the SharedElementGroup on the screen that you are navigating away from.

Example

import React from 'react';
import { View, Image } from 'react-native';
import Navigator, { SharedElement, SharedElementGroup } from 'native-navigation';

const onPress = id => () => 
 Navigator.push('ToScreen', { id }, {
  transitionGroup: id,
 });

class FromScreen extends React.Component {
 render() {
  const { posters } = this.props;
  return (
   <View>
    {posters.map(poster => (
     <SharedElementGroup id={poster.id}>
      <Touchable onPress={onPress(poster.id)}>
       <SharedElement
        type="poster"
        typeId={poster.id}
       >
        <Image source={{ uri: poster.url }} />
       </SharedElement>
      </Touchable>
     </SharedElementGroup>
    ))}
   </View>
  );
 }
}
import React from 'react';
import { View, Image } from 'react-native';
import Navigator, { SharedElement, SharedElementGroup } from 'native-navigation';

class ToScreen extends React.Component {
 render() {
  return (
   <View>
    {/* ... */}
    <SharedElement
     type="poster"
     typeId={poster.id}
    >
     <Image source={{ uri: poster.url }} />
    </SharedElement>
    {/* ... */}
   </View>
  );
 }
}

Shared Element Identifiers

This section has not yet been filled out

How it works (iOS)

The animation is executed by creating a number of snapshots (using snapshotViewAfterScreenUpdates ) and animating them inside of an animation container using a UIViewControllerTransitioningDelegate

The snapshot itself is several steps, as outlined below. Provided you have the following three inputs:

  1. fromViewController: The view controller you are navigating away from
  2. toViewController: The view controller you are navigating to
  3. transitionGroup: the id of the transition group in the fromViewController that is going to be involved in the transition.

With those parameters, we do the following:

  1. Snapshot fromViewController and put the snapshot on top of the view hierarchy to cover the screen
  2. In fromViewController collect snapshots of every SharedElement inside of the SharedElementGroup with id matching transitionGroup.
  3. Snapshot the entire fromViewController view hierarchy with each SharedElement from #2 having alpha set to 0.
  4. In toViewController collect snapshots of every SharedElement.
  5. Snapshot the entire toViewController view hierarchy with each SharedElement from #4 having alpha set to 0.
  6. Find all of the matching SharedElements in toViewController and fromViewController, matching by typeId etc.
  7. Remove snapshot from #1. Insert snapshots into the animation container in the following order:
  • full screen snapshot of fromViewController minus shared elements
  • all shared elements from fromViewController without a match
  • full screen snapshot of toViewController minus shared elements (set alpha to 0)
  • all shared elements from toViewController without a match (set alpha to 0)
  • all shared elements from fromViewController with matches in toViewController
  1. Execute a UIView.animateWithDuration(...) and set the following things in the animation:
  • full screen snapshot of toViewController minus shared elements (set alpha to 1)
  • all shared elements from toViewController without a match (set alpha to 1)
  • all shared elements from fromViewController with matches in toViewController:
    • set .center to the .center of the corresponding matched element
    • set .transform to be an XY scale transform that matches the ratio of the widths/heights of the element relative to its matched pair.
  1. On completion of animation, remove the animation container completely.

How it works (Android)

This section has not yet been filled out

Common Issues

White screen on iPhone 7 Simulators:

Shared Element Transitions use custom transition coordinators and view snapshotting on iOS. There is a known issue with these on iPhone 7 Simulators. In these simulators, the whole screen will appear white and not work. This affects simulators only and not real devices.

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