Flutter Hero Animation Unmasked - Part 2/2

flutter

dart

hero animation

Guillaume Diallo-Mulliez

8min à lire ⬝ 30 juin 2021

Welcome back to “Hero Animation Unmasked”, our little adventure to recode the flutter Shared Element Transition called “Hero Animation”.

Previously in Hero Animation Unmasked

In the previous part: Hero Animation Unmasked - Part 1, we have …

1. understood the general mechanism of the Hero Animation
2. created our own UnmaskedHero widget and its UnmaskedHeroController
3. hooked ourselves to the didPush Navigation method to react to navigation
4. browsed the Elements Tree to look for interesting Widget instances
5. displayed our UnmaskedHero into the screen overlay

Now that we’ve managed to find our Hero-wrapped widgets and display them onto the screen…

Display Hero on Overlay

Let’s make them Fly!

In order to so, we’ll implement the following steps:

1. Display our UnmaskedHeroes at their initial position on screen
2. Animate them from their initial to their final positions
3. Hide the source & destination widgets during the flight


Eventually, we’ll buy our UnmaskedHero a return ticket by making sure they can fly back when we navigate back

1. 🛫 Ready for Take-off?

Ready for Take-off

Compute Hero from/to locations on screen

Commit: 7f37e14b1d45335f9044fba6187d83ec3ccb0350

In order to make our Hero fly, we first need to compute the locations on the screen from and to which they should be flying.
To do so, in the UnmaskedHeroController class, we create a _locateHero method that, given the UnmaskedHeroState and a BuildContext, will return a Rect object holding the actual onscreen position of the associated element.

 /// lib/packages/unmasked_hero/unmasked_hero_controller.dart

  /// Locate Hero from within a given context
  /// returns a [Rect] that will hold the hero's position & size in the context's frame of reference
  Rect _locateHero({
    required UnmaskedHeroState hero,
    required BuildContext context,
  }) {
    final heroRenderBox = (hero.context.findRenderObject() as RenderBox);
    final RenderBox ancestorRenderBox = context.findRenderObject() as RenderBox;
    assert(heroRenderBox.hasSize && heroRenderBox.size.isFinite);

    return MatrixUtilstransformRect(
      heroRenderBox.getTransformTo(ancestorRenderBox),
      Offset.zero & heroRenderBox.size,
    );
  }

Here, we access the RenderObject of the hero by calling hero.context.findRenderObject() and the RenderObject of the global context (which we refer to as its ancestor): context.findRenderObject().
Then we compute the transformation matrix that describes the geometric transformation between 2 RenderObject using the getTransformTo method.
Finally, we apply this transformation using MatrixUtils.transformRect to return our hero’s location in the frame of reference of the given context.

In the didPush method, let’s now call the _locateHero method for both source and destination to compute the from and to position:

for (UnmaskedHeroState hero in destinationHeroes.values) {
        final UnmaskedHeroState? sourceHero = sourceHeroes[hero.widget.tag];
        final UnmaskedHeroState destinationHero = hero;
        if (sourceHero == null) {
          print(
              'No source Hero could be found for destination Hero with tag: ${hero.widget.tag}');
          continue;
        }

        final Rect fromPosition =
            _locateHero(hero: sourceHero, context: fromContext);
        final Rect toPosition =
            _locateHero(hero: destinationHero, context: toContext);
        print(fromPosition);
        print(toPosition);
        _displayFlyingHero(hero);
}
Compute locations

Display flying Hero at source position

Commit: ef3c500cd5271cbd30f785d541f2e78108a42040

Now that our Hero’s initial and final positions are computed. We need to make them initially appear at the “from” position. For that, let’s rename the displayFlyingHero method with a more explanatory name: _displayFlyingHeroAtPosition and pass it an additional Rect parameter that will hold the position at which we want to display the Hero:

/// lib/packages/unmasked_hero/unmasked_hero_controller.dart

 /// Display Hero on Overlay at the given position
  void _displayFlyingHeroAtPosition({
    required UnmaskedHeroState hero,
    required Rect position,
  }) {
    ...
  }

  @override
  void didPush(Route<dynamic> toRoute, Route<dynamic>? fromRoute) {
    ...

    final Rect fromPosition =_locateHero(hero:: sourceHero, context: fromContext);
    final Rect toPosition = _locateHero(hero: destinationHero, context: toContext);
    _displayFlyingHeroAtPosition(hero: hero, position: fromPosition);
  }

Next, we replace the Container returned by the builder of the OverlayEntry with a Positioned widget and pass it the given position:

/// lib/packages/unmasked_hero/unmasked_hero_controller.dart

overlayEntry = OverlayEntry(
  builder: (BuildContext context) => Positioned(
    child: hero.widget.child,
    top: position.top,
    left: position.left,
    width: position.width,
    height: position.height,
  ),
);

Unmasked_Hero_Initial_Position-1

The overlayed Hero now appears at the same position as the source Hero widget.

2. 🛩 Animate hero between from/to positions

Compute locations

Commit: daa20dd0837a87659885411c8de0c591643d7bd8

Eventually, here comes the actual “Flying” part we’ve been waiting for 😅!
To do so, we’ll create a widget responsible for the animation and display it on the overlay:

First, in the unmasked_hero folder, create a FlyingUnmaskedHero widget:

/// lib/packages/unmasked_hero/flying_unmasked_hero.dart

import 'dart:async';
import 'package:flutter/widgets.dart';

class FlyingUnmaskedHero extends StatefulWidget {
  final Rect fromPosition;
  final Rect toPosition;
  final Widget child;

  FlyingUnmaskedHero({
    required this.fromPosition,
    required this.toPosition,
    required this.child,
  });

  @override
  FlyingUnmaskedHeroState createState() => FlyingUnmaskedHeroState();
}

class FlyingUnmaskedHeroState extends State<FlyingUnmaskedHero> {
  bool flying = false;

  @override
  void initState() {
    Timer(Duration(milliseconds: 0), () {
      setState(() {
        flying = true;
      });
    });
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    final Rect fromPosition = widget.fromPosition;
    final Rect toPosition = widget.toPosition;
    return AnimatedPositioned(
      child: widget.child,
      duration: Duration(milliseconds: 200),
      top: flying ? toPosition.top : fromPosition.top,
      left: flying ? toPosition.left : fromPosition.left,
      height: flying ? toPosition.height : fromPosition.height,
      width: flying ? toPosition.width : fromPosition.width,
    );
  }
}

This widget is a simple StatefulWidget responsible for handling the animation between the initial and final position of our hero that we pass as parameters.

For the sake of simplicity, we use an AnimatedPositioned widget to handle the animation between the 2 positions. The actual Hero widget uses the lower-level API RectTween. This induces a couple of notable things here:

1. We hardcode the duration of the animation to 200. In real life, we would want to ensure the animation duration matches the animation of the navigation between the 2 pages.

2. We use a Timer of 0 milliseconds in the initState method to ensure the widget is built once with the flying state set to false, which initialize the position to toPosition before being animated to toward the fromPosition.


Next, we rename the _displayFlyingHeroAtPosition to _startFlying and pass it both the from and to positions:

/// lib/packages/unmasked_hero/unmasked_hero_controller.dart

  void _startFlying({
    required UnmaskedHeroState hero,
    required Rect fromPosition,
    required Rect toPosition,
  }) {
    ...
  }

  ...

    _startFlying(hero: hero, fromPosition: fromPosition, toPosition: toPosition);

Finally, we can replace the widget built by the OverlayEntry to use our FlyingUnmaskedHero:

/// lib/packages/unmasked_hero/unmasked_hero_controller.dart

overlayEntry = OverlayEntry(
  builder: (BuildContext context) => FlyingUnmaskedHero(
    fromPosition: fromPosition,
    toPosition: toPosition,
    child: hero.widget.child,
  ),
);

… is it a bird 🦅 ? is it a plane 🛩 ? 🤩

Flying_Unmasked_Hero_Raw-1

Our hero animates nicely between the initial and final positions 💪.

3. 🧹 Clean up

We’re almost there. Now we just need to make sure that the original widgets and the one flying onto the overlay are not displayed simultaneously to produce the illusion that they are the same widget.
In order to produce this illusion, there are 2 things left to do:

1. Remove the widget from the overlay when the animation ends
2. Hide our UnmaskedHero widget’s child during the animation


Compute locations

Remove the widget from the overlay when the animation ends

Commit: dc3e4b6b222d6c0ad004b958464ae5e14466b714

Add a onFlyingEnded callback property to the FlyingUnmaskedHero widget to be able to listen to this event:

/// lib/packages/unmasked_hero/flying_unmasked_hero.dart

import 'dart:async';
import 'package:flutter/widgets.dart';

class FlyingUnmaskedHero extends StatefulWidget {
  final Rect fromPosition;
  final Rect toPosition;
  final Widget child;
  final VoidCallback? onFlyingEnded; /// <--- Add this line


  FlyingUnmaskedHero({
    required this.fromPosition,
    required this.toPosition,
    required this.child,
    this.onFlyingEnded, /// <--- Add this line
  });

  ...


  @override
  Widget build(BuildContext context) {
      ...

    return AnimatedPositioned(
      child: widget.child,
      duration: Duration(milliseconds: 200),
      top: flying ? toPosition.top : fromPosition.top,
      left: flying ? toPosition.left : fromPosition.left,
      height: flying ? toPosition.height : fromPosition.height,
      width: flying ? toPosition.width : fromPosition.width,
      onEnd: widget.onFlyingEnded,  /// <--- Add this line
    );
  }
}

and pass it a function that removes the overlayEntry:

Hide hero’s child during animation

Commit: 7836d20d8fa1ddba160aab782aaf689c92899b3e

We first modify our UnmaskedHeroState to add the capibility to hide or show its child:

/// lib/packages/unmasked_hero/unmasked_hero.dart
...
class UnmaskedHeroState extends State<UnmaskedHero> {
  bool hidden = false;

  void hide() {
    setState(() {
      hidden = true;
    });
  }

  void show() {
    setState(() {
      hidden = false;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Opacity(opacity: hidden ? 0.0 : 1.0, child: widget.child);
  }
}

We now modify the startFlying method to pass both sourceHero and destinationHero

/// lib/packages/unmasked_hero/unmasked_hero_controller.dart

  void _startFlying({
    required UnmaskedHeroState sourceHero,      /// <--- Replace hero by sourceHero here 
    required UnmaskedHeroState destinationHero, /// <--- Add this line 
    required Rect fromPosition,
    required Rect toPosition,
  }) {
    ...
  }

  ...

    _startFlying(
        sourceHero: sourceHero,           /// <--- Replace hero by sourceHero here 
        destinationHero: destinationHero, /// <--- Add this line 
        fromPosition: fromPosition,
        toPosition: toPosition
    );

And finally, we add the logic:

1. Hide the widgets before the animation starts
2. Show the widgets once it ended

/// lib/packages/unmasked_hero/unmasked_hero_controller.dart

    ...

    /// Hide source & destination heroes during flight animation
    sourceHerohide();
    destinationHerohide();

    OverlayEntry? overlayEntry;
    overlayEntry = OverlayEntry(
        builder: (BuildContext context) => FlyingUnmaskedHero(
            fromPosition: fromPosition,
            toPosition: toPosition,
            child: hero.widget.child,
            onFlyingEnded: () {
                /// Show source & destination heroes at the end of flight animation
                sourceHero.show();
                destinationHero.show();
                overlayEntry?.remove();
            }),
        ),
    );

Unmasked_Hero_final_forward_only-1

Well… this starts to look very much like a proper Shared Element Transition doesn’t it?

4. ⏮ Be kind, rewind

Alright, I heard you… it does not quite because the animation is not performed backward when we navigate back to the source page 😏.

Compute locations

Let’s take this last step together:

Refactor hero flying logic to use didPop

Commit: add9e0e1fcbf282f1ce817d8eb86f34a0c64311c

In the UnmaskedHeroController, proceed to an “extract method” type of refactoring to move the entire content of the didPush method inside a _flyFromTo method:

/// lib/packages/unmasked_hero/unmasked_hero_controller.dart

    void _flyFromTo( /// <--- Replace didPush by _flyFromTo
        Route? fromRoute,
        Route toRoute, 
    ) {

    ...

    @override
    void didPush(Route? fromRoute, Route toRoute) {
        _flyFromTo(fromRoute, toRoute); /// <--- Call it here
        super.didPush(toRoute, fromRoute);
    }

and eventually use it inside the overriden didPop method of the controller:

    @override
    void didPop(Route fromRoute, Route? toRoute) {
        if(toRoute == null) {
            return;
        }
        _flyFromTo(fromRoute, toRoute); /// <--- Call it here
        super.didPop(fromRoute, toRoute);
    }

And there you go…

Unmasked_Hero_Animation-1

Our UnmaskedHero smoothly flies back and forth from one page to the other, just as the original Hero widget.

🌯 Let’s wrap it up

Our great adventure of recoding the Flutter Hero animation comes to an end. Let’s take a look back at our journey:

In the 1st part of this article: Flutter Hero Animation Unmasked - Part 1/2, we have:

1. understood the general mechanism of the Hero Animation
2. created our own UnmaskedHero widget and its UnmaskedHeroController
3. hooked ourselves to the didPush Navigation method to react to navigation
4. browsed the Elements Tree to look for interesting Widget instances
5. displayed our UnmaskedHero into the screen overlay

in this 2nd part, we have:
6. computed the initial and final position on screen of our widget
7. animated them onto the overlay from initial to final position
8. produced the illusion of the widget “moving” by hiding the original ones during the animation and removing the overlayed one afterward
9. added support for the backward animation when navigating back from the destination page

 

I really hope that you enjoyed this journey to Unmasked the Hero Animation as much as I did and that you learned some things.
The Hero widget has no secrets for you now 😉.

If you want to dive even deeper in your understanding of this animation, check out the source code of the actual Hero widget.

If you have any questions, if you find any mistakes, inaccuracies, or if you just want to have a chat about Flutter, I’d be very happy to. You can find me on Twitter @Guitoof or on the Flutter Community Slack where I also go by the name Guitoof.
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