mimic.dart

// Copyright 2015 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

import 'dart:async';

import 'package:flutter/rendering.dart' show RenderStack;

import 'basic.dart';
import 'framework.dart';
import 'overlay.dart';

/// An opaque reference to a widget that can be mimicked.
class MimicableHandle {
  MimicableHandle._(this._state);

  final MimicableState _state;

  /// The size and position of the original widget in global coordinates.
  Rect get globalBounds => _state._globalBounds;

  /// Stop the mimicking process, restoring the widget to its original location in the tree.
  void stopMimic() {
    _state._stopMimic();
  }
}

/// An overlay entry that is mimicking another widget.
class MimicOverlayEntry {
  MimicOverlayEntry._(this._handle) {
    _overlayEntry = new OverlayEntry(builder: _build);
    _initialGlobalBounds = _handle.globalBounds;
  }

  Rect _initialGlobalBounds;

  MimicableHandle _handle;
  OverlayEntry _overlayEntry;

  // Animation state
  GlobalKey _targetKey;
  Curve _curve;
  AnimationController _controller;

  /// Animate the entry to the location of the widget that has the given target key.
  ///
  /// The animation will take place over the given duration and will apply the
  /// given curve.
  ///
  /// This function can only be called once per overlay entry.
  Future<Null> animateTo({
    GlobalKey targetKey,
    Duration duration,
    Curve curve: Curves.linear
  }) {
    assert(_handle != null);
    assert(_overlayEntry != null);
    assert(targetKey != null);
    assert(duration != null);
    assert(curve != null);
    _targetKey = targetKey;
    _curve = curve;
    // TODO(abarth): Support changing the animation target when in flight.
    assert(_controller == null);
    _controller = new AnimationController(duration: duration)
      ..addListener(_overlayEntry.markNeedsBuild);
    return _controller.forward();
  }

  /// Cause the overlay entry to rebuild during the next pipeline flush.
  ///
  /// You need to call this function if you rebuild the widget that this entry
  /// is mimicking in order for the overlay entry to pick up the changes that
  /// you've made to the [Mimicable].
  void markNeedsBuild() {
   _overlayEntry?.markNeedsBuild();
 }

  /// Remove this entry from the overlay and restore the widget to its original place in the tree.
  ///
  /// Once removed, the overlay entry cannot be used further.
  void dispose() {
    _targetKey = null;
    _curve = null;
    _controller?.stop();
    _controller = null;
    _handle.stopMimic();
    _handle = null;
    _overlayEntry.remove();
    _overlayEntry = null;
  }

  Widget _build(BuildContext context) {
    assert(_handle != null);
    assert(_overlayEntry != null);
    Rect globalBounds = _initialGlobalBounds;
    Point globalPosition = globalBounds.topLeft;
    if (_targetKey != null) {
      assert(_controller != null);
      assert(_curve != null);
      RenderBox box = _targetKey.currentContext?.findRenderObject();
      if (box != null) {
        // TODO(abarth): Handle the case where the transform here isn't just a translation.
        Point localPosition = box.localToGlobal(Point.origin);
        double t = _curve.transform(_controller.value);
        globalPosition = Point.lerp(globalPosition, localPosition, t);
      }
    }

    RenderBox stack = context.ancestorRenderObjectOfType(const TypeMatcher<RenderStack>());
    // TODO(abarth): Handle the case where the transform here isn't just a translation.
    // TODO(ianh): We should probably be getting the overlay's render object rather than looking for a RenderStack.
    assert(stack != null);
    Point localPosition = stack.globalToLocal(globalPosition);
    return new Positioned(
      left: localPosition.x,
      top: localPosition.y,
      width: globalBounds.width,
      height: globalBounds.height,
      child: new Mimic(original: _handle)
    );
  }
}

/// A widget that copies the appearance of another widget.
class Mimic extends StatelessWidget {
  Mimic({ Key key, this.original }) : super(key: key);

  /// A handle to the widget that this widget should copy.
  final MimicableHandle original;

  Widget build(BuildContext context) {
    if (original != null && original._state.mounted && original._state._placeholderSize != null)
      return original._state.config.child;
    return new Container();
  }
}

/// A widget that can be copied by a [Mimic].
///
/// This widget's State, [MimicableState], contains an API for initiating the
/// mimic operation.
class Mimicable extends StatefulWidget {
  Mimicable({ Key key, this.child }) : super(key: key);

  final Widget child;

  MimicableState createState() => new MimicableState();
}

/// The state for a [Mimicable].
///
/// Exposes an API for starting and stopping mimicking.
class MimicableState extends State<Mimicable> {
  Size _placeholderSize;

  Rect get _globalBounds {
    assert(mounted);
    RenderBox box = context.findRenderObject();
    assert(box != null);
    assert(box.hasSize);
    assert(!box.needsLayout);
    // TODO(abarth): The bounds will be wrong if there's a scale or rotation transform involved
    return box.localToGlobal(Point.origin) & box.size;
  }

  /// Start the mimicking process.
  ///
  /// The child of this object will no longer be built at this
  /// location in the tree. Instead, this widget will build a
  /// transparent placeholder with the same dimensions as the widget
  /// had when the mimicking process started.
  ///
  /// If you use startMimic(), it is your responsibility to do
  /// something with the returned [MimicableHandle]; typically,
  /// passing it to a [Mimic] widget. To mimic the child in the
  /// [Overlay], consider using [liftToOverlay()] instead.
  MimicableHandle startMimic() {
    assert(() {
      if (_placeholderSize != null) {
        throw new WidgetError(
          'Mimicable started while already active.\n'
          'When startMimic() or liftToOverlay() is called on a MimicableState, the mimic becomes active. '
          'While active, it cannot be reactivated until it is stopped. '
          'To stop a Mimicable started with startMimic(), call the MimicableHandle object\'s stopMimic() method. '
          'To stop a Mimicable started with liftToOverlay(), call dispose() on the MimicOverlayEntry.'
        );
      }
      return true;
    });
    RenderBox box = context.findRenderObject();
    assert(box != null);
    assert(box.hasSize);
    assert(!box.needsLayout);
    setState(() {
      _placeholderSize = box.size;
    });
    return new MimicableHandle._(this);
  }

  /// Start the mimicking process and mimic this object in the
  /// enclosing [Overlay].
  ///
  /// The child of this object will no longer be built at this
  /// location in the tree. Instead, (1) this widget will build a
  /// transparent placeholder with the same dimensions as the widget
  /// had when the mimicking process started and (2) the child will be
  /// placed in the enclosing overlay.
  MimicOverlayEntry liftToOverlay() {
    OverlayState overlay = Overlay.of(context, debugRequiredFor: config);
    MimicOverlayEntry entry = new MimicOverlayEntry._(startMimic());
    overlay.insert(entry._overlayEntry);
    return entry;
  }

  void _stopMimic() {
    assert(_placeholderSize != null);
    if (mounted) {
      setState(() {
        _placeholderSize = null;
      });
    }
  }

  Widget build(BuildContext context) {
    if (_placeholderSize != null) {
      return new ConstrainedBox(
        constraints: new BoxConstraints.tight(_placeholderSize)
      );
    }
    return config.child;
  }
}