• Michael Goderbauer's avatar
    Stand-alone widget tree with multiple render trees to enable multi-view rendering (#125003) · 6f09064e
    Michael Goderbauer authored
    This change enables Flutter to generate multiple Scenes to be rendered into separate FlutterViews from a single widget tree. Each Scene is described by a separate render tree, which are all associated with the single widget tree.
    
    This PR implements the framework-side mechanisms to describe the content to be rendered into multiple views. Separate engine-side changes are necessary to provide these views to the framework and to draw the framework-generated Scene into them.
    
    ## Summary of changes
    
    The details of this change are described in [flutter.dev/go/multiple-views](https://flutter.dev/go/multiple-views). Below is a high-level summary organized by layers.
    
    ### Rendering layer changes
    
    * The `RendererBinding` no longer owns a single `renderView`. In fact, it doesn't OWN any `RenderView`s at all anymore. Instead, it offers an API (`addRenderView`/`removeRenderView`) to add and remove `RenderView`s that then will be MANAGED by the binding. The `RenderView` itself is now owned by a higher-level abstraction (e.g. the `RawView` Element of the widgets layer, see below), who is also in charge of adding it to the binding. When added, the binding will interact with the `RenderView` to produce a frame (e.g. by calling `compositeFrame` on it) and to perform hit tests for incoming pointer events. Multiple `RenderView`s can be added to the binding (typically one per `FlutterView`) to produce multiple Scenes.
    * Instead of owning a single `pipelineOwner`, the `RendererBinding` now owns the root of the `PipelineOwner` tree (exposed as `rootPipelineOwner` on the binding). Each `PipelineOwner` in that tree (except for the root) typically manages its own render tree typically rooted in one of the `RenderView`s mentioned in the previous bullet. During frame production, the binding will instruct each `PipelineOwner` of that tree to flush layout, paint, semantics etc. A higher-level abstraction (e.g. the widgets layer, see below) is in charge of adding `PipelineOwner`s to this tree.
    * Backwards compatibility: The old `renderView` and `pipelineOwner` properties of the `RendererBinding` are retained, but marked as deprecated. Care has been taken to keep their original behavior for the deprecation period, i.e. if you just call `runApp`, the render tree bootstrapped by this call is rooted in the deprecated `RendererBinding.renderView` and managed by the deprecated `RendererBinding.pipelineOwner`.
    
    ### Widgets layer changes
    
    * The `WidgetsBinding` no longer attaches the widget tree to an existing render tree. Instead, it bootstraps a stand-alone widget tree that is not backed by a render tree. For this, `RenderObjectToWidgetAdapter` has been replaced by `RootWidget`.
    * Multiple render trees can be bootstrapped and attached to the widget tree with the help of the `View` widget, which internally is backed by a `RawView` widget. Configured with a `FlutterView` to render into, the `RawView` creates a new `PipelineOwner` and a new `RenderView` for the new render tree. It adds the new `RenderView` to the `RendererBinding` and its `PipelineOwner` to the pipeline owner tree.
    * The `View` widget can only appear in certain well-defined locations in the widget tree since it bootstraps a new render tree and does not insert a `RenderObject` into an ancestor. However, almost all Elements expect that their children insert `RenderObject`s, otherwise they will not function properly. To produce a good error message when the `View` widget is used in an illegal location, the `debugMustInsertRenderObjectIntoSlot` method has been added to Element, where a child can ask whether a given slot must insert a RenderObject into its ancestor or not. In practice, the `View` widget can be used as a child of the `RootWidget`, inside the `view` slot of the `ViewAnchor` (see below) and inside a `ViewCollection` (see below). In those locations, the `View` widget may be wrapped in other non-RenderObjectWidgets (e.g. InheritedWidgets).
    * The new `ViewAnchor` can be used to create a side-view inside a parent `View`. The `child` of the `ViewAnchor` widget renders into the parent `View` as usual, but the `view` slot can take on another `View` widget, which has access to all inherited widgets above the `ViewAnchor`. Metaphorically speaking, the view is anchored to the location of the `ViewAnchor` in the widget tree.
    * The new `ViewCollection` widget allows for multiple sibling views as it takes a list of `View`s as children. It can be used in all the places that accept a `View` widget.
    
    ## Google3
    
    As of July 5, 2023 this change passed a TAP global presubmit (TGP) in google3: tap/OCL:544707016:BASE:545809771:1688597935864:e43dd651
    
    ## Note to reviewers
    
    This change is big (sorry). I suggest focusing the initial review on the changes inside of `packages/flutter` first. The majority of the changes describe above are implemented in (listed in suggested review order):
    
    * `rendering/binding.dart`
    * `widgets/binding.dart`
    * `widgets/view.dart`
    * `widgets/framework.dart`
    
    All other changes included in the PR are basically the fallout of what's implemented in those files. Also note that a lot of the lines added in this PR are documentation and tests.
    
    I am also very happy to walk reviewers through the code in person or via video call, if that is helpful.
    
    I appreciate any feedback.
    
    ## Feedback to address before submitting ("TODO")
    Unverified
    6f09064e
binding.dart 2.29 KB
// Copyright 2014 The Flutter 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:ui';

import 'package:flutter/rendering.dart';

/// An extension of [RenderingFlutterBinding] that owns and manages a
/// [renderView].
///
/// Unlike [RenderingFlutterBinding], this binding also creates and owns a
/// [renderView] to simplify bootstrapping for apps that have a dedicated main
/// view.
class ViewRenderingFlutterBinding extends RenderingFlutterBinding {
  /// Creates a binding for the rendering layer.
  ///
  /// The `root` render box is attached directly to the [renderView] and is
  /// given constraints that require it to fill the window. The [renderView]
  /// itself is attached to the [rootPipelineOwner].
  ///
  /// This binding does not automatically schedule any frames. Callers are
  /// responsible for deciding when to first call [scheduleFrame].
  ViewRenderingFlutterBinding({ RenderBox? root }) : _root = root;

  @override
  void initInstances() {
    super.initInstances();
    // TODO(goderbauer): Create window if embedder doesn't provide an implicit view.
    assert(PlatformDispatcher.instance.implicitView != null);
    _renderView = initRenderView(PlatformDispatcher.instance.implicitView!);
    _renderView.child = _root;
    _root = null;
  }

  RenderBox? _root;

  @override
  RenderView get renderView => _renderView;
  late RenderView _renderView;

  /// Creates a [RenderView] object to be the root of the
  /// [RenderObject] rendering tree, and initializes it so that it
  /// will be rendered when the next frame is requested.
  ///
  /// Called automatically when the binding is created.
  RenderView initRenderView(FlutterView view) {
    final RenderView renderView = RenderView(view: view);
    rootPipelineOwner.rootNode = renderView;
    addRenderView(renderView);
    renderView.prepareInitialFrame();
    return renderView;
  }

  @override
  PipelineOwner createRootPipelineOwner() {
    return PipelineOwner(
      onSemanticsOwnerCreated: () {
        renderView.scheduleInitialSemantics();
      },
      onSemanticsUpdate: (SemanticsUpdate update) {
        renderView.updateSemantics(update);
      },
      onSemanticsOwnerDisposed: () {
        renderView.clearSemantics();
      },
    );
  }
}