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[web] remove obsolete object caches; simplify native object management (
#40894) (this is attempt 3; details below) Remove obsolete object caches and introduce a simpler way to manage native objects: * Remove the unused `SynchronousSkiaObjectCache`. * Introduce new library `native_memory.dart` that's smaller and simpler than `skia_object_cache.dart`. * Introduce two types of native object references: * `UniqueRef` a reference with a unique Dart object owner. * `CountedRef` a ref-counted reference with multiple Dart object owners. * All native references use GC (via `FinalizationRegistry`) as a back-up. * The new library removes everything related to object resurrection that was needed only in browsers that didn't support `FinalizationRegistry`. All browsers support it now. * Remove the ad hoc `SkParagraph` cache that predates the introduction of `Paragraph.dispose`. * Rewrite `CkParagraph` in terms of `UniqueRef`. * Rewrite `CkImage` in terms of `CountedRef`; delete `SkiaObjectBox`. This PR does not migrate all objects from the old `skia_object_cache.dart` to `native_memory.dart`. That would be too big of a change. The migration can be done in multiple smaller PRs. This also removes a few unnecessary relayouts observed in flutter/flutter#120921, but not all of them (more details in flutter/flutter#120921 (comment)) ## About attempt 3 More about [attempt 2 here](#40862). In this attempt 3 I'm replacing the `factory` with a top-level function.
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| Original file line number | Diff line number | Diff line change |
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| // Copyright 2013 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. | ||
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| import 'dart:js_interop'; | ||
| import 'package:meta/meta.dart'; | ||
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| import '../../engine.dart' show Instrumentation; | ||
| import '../util.dart'; | ||
| import 'canvaskit_api.dart'; | ||
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| /// Collects native objects that weren't explicitly disposed of using | ||
| /// [UniqueRef.dispose] or [CountedRef.unref]. | ||
| SkObjectFinalizationRegistry _finalizationRegistry = createSkObjectFinalizationRegistry( | ||
| (UniqueRef<Object> uniq) { | ||
| uniq.collect(); | ||
| }.toJS | ||
| ); | ||
|
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| NativeMemoryFinalizationRegistry nativeMemoryFinalizationRegistry = NativeMemoryFinalizationRegistry(); | ||
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| /// An indirection to [SkObjectFinalizationRegistry] to enable tests provide a | ||
| /// mock implementation of a finalization registry. | ||
| class NativeMemoryFinalizationRegistry { | ||
| void register(Object owner, UniqueRef<Object> ref) { | ||
| _finalizationRegistry.register(owner, ref); | ||
| } | ||
| } | ||
|
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| /// Manages the lifecycle of a C++ object referenced by a single Dart object. | ||
| /// | ||
| /// It is expected that when the C++ object is no longer needed [dispose] is | ||
| /// called. | ||
| /// | ||
| /// To prevent memory leaks, the underlying C++ object is deleted by the GC if | ||
| /// it wasn't previously disposed of explicitly. | ||
| class UniqueRef<T extends Object> { | ||
| UniqueRef(Object owner, T nativeObject, this._debugOwnerLabel) { | ||
| _nativeObject = nativeObject; | ||
| if (Instrumentation.enabled) { | ||
| Instrumentation.instance.incrementCounter('$_debugOwnerLabel Created'); | ||
| } | ||
| nativeMemoryFinalizationRegistry.register(owner, this); | ||
| } | ||
|
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| T? _nativeObject; | ||
| final String _debugOwnerLabel; | ||
|
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| /// Returns the underlying native object reference, if it has not been | ||
| /// disposed of yet. | ||
| /// | ||
| /// The returned reference must not be stored. I should only be borrowed | ||
| /// temporarily. Storing this reference may result in dangling pointer errors. | ||
| T get nativeObject { | ||
| assert(!isDisposed, 'Native object was disposed.'); | ||
| return _nativeObject!; | ||
| } | ||
|
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| /// Returns whether the underlying native object has been disposed and | ||
| /// therefore can no longer be used. | ||
| bool get isDisposed => _nativeObject == null; | ||
|
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| /// Disposes the underlying native object. | ||
| /// | ||
| /// The underlying object may be deleted or its ref count may be bumped down. | ||
| /// The exact action taken depends on the sharing model of that particular | ||
| /// object. For example, an [SkImage] may not be immediately deleted if a | ||
| /// [SkPicture] exists that still references it. On the other hand, [SkPaint] | ||
| /// is deleted eagerly. | ||
| void dispose() { | ||
| assert(!isDisposed, 'A native object reference cannot be disposed more than once.'); | ||
| if (Instrumentation.enabled) { | ||
| Instrumentation.instance.incrementCounter('$_debugOwnerLabel Deleted'); | ||
| } | ||
| final SkDeletable object = nativeObject as SkDeletable; | ||
| if (!object.isDeleted()) { | ||
| object.delete(); | ||
| } | ||
| _nativeObject = null; | ||
| } | ||
|
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| /// Called by the garbage [Collector] when the owner of this handle is | ||
| /// collected. | ||
| /// | ||
| /// Garbage collection is used as a back-up for the cases when the handle | ||
| /// isn't disposed of explicitly by calling [dispose]. It most likely | ||
| /// indicates a memory leak or inefficiency in the framework or application | ||
| /// code. | ||
| @visibleForTesting | ||
| void collect() { | ||
| if (!isDisposed) { | ||
| if (Instrumentation.enabled) { | ||
| Instrumentation.instance.incrementCounter('$_debugOwnerLabel Leaked'); | ||
| } | ||
| dispose(); | ||
| } | ||
| } | ||
| } | ||
|
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| /// Interface that classes wrapping [UniqueRef] must implement. | ||
| /// | ||
| /// Used to collect stack traces in debug mode. | ||
| abstract class StackTraceDebugger { | ||
| /// The stack trace pointing to code location that created or upreffed a | ||
| /// [CountedRef]. | ||
| StackTrace get debugStackTrace; | ||
| } | ||
|
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| /// Manages the lifecycle of a C++ object referenced by multiple Dart objects. | ||
| /// | ||
| /// Uses reference counting to manage the lifecycle of the C++ object. | ||
| /// | ||
| /// If the C++ object has a unique owner, use [UniqueRef] instead. | ||
| /// | ||
| /// The [ref] method can be used to increment the refcount to tell this box to | ||
| /// keep the underlying C++ object alive. | ||
| /// | ||
| /// The [unref] method can be used to decrement the refcount indicating that a | ||
| /// referring object no longer needs it. When the refcount drops to zero the | ||
| /// underlying C++ object is deleted. | ||
| /// | ||
| /// In addition to ref counting, this object is also managed by GC. When this | ||
| /// reference is garbage collected, the underlying C++ object is automatically | ||
| /// deleted. This is mostly done to prevent memory leaks in production. Well | ||
| /// behaving framework and app code are expected to rely on [ref] and [unref] | ||
| /// for timely collection of resources. | ||
| class CountedRef<R extends StackTraceDebugger, T extends Object> { | ||
| /// Creates a counted reference. | ||
| CountedRef(T nativeObject, R debugReferrer, String debugLabel) { | ||
| _ref = UniqueRef<T>(this, nativeObject, debugLabel); | ||
| if (assertionsEnabled) { | ||
| debugReferrers.add(debugReferrer); | ||
| } | ||
| assert(refCount == debugReferrers.length); | ||
| } | ||
|
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| /// The native object reference whose lifecycle is being managed by this ref | ||
| /// count. | ||
| /// | ||
| /// Do not store this value outside this class. | ||
| late final UniqueRef<T> _ref; | ||
|
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| /// Returns the underlying native object reference, if it has not been | ||
| /// disposed of yet. | ||
| /// | ||
| /// The returned reference must not be stored. I should only be borrowed | ||
| /// temporarily. Storing this reference may result in dangling pointer errors. | ||
| T get nativeObject => _ref.nativeObject; | ||
|
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| /// The number of objects sharing references to this box. | ||
| /// | ||
| /// When this count reaches zero, the underlying [nativeObject] is scheduled | ||
| /// for deletion. | ||
| int get refCount => _refCount; | ||
| int _refCount = 1; | ||
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| /// Whether the underlying [nativeObject] has been disposed and is no longer | ||
| /// accessible. | ||
| bool get isDisposed => _ref.isDisposed; | ||
|
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| /// When assertions are enabled, stores all objects that share this box. | ||
| /// | ||
| /// The length of this list is always identical to [refCount]. | ||
| /// | ||
| /// This list can be used for debugging ref counting issues. | ||
| final Set<R> debugReferrers = <R>{}; | ||
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| /// If asserts are enabled, the [StackTrace]s representing when a reference | ||
| /// was created. | ||
| List<StackTrace> debugGetStackTraces() { | ||
| if (assertionsEnabled) { | ||
| return debugReferrers | ||
| .map<StackTrace>((R referrer) => referrer.debugStackTrace) | ||
| .toList(); | ||
| } | ||
| throw UnsupportedError(''); | ||
| } | ||
|
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| /// Increases the reference count of this box because a new object began | ||
| /// sharing ownership of the underlying [nativeObject]. | ||
| void ref(R debugReferrer) { | ||
| assert( | ||
| !_ref.isDisposed, | ||
| 'Cannot increment ref count on a deleted handle.', | ||
| ); | ||
| assert(_refCount > 0); | ||
| assert( | ||
| debugReferrers.add(debugReferrer), | ||
| 'Attempted to increment ref count by the same referrer more than once.', | ||
| ); | ||
| _refCount += 1; | ||
| assert(refCount == debugReferrers.length); | ||
| } | ||
|
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| /// Decrements the reference count for the [nativeObject]. | ||
| /// | ||
| /// Does nothing if the object has already been deleted. | ||
| /// | ||
| /// If this causes the reference count to drop to zero, deletes the | ||
| /// [nativeObject]. | ||
| void unref(R debugReferrer) { | ||
| assert( | ||
| !_ref.isDisposed, | ||
| 'Attempted to unref an already deleted native object.', | ||
| ); | ||
| assert( | ||
| debugReferrers.remove(debugReferrer), | ||
| 'Attempted to decrement ref count by the same referrer more than once.', | ||
| ); | ||
| _refCount -= 1; | ||
| assert(refCount == debugReferrers.length); | ||
| if (_refCount == 0) { | ||
| _ref.dispose(); | ||
| } | ||
| } | ||
| } |
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