Add documentation for platform/bindings

third_party/WebKit/Source/bindings/core/v8/TraceWrapperReference.md

Index: third_party/WebKit/Source/bindings/core/v8/TraceWrapperReference.md
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-# Wrapper Tracing Reference
-
-This document describes wrapper tracing and how its API is supposed to be used.
-
-[TOC]
-
-## Quickstart guide
-
-Wrapper tracing is used to represent reachability across V8 and Blink. The
-following checklist highlights the modifications needed to make a class
-participate in wrapper tracing.
-
-1. Make sure that objects participating in tracing either inherit from
-``ScriptWrappable`` (if they can reference wrappers) or ``TraceWrapperBase``
-(transitively holding wrappers alive).
-2. Use ``TraceWrapperMember<T>`` to annotate fields that need to be followed to
-find other wrappers that this object should keep alive.
-3. Use ``TraceWrapperV8Reference<T>`` to annotate references to V8 that this
-object should keep alive.
-4. Declare a method to trace other wrappers using
-``DECLARE_VIRTUAL_TRACE_WRAPPERS()``.
-5. Define the method using ``DEFINE_TRACE_WRAPPERS(ClassName)``.
-6. Trace all fields that received a wrapper tracing type in (1) and (2) using
-``visitor->traceWrapers(<m_field>)`` in the body of ``DEFINE_TRACE_WRAPPERS``.
-
-The following example illustrates these steps:
-
-```c++
-#include "bindings/core/v8/ScriptWrappable.h"
-#include "bindings/core/v8/TraceWrapperMember.h"
-#include "bindings/core/v8/TraceWrapperV8Reference.h"
-
-class SomeDOMObject : public ScriptWrappable {          // (1)
- public:
-  DECLARE_VIRTUAL_TRACE_WRAPPERS();                     // (4)
-
- private:
-  TraceWrapperMember<OtherWrappable> m_otherWrappable;  // (2)
-  Member<NonWrappable> m_nonWrappable;
-  TraceWrapperV8Reference<v8::Value> m_v8object;        // (3)
-  // ...
-};
-
-DEFINE_TRACE_WRAPPERS(SomeDOMObject) {                  // (5)
-  visitor->traceWrappers(m_otherWrappable);             // (6)
-  visitor->traceWrappers(m_v8object);                   // (6)
-}
-```
-
-For more in-depth information and how to deal with corner cases continue on reading.
-
-## Background
-
-Blink and V8 need to cooperate to collect JavaScript *wrappers*. Each V8
-*wrapper* object (*W*) in JavaScript is assigned a C++ *DOM object* (*D*) in
-Blink. A single C++ *DOM object* can hold onto one or many *wrapper* objects.
-During a garbage collection initiated from JavaScript, a *wrapper* then keeps
-the  C++ *DOM object* and all its transitive dependencies, including other
-*wrappers*, alive, effectively tracing paths like
-*W<sub>x</sub> -> D<sub>1</sub>  -> ⋯ -> D<sub>n</sub> -> W<sub>y</sub>*.
-
-Previously, this relationship was expressed using so-called object groups,
-effectively assigning all C++ *DOM objects* in a given DOM tree the same group.
-The group would only die if all objects belonging to the same group die. A brief
-introduction on the limitations on this approach can be found in
-[this slide deck][object-grouping-slides].
-
-Wrapper tracing solves this problem by determining liveness based on
-reachability by tracing through the C++ *DOM objects*. The rest of this document
-describes how the API is used to keep JavaScript wrapper objects alive.
-
-Note that *wrappables* have to be *on-heap objects* and thus all
-[Oilpan-related rules][oilpan-docs] apply.
-
-[object-grouping-slides]: https://docs.google.com/presentation/d/1I6leiRm0ysSTqy7QWh33Gfp7_y4ngygyM2tDAqdF0fI/
-[oilpan-docs]: https://chromium.googlesource.com/chromium/src/+/master/third_party/WebKit/Source/platform/heap/BlinkGCAPIReference.md
-
-## Basic usage
-
-The annotations that are required can be found in the following header files.
-Pick the header file depending on what types are needed.
-
-```c++
-#include "bindings/core/v8/ScriptWrappable.h"
-#include "bindings/core/v8/TraceWrapperBase.h"
-#include "bindings/core/v8/TraceWrapperMember.h"
-#include "bindings/core/v8/TraceWrapperV8Reference.h"
-```
-
-The following example will guide through the modifications that are needed to
-adjust a given class ``SomeDOMObject`` to participate in wrapper tracing.
-
-```c++
-class SomeDOMObject : public ScriptWrappable {
-  // ...
- private:
-  Member<OtherWrappable /* extending ScriptWrappable */> m_otherWrappable;
-  Member<NonWrappable> m_nonWrappable;
-};
-```
-
-In this scenario ``SomeDOMObject`` is the object that is wrapped by an object on
-the JavaScript side. The next step is to identify the paths that lead to other
-wrappables. In this case, only ``m_otherWrappable`` needs to be traced to find
-other *wrappers* in V8.
-
-As wrapper tracing only traces a subset of members residing on the Oilpan heap,
-it requires its own tracing method. The macros are as follows:
-
-* ``DECLARE_VIRTUAL_TRACE_WRAPPERS();``: Use in the class declaration to declare
-the needed wrapper tracing method.
-* ``DEFINE_TRACE_WRAPPERS(ClassName)``: Use to define the implementation of
-wrapper tracing.
-
-Many more convenience wrappers, like inline definitions, can be found in
-``platform/heap/WrapperVisitor.h``.
-
-```c++
-class SomeDOMObject : public ScriptWrappable {
- public:
-  DECLARE_VIRTUAL_TRACE_WRAPPERS();
-
- private:
-  Member<OtherWrappable> m_otherWrappable;
-  Member<NonWrappable> m_nonWrappable;
-};
-
-DEFINE_TRACE_WRAPPERS(SomeDOMObject) {
-  visitor->traceWrappers(m_otherWrappable);
-}
-```
-
-
-Blink and V8 implement *incremental* wrapper tracing, which means that marking
-can be interleaved with JavaScript or even DOM operations. This poses a
-challenge, because already marked objects will not be considered again if they
-are reached through some other path.
-
-For example, consider an object ``A`` that has already been marked and a write
-to a field ``A.x`` setting ``x`` to an unmarked object ``Y``.  Since ``A.x`` is
-the only reference keeping ``Y``, and ``A`` has already been marked, the garbage
-collector will not find ``Y`` and reclaim it.
-
-To overcome this problem we require all writes of interesting objects, i.e.,
-writes to traced fields, to go through a write barrier.  Repeat for simpler
-readers: The write barrier will check for the problem case above and make sure
-``Y`` will be marked. In order to automatically issue a write barrier
-``m_otherWrappable`` needs ``TraceWrapperMember`` type.
-
-```c++
-class SomeDOMObject : public ScriptWrappable {
- public:
-  SomeDOMObject() : m_otherWrappable(this, nullptr) {}
-  DECLARE_VIRTUAL_TRACE_WRAPPERS();
-
- private:
-  TraceWrapperMember<OtherWrappable> m_otherWrappable;
-  Member<NonWrappable> m_nonWrappable;
-};
-
-DEFINE_TRACE_WRAPPERS(SomeDOMObject) {
-  visitor->traceWrappers(m_otherWrappable);
-}
-```
-
-``TraceWrapperMember`` makes sure that any write to ``m_otherWrappable`` will
-consider doing a write barrier. Using the proper type, the write barrier is
-correct by construction, i.e., it will never be missed.
-
-## Heap collections
-
-The proper type usage for collections, e.g. ``HeapVector`` looks like the
-following.
-
-```c++
-class SomeDOMObject : public ScriptWrappable {
- public:
-  // ...
-  void AppendNewValue(OtherWrappable* newValue);
-  DECLARE_VIRTUAL_TRACE_WRAPPERS();
-
- private:
-  HeapVector<TraceWrapperMember<OtherWrappable>> m_otherWrappables;
-};
-
-DEFINE_TRACE_WRAPPERS(SomeDOMObject) {
-  for (auto other : m_otherWrappables)
-    visitor->traceWrappers(other);
-}
-```
-
-Note that this is different to Oilpan which can just trace the whole collection.
-Whenever an element is added through ``append()`` the value needs to be
-constructed using ``TraceWrapperMember``, e.g.
-
-```c++
-void SomeDOMObject::AppendNewValue(OtherWrappable* newValue) {
-  m_otherWrappables.append(TraceWrapperMember(this, newValue));
-}
-```
-
-The compiler will throw an error for each ommitted ``TraceWrapperMember``
-construction.
-
-### Corner-case: Pre-sized containers
-
-Containers know how to construct an empty ``TraceWrapperMember`` slot. This
-allows simple creation of containers at the cost of loosing the compile-time
-check for assigning a raw value.
-
-```c++
-class SomeDOMObject : public ScriptWrappable {
- public:
-  SomeDOMObject() {
-    m_otherWrappables.resize(5);
-  }
-
-  void writeAt(int i, OtherWrappable* other) {
-    m_otherWrappables[i] = other;
-  }
-
-  DECLARE_VIRTUAL_TRACE_WRAPPERS();
- private:
-  HeapVector<TraceWrapperMember<OtherWrappable>> m_otherWrappables;
-};
-
-DEFINE_TRACE_WRAPPERS(SomeDOMObject) {
-  for (auto other : m_otherWrappables)
-    visitor->traceWrappers(other);
-}
-```
-
-In this example, the compiler will not warn you on
-``m_otherWrappables[i] = other``, but an assertion will throw at runtime as long
-as there exists a test covering that branch.
-
-The correct assignment looks like
-
-```c++
-m_otherWrappables[i] = TraceWrapperMember<OtherWrappable>(this, other);
-```
-
-Note that the assertion that triggers when the annotation is not present does
-not require wrapper tracing to be enabled.
-
-## Tracing through non-``ScriptWrappable`` types
-
-Sometimes it is necessary to trace through types that do not inherit from
-``ScriptWrappable``. For example, consider the object graph
-``A -> B -> C`` where both ``A`` and ``C`` are ``ScriptWrappable``s that
-need to be traced.
-
-In this case, the same rules as with ``ScriptWrappables`` apply, except for the
-difference that these classes need to inherit from ``TraceWrapperBase``.
-
-### Memory-footprint critical uses
-
-In the case we cannot afford inheriting from ``TraceWrapperBase``, which will
-add a vtable pointer for tracing wrappers, use
-``DEFINE_TRAIT_FOR_TRACE_WRAPPERS(ClassName)`` after defining
-``ClassName`` to define the proper tracing specializations.
-
-## Explicit write barriers
-
-Sometimes it is necessary to stick with the regular types and issue the write
-barriers explicitly. For example, if memory footprint is really important and
-it's not possible to use ``TraceWrapperMember`` which adds another pointer
-field. In this case, tracing needs to be adjusted to tell the system that all
-barriers will be done manually.
-
-```c++
-class ManualWrappable : public ScriptWrappable {
- public:
-  void setNew(OtherWrappable* newValue) {
-    m_otherWrappable = newValue;
-    SriptWrappableVisitor::writeBarrier(this, m_otherWrappable);
-  }
-
-  DECLARE_VIRTUAL_TRACE_WRAPPERS();
- private:
-  Member<OtherWrappable>> m_otherWrappable;
-};
-
-DEFINE_TRACE_WRAPPERS(ManualWrappable) {
-  for (auto other : m_otherWrappables)
-    visitor->traceWrappersWithManualWriteBarrier(other);
-}
-```