gecko-dev/layout/base/FramePropertyTable.h

/* -*- Mode: C++; tab-width: 20; indent-tabs-mode: nil; c-basic-offset: 2 -*-
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#ifndef FRAMEPROPERTYTABLE_H_
#define FRAMEPROPERTYTABLE_H_

#include "nsTHashtable.h"
#include "nsHashKeys.h"

class nsIFrame;

namespace mozilla {

struct FramePropertyDescriptor;

typedef void (*FramePropertyDestructor)(void* aPropertyValue);
typedef void (*FramePropertyDestructorWithFrame)(nsIFrame* aFrame,
                                                 void* aPropertyValue);

/**
 * A pointer to a FramePropertyDescriptor serves as a unique property ID.
 * The FramePropertyDescriptor stores metadata about the property.
 * Currently the only metadata is a destructor function. The destructor
 * function is called on property values when they are overwritten or
 * deleted.
 * 
 * To use this class, declare a global (i.e., file, class or function-scope
 * static member) FramePropertyDescriptor and pass its address as
 * aProperty in the FramePropertyTable methods.
 */
struct FramePropertyDescriptor {
  /**
   * mDestructor will be called if it's non-null.
   */
  FramePropertyDestructor          mDestructor;
  /**
   * mDestructorWithFrame will be called if it's non-null and mDestructor
   * is null. WARNING: The frame passed to mDestructorWithFrame may
   * be a dangling frame pointer, if this is being called during
   * presshell teardown. Do not use it except to compare against
   * other frame pointers. No frame will have been allocated with
   * the same address yet.
   */
  FramePropertyDestructorWithFrame mDestructorWithFrame;
  /**
   * mDestructor and mDestructorWithFrame may both be null, in which case
   * no value destruction is a no-op.
   */
};

/**
 * The FramePropertyTable is optimized for storing 0 or 1 properties on
 * a given frame. Storing very large numbers of properties on a single
 * frame will not be efficient.
 * 
 * Property values are passed as void* but do not actually have to be
 * valid pointers. You can use NS_INT32_TO_PTR/NS_PTR_TO_INT32 to
 * store int32_t values. Null/zero values can be stored and retrieved.
 * Of course, the destructor function (if any) must handle such values
 * correctly.
 */
class FramePropertyTable {
public:
  FramePropertyTable() : mLastFrame(nullptr), mLastEntry(nullptr)
  {
    mEntries.Init();
  }
  ~FramePropertyTable()
  {
    DeleteAll();
  }

  /**
   * Set a property value on a frame. This requires one hashtable
   * lookup (using the frame as the key) and a linear search through
   * the properties of that frame. Any existing value for the property
   * is destroyed.
   */
  void Set(nsIFrame* aFrame, const FramePropertyDescriptor* aProperty,
           void* aValue);
  /**
   * Get a property value for a frame. This requires one hashtable
   * lookup (using the frame as the key) and a linear search through
   * the properties of that frame. If the frame has no such property,
   * returns null.
   * @param aFoundResult if non-null, receives a value 'true' iff
   * the frame has a value for the property. This lets callers
   * disambiguate a null result, which can mean 'no such property' or
   * 'property value is null'.
   */
  void* Get(const nsIFrame* aFrame, const FramePropertyDescriptor* aProperty,
            bool* aFoundResult = nullptr);
  /**
   * Remove a property value for a frame. This requires one hashtable
   * lookup (using the frame as the key) and a linear search through
   * the properties of that frame. The old property value is returned
   * (and not destroyed). If the frame has no such property,
   * returns null.
   * @param aFoundResult if non-null, receives a value 'true' iff
   * the frame had a value for the property. This lets callers
   * disambiguate a null result, which can mean 'no such property' or
   * 'property value is null'.
   */
  void* Remove(nsIFrame* aFrame, const FramePropertyDescriptor* aProperty,
               bool* aFoundResult = nullptr);
  /**
   * Remove and destroy a property value for a frame. This requires one
   * hashtable lookup (using the frame as the key) and a linear search
   * through the properties of that frame. If the frame has no such
   * property, nothing happens.
   */
  void Delete(nsIFrame* aFrame, const FramePropertyDescriptor* aProperty);
  /**
   * Remove and destroy all property values for a frame. This requires one
   * hashtable lookup (using the frame as the key).
   */
  void DeleteAllFor(nsIFrame* aFrame);
  /**
   * Remove and destroy all property values for all frames.
   */
  void DeleteAll();

  size_t SizeOfExcludingThis(nsMallocSizeOfFun aMallocSizeOf) const;

protected:
  /**
   * Stores a property descriptor/value pair. It can also be used to
   * store an nsTArray of PropertyValues.
   */
  struct PropertyValue {
    PropertyValue() : mProperty(nullptr), mValue(nullptr) {}
    PropertyValue(const FramePropertyDescriptor* aProperty, void* aValue)
      : mProperty(aProperty), mValue(aValue) {}

    bool IsArray() { return !mProperty && mValue; }
    nsTArray<PropertyValue>* ToArray()
    {
      NS_ASSERTION(IsArray(), "Must be array");
      return reinterpret_cast<nsTArray<PropertyValue>*>(&mValue);
    }

    void DestroyValueFor(nsIFrame* aFrame) {
      if (mProperty->mDestructor) {
        mProperty->mDestructor(mValue);
      } else if (mProperty->mDestructorWithFrame) {
        mProperty->mDestructorWithFrame(aFrame, mValue);
      }
    }

    size_t SizeOfExcludingThis(nsMallocSizeOfFun aMallocSizeOf) {
      size_t n = 0;
      // We don't need to measure mProperty because it always points to static
      // memory.  As for mValue:  if it's a single value we can't measure it,
      // because the type is opaque;  if it's an array, we measure the array
      // storage, but we can't measure the individual values, again because
      // their types are opaque.
      if (IsArray()) {
        nsTArray<PropertyValue>* array = ToArray();
        n += array->SizeOfExcludingThis(aMallocSizeOf);
      }
      return n;
    }

    const FramePropertyDescriptor* mProperty;
    void* mValue;
  };

  /**
   * Used with an array of PropertyValues to allow lookups that compare
   * only on the FramePropertyDescriptor.
   */
  class PropertyComparator {
  public:
    bool Equals(const PropertyValue& a, const PropertyValue& b) const {
      return a.mProperty == b.mProperty;
    }
    bool Equals(const FramePropertyDescriptor* a, const PropertyValue& b) const {
      return a == b.mProperty;
    }
    bool Equals(const PropertyValue& a, const FramePropertyDescriptor* b) const {
      return a.mProperty == b;
    }
  };

  /**
   * Our hashtable entry. The key is an nsIFrame*, the value is a
   * PropertyValue representing one or more property/value pairs.
   */
  class Entry : public nsPtrHashKey<nsIFrame>
  {
  public:
    Entry(KeyTypePointer aKey) : nsPtrHashKey<nsIFrame>(aKey) {}
    Entry(const Entry &toCopy) :
      nsPtrHashKey<nsIFrame>(toCopy), mProp(toCopy.mProp) {}

    PropertyValue mProp;
  };

  static void DeleteAllForEntry(Entry* aEntry);
  static PLDHashOperator DeleteEnumerator(Entry* aEntry, void* aArg);

  static size_t SizeOfPropertyTableEntryExcludingThis(Entry* aEntry,
                  nsMallocSizeOfFun aMallocSizeOf, void *);

  nsTHashtable<Entry> mEntries;
  nsIFrame* mLastFrame;
  Entry* mLastEntry;
};

/**
 * This class encapsulates the properties of a frame.
 */
class FrameProperties {
public:
  FrameProperties(FramePropertyTable* aTable, nsIFrame* aFrame)
    : mTable(aTable), mFrame(aFrame) {}
  FrameProperties(FramePropertyTable* aTable, const nsIFrame* aFrame)
    : mTable(aTable), mFrame(const_cast<nsIFrame*>(aFrame)) {}

  void Set(const FramePropertyDescriptor* aProperty, void* aValue) const
  {
    mTable->Set(mFrame, aProperty, aValue);
  }
  void* Get(const FramePropertyDescriptor* aProperty,
            bool* aFoundResult = nullptr) const
  {
    return mTable->Get(mFrame, aProperty, aFoundResult);
  }
  void* Remove(const FramePropertyDescriptor* aProperty,
               bool* aFoundResult = nullptr) const
  {
    return mTable->Remove(mFrame, aProperty, aFoundResult);
  }
  void Delete(const FramePropertyDescriptor* aProperty)
  {
    mTable->Delete(mFrame, aProperty);
  }

private:
  FramePropertyTable* mTable;
  nsIFrame* mFrame;
};

}

#endif /* FRAMEPROPERTYTABLE_H_ */