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Merge pull request #964 from CesiumGS/shared-assets-lifetime
Fix asset lifetime, avoid circular reference counting.
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| Original file line number | Diff line number | Diff line change |
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| #pragma once | ||
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| #include <CesiumAsync/SharedAssetDepot.h> | ||
| #include <CesiumUtility/ExtensibleObject.h> | ||
| #include <CesiumUtility/IntrusivePointer.h> | ||
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| #include <atomic> | ||
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| namespace CesiumAsync { | ||
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| /** | ||
| * @brief An asset that is potentially shared between multiple objects, such as | ||
| * an image shared between multiple glTF models. This is intended to be the base | ||
| * class for such assets. | ||
| * | ||
| * The lifetime of instances of this class should be managed by reference | ||
| * counting with {@link IntrusivePointer}. | ||
| * | ||
| * @tparam T The type that is _deriving_ from this class. For example, you | ||
| * should declare your class as | ||
| * `class MyClass : public SharedAsset<MyClass> { ... };` | ||
| * | ||
| * @remarks @parblock | ||
| * A `SharedAsset` can be in one of three possible states: | ||
| * | ||
| * **Independent Asset** | ||
| * An independent asset isn't affiliated with an asset depot at all. | ||
| * Its lifetime is controlled exclusively by IntrusivePointer / reference | ||
| * counting. When the asset's reference count goes to zero, it deletes itself. | ||
| * An independent asset's `_pDepot` is nullptr. | ||
| * | ||
| * **Active Depot Asset** | ||
| * This is an asset that is owned by an asset depot and that is in use, meaning | ||
| * it has a reference count greater than zero. The asset depot owns the asset | ||
| * via an `std::unique_ptr`, not via adding to the reference count. So when the | ||
| * reference count goes to zero, only the asset depot itself still has a | ||
| * reference to it, so it becomes an inactive depot asset. | ||
| * | ||
| * **Inactive Depot Asset** | ||
| * This is also an asset that is owned by the asset depot, but there are no | ||
| * other references to it (it has a reference count of zero). It is found in the | ||
| * asset depot's `deletionCandidates` list. When a reference to it is added, it | ||
| * is removed from `deletionCandidates` and it becomes an active depot asset. | ||
| * @endparblock | ||
| */ | ||
| template <typename T> | ||
| class CESIUMASYNC_API SharedAsset : public CesiumUtility::ExtensibleObject { | ||
| public: | ||
| /** | ||
| * Assets can be copied, but the fresh instance has no references and is not | ||
| * in the asset depot. | ||
| */ | ||
| SharedAsset(const SharedAsset& rhs) | ||
| : ExtensibleObject(rhs), | ||
| _referenceCount(0), | ||
| _pDepot(nullptr), | ||
| _uniqueAssetId() {} | ||
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| /** | ||
| * After a move construction, the content of the asset is moved to the new | ||
| * instance, but the asset depot still references the old instance. | ||
| */ | ||
| SharedAsset(SharedAsset&& rhs) | ||
| : ExtensibleObject(std::move(rhs)), | ||
| _referenceCount(0), | ||
| _pDepot(nullptr), | ||
| _uniqueAssetId() {} | ||
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| /** | ||
| * Assignment does not affect the asset's relationship with the depot, but is | ||
| * useful to assign the data in derived classes. | ||
| */ | ||
| SharedAsset& operator=(const SharedAsset& rhs) { | ||
| CesiumUtility::ExtensibleObject::operator=(rhs); | ||
| return *this; | ||
| } | ||
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| SharedAsset& operator=(SharedAsset&& rhs) { | ||
| CesiumUtility::ExtensibleObject::operator=(std::move(rhs)); | ||
| return *this; | ||
| } | ||
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| /** | ||
| * @brief Adds a counted reference to this object. Use | ||
| * {@link CesiumUtility::IntrusivePointer} instead of calling this method | ||
| * directly. | ||
| */ | ||
| void addReference() const /*noexcept*/ { | ||
| const int32_t prevReferences = this->_referenceCount++; | ||
| if (this->_pDepot && prevReferences <= 0) { | ||
| this->_pDepot->unmarkDeletionCandidate(static_cast<const T*>(this)); | ||
| } | ||
| } | ||
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| /** | ||
| * @brief Removes a counted reference from this object. When the last | ||
| * reference is removed, this method will delete this instance. Use | ||
| * {@link CesiumUtility::IntrusivePointer} instead of calling this method | ||
| * directly. | ||
| */ | ||
| void releaseReference() const /*noexcept*/ { | ||
| CESIUM_ASSERT(this->_referenceCount > 0); | ||
| const int32_t references = --this->_referenceCount; | ||
| if (references == 0) { | ||
| SharedAssetDepot<T>* pDepot = this->_pDepot; | ||
| if (pDepot) { | ||
| // Let the depot manage this object's lifetime. | ||
| pDepot->markDeletionCandidate(static_cast<const T*>(this)); | ||
| } else { | ||
| // No depot, so destroy this object directly. | ||
| delete static_cast<const T*>(this); | ||
| } | ||
| } | ||
| } | ||
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| /** | ||
| * @brief Gets the shared asset depot that owns this asset, or nullptr if this | ||
| * asset is independent of an asset depot. | ||
| */ | ||
| const SharedAssetDepot<T>* getDepot() const { return this->_pDepot; } | ||
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| /** | ||
| * @brief Gets the shared asset depot that owns this asset, or nullptr if this | ||
| * asset is independent of an asset depot. | ||
| */ | ||
| SharedAssetDepot<T>* getDepot() { return this->_pDepot; } | ||
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| /** | ||
| * @brief Gets the unique ID of this asset, if it {@link isShareable}. | ||
| * | ||
| * If this asset is not shareable, this method will return an empty string. | ||
| */ | ||
| const std::string& getUniqueAssetId() const { return this->_uniqueAssetId; } | ||
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| protected: | ||
| SharedAsset() = default; | ||
| ~SharedAsset() { CESIUM_ASSERT(this->_referenceCount == 0); } | ||
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| private: | ||
| mutable std::atomic<std::int32_t> _referenceCount{0}; | ||
| SharedAssetDepot<T>* _pDepot{nullptr}; | ||
| std::string _uniqueAssetId{}; | ||
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| // The size of this asset when it was counted by the depot. This is stored so | ||
| // that the exact same size can be subtracted later. | ||
| int64_t _sizeInDepot{0}; | ||
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| // To allow the depot to modify _pDepot, _uniqueAssetId, and _sizeInDepot. | ||
| friend class SharedAssetDepot<T>; | ||
| }; | ||
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| } // namespace CesiumAsync |
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