#pragma once
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#include "gc/GCHandle.h"
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#include "vm/Atomic.h"
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#include "vm/CCWBase.h"
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#include "utils/Memory.h"
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#include "utils/TemplateUtils.h"
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namespace il2cpp
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{
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namespace vm
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{
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// Alright, so the lifetime of this guy is pretty weird
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// For a single managed object, the IUnknown of its COM Callable Wrapper must always be the same
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// That means that we have to keep the same COM Callable Wrapper alive for an object once we create it
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// They are cached in il2cpp::vm::g_CCWCache, which is managed by il2cpp::vm::CCW class
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//
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// Here comes the tricky part: when a native object has a reference to the COM Callable Wrapper,
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// the managed object is not supposed to be garbage collected. However, when no native objects are referencing
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// it, it should not prevent the GC from collecting the managed object. We implement this by keeping a GC handle
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// on the managed object if our reference count is 1 or more. We acquire it when it gets increased from 0 (this
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// is safe because such AddRef can only come when this object is retrieved from CCW Cache) and release the GC
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// handle when our reference count gets decreased to 0. Here's a kicker: we don't destroy the COM Callable Wrapper
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// when the reference count reaches 0; we instead rely on GC finalizer of the managed object to both remove it from
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// CCW cache and also destroy it.
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template<typename TDerived>
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struct NOVTABLE CachedCCWBase : CCWBase
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{
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private:
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volatile uint32_t m_RefCount;
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uint32_t m_GCHandle;
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public:
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inline CachedCCWBase(Il2CppObject* obj) :
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CCWBase(obj),
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m_RefCount(0), // We do not hold any references upon its creation
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m_GCHandle(0)
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{
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Il2CppStaticAssert(utils::TemplateUtils::IsBaseOf<CachedCCWBase<TDerived>, TDerived>::value);
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}
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virtual uint32_t STDCALL AddRef() IL2CPP_OVERRIDE
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{
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return AddRefImpl();
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}
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virtual uint32_t STDCALL Release() IL2CPP_OVERRIDE
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{
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return ReleaseImpl();
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}
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// AddRef can be called at any time whatsoever, as it's called when
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// managed objects are passed to native code
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IL2CPP_NO_INLINE uint32_t AddRefImpl()
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{
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const uint32_t refCount = Atomic::Increment(&m_RefCount);
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if (refCount == 1)
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{
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// Since AddRef can be called at any time, it's possible that
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// at this point we're in middle of ReleaseImpl call just after
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// it decrements the gccount to 0 but hasn't released m_GCHandle
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// yet. We spin until it is released.
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uint32_t gcHandle = gc::GCHandle::New(GetManagedObjectInline(), false);
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while (Atomic::CompareExchange(&m_GCHandle, gcHandle, 0) != 0) {}
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}
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return refCount;
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}
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// Release can be called only if m_RefCount is greater than 0,
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// and the AddRef call that has increased the ref count above 0 has returned
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IL2CPP_NO_INLINE uint32_t ReleaseImpl()
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{
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const uint32_t count = Atomic::Decrement(&m_RefCount);
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if (count == 0)
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{
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// We decreased the ref count to 0, so we are responsible
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// for freeing the handle. Only one ReleaseImpl that reduced
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// ref count to 0 will ever be in flight at the same time
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// because AddRefImpl that takes us out of this state halts until
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// we set m_GCHandle to zero.
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uint32_t gcHandle = Atomic::Exchange(&m_GCHandle, 0);
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IL2CPP_ASSERT(gcHandle != 0);
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gc::GCHandle::Free(gcHandle);
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}
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return count;
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}
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virtual void STDCALL Destroy() IL2CPP_FINAL IL2CPP_OVERRIDE
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{
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IL2CPP_ASSERT(m_RefCount == 0);
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TDerived* instance = static_cast<TDerived*>(this);
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instance->~TDerived();
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utils::Memory::Free(instance);
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}
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};
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}
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}
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