Remove the PrefabEditManager. Introduce the FocusMode system on the Editor side, and a PrefabFocusHandler on the Prefab side to handle focus.

Signed-off-by: Danilo Aimini <82231674+AMZN-daimini@users.noreply.github.com>

Code/Framework/AzToolsFramework/AzToolsFramework/Prefab/PrefabFocusHandler.cpp

@@ -0,0 +1,103 @@
+/*
+ * Copyright (c) Contributors to the Open 3D Engine Project.
+ * For complete copyright and license terms please see the LICENSE at the root of this distribution.
+ *
+ * SPDX-License-Identifier: Apache-2.0 OR MIT
+ *
+ */
+
+#include <AzToolsFramework/Prefab/PrefabFocusHandler.h>
+
+#include <AzToolsFramework/Entity/PrefabEditorEntityOwnershipInterface.h>
+#include <AzToolsFramework/Prefab/Instance/Instance.h>
+#include <AzToolsFramework/Prefab/Instance/InstanceEntityMapperInterface.h>
+
+namespace AzToolsFramework::Prefab
+{
+    FocusModeFramework::FocusModeInterface* PrefabFocusHandler::s_focusModeInterface = nullptr;
+    InstanceEntityMapperInterface* PrefabFocusHandler::s_instanceEntityMapperInterface = nullptr;
+    PrefabEditorEntityOwnershipInterface* PrefabFocusHandler::s_prefabEditorEntityOwnershipInterface = nullptr;
+
+    PrefabFocusHandler::~PrefabFocusHandler()
+    {
+        AZ::Interface<PrefabFocusInterface>::Unregister(this);
+    }
+
+    void PrefabFocusHandler::Initialize()
+    {
+        s_focusModeInterface = AZ::Interface<FocusModeFramework::FocusModeInterface>::Get();
+        AZ_Assert(
+            s_focusModeInterface,
+            "Prefab - PrefabFocusHandler - "
+            "Focus Mode Interface could not be found. "
+            "Check that it is being correctly initialized.");
+
+        s_instanceEntityMapperInterface = AZ::Interface<InstanceEntityMapperInterface>::Get();
+        AZ_Assert(
+            s_instanceEntityMapperInterface,
+            "Prefab - PrefabFocusHandler - "
+            "Instance Entity Mapper Interface could not be found. "
+            "Check that it is being correctly initialized.");
+
+        s_prefabEditorEntityOwnershipInterface = AZ::Interface<PrefabEditorEntityOwnershipInterface>::Get();
+        AZ_Assert(
+            s_prefabEditorEntityOwnershipInterface,
+            "Prefab - PrefabFocusHandler - "
+            "Prefab Editor Entity Ownership Interface could not be found. "
+            "Check that it is being correctly initialized.");
+
+        AZ::Interface<PrefabFocusInterface>::Register(this);
+    }
+
+    void PrefabFocusHandler::FocusOnOwningPrefab(AZ::EntityId entityId)
+    {
+        InstanceOptionalReference focusedInstance;
+
+        if (entityId == AZ::EntityId())
+        {
+            focusedInstance = s_prefabEditorEntityOwnershipInterface->GetRootPrefabInstance();
+        }
+        else
+        {
+            focusedInstance = s_instanceEntityMapperInterface->FindOwningInstance(entityId);
+        }
+
+        if (!focusedInstance.has_value())
+        {
+            // TODO - ERROR
+        }
+
+        m_focusedInstance = focusedInstance;
+        m_focusedTemplateId = focusedInstance->get().GetTemplateId();
+        s_focusModeInterface->SetFocusRoot(focusedInstance->get().GetContainerEntityId());
+    }
+    
+    TemplateId PrefabFocusHandler::GetFocusedPrefabTemplateId()
+    {
+        return m_focusedTemplateId;
+    }
+
+    InstanceOptionalReference PrefabFocusHandler::GetFocusedPrefabInstance()
+    {
+        return m_focusedInstance;
+    }
+
+    bool PrefabFocusHandler::IsOwningPrefabBeingFocused(AZ::EntityId entityId)
+    {
+        if (entityId == AZ::EntityId())
+        {
+            // TODO - Warn?
+            return false;
+        }
+
+        InstanceOptionalReference instance = s_instanceEntityMapperInterface->FindOwningInstance(entityId);
+
+        if (!instance.has_value())
+        {
+            // TODO - ERROR
+        }
+
+        return (&instance->get() == &m_focusedInstance->get());
+    }
+
+} // namespace AzToolsFramework::Prefab

Code/Framework/AzToolsFramework/AzToolsFramework/Prefab/PrefabFocusHandler.h

@@ -0,0 +1,56 @@
+/*
+ * Copyright (c) Contributors to the Open 3D Engine Project. For complete copyright and license terms please see the LICENSE at the root of
+ * this distribution.
+ *
+ * SPDX-License-Identifier: Apache-2.0 OR MIT
+ *
+ */
+
+#pragma once
+
+#include <AzCore/Memory/SystemAllocator.h>
+
+#include <AzToolsFramework/FocusMode/FocusModeInterface.h>
+#include <AzToolsFramework/Prefab/PrefabFocusInterface.h>
+#include <AzToolsFramework/Prefab/Template/Template.h>
+
+namespace AzToolsFramework
+{
+    class PrefabEditorEntityOwnershipInterface;
+}
+
+namespace AzToolsFramework::FocusModeFramework
+{
+    class FocusModeInterface;
+}
+
+namespace AzToolsFramework::Prefab
+{
+    class InstanceEntityMapperInterface;
+
+    class PrefabFocusHandler final
+        : private PrefabFocusInterface
+    {
+    public:
+        AZ_CLASS_ALLOCATOR(PrefabFocusHandler, AZ::SystemAllocator, 0);
+
+        ~PrefabFocusHandler();
+
+        void Initialize();
+
+        // PrefabFocusInterface...
+        void FocusOnOwningPrefab(AZ::EntityId entityId) override;
+        TemplateId GetFocusedPrefabTemplateId() override;
+        InstanceOptionalReference GetFocusedPrefabInstance() override;
+        bool IsOwningPrefabBeingFocused(AZ::EntityId entityId) override;
+
+    private:
+        InstanceOptionalReference m_focusedInstance;
+        TemplateId m_focusedTemplateId;
+
+        static FocusModeFramework::FocusModeInterface* s_focusModeInterface;
+        static InstanceEntityMapperInterface* s_instanceEntityMapperInterface;
+        static PrefabEditorEntityOwnershipInterface* s_prefabEditorEntityOwnershipInterface;
+    };
+
+} // namespace AzToolsFramework::Prefab

Code/Framework/AzToolsFramework/AzToolsFramework/Prefab/PrefabFocusInterface.h

@@ -0,0 +1,40 @@
+/*
+ * Copyright (c) Contributors to the Open 3D Engine Project. For complete copyright and license terms please see the LICENSE at the root of
+ * this distribution.
+ *
+ * SPDX-License-Identifier: Apache-2.0 OR MIT
+ *
+ */
+
+#pragma once
+
+#include <AzCore/Interface/Interface.h>
+#include <AzCore/Serialization/SerializeContext.h>
+
+#include <AzToolsFramework/Prefab/Instance/Instance.h>
+#include <AzToolsFramework/Prefab/Template/Template.h>
+
+namespace AzToolsFramework::Prefab
+{
+    /*!
+     * PrefabFocusInterface
+     */
+    class PrefabFocusInterface
+    {
+    public:
+        AZ_RTTI(PrefabFocusInterface, "{F3CFA37B-5FD8-436A-9C30-60EB54E350E1}");
+
+        // TODO - Add comment
+        virtual void FocusOnOwningPrefab(AZ::EntityId entityId) = 0;
+
+        // TODO - Add comment
+        virtual TemplateId GetFocusedPrefabTemplateId() = 0;
+
+        // TODO - Add comment
+        virtual InstanceOptionalReference GetFocusedPrefabInstance() = 0;
+
+        // TODO - Add comment
+        virtual bool IsOwningPrefabBeingFocused(AZ::EntityId entityId) = 0;
+    };
+
+} // namespace AzToolsFramework::Prefab

Code/Framework/AzToolsFramework/AzToolsFramework/Prefab/PrefabSystemComponent.cpp

@@ -34,6 +34,7 @@ namespace AzToolsFramework
             m_prefabLoader.RegisterPrefabLoaderInterface();
             m_instanceUpdateExecutor.RegisterInstanceUpdateExecutorInterface();
             m_instanceToTemplatePropagator.RegisterInstanceToTemplateInterface();
+            m_prefabFocusHandler.Initialize();
             m_prefabPublicHandler.RegisterPrefabPublicHandlerInterface();
             m_prefabPublicRequestHandler.Connect();
             AZ::SystemTickBus::Handler::BusConnect();

Code/Framework/AzToolsFramework/AzToolsFramework/Prefab/PrefabSystemComponent.h

@@ -21,6 +21,7 @@
 #include <AzToolsFramework/Prefab/Instance/InstanceToTemplatePropagator.h>
 #include <AzToolsFramework/Prefab/Instance/TemplateInstanceMapper.h>
 #include <AzToolsFramework/Prefab/Link/Link.h>
+#include <AzToolsFramework/Prefab/PrefabFocusHandler.h>
 #include <AzToolsFramework/Prefab/PrefabIdTypes.h>
 #include <AzToolsFramework/Prefab/PrefabLoader.h>
 #include <AzToolsFramework/Prefab/PrefabPublicHandler.h>
@@ -369,7 +370,7 @@ namespace AzToolsFramework
             // A counter for generating unique Link Ids.
             AZStd::atomic<LinkId> m_linkIdCounter = 0u;
 
-            // Used for finding the owning instance of an arbitrary entity
+            // Used for finding the owning instance of an arbitrary entity.
             InstanceEntityMapper m_instanceEntityMapper;
 
             // Used for finding the Instances owned by an arbitrary Template.
@@ -378,16 +379,19 @@ namespace AzToolsFramework
             // Used for loading/saving Prefab Template files.
             PrefabLoader m_prefabLoader;
 
-            // Handler the public Prefab API used by UI and scripting
+            // Handles the Prefab Focus API that determines what prefab is being edited.
+            PrefabFocusHandler m_prefabFocusHandler;
+
+            // Handles the public Prefab API used by UI and scripting.
             PrefabPublicHandler m_prefabPublicHandler;
 
             // Used for updating Instances of Prefab Template.
             InstanceUpdateExecutor m_instanceUpdateExecutor;
 
-            // Used for updating Templates when Instances are modified
+            // Used for updating Templates when Instances are modified.
             InstanceToTemplatePropagator m_instanceToTemplatePropagator;
 
-            // Handler of the public Prefab requests
+            // Handler of the public Prefab requests.
             PrefabPublicRequestHandler m_prefabPublicRequestHandler;
         };
     } // namespace Prefab