Add API writers

lib/mayaUsd/fileio/CMakeLists.txt

@@ -3,6 +3,8 @@
 # -----------------------------------------------------------------------------
 target_sources(${PROJECT_NAME} 
     PRIVATE
+        apiWriterContext.cpp
+        apiWriterRegistry.cpp
         fallbackPrimReader.cpp
         functorPrimReader.cpp
         functorPrimWriter.cpp
@@ -42,6 +44,8 @@ if(CMAKE_UFE_V3_FEATURES_AVAILABLE)
 endif()
 
 set(HEADERS
+    apiWriterContext.h
+    apiWriterRegistry.h
     fallbackPrimReader.h
     functorPrimReader.h
     functorPrimWriter.h

lib/mayaUsd/fileio/apiWriterContext.cpp

@@ -0,0 +1,37 @@
+//
+// Copyright 2022 Pixar
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+#include "apiWriterContext.h"
+
+PXR_NAMESPACE_OPEN_SCOPE
+
+UsdMayaApiWriterContext::UsdMayaApiWriterContext(
+    const MDagPath&    dagPath,
+    const UsdPrim&     usdPrim,
+    const UsdTimeCode& timeCode)
+    : _dagPath(dagPath)
+    , _usdPrim(usdPrim)
+    , _timeCode(timeCode)
+{
+}
+
+const MDagPath& UsdMayaApiWriterContext::GetMDagPath() const { return _dagPath; }
+
+const UsdPrim& UsdMayaApiWriterContext::GetUsdPrim() const { return _usdPrim; }
+
+const UsdTimeCode& UsdMayaApiWriterContext::GetTimeCode() const { return _timeCode; }
+
+PXR_NAMESPACE_CLOSE_SCOPE

lib/mayaUsd/fileio/apiWriterContext.h

@@ -0,0 +1,62 @@
+//
+// Copyright 2022 Pixar
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+#ifndef PXRUSDMAYA_APIWRITERCONTEXT_H
+#define PXRUSDMAYA_APIWRITERCONTEXT_H
+
+#include <mayaUsd/base/api.h>
+
+#include <pxr/pxr.h>
+
+#include <maya/MDagPath.h>
+
+PXR_NAMESPACE_OPEN_SCOPE
+
+class UsdPrim;
+class UsdTimeCode;
+
+/// Class that provides an interface for API writer plugins.
+/// \sa UsdMayaApiWriterRegistry
+class UsdMayaApiWriterContext
+{
+public:
+    MAYAUSD_CORE_PUBLIC
+    UsdMayaApiWriterContext(
+        const MDagPath&    dagPath,
+        const UsdPrim&     usdPrim,
+        const UsdTimeCode& timeCode);
+
+    /// Returns the MDagPath that corresponds to the \p usdPrim that was
+    /// exported that the API is being applied to.
+    MAYAUSD_CORE_PUBLIC
+    const MDagPath& GetMDagPath() const;
+
+    /// Returns the UsdPrim that was just exported.
+    MAYAUSD_CORE_PUBLIC
+    const UsdPrim& GetUsdPrim() const;
+
+    /// Returns the time code.
+    MAYAUSD_CORE_PUBLIC
+    const UsdTimeCode& GetTimeCode() const;
+
+private:
+    const MDagPath&    _dagPath;
+    const UsdPrim&     _usdPrim;
+    const UsdTimeCode& _timeCode;
+};
+
+PXR_NAMESPACE_CLOSE_SCOPE
+
+#endif

lib/mayaUsd/fileio/apiWriterRegistry.cpp

@@ -0,0 +1,39 @@
+//
+// Copyright 2022 Pixar
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+#include "apiWriterRegistry.h"
+
+#include <pxr/pxr.h>
+
+PXR_NAMESPACE_OPEN_SCOPE
+
+static std::map<std::string, UsdMayaApiWriterRegistry::WriterFn> _reg;
+
+void UsdMayaApiWriterRegistry::Register(const std::string& writerId, const WriterFn& fn)
+{
+    const bool inserted = _reg.insert({ writerId, fn }).second;
+    if (!inserted) {
+        TF_CODING_ERROR("Duplicate registration for %s.", writerId.c_str());
+    }
+}
+
+const std::map<std::string, UsdMayaApiWriterRegistry::WriterFn>& UsdMayaApiWriterRegistry::GetAll()
+{
+    TfRegistryManager::GetInstance().SubscribeTo<UsdMayaApiWriterRegistry>();
+    return _reg;
+}
+
+PXR_NAMESPACE_CLOSE_SCOPE

lib/mayaUsd/fileio/apiWriterRegistry.h

@@ -0,0 +1,68 @@
+//
+// Copyright 2022 Pixar
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+#ifndef PXRUSDMAYA_API_WRITER_REGISTRY_H
+#define PXRUSDMAYA_API_WRITER_REGISTRY_H
+
+#include <mayaUsd/fileio/primWriterRegistry.h>
+
+#include <pxr/pxr.h>
+
+#include <string>
+
+PXR_NAMESPACE_OPEN_SCOPE
+
+class UsdMayaApiWriterContext;
+
+/// API writers are plugins that can apply API schemas to exported UsdPrims.
+///
+/// For each prim that is exported, we will then run all of the API Writers (in
+/// writerId order).
+class UsdMayaApiWriterRegistry
+{
+public:
+    using WriterFn = std::function<bool(UsdMayaApiWriterContext*)>;
+
+    /// Registers \p fn as an API writer.  This will be identified by \p
+    /// writerId.
+    static void Register(const std::string& writerId, const WriterFn& fn);
+
+    /// Returns all a map of all the API writers.
+    static const std::map<std::string, WriterFn>& GetAll();
+
+private:
+};
+
+/// This macro is for handling the case of a maya type that corresponds to an
+/// API schema.  In particular, the mayaTypeName should *not* result in a new
+/// UsdPrim.
+#define PXRUSDMAYA_DEFINE_API_WRITER(mayaTypeName, ctxVarName)                               \
+    struct UsdMayaApiWriter_##mayaTypeName                                                   \
+    {                                                                                        \
+    };                                                                                       \
+    static bool UsdMaya_ApiWriter_##mayaTypeName(UsdMayaApiWriterContext*);                  \
+    TF_REGISTRY_FUNCTION_WITH_TAG(UsdMayaPrimWriterRegistry, UsdMayaWriter_##mayaTypeName)   \
+    {                                                                                        \
+        UsdMayaPrimWriterRegistry::RegisterPrimless(#mayaTypeName);                          \
+    }                                                                                        \
+    TF_REGISTRY_FUNCTION_WITH_TAG(UsdMayaApiWriterRegistry, UsdMayaWriter_##mayaTypeName)    \
+    {                                                                                        \
+        UsdMayaApiWriterRegistry::Register(#mayaTypeName, UsdMaya_ApiWriter_##mayaTypeName); \
+    }                                                                                        \
+    bool UsdMaya_ApiWriter_##mayaTypeName(UsdMayaApiWriterContext* ctxVarName)
+
+PXR_NAMESPACE_CLOSE_SCOPE
+
+#endif

lib/mayaUsd/fileio/primWriter.cpp

@@ -15,6 +15,8 @@
 //
 #include "primWriter.h"
 
+#include <mayaUsd/fileio/apiWriterContext.h>
+#include <mayaUsd/fileio/apiWriterRegistry.h>
 #include <mayaUsd/fileio/jobs/jobArgs.h>
 #include <mayaUsd/fileio/translators/translatorGprim.h>
 #include <mayaUsd/fileio/utils/adaptor.h>
@@ -106,6 +108,20 @@ static bool _GetClassNamesToWrite(const MObject& mObj, std::vector<std::string>*
         MFnDependencyNode(mObj), MString(_tokens->USD_inheritClassNames.GetText()), outClassNames);
 }
 
+static void _WriteAPISchemasFromRegistry(UsdMayaApiWriterContext* ctx)
+{
+    for (const auto& idAndFn : UsdMayaApiWriterRegistry::GetAll()) {
+        const std::string&                        writerId = idAndFn.first;
+        const UsdMayaApiWriterRegistry::WriterFn& writerFn = idAndFn.second;
+        if (!writerFn(ctx)) {
+            TF_WARN(
+                "API writer '%s' did not export properly at %s.",
+                writerId.c_str(),
+                ctx->GetUsdPrim().GetPath().GetText());
+        }
+    }
+}
+
 /* virtual */
 void UsdMayaPrimWriter::Write(const UsdTimeCode& usdTime)
 {
@@ -203,6 +219,12 @@ void UsdMayaPrimWriter::Write(const UsdTimeCode& usdTime)
         UsdMayaWriteUtil::WriteMetadataToPrim(GetMayaObject(), _usdPrim);
     }
 
+    // Write any applied API schemas via the API writer registry.
+    {
+        UsdMayaApiWriterContext ctx(GetDagPath(), _usdPrim, usdTime);
+        _WriteAPISchemasFromRegistry(&ctx);
+    }
+
     UsdMayaWriteUtil::WriteAPISchemaAttributesToPrim(
         GetMayaObject(), _usdPrim, _GetExportArgs(), usdTime, _GetSparseValueWriter());
 

lib/mayaUsd/fileio/primWriterRegistry.cpp

@@ -44,6 +44,7 @@ TF_DEFINE_PRIVATE_TOKENS(
 
 typedef std::map<std::string, UsdMayaPrimWriterRegistry::WriterFactoryFn> _Registry;
 static _Registry                                                          _reg;
+static std::set<std::string> _mayaTypesThatDoNotCreatePrims;
 
 /* static */
 void UsdMayaPrimWriterRegistry::Register(
@@ -101,4 +102,17 @@ UsdMayaPrimWriterRegistry::Find(const std::string& mayaTypeName)
     return ret;
 }
 
+/* static */
+void UsdMayaPrimWriterRegistry::RegisterPrimless(const std::string& mayaTypeName)
+{
+    TfRegistryManager::GetInstance().SubscribeTo<UsdMayaPrimWriterRegistry>();
+    _mayaTypesThatDoNotCreatePrims.insert(mayaTypeName);
+}
+
+/* static */
+bool UsdMayaPrimWriterRegistry::IsPrimless(const std::string& mayaTypeName)
+{
+    return _mayaTypesThatDoNotCreatePrims.count(mayaTypeName) == 0;
+}
+
 PXR_NAMESPACE_CLOSE_SCOPE

lib/mayaUsd/fileio/primWriterRegistry.h

@@ -110,6 +110,33 @@ struct UsdMayaPrimWriterRegistry
     /// If there is no writer plugin for \p mayaTypeName, returns nullptr.
     MAYAUSD_CORE_PUBLIC
     static WriterFactoryFn Find(const std::string& mayaTypeName);
+
+    /// \brief Registers a maya node type to *not* create a new prim.
+    ///
+    /// This is relevant for Maya nodes that may not result in a new prim in
+    /// Usd.  For example, the Maya node may be exported as an applied API
+    /// schema on an exported UsdPrim.
+    ///
+    /// This could be relevant when determining whether or not a transform can
+    /// be collapsed.  For example:
+    /// ```
+    /// |Mesh             (transform)
+    ///   |MeshShape      (mesh)
+    ///   |MyNode         (transform)
+    ///     |MyNodeShape  (typeThatShouldNotBeExported)
+    /// ```
+    /// The "Mesh" and "MeshShape" nodes are collapsible because "MyNode" should
+    /// not result in a prim.
+    MAYAUSD_CORE_PUBLIC
+    static void RegisterPrimless(const std::string& mayaTypeName);
+
+    /// \brief Returns true if \p mayaTypeName nodes should result in a prim in
+    /// Usd.
+    ///
+    /// This typically returns true unless \p mayaTypeName was explicitly
+    /// registered to not be exported.
+    MAYAUSD_CORE_PUBLIC
+    static bool IsPrimless(const std::string& mayaTypeName);
 };
 
 // Note, TF_REGISTRY_FUNCTION_WITH_TAG needs a type to register with so we