Move mjSpec/mjModel/mjData management into a separate class.

PiperOrigin-RevId: 866327584
Change-Id: I72f6ec656e193f67dee22214fe01c517432f87e5

src/experimental/platform/CMakeLists.txt

@@ -48,6 +48,8 @@ target_sources(${MUJOCO_PLATFORM_TARGET_NAME}
         imgui_widgets.h
         interaction.cc
         interaction.h
+        model_holder.cc
+        model_holder.h
         picture_gui.h
         picture_gui.cc
         plugin.cc

src/experimental/platform/model_holder.cc

@@ -0,0 +1,140 @@
+// Copyright 2026 DeepMind Technologies Limited
+//
+// 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 "experimental/platform/model_holder.h"
+
+#include <cstddef>
+#include <cstring>
+#include <memory>
+#include <span>
+#include <string_view>
+
+#include <mujoco/mujoco.h>
+#include "user/user_resource.h"
+
+namespace mujoco::platform {
+
+struct BufferProvider : public mjpResourceProvider {
+  BufferProvider(std::span<const std::byte> buffer) : buffer(buffer) {
+    mjp_defaultResourceProvider(this);
+    open = [](mjResource* resource) {
+      return 1;
+    };
+    read = [](mjResource* resource, const void** buffer) {
+      BufferProvider* self = (BufferProvider*)resource->provider;
+      *buffer = self->buffer.data();
+      return static_cast<int>(self->buffer.size());
+    };
+    close = [](mjResource* resource) {};
+  }
+  std::span<const std::byte> buffer;
+};
+
+std::unique_ptr<ModelHolder> ModelHolder::FromSpec(mjSpec* spec) {
+  auto mh = std::unique_ptr<ModelHolder>(new ModelHolder());
+  mh->InitFromSpec(spec);
+  return mh;
+}
+
+std::unique_ptr<ModelHolder> ModelHolder::FromFile(std::string_view filepath) {
+  auto mh = std::unique_ptr<ModelHolder>(new ModelHolder());
+  mh->InitFromFile(filepath);
+  return mh;
+}
+
+std::unique_ptr<ModelHolder> ModelHolder::FromBuffer(
+    std::span<const std::byte> buffer, std::string_view content_type,
+    std::string_view filename) {
+  auto mh = std::unique_ptr<ModelHolder>(new ModelHolder());
+  mh->InitFromBuffer(buffer, content_type, filename);
+  return mh;
+}
+
+ModelHolder::~ModelHolder() {
+  if (data_) {
+    mj_deleteData(data_);
+  }
+  if (model_) {
+    mj_deleteModel(model_);
+  }
+  if (spec_) {
+    mj_deleteSpec(spec_);
+  }
+  mj_deleteVFS(&vfs_);
+}
+
+void ModelHolder::PostInit() {
+  if (spec_ && !model_) {
+    model_ = mj_compile(spec_, &vfs_);
+    if (!model_) {
+      SetLoadError("Error compiling model from spec.");
+      return;
+    }
+  }
+  data_ = mj_makeData(model_);
+  if (!data_) {
+    SetLoadError("Error making data for model.");
+  }
+}
+
+void ModelHolder::InitFromSpec(mjSpec* spec) {
+  spec_ = spec;
+  PostInit();
+}
+
+void ModelHolder::InitFromFile(std::string_view filepath) {
+  mj_defaultVFS(&vfs_);
+  if (filepath.ends_with(".mjb")) {
+    model_ = mj_loadModel(filepath.data(), &vfs_);
+  } else {
+    spec_ = mj_parse(filepath.data(), nullptr, &vfs_, error_, sizeof(error_));
+  }
+  if (error_[0] == 0) {
+    PostInit();
+  }
+}
+
+void ModelHolder::InitFromBuffer(std::span<const std::byte> buffer,
+                                 std::string_view content_type,
+                                 std::string_view filename) {
+  mj_defaultVFS(&vfs_);
+
+  if (content_type == "text/xml") {
+    const char* ptr = reinterpret_cast<const char*>(buffer.data());
+    spec_ = mj_parseXMLString(ptr, nullptr, error_, sizeof(error_));
+  } else if (content_type == "application/mjb") {
+    model_ = mj_loadModelBuffer(buffer.data(), buffer.size());
+  } else if (content_type == "application/zip") {
+    BufferProvider provider(buffer);
+    mjResource resource;
+    std::memset(&resource, 0, sizeof(mjResource));
+    resource.vfs = &vfs_;
+    resource.provider = &provider;
+    resource.name = const_cast<char*>(filename.data());
+    spec_ = mju_decodeResource(&resource, content_type.data(), &vfs_);
+  } else {
+    SetLoadError(
+        "Unknown content type; expected text/xml or application/mjb");
+  }
+  if (error_[0] == 0) {
+    PostInit();
+  }
+}
+
+void ModelHolder::SetLoadError(std::string_view error) {
+  strncpy(error_, error.data(), sizeof(error_) - 1);
+  error_[sizeof(error_) - 1] = 0;
+}
+
+}  // namespace mujoco::platform

src/experimental/platform/model_holder.h

@@ -0,0 +1,77 @@
+// Copyright 2026 DeepMind Technologies Limited
+//
+// 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 MUJOCO_SRC_EXPERIMENTAL_PLATFORM_MODEL_HOLDER_H_
+#define MUJOCO_SRC_EXPERIMENTAL_PLATFORM_MODEL_HOLDER_H_
+
+#include <memory>
+#include <cstddef>
+#include <span>
+#include <string_view>
+
+#include <mujoco/mujoco.h>
+
+namespace mujoco::platform {
+
+// Container for storing an mjModel as well as its associated mjData. Also, if
+// applicable, stores the mjSpec and mjVFS from which the model was constructed.
+class ModelHolder {
+ public:
+  ModelHolder(const ModelHolder&) = delete;
+  ModelHolder& operator=(const ModelHolder&) = delete;
+  ~ModelHolder();
+
+  // Creates an mjModel from an existing mjSpec. This class takes ownership of
+  // the spec and will `mj_deleteSpec` it on destruction.
+  static std::unique_ptr<ModelHolder> FromSpec(mjSpec* spec);
+
+  // Creates an mjModel by attempting to open and parse the give file.
+  static std::unique_ptr<ModelHolder> FromFile(std::string_view filepath);
+
+  // Creates an mjModel by attempting to decode the given buffer.
+  static std::unique_ptr<ModelHolder> FromBuffer(
+      std::span<const std::byte> buffer, std::string_view content_type,
+      std::string_view filename);
+
+  // Accessors to the MuJoCo structures managed by this object.
+  mjVFS* vfs() { return &vfs_; }
+  mjSpec* spec() { return spec_; }
+  mjData* data() { return data_; }
+  mjModel* model() { return model_; }
+
+  // Returns true if the holder holds a valid model (and data).
+  bool ok() const { return error_[0] == 0 && model_ && data_; }
+
+  // Returns the error message if the model failed to load.
+  std::string_view error() const { return error_; }
+
+ private:
+  ModelHolder() = default;
+  void InitFromSpec(mjSpec* spec);
+  void InitFromFile(std::string_view filepath);
+  void InitFromBuffer(std::span<const std::byte> buffer,
+                      std::string_view content_type,
+                      std::string_view filename);
+  void PostInit();
+  void SetLoadError(std::string_view error);
+
+  mjVFS vfs_;
+  mjSpec* spec_ = nullptr;
+  mjModel* model_ = nullptr;
+  mjData* data_ = nullptr;
+  char error_[1000] = "";
+};
+}  // namespace mujoco::platform
+
+#endif  // MUJOCO_SRC_EXPERIMENTAL_PLATFORM_MODEL_HOLDER_H_

src/experimental/studio/app.cc

@@ -37,12 +37,12 @@
 #include "experimental/platform/helpers.h"
 #include "experimental/platform/imgui_widgets.h"
 #include "experimental/platform/interaction.h"
+#include "experimental/platform/model_holder.h"
 #include "experimental/platform/picture_gui.h"
 #include "experimental/platform/plugin.h"
 #include "experimental/platform/renderer.h"
 #include "experimental/platform/step_control.h"
 #include "experimental/platform/window.h"
-#include "user/user_resource.h"
 
 namespace mujoco::studio {
 
@@ -120,20 +120,8 @@ App::App(Config config)
 }
 
 void App::ClearModel() {
-  if (model_) {
-    mj_deleteData(data_);
-    data_ = nullptr;
-    mj_deleteModel(model_);
-    model_ = nullptr;
-
-    if (spec_) {
-      mj_deleteSpec(spec_);
-      spec_ = nullptr;
-    }
-  }
-
+  model_holder_.reset();
   window_->SetTitle("MuJoCo Studio");
-
   step_control_.SetSpeed(100.f);
   profiler_.Clear();
   tmp_ = UiTempState();
@@ -152,138 +140,61 @@ void App::RequestModelReload() {
 }
 
 void App::InitEmptyModel() {
-  mjSpec* spec = mj_makeSpec();
-  mjModel* model = mj_compile(spec, nullptr);
-  InitModel(model, spec, nullptr, "", kEmptyModel);
+  model_holder_ = platform::ModelHolder::FromSpec(mj_makeSpec());
+  OnModelLoaded("", kEmptyModel);
 }
 
 void App::LoadModelFromFile(const std::string& filepath) {
-  mjModel* model = nullptr;
-  mjSpec* spec = nullptr;
-  mjVFS vfs;
-  mj_defaultVFS(&vfs);
 
   const std::string resolved_file =
       platform::ResolveFile(filepath, search_paths_);
-
   if (resolved_file.empty()) {
     SetLoadError("File not found: " + filepath);
     return;
   }
 
-  char err[1000] = "";
-  if (resolved_file.ends_with(".mjb")) {
-    model = mj_loadModel(resolved_file.c_str(), &vfs);
+  model_holder_ = platform::ModelHolder::FromFile(resolved_file);
+  if (model_holder_->ok()) {
+    OnModelLoaded(filepath, kModelFromFile);
+    UpdateFilePaths(resolved_file);
+    window_->SetTitle("MuJoCo Studio : " + filepath);
   } else {
-    spec = mj_parse(resolved_file.c_str(), nullptr, &vfs, err, sizeof(err));
+    SetLoadError(std::string(model_holder_->error()));
   }
-  if (err[0]) {
-    SetLoadError(err);
-    return;
-  }
-
-  InitModel(model, spec, &vfs, filepath, kModelFromFile);
-  UpdateFilePaths(resolved_file);
-  window_->SetTitle("MuJoCo Studio : " + filepath);
-
-  mj_deleteVFS(&vfs);
 }
 
-struct BufferProvider : public mjpResourceProvider {
-  BufferProvider(std::span<const std::byte> buffer) : buffer(buffer) {
-    mjp_defaultResourceProvider(this);
-    open = [](mjResource* resource) {
-      return 1;
-    };
-    read = [](mjResource* resource, const void** buffer) {
-      BufferProvider* self = (BufferProvider*)resource->provider;
-      *buffer = self->buffer.data();
-      return static_cast<int>(self->buffer.size());
-    };
-    close = [](mjResource* resource) {};
-  }
-  std::span<const std::byte> buffer;
-};
-
 void App::LoadModelFromBuffer(std::span<const std::byte> buffer,
                               std::string_view content_type,
                               std::string_view filename) {
-  mjModel* model = nullptr;
-  mjSpec* spec = nullptr;
-
-  mjVFS vfs;
-  mj_defaultVFS(&vfs);
-
-  char err[1000] = "";
-  if (content_type == "text/xml") {
-    const char* ptr = reinterpret_cast<const char*>(buffer.data());
-    spec = mj_parseXMLString(ptr, nullptr, err, sizeof(err));
-  } else if (content_type == "application/mjb") {
-    model = mj_loadModelBuffer(buffer.data(), buffer.size());
-  } else if (content_type == "application/zip") {
-    BufferProvider provider(buffer);
-    mjResource resource;
-    memset(&resource, 0, sizeof(mjResource));
-    resource.vfs = &vfs;
-    resource.provider = &provider;
-    resource.name = (char*)filename.data();
-    spec = mju_decodeResource(&resource, content_type.data(), &vfs);
+  model_holder_ =
+      platform::ModelHolder::FromBuffer(buffer, content_type, filename);
+  if (model_holder_->ok()) {
+    OnModelLoaded(std::string(filename), kModelFromFile);
   } else {
-    SetLoadError("Unknown content type; expected text/xml or application/mjb");
-    return;
+    SetLoadError(std::string(model_holder_->error()));
   }
-  if (err[0]) {
-    SetLoadError(err);
-    return;
-  }
-
-  InitModel(model, spec, &vfs, std::string(filename), kModelFromBuffer);
-
-  mj_deleteVFS(&vfs);
 }
 
-void App::InitModel(mjModel* model, mjSpec* spec, mjVFS* vfs,
-                    std::string filename, ModelKind model_kind) {
+void App::OnModelLoaded(std::string filename, ModelKind model_kind) {
+  model_path_ = std::move(filename);
+
   if (model_kind_ == kEmptyModel) {
     step_control_.Unpause();
   }
-  ClearModel();
-
-  model_path_ = std::move(filename);
   model_kind_ = model_kind;
   if (model_kind_ == kEmptyModel) {
     step_control_.Pause();
   }
 
-  spec_ = spec;
-  model_ = model;
-
-  // If we have a spec but not a model, we need to compile the model from the spec.
-  if (spec_ && !model_) {
-    model_ = mj_compile(spec_, vfs);
-    if (!model_) {
-      SetLoadError("Error compiling model from spec.");
-      return;
-    }
-  }
-
-  if (!model_) {
-    mju_error("Error making data for model: %s", model_path_.c_str());
-  }
-
-  data_ = mj_makeData(model_);
-  if (!data_) {
-    mju_error("Error making data for model: %s", model_path_.c_str());
-  }
-
   // Reset/reinitialize everything that depends on the new mjModel.
-  renderer_->Init(model_);
-  const int state_size = mj_stateSize(model_, mjSTATE_INTEGRATION);
+  mjModel* model = model_holder_->model();
+  renderer_->Init(model);
+  const int state_size = mj_stateSize(model, mjSTATE_INTEGRATION);
   history_.Init(state_size);
 
   // Initialize the speed based on the model's default real-time setting.
   float min_error = FLT_MAX;
-  const float desired = mju_log(100 * model_->vis.global.realtime);
+  const float desired = mju_log(100 * model->vis.global.realtime);
   for (int i = 0; i < kPercentRealTime.size(); ++i) {
     const float speed = std::stof(kPercentRealTime[i]);
     const float error = mju_abs(mju_log(speed) - desired);

src/experimental/studio/app.h

@@ -27,6 +27,7 @@
 #include <mujoco/mujoco.h>
 #include "experimental/platform/gui.h"
 #include "experimental/platform/interaction.h"
+#include "experimental/platform/model_holder.h"
 #include "experimental/platform/picture_gui.h"
 #include "experimental/platform/renderer.h"
 #include "experimental/platform/sim_history.h"
@@ -168,8 +169,7 @@ class App {
 
   // Updates the currently loaded model to the given model. If model is null,
   // then compile the spec to a model.
-  void InitModel(mjModel* model, mjSpec* spec, mjVFS* vfs, std::string filename,
-                 ModelKind model_kind);
+  void OnModelLoaded(std::string filename, ModelKind model_kind);
 
   void SetLoadError(std::string error);
   void UpdateFilePaths(const std::string& resolved_path);
@@ -207,12 +207,13 @@ class App {
   float GetExpectedLabelWidth();
   std::vector<const char*> GetCameraNames();
 
-  mjSpec* spec() { return spec_; }
-  mjModel* model() { return model_; }
-  mjData* data() { return data_; }
-  bool has_spec() const { return spec_ != nullptr; }
-  bool has_model() const { return model_ != nullptr; }
-  bool has_data() const { return data_ != nullptr; }
+  mjSpec* spec() { return model_holder_->spec(); }
+  mjModel* model() { return model_holder_->model(); }
+  mjData* data() { return model_holder_->data(); }
+  bool has_spec() const { return model_holder_ && model_holder_->spec(); }
+  bool has_model() const { return model_holder_ && model_holder_->model(); }
+  bool has_data() const { return model_holder_ && model_holder_->data(); }
+
 
   std::string ini_path_;
   std::string model_name_;  // Used if model_kind_ is kModelFromBuffer.
@@ -224,15 +225,11 @@ class App {
 
   std::unique_ptr<platform::Window> window_;
   std::unique_ptr<platform::Renderer> renderer_;
+  std::unique_ptr<platform::ModelHolder> model_holder_;
   platform::StepControl step_control_;
   platform::SimProfiler profiler_;
   platform::SimHistory history_;
-
   std::vector<std::string> search_paths_;
-
-  mjSpec* spec_ = nullptr;
-  mjModel* model_ = nullptr;
-  mjData* data_ = nullptr;
   std::vector<std::byte> pixels_;
 
   mjvCamera camera_;