Deprecate ResourceManager::instance().

If I would have done this a year ago, I could have it removed by now.
Well. Gotta look forward to 2020, then.

doc/changelog.dox

@@ -585,6 +585,9 @@ See also:
 -   Passing @cpp nullptr @ce to @ref ImageView constructors is deprecated and
     will print a warning at runtime. Use a constructor without the @p data
     parameter instead.
+-   The @cpp ResourceManager::instance() @ce singleton is deprecated as it
+    makes some use cases harder than they should be. Make your own singleton
+    or explicitly pass a @ref ResourceManager reference around instead.
 -   @ref Platform::BasicScreen::application() now returns a reference instead
     of a pointer and together with @ref Platform::BasicScreen::redraw() checks
     that the screen is actually added to the application instead of returning

doc/snippets/Magnum.cpp

@@ -233,7 +233,7 @@ struct MyShader: GL::AbstractShaderProgram {
     void bindTexture(GL::Texture2D&) {}
 };
 /* [ResourceManager-fill] */
-MyResourceManager& manager = MyResourceManager::instance();
+MyResourceManager manager;
 Resource<GL::Texture2D> texture{manager.get<GL::Texture2D>("texture")};
 Resource<GL::AbstractShaderProgram, MyShader> shader =
     manager.get<GL::AbstractShaderProgram, MyShader>("shader");

src/Magnum/DebugTools/ResourceManager.cpp

@@ -41,11 +41,32 @@ namespace Implementation {
 
 namespace DebugTools {
 
+namespace {
+    #ifdef CORRADE_BUILD_MULTITHREADED
+    CORRADE_THREAD_LOCAL
+    #endif
+    ResourceManager* resourceManagerInstance = nullptr;
+}
+
+ResourceManager& ResourceManager::instance() {
+    CORRADE_ASSERT(resourceManagerInstance,
+        "DebugTools::ResourceManager::instance(): no instance exists",
+        *resourceManagerInstance);
+    return *resourceManagerInstance;
+}
+
 ResourceManager::ResourceManager() {
+    CORRADE_ASSERT(!resourceManagerInstance,
+        "DebugTools::ResourceManager: another instance is already created", );
+    resourceManagerInstance = this;
+
     setFallback(new ForceRendererOptions);
     setFallback(new ObjectRendererOptions);
 }
 
-ResourceManager::~ResourceManager() = default;
+ResourceManager::~ResourceManager() {
+    CORRADE_INTERNAL_ASSERT(resourceManagerInstance == this);
+    resourceManagerInstance = nullptr;
+}
 
 }}

src/Magnum/DebugTools/ResourceManager.h

@@ -65,6 +65,13 @@ information.
 class MAGNUM_DEBUGTOOLS_EXPORT ResourceManager: public Magnum::ResourceManager<Magnum::Implementation::ResourceManagerLocalInstance, GL::AbstractShaderProgram, GL::Buffer, GL::Mesh, GL::MeshView, DebugTools::ForceRendererOptions, DebugTools::ObjectRendererOptions>
 {
     public:
+        /**
+         * @brief Global instance
+         *
+         * Assumes that the instance exists.
+         */
+        static ResourceManager& instance();
+
         explicit ResourceManager();
         ~ResourceManager();
 };

src/Magnum/ResourceManager.h

@@ -246,12 +246,14 @@ Basic usage is:
    template implementation file. */
 template<class... Types> class ResourceManager: private Implementation::ResourceManagerImplementation<Types>... {
     public:
+        #ifdef MAGNUM_BUILD_DEPRECATED
         /**
          * @brief Global instance
          *
          * Assumes that the instance exists.
          */
-        static ResourceManager<Types...>& instance();
+        static CORRADE_DEPRECATED("implicit ResourceManager singleton is deprecated, make your own or pass a reference around instead") ResourceManager<Types...>& instance();
+        #endif
 
         /**
          * @brief Constructor
@@ -625,23 +627,29 @@ template<class T> inline ResourceManagerData<T>::Data::~Data() {
 
 }
 
+#ifdef MAGNUM_BUILD_DEPRECATED
 template<class ...Types> ResourceManager<Types...>& ResourceManager<Types...>::instance() {
     CORRADE_ASSERT(Implementation::ResourceManagerImplementation<Types...>::internalInstance(),
         "ResourceManager::instance(): no instance exists",
         static_cast<ResourceManager<Types...>&>(*Implementation::ResourceManagerImplementation<Types...>::internalInstance()));
     return static_cast<ResourceManager<Types...>&>(*Implementation::ResourceManagerImplementation<Types...>::internalInstance());
 }
+#endif
 
 template<class ...Types> ResourceManager<Types...>::ResourceManager() {
+    #ifdef MAGNUM_BUILD_DEPRECATED
     CORRADE_ASSERT(!Implementation::ResourceManagerImplementation<Types...>::internalInstance(),
         "ResourceManager::ResourceManager(): another instance is already created", );
     Implementation::ResourceManagerImplementation<Types...>::internalInstance() = this;
+    #endif
 }
 
 template<class ...Types> ResourceManager<Types...>::~ResourceManager() {
     freeLoaders(typename Implementation::ResourceManagerImplementation<Types...>::TypePack{});
+    #ifdef MAGNUM_BUILD_DEPRECATED
     CORRADE_INTERNAL_ASSERT(Implementation::ResourceManagerImplementation<Types...>::internalInstance() == this);
     Implementation::ResourceManagerImplementation<Types...>::internalInstance() = nullptr;
+    #endif
 }
 
 }

src/Magnum/ResourceManager.hpp

@@ -40,6 +40,7 @@
 
 namespace Magnum { namespace Implementation {
 
+#ifdef MAGNUM_BUILD_DEPRECATED
 template<class ...Types>
 #ifndef _MSC_VER
 CORRADE_VISIBILITY_EXPORT
@@ -48,6 +49,7 @@ ResourceManager<Types...>*& ResourceManagerLocalInstanceImplementation<Types...>
     static ResourceManager<Types...>* _instance(nullptr);
     return _instance;
 }
+#endif
 
 }}
 

src/Magnum/Test/CMakeLists.txt

@@ -35,13 +35,6 @@ corrade_add_test(ResourceManagerTest ResourceManagerTest.cpp LIBRARIES Magnum)
 target_compile_definitions(ResourceManagerTest PRIVATE "CORRADE_GRACEFUL_ASSERT")
 corrade_add_test(SamplerTest SamplerTest.cpp LIBRARIES MagnumTestLib)
 
-add_library(ResourceManagerLocalInstanceTestLib ${SHARED_OR_STATIC} ResourceManagerLocalInstanceTestLib.cpp)
-target_link_libraries(ResourceManagerLocalInstanceTestLib Magnum)
-if(NOT BUILD_STATIC)
-    target_compile_definitions(ResourceManagerLocalInstanceTestLib PRIVATE "ResourceManagerLocalInstanceTestLib_EXPORTS")
-endif()
-corrade_add_test(ResourceManagerLocalInstanceTest ResourceManagerLocalInstanceTest.cpp LIBRARIES Magnum ResourceManagerLocalInstanceTestLib)
-
 corrade_add_test(TagsTest TagsTest.cpp LIBRARIES Magnum)
 
 set_target_properties(
@@ -52,8 +45,20 @@ set_target_properties(
     PixelFormatTest
     PixelStorageTest
     ResourceManagerTest
-    ResourceManagerLocalInstanceTestLib
-    ResourceManagerLocalInstanceTest
     SamplerTest
     TagsTest
     PROPERTIES FOLDER "Magnum/Test")
+
+if(MAGNUM_BUILD_DEPRECATED)
+    add_library(ResourceManagerLocalInstanceTestLib ${SHARED_OR_STATIC} ResourceManagerLocalInstanceTestLib.cpp)
+    target_link_libraries(ResourceManagerLocalInstanceTestLib Magnum)
+    if(NOT BUILD_STATIC)
+        target_compile_definitions(ResourceManagerLocalInstanceTestLib PRIVATE "ResourceManagerLocalInstanceTestLib_EXPORTS")
+    endif()
+    corrade_add_test(ResourceManagerLocalInstanceTest ResourceManagerLocalInstanceTest.cpp LIBRARIES Magnum ResourceManagerLocalInstanceTestLib)
+
+    set_target_properties(
+        ResourceManagerLocalInstanceTestLib
+        ResourceManagerLocalInstanceTest
+        PROPERTIES FOLDER "Magnum/Test")
+endif()

src/Magnum/Test/ResourceManagerLocalInstanceTest.cpp

@@ -42,9 +42,10 @@ ResourceManagerLocalInstanceTest::ResourceManagerLocalInstanceTest() {
 }
 
 void ResourceManagerLocalInstanceTest::instance() {
+    CORRADE_IGNORE_DEPRECATED_PUSH
     ResourceManagerWithLocalInstance::instance().set("another", 13);
-
     CORRADE_COMPARE(&manager.staticInstance, &manager.instance());
+    CORRADE_IGNORE_DEPRECATED_POP
     CORRADE_COMPARE(manager.count<Int>(), 2);
     CORRADE_COMPARE(manager.state<Int>("integer"), ResourceState::Final);
 }

src/Magnum/Test/ResourceManagerLocalInstanceTestLib.cpp

@@ -35,9 +35,11 @@ namespace Implementation {
 
 namespace Test {
 
+CORRADE_IGNORE_DEPRECATED_PUSH
 ResourceManagerWithLocalInstance::ResourceManagerWithLocalInstance(): staticInstance(static_cast<ResourceManagerWithLocalInstance&>(instance())) {
     /* Add some stuff */
     set("integer", 42);
 }
+CORRADE_IGNORE_DEPRECATED_POP
 
 }}

src/Magnum/Test/ResourceManagerTest.cpp

@@ -316,7 +316,7 @@ void ResourceManagerTest::clearWhileReferenced() {
 void ResourceManagerTest::loader() {
     class IntResourceLoader: public AbstractResourceLoader<Int> {
         public:
-            IntResourceLoader(): resource(ResourceManager::instance().get<Data>("data")) {}
+            IntResourceLoader(ResourceManager& instance): resource{instance.get<Data>("data")} {}
 
             void load() {
                 set("hello", Containers::pointer<Int>(773), ResourceDataState::Final, ResourcePolicy::Resident);
@@ -338,7 +338,7 @@ void ResourceManagerTest::loader() {
 
     {
         ResourceManager rm;
-        Containers::Pointer<IntResourceLoader> loaderPtr{Containers::InPlaceInit};
+        Containers::Pointer<IntResourceLoader> loaderPtr{Containers::InPlaceInit, rm};
         IntResourceLoader& loader = *loaderPtr;
         rm.setLoader<Int>(std::move(loaderPtr));