add openState to AbstractImporter to allow setting directly the state

src/Magnum/Trade/AbstractImporter.cpp

@@ -62,6 +62,19 @@ void AbstractImporter::doOpenData(Containers::ArrayView<const char>) {
     CORRADE_ASSERT(false, "Trade::AbstractImporter::openData(): feature advertised but not implemented", );
 }
 
+bool AbstractImporter::openState(const void* state, const std::string& filePath) {
+    CORRADE_ASSERT(features() & Feature::OpenState,
+        "Trade::AbstractImporter::OpenState(): feature not supported", {});
+
+    close();
+    doOpenState(state, filePath);
+    return isOpened();
+}
+
+void AbstractImporter::doOpenState(const void*, const std::string&) {
+    CORRADE_ASSERT(false, "Trade::AbstractImporter::openState(): feature advertised but not implemented", );
+}
+
 bool AbstractImporter::openFile(const std::string& filename) {
     close();
     doOpenFile(filename);

src/Magnum/Trade/AbstractImporter.h

@@ -97,7 +97,9 @@ class MAGNUM_EXPORT AbstractImporter: public PluginManager::AbstractManagingPlug
          */
         enum class Feature: UnsignedByte {
             /** Opening files from raw data using @ref openData() */
-            OpenData = 1 << 0
+            OpenData = 1 << 0,
+            /** Opening already loaded state using @ref openState() */
+            OpenState = 1 << 1
         };
 
         /** @brief Set of features supported by this importer */
@@ -128,6 +130,17 @@ class MAGNUM_EXPORT AbstractImporter: public PluginManager::AbstractManagingPlug
          */
         bool openData(Containers::ArrayView<const char> data);
 
+
+        /**
+         * @brief Open already loaded state
+         *
+         * Closes previous file, if it was opened, and tries to open given
+         * state. Available only if @ref Feature::OpenState is supported. Returns
+         * `true` on success, `false` otherwise.
+         * @see @ref features(), @ref openData()
+         */
+        bool openState(const void* state, const std::string& filePath = {});
+
         /**
          * @brief Open file
          *
@@ -521,6 +534,9 @@ class MAGNUM_EXPORT AbstractImporter: public PluginManager::AbstractManagingPlug
         /** @brief Implementation for @ref openData() */
         virtual void doOpenData(Containers::ArrayView<const char> data);
 
+        /** @brief Implementation for @ref openState() */
+        virtual void doOpenState(const void* state, const std::string& filePath = {});
+
         /** @brief Implementation for @ref close() */
         virtual void doClose() = 0;