Trade,Text: this is a much better example for template file callbacks.

doc/snippets/MagnumText.cpp

@@ -95,7 +95,7 @@ Containers::Pointer<Text::AbstractFont> font;
 /* [AbstractFont-setFileCallback] */
 font->setFileCallback([](const std::string& filename,
     InputFileCallbackPolicy, void*) {
-        Utility::Resource rs("data");
+        Utility::Resource rs{"data"};
         return Containers::optional(rs.getRaw(filename));
     });
 /* [AbstractFont-setFileCallback] */
@@ -104,22 +104,11 @@ font->setFileCallback([](const std::string& filename,
 {
 Containers::Pointer<Text::AbstractFont> font;
 /* [AbstractFont-setFileCallback-template] */
-struct Data {
-    std::unordered_map<std::string, Containers::Array<char>> files;
-} data;
-
+const Utility::Resource rs{"data"};
 font->setFileCallback([](const std::string& filename,
-    InputFileCallbackPolicy, Data& data)
-        -> Containers::Optional<Containers::ArrayView<const char>>
-    {
-        auto found = data.files.find(filename);
-        if(found == data.files.end()) {
-            if(!Utility::Directory::exists(filename))
-                return Containers::NullOpt;
-            found = data.files.emplace(filename, Utility::Directory::read(filename)).first;
-        }
-        return Containers::ArrayView<const char>{found->second};
-    }, data);
+    InputFileCallbackPolicy, const Utility::Resource& rs) {
+        return Containers::optional(rs.getRaw(filename));
+    }, rs);
 /* [AbstractFont-setFileCallback-template] */
 }
 

doc/snippets/MagnumTrade.cpp

@@ -142,7 +142,7 @@ Containers::Pointer<Trade::AbstractImporter> importer;
 /* [AbstractImporter-setFileCallback] */
 importer->setFileCallback([](const std::string& filename,
     InputFileCallbackPolicy, void*) {
-        Utility::Resource rs("data");
+        Utility::Resource rs{"data"};
         return Containers::optional(rs.getRaw(filename));
     });
 /* [AbstractImporter-setFileCallback] */
@@ -151,22 +151,11 @@ importer->setFileCallback([](const std::string& filename,
 {
 Containers::Pointer<Trade::AbstractImporter> importer;
 /* [AbstractImporter-setFileCallback-template] */
-struct Data {
-    std::unordered_map<std::string, Containers::Array<char>> files;
-} data;
-
+const Utility::Resource rs{"data"};
 importer->setFileCallback([](const std::string& filename,
-    InputFileCallbackPolicy, Data& data)
-        -> Containers::Optional<Containers::ArrayView<const char>>
-    {
-        auto found = data.files.find(filename);
-        if(found == data.files.end()) {
-            if(!Utility::Directory::exists(filename))
-                return Containers::NullOpt;
-            found = data.files.emplace(filename, Utility::Directory::read(filename)).first;
-        }
-        return Containers::ArrayView<const char>{found->second};
-    }, data);
+    InputFileCallbackPolicy, const Utility::Resource& rs) {
+        return Containers::optional(rs.getRaw(filename));
+    }, rs);
 /* [AbstractImporter-setFileCallback-template] */
 }
 

src/Magnum/Text/AbstractFont.h

@@ -286,7 +286,9 @@ class MAGNUM_TEXT_EXPORT AbstractFont: public PluginManager::AbstractPlugin {
          *
          * Equivalent to calling the above with a lambda wrapper that casts
          * @cpp void* @ce back to @cpp T* @ce and dereferences it in order to
-         * pass it to @p callback. Example usage:
+         * pass it to @p callback. Example usage --- this reuses an existing
+         * @ref Corrade::Utility::Resource instance to avoid a potentially slow
+         * resource group lookup every time:
          *
          * @snippet MagnumText.cpp AbstractFont-setFileCallback-template
          *

src/Magnum/Trade/AbstractImporter.h

@@ -457,7 +457,9 @@ class MAGNUM_TRADE_EXPORT AbstractImporter: public PluginManager::AbstractManagi
          *
          * Equivalent to calling the above with a lambda wrapper that casts
          * @cpp void* @ce back to @cpp T* @ce and dereferences it in order to
-         * pass it to @p callback. Example usage:
+         * pass it to @p callback. Example usage --- this reuses an existing
+         * @ref Corrade::Utility::Resource instance to avoid a potentially slow
+         * resource group lookup every time:
          *
          * @snippet MagnumTrade.cpp AbstractImporter-setFileCallback-template
          *