GL: added Context::makeCurrent().

doc/changelog.dox

@@ -122,6 +122,9 @@ See also:
 -   It's now possible to pass @ref std::initializer_list to
     @ref GL::Buffer::setData() / @ref GL::Buffer::setSubData() for a succint
     upload of basic types
+-   Added an ability to use Magnum with multiple OpenGL contexts using
+    @ref GL::Context::makeCurrent(), see @ref GL-Context-multiple for more
+    information
 
 @subsubsection changelog-latest-new-math Math library
 

doc/snippets/MagnumGL-framebuffer.cpp

@@ -23,9 +23,11 @@
     DEALINGS IN THE SOFTWARE.
 */
 
+#include "Magnum/GL/Buffer.h"
 #include "Magnum/GL/DefaultFramebuffer.h"
 #include "Magnum/GL/Framebuffer.h"
 #include "Magnum/Platform/Sdl2Application.h"
+#include "Magnum/Platform/GLContext.h"
 
 using namespace Magnum;
 
@@ -69,3 +71,30 @@ void drawEvent() override {
 }
 /* [Framebuffer-usage-draw] */
 };
+
+int main() {
+
+{
+SDL_Window* _window{};
+SDL_GLContext* _otherGLContext{};
+/* [Context-makeCurrent-nullptr] */
+Platform::GLContext context;
+
+SDL_GL_MakeCurrent(_window, _otherGLContext); // or other platform-specific API
+Platform::GLContext::makeCurrent(nullptr);
+
+Platform::GLContext other;
+/* [Context-makeCurrent-nullptr] */
+
+/* [Context-makeCurrent] */
+Platform::GLContext::makeCurrent(&context);
+
+GL::Buffer a; // implicitly tied to `context`
+
+Platform::GLContext::makeCurrent(&other);
+
+GL::Buffer b; // implicitly tied to `other`
+/* [Context-makeCurrent] */
+}
+
+}

src/Magnum/GL/Context.cpp

@@ -456,6 +456,8 @@ Context& Context::current() {
     return *currentContext;
 }
 
+void Context::makeCurrent(Context* context) { currentContext = context; }
+
 Context::Context(NoCreateT, Int argc, const char** argv, void functionLoader(Context&)): Context{NoCreate, Utility::Arguments{"magnum"}, argc, argv, functionLoader} {}
 
 Context::Context(NoCreateT, Utility::Arguments& args, Int argc, const char** argv, void functionLoader(Context&)): _functionLoader{functionLoader}, _version{Version::None} {

src/Magnum/GL/Context.h

@@ -154,6 +154,28 @@ If Corrade is compiled with @ref CORRADE_BUILD_MULTITHREADED (the default), the
 OpenGL context thread locality. This might cause some performance penalties ---
 if you are sure that you never need to have multiple independent thread-local
 Magnum context, build Corrade with the option disabled.
+
+@section GL-Context-multiple Using multiple OpenGL contexts
+
+By default, Magnum assumes you have one OpenGL context active at all times, and
+all state tracking is done by the @ref Context instance that's associated with
+it. When you are using multiple OpenGL contexts, each of them needs to have a
+corresponding @ref Context instance active at the same time, and you need to
+ensure you only access OpenGL objects that were created by the same context as
+is currently active.
+
+To prevent accidents in common cases, the @ref Context class expects that no
+other instance is active during its creation. In order to create additional
+instances for other OpenGL contexts, *first* you need to "unset" the current one
+with @ref makeCurrent() and *then* create another instance, which will then
+become implicitly active:
+
+@snippet MagnumGL-framebuffer.cpp Context-makeCurrent-nullptr
+
+Once all needed instances are created, switch between them right after making
+the underlying GL context current:
+
+@snippet MagnumGL-framebuffer.cpp Context-makeCurrent
 */
 class MAGNUM_GL_EXPORT Context {
     public:
@@ -420,6 +442,14 @@ class MAGNUM_GL_EXPORT Context {
          */
         static Context& current();
 
+        /**
+         * @brief Make a context current
+         *
+         * To be used when you need to manage multiple OpenGL contexts. See
+         * @ref GL-Context-multiple for more information.
+         */
+        static void makeCurrent(Context* context);
+
         /** @brief Copying is not allowed */
         Context(const Context&) = delete;
 

src/Magnum/GL/Test/ContextGLTest.cpp

@@ -38,6 +38,8 @@ namespace Magnum { namespace GL { namespace Test { namespace {
 struct ContextGLTest: OpenGLTester {
     explicit ContextGLTest();
 
+    void makeCurrent();
+
     #ifndef CORRADE_TARGET_EMSCRIPTEN
     void multithreaded();
     #endif
@@ -53,6 +55,8 @@ struct ContextGLTest: OpenGLTester {
 
 ContextGLTest::ContextGLTest() {
     addTests({
+        &ContextGLTest::makeCurrent,
+
         #ifndef CORRADE_TARGET_EMSCRIPTEN
         &ContextGLTest::multithreaded,
         #endif
@@ -66,6 +70,20 @@ ContextGLTest::ContextGLTest() {
         &ContextGLTest::isExtensionDisabled});
 }
 
+void ContextGLTest::makeCurrent() {
+    CORRADE_VERIFY(Context::hasCurrent());
+
+    Context& current = Context::current();
+    Context::makeCurrent(nullptr);
+
+    CORRADE_VERIFY(!Context::hasCurrent());
+
+    Context::makeCurrent(&current);
+
+    CORRADE_VERIFY(Context::hasCurrent());
+    CORRADE_COMPARE(&Context::current(), &current);
+}
+
 #ifndef CORRADE_TARGET_EMSCRIPTEN
 void ContextGLTest::multithreaded() {
     CORRADE_VERIFY(Context::hasCurrent());

src/Magnum/GL/Test/ContextTest.cpp

@@ -38,6 +38,8 @@ struct ContextTest: TestSuite::Tester {
     void constructNoCreate();
     void constructCopyMove();
 
+    void makeCurrentNoOp();
+
     void extensions();
 
     void debugFlag();
@@ -51,6 +53,8 @@ ContextTest::ContextTest() {
     addTests({&ContextTest::constructNoCreate,
               &ContextTest::constructCopyMove,
 
+              &ContextTest::makeCurrentNoOp,
+
               &ContextTest::extensions,
 
               &ContextTest::debugFlag,
@@ -82,6 +86,12 @@ void ContextTest::constructCopyMove() {
     CORRADE_VERIFY(!(std::is_assignable<Context, Context&&>{}));
 }
 
+void ContextTest::makeCurrentNoOp() {
+    CORRADE_VERIFY(!Context::hasCurrent());
+    Context::makeCurrent(nullptr);
+    CORRADE_VERIFY(!Context::hasCurrent());
+}
+
 void ContextTest::extensions() {
     const char* used[GL::Implementation::ExtensionCount]{};