wasm: rewrite/simplify OpenglContext

We don’t need the contextLost callback since we can
poll for the “lost” status in isValid()

Recreating the native context is not very helpful,
since it destroys all current context state. Remove
this logic.

Support makeCurrent() on different surfaces, as long
as they refer to the same screen. Create the native
context (and record which screen) on the first call
to makeCurrent()

Task-number: QTBUG-75463
Change-Id: I6eb830df14578ffdbed5b0505fe860ce433e4f9b
Reviewed-by: Lorn Potter <lorn.potter@gmail.com>

2 files changed, 30 insertions, 35 deletions

diff --git a/src/plugins/platforms/wasm/qwasmopenglcontext.cpp b/src/plugins/platforms/wasm/qwasmopenglcontext.cpp
index ae43e2ebf0..0eaffdb77e 100644
--- a/src/plugins/platforms/wasm/qwasmopenglcontext.cpp
+++ b/src/plugins/platforms/wasm/qwasmopenglcontext.cpp
@@ -41,40 +41,31 @@ QWasmOpenGLContext::QWasmOpenGLContext(const QSurfaceFormat &format)
 
 QWasmOpenGLContext::~QWasmOpenGLContext()
 {
-    if (m_context)
+    if (m_context) {
         emscripten_webgl_destroy_context(m_context);
+        m_context = 0;
+    }
 }
 
-void QWasmOpenGLContext::maybeRecreateEmscriptenContext(QPlatformSurface *surface)
+bool QWasmOpenGLContext::maybeCreateEmscriptenContext(QPlatformSurface *surface)
 {
-    // Native emscripten contexts are tied to a single surface. Recreate
-    // the context if the surface is changed.
-    if (surface != m_surface) {
-        m_surface = surface;
-
-        // Destroy existing context
-        if (m_context)
-            emscripten_webgl_destroy_context(m_context);
-
-        // Create new context
-        const QString canvasId = QWasmScreen::get(surface->screen())->canvasId();
-        m_context = createEmscriptenContext(canvasId, m_requestedFormat);
-
-        // Register context-lost callback.
-        auto callback = [](int eventType, const void *reserved, void *userData) -> EM_BOOL
-        {
-            Q_UNUSED(eventType);
-            Q_UNUSED(reserved);
-            // The application may get contex-lost if e.g. moved to the background. Set
-            // m_contextLost which will make isValid() return false. Application code will
-            // then detect this and recrate the the context, resulting in a new QWasmOpenGLContext
-            // instance.
-            reinterpret_cast<QWasmOpenGLContext *>(userData)->m_contextLost = true;
-            return true;
-        };
-        bool capture = true;
-        emscripten_set_webglcontextlost_callback(canvasId.toLocal8Bit().constData(), this, capture, callback);
-    }
+    // Native emscripten/WebGL contexts are tied to a single screen/canvas. The first
+    // call to this function creates a native canvas for the given screen, subsequent
+    // calls verify that the surface is on/off the same screen.
+    QPlatformScreen *screen = surface->screen();
+    if (m_context && !screen)
+        return false; // Alternative: return true to support makeCurrent on QOffScreenSurface with
+                      // no screen. However, Qt likes to substitute QGuiApplication::primaryScreen()
+                      // for null screens, which foils this plan.
+    if (!screen)
+        return false;
+    if (m_context)
+        return m_screen == screen;
+
+    QString canvasId = QWasmScreen::get(screen)->canvasId();
+    m_context = createEmscriptenContext(canvasId, m_requestedFormat);
+    m_screen = screen;
+    return true;
 }
 
 EMSCRIPTEN_WEBGL_CONTEXT_HANDLE QWasmOpenGLContext::createEmscriptenContext(const QString &canvasId, QSurfaceFormat format)
@@ -113,7 +104,9 @@ GLuint QWasmOpenGLContext::defaultFramebufferObject(QPlatformSurface *surface) c
 
 bool QWasmOpenGLContext::makeCurrent(QPlatformSurface *surface)
 {
-    maybeRecreateEmscriptenContext(surface);
+    bool ok = maybeCreateEmscriptenContext(surface);
+    if (!ok)
+        return false;
 
     return emscripten_webgl_make_context_current(m_context) == EMSCRIPTEN_RESULT_SUCCESS;
 }
@@ -136,7 +129,9 @@ bool QWasmOpenGLContext::isSharing() const
 
 bool QWasmOpenGLContext::isValid() const
 {
-    return (m_contextLost == false);
+    // Note: we get isValid() calls before we see the surface and can
+    // create a native context, so no context is also a valid state.
+    return !m_context || !emscripten_is_webgl_context_lost(m_context);
 }
 
 QFunctionPointer QWasmOpenGLContext::getProcAddress(const char *procName)
diff --git a/src/plugins/platforms/wasm/qwasmopenglcontext.h b/src/plugins/platforms/wasm/qwasmopenglcontext.h
index 126b596a7e..d27007e8ea 100644
--- a/src/plugins/platforms/wasm/qwasmopenglcontext.h
+++ b/src/plugins/platforms/wasm/qwasmopenglcontext.h
@@ -34,6 +34,7 @@
 
 QT_BEGIN_NAMESPACE
 
+class QPlatformScreen;
 class QWasmOpenGLContext : public QPlatformOpenGLContext
 {
 public:
@@ -50,12 +51,11 @@ public:
     QFunctionPointer getProcAddress(const char *procName) override;
 
 private:
-    void maybeRecreateEmscriptenContext(QPlatformSurface *surface);
+    bool maybeCreateEmscriptenContext(QPlatformSurface *surface);
     static EMSCRIPTEN_WEBGL_CONTEXT_HANDLE createEmscriptenContext(const QString &canvasId, QSurfaceFormat format);
 
-    bool m_contextLost = false;
     QSurfaceFormat m_requestedFormat;
-    QPlatformSurface *m_surface = nullptr;
+    QPlatformScreen *m_screen = nullptr;
     EMSCRIPTEN_WEBGL_CONTEXT_HANDLE m_context = 0;
 };