Synchronous rendering on Metal when annotation views are visible (#2334)

platform/ios/src/MLNMapView+Impl.h

@@ -57,6 +57,10 @@ class MLNMapViewImpl : public mbgl::MapObserver {
     void render();
 
     virtual MLNBackendResource getObject() = 0;
+
+    virtual void setSynchronous(bool value) {};
+    virtual bool getSynchronous() const { return false; };
+
 
     // mbgl::MapObserver implementation
     void onCameraWillChange(mbgl::MapObserver::CameraChangeMode) override;

platform/ios/src/MLNMapView+Metal.h

@@ -50,6 +50,9 @@ class MLNMapViewMetalImpl final : public MLNMapViewImpl,
     MLNBackendResource getObject() override;
     // End implementation of MLNMapViewImpl
 
+    void setSynchronous(bool value) override { synchronousFrame = value; };
+    bool getSynchronous() const override { return synchronousFrame; };
 private:
     bool presentsWithTransaction = false;
+    bool synchronousFrame = false;
 };

platform/ios/src/MLNMapView+Metal.mm

@@ -84,9 +84,15 @@ void swap() override {
         }
         [commandBuffer commit];
 
-        // Un-comment for synchronous, which can help troubleshoot rendering problems,
+        // Synchronous rendering can help troubleshoot rendering problems,
         // particularly those related to resource tracking and multiple queued buffers.
-        //[commandBuffer waitUntilCompleted];
+        // The conditional logic for synchronous rendering is commanded from MLNMapView.updateAnnotationViews:
+        // `_mbglView->setSynchronous(haveVisibleAnnotationViews);`
+        // and fixes the desynchronization of UIView annotation when the map is rendered on the Metal backend
+        // Issue: https://github.com/maplibre/maplibre-native/issues/2053
+        if (backend.getSynchronous()) {
+            [commandBuffer waitUntilCompleted];
+        }
 
         commandBuffer = nil;
         commandBufferPtr.reset();

platform/ios/src/MLNMapView.mm

@@ -6917,6 +6917,9 @@ - (void)updateAnnotationViews
     NSMutableArray *offscreenAnnotations = [self.annotations mutableCopy];
     [offscreenAnnotations removeObjectsInArray:visibleAnnotations];
 
+    // Dynamic flag used to drive the backend's synchronous frame rendering (see comment below)
+    bool haveVisibleAnnotationViews = false;
+
     // Update the center of visible annotation views
     for (id<MLNAnnotation> annotation in visibleAnnotations)
     {
@@ -6953,6 +6956,7 @@ - (void)updateAnnotationViews
         if (annotationView)
         {
             annotationView.center = MLNPointRounded([self convertCoordinate:annotationContext.annotation.coordinate toPointToView:self]);
+            haveVisibleAnnotationViews = true;
         }
     }
 
@@ -7000,6 +7004,14 @@ - (void)updateAnnotationViews
             }
         }
     }
+
+    // Switch synchronous frame rendering on if we have visible annotation views.
+    // Only implemented on Metal, just a stub on OpenGL.
+    // This logic is needed to fix the desynchronization of UIView annotations when the map is rendered on the Metal backend with asynchronous frames.
+    // Root cause: the Map transform is updated immediately and the annotations are positioned on the screen using it,
+    // while the map rendered surface catches up when the frame is complete.
+    // Issue: https://github.com/maplibre/maplibre-native/issues/2053
+    _mbglView->setSynchronous(haveVisibleAnnotationViews);
 }
 
 - (BOOL)hasAnAnchoredAnnotationCalloutView