[core] Introduce Scheduler::bindOnce() and use it in ImageManager

include/mbgl/actor/scheduler.hpp

@@ -40,6 +40,16 @@ public:
     // Makes a weak pointer to this Scheduler.
     virtual mapbox::base::WeakPtr<Scheduler> makeWeakPtr() = 0;
 
+    // Returns a closure wrapping the given one.
+    //
+    // When the returned closure is invoked for the first time, it schedules
+    // the given closure to this scheduler, the consequent calls of the
+    // returned closure are ignored.
+    //
+    // If this scheduler is already deleted by the time the returnded closure is
+    // first invoked, the call is ignored.
+    std::function<void()> bindOnce(std::function<void()>);
+
     // Set/Get the current Scheduler for this thread
     static Scheduler* GetCurrent();
     static void SetCurrent(Scheduler*);

src/mbgl/actor/scheduler.cpp

@@ -4,7 +4,14 @@
 
 namespace mbgl {
 
-util::ThreadLocal<Scheduler> g_currentScheduler;
+std::function<void()> Scheduler::bindOnce(std::function<void()> fn) {
+    assert(fn);
+    return [scheduler = makeWeakPtr(), scheduled = std::move(fn)]() mutable {
+        if (!scheduled) return; // Repeated call.
+        auto schedulerGuard = scheduler.lock();
+        if (scheduler) scheduler->schedule(std::move(scheduled));
+    };
+}
 
 static auto& current() {
     static util::ThreadLocal<Scheduler> scheduler;

src/mbgl/renderer/image_manager.cpp

@@ -204,6 +204,7 @@ void ImageManager::checkMissingAndNotify(ImageRequestor& requestor, const ImageR
             }
             requestedImages[missingImage].emplace(requestorPtr);
             requestor.addPendingRequest(missingImage);
+
             auto removePendingRequests = [this, missingImage] {
                 auto existingRequest = requestedImages.find(missingImage);
                 if (existingRequest == requestedImages.end()) {
@@ -214,11 +215,8 @@ void ImageManager::checkMissingAndNotify(ImageRequestor& requestor, const ImageR
                     req->removePendingRequest(missingImage);
                 }
             };
-
-            Scheduler& scheduler = *Scheduler::GetCurrent();
-            auto callback = [&scheduler, removePendingRequests]() { scheduler.schedule(removePendingRequests); };
-
-            observer->onStyleImageMissing(missingImage, std::move(callback));
+            observer->onStyleImageMissing(missingImage,
+                                          Scheduler::GetCurrent()->bindOnce(std::move(removePendingRequests)));
         }
     } else {
         // Associate requestor with an image that was provided by the client.