[core] Decouple Scheduler interface from actor model

So that it is possible to schedule normal `std::function` and use `mapbox::base::WeakPtr`.

include/mbgl/actor/mailbox.hpp

@@ -2,6 +2,7 @@
 
 #include <mbgl/util/optional.hpp>
 
+#include <functional>
 #include <memory>
 #include <mutex>
 #include <queue>
@@ -34,6 +35,7 @@ public:
     void receive();
 
     static void maybeReceive(std::weak_ptr<Mailbox>);
+    static std::function<void()> makeClosure(std::weak_ptr<Mailbox>);
 
 private:
     optional<Scheduler*> scheduler;

include/mbgl/actor/scheduler.hpp

@@ -1,5 +1,6 @@
 #pragma once
 
+#include <functional>
 #include <memory>
 
 namespace mbgl {
@@ -31,12 +32,9 @@ class Mailbox;
 class Scheduler {
 public:
     virtual ~Scheduler() = default;
-    
-    // Used by a Mailbox when it has a message in its queue to request that it
-    // be scheduled. Specifically, the scheduler is expected to asynchronously
-    // call `receive() on the given mailbox, provided it still exists at that
-    // time.
-    virtual void schedule(std::weak_ptr<Mailbox>) = 0;
+
+    // Enqueues a function for execution.
+    virtual void schedule(std::function<void()>) = 0;
 
     // Set/Get the current Scheduler for this thread
     static Scheduler* GetCurrent();

include/mbgl/util/run_loop.hpp

@@ -72,12 +72,8 @@ public:
         push(Priority::Default, task);
         return std::make_unique<WorkRequest>(task);
     }
-                    
-    void schedule(std::weak_ptr<Mailbox> mailbox) override {
-        invoke([mailbox] () {
-            Mailbox::maybeReceive(mailbox);
-        });
-    }
+
+    void schedule(std::function<void()> fn) override { invoke(std::move(fn)); }
 
     class Impl;
 

platform/android/src/map_renderer.cpp

@@ -43,7 +43,7 @@ ActorRef<Renderer> MapRenderer::actor() const {
     return *rendererRef;
 }
 
-void MapRenderer::schedule(std::weak_ptr<Mailbox> scheduled) {
+void MapRenderer::schedule(std::function<void()> scheduled) {
     // Create a runnable
     android::UniqueEnv _env = android::AttachEnv();
     auto runnable = std::make_unique<MapRendererRunnable>(*_env, std::move(scheduled));

platform/android/src/map_renderer.hpp

@@ -67,7 +67,7 @@ public:
 
     // From Scheduler. Schedules by using callbacks to the
     // JVM to process the mailbox on the right thread.
-    void schedule(std::weak_ptr<Mailbox> scheduled) override;
+    void schedule(std::function<void()> scheduled) override;
 
     void requestRender();
 

platform/android/src/map_renderer_runnable.cpp

@@ -5,9 +5,8 @@
 namespace mbgl {
 namespace android {
 
-MapRendererRunnable::MapRendererRunnable(jni::JNIEnv& env, std::weak_ptr<Mailbox> mailbox_)
-    : mailbox(std::move(mailbox_)) {
-
+MapRendererRunnable::MapRendererRunnable(jni::JNIEnv& env, std::function<void()> function_)
+    : function(std::move(function_)) {
     // Create the Java peer and hold on to a global reference
     // Not using a weak reference here as this might oerflow
     // the weak reference table on some devices
@@ -21,7 +20,7 @@ MapRendererRunnable::MapRendererRunnable(jni::JNIEnv& env, std::weak_ptr<Mailbox
 MapRendererRunnable::~MapRendererRunnable() = default;
 
 void MapRendererRunnable::run(jni::JNIEnv&) {
-    Mailbox::maybeReceive(mailbox);
+    if (function) function();
 }
 
 jni::Global<jni::Object<MapRendererRunnable>> MapRendererRunnable::peer() {

platform/android/src/map_renderer_runnable.hpp

@@ -24,7 +24,7 @@ public:
 
     static void registerNative(jni::JNIEnv&);
 
-    MapRendererRunnable(jni::JNIEnv&, std::weak_ptr<Mailbox>);
+    MapRendererRunnable(jni::JNIEnv&, std::function<void()>);
 
     // Only for jni registration, unused
     MapRendererRunnable(jni::JNIEnv&) {
@@ -40,7 +40,7 @@ public:
 
 private:
     jni::Global<jni::Object<MapRendererRunnable>> javaPeer;
-    std::weak_ptr<Mailbox> mailbox;
+    std::function<void()> function;
 };
 
 } // namespace android

platform/qt/src/qmapboxgl_scheduler.cpp

@@ -13,10 +13,9 @@ QMapboxGLScheduler::~QMapboxGLScheduler()
     MBGL_VERIFY_THREAD(tid);
 }
 
-void QMapboxGLScheduler::schedule(std::weak_ptr<mbgl::Mailbox> mailbox)
-{
+void QMapboxGLScheduler::schedule(std::function<void()> function) {
     std::lock_guard<std::mutex> lock(m_taskQueueMutex);
-    m_taskQueue.push(mailbox);
+    m_taskQueue.push(std::move(function));
 
     // Need to force the main thread to wake
     // up this thread and process the events.
@@ -25,14 +24,15 @@ void QMapboxGLScheduler::schedule(std::weak_ptr<mbgl::Mailbox> mailbox)
 
 void QMapboxGLScheduler::processEvents()
 {
-    std::queue<std::weak_ptr<mbgl::Mailbox>> taskQueue;
+    std::queue<std::function<void()>> taskQueue;
     {
         std::unique_lock<std::mutex> lock(m_taskQueueMutex);
         std::swap(taskQueue, m_taskQueue);
     }
 
     while (!taskQueue.empty()) {
-        mbgl::Mailbox::maybeReceive(taskQueue.front());
+        auto& function = taskQueue.front();
+        if (function) function();
         taskQueue.pop();
     }
 }

platform/qt/src/qmapboxgl_scheduler.hpp

@@ -1,6 +1,5 @@
 #pragma once
 
-#include <mbgl/actor/mailbox.hpp>
 #include <mbgl/actor/scheduler.hpp>
 #include <mbgl/util/util.hpp>
 
@@ -19,7 +18,7 @@ public:
     virtual ~QMapboxGLScheduler();
 
     // mbgl::Scheduler implementation.
-    void schedule(std::weak_ptr<mbgl::Mailbox> scheduled) final;
+    void schedule(std::function<void()> scheduled) final;
 
     void processEvents();
 
@@ -30,5 +29,5 @@ private:
     MBGL_STORE_THREAD(tid);
 
     std::mutex m_taskQueueMutex;
-    std::queue<std::weak_ptr<mbgl::Mailbox>> m_taskQueue;
+    std::queue<std::function<void()>> m_taskQueue;
 };

src/mbgl/actor/mailbox.cpp

@@ -27,7 +27,7 @@ void Mailbox::open(Scheduler& scheduler_) {
     }
     
     if (!queue.empty()) {
-        (*scheduler)->schedule(shared_from_this());
+        (*scheduler)->schedule(makeClosure(shared_from_this()));
     }
 }
 
@@ -57,7 +57,7 @@ void Mailbox::push(std::unique_ptr<Message> message) {
     bool wasEmpty = queue.empty();
     queue.push(std::move(message));
     if (wasEmpty && scheduler) {
-        (*scheduler)->schedule(shared_from_this());
+        (*scheduler)->schedule(makeClosure(shared_from_this()));
     }
 }
 
@@ -84,14 +84,20 @@ void Mailbox::receive() {
     (*message)();
 
     if (!wasEmpty) {
-        (*scheduler)->schedule(shared_from_this());
+        (*scheduler)->schedule(makeClosure(shared_from_this()));
     }
 }
 
+// static
 void Mailbox::maybeReceive(std::weak_ptr<Mailbox> mailbox) {
     if (auto locked = mailbox.lock()) {
         locked->receive();
     }
 }
 
+// static
+std::function<void()> Mailbox::makeClosure(std::weak_ptr<Mailbox> mailbox) {
+    return [mailbox]() { maybeReceive(mailbox); };
+}
+
 } // namespace mbgl

src/mbgl/util/thread_pool.cpp

@@ -26,11 +26,10 @@ ThreadPool::ThreadPool(std::size_t count) {
                     return;
                 }
 
-                auto mailbox = queue.front();
+                auto function = std::move(queue.front());
                 queue.pop();
                 lock.unlock();
-
-                Mailbox::maybeReceive(mailbox);
+                if (function) function();
             }
         });
     }
@@ -49,10 +48,10 @@ ThreadPool::~ThreadPool() {
     }
 }
 
-void ThreadPool::schedule(std::weak_ptr<Mailbox> mailbox) {
+void ThreadPool::schedule(std::function<void()> fn) {
     {
         std::lock_guard<std::mutex> lock(mutex);
-        queue.push(mailbox);
+        queue.push(std::move(fn));
     }
 
     cv.notify_one();

src/mbgl/util/thread_pool.hpp

@@ -15,11 +15,11 @@ public:
     explicit ThreadPool(std::size_t count);
     ~ThreadPool() override;
 
-    void schedule(std::weak_ptr<Mailbox>) override;
+    void schedule(std::function<void()>) override;
 
 private:
     std::vector<std::thread> threads;
-    std::queue<std::weak_ptr<Mailbox>> queue;
+    std::queue<std::function<void()>> queue;
     std::mutex mutex;
     std::condition_variable cv;
     bool terminate{ false };

test/actor/actor.test.cpp

@@ -101,7 +101,7 @@ TEST(Actor, DestructionBlocksOnSend) {
             EXPECT_TRUE(waited.load());
         }
 
-        void schedule(std::weak_ptr<Mailbox>) final {
+        void schedule(std::function<void()>) final {
             promise.set_value();
             future.wait();
             std::this_thread::sleep_for(1ms);