[core] Throttle tiles to redo symbol placement at most once every 300ms.

Fixes issue #8435 and prepares for pitch-scaling changes in issue #8967.

cmake/core-files.cmake

@@ -593,6 +593,8 @@ set(MBGL_CORE_FILES
     src/mbgl/util/thread_context.cpp
     src/mbgl/util/thread_context.hpp
     src/mbgl/util/thread_local.hpp
+    src/mbgl/util/throttler.cpp
+    src/mbgl/util/throttler.hpp
     src/mbgl/util/tile_coordinate.hpp
     src/mbgl/util/tile_cover.cpp
     src/mbgl/util/tile_cover.hpp

src/mbgl/tile/geometry_tile.cpp

@@ -18,6 +18,7 @@
 #include <mbgl/map/query.hpp>
 #include <mbgl/util/run_loop.hpp>
 #include <mbgl/style/filter_evaluator.hpp>
+#include <mbgl/util/chrono.hpp>
 #include <mbgl/util/logging.hpp>
 
 #include <iostream>
@@ -41,7 +42,8 @@ GeometryTile::GeometryTile(const OverscaledTileID& id_,
              obsolete,
              parameters.mode),
       glyphAtlas(glyphAtlas_),
-      spriteAtlas(spriteAtlas_) {
+      spriteAtlas(spriteAtlas_),
+      placementThrottler(Milliseconds(300), [this] { invokePlacement(); }) {
 }
 
 GeometryTile::~GeometryTile() {
@@ -85,7 +87,13 @@ void GeometryTile::setPlacementConfig(const PlacementConfig& desiredConfig) {
 
     ++correlationID;
     requestedConfig = desiredConfig;
-    worker.invoke(&GeometryTileWorker::setPlacementConfig, desiredConfig, correlationID);
+    placementThrottler.invoke();
+}
+
+void GeometryTile::invokePlacement() {
+    if (requestedConfig) {
+        worker.invoke(&GeometryTileWorker::setPlacementConfig, *requestedConfig, correlationID);
+    }
 }
 
 void GeometryTile::redoLayout() {

src/mbgl/tile/geometry_tile.hpp

@@ -6,6 +6,7 @@
 #include <mbgl/text/glyph_atlas.hpp>
 #include <mbgl/text/placement_config.hpp>
 #include <mbgl/util/feature.hpp>
+#include <mbgl/util/throttler.hpp>
 #include <mbgl/actor/actor.hpp>
 
 #include <atomic>
@@ -88,6 +89,7 @@ class GeometryTile : public Tile, public GlyphRequestor, IconRequestor {
 
 private:
     void markObsolete();
+    void invokePlacement();
 
     const std::string sourceID;
     style::Style& style;
@@ -110,6 +112,8 @@ class GeometryTile : public Tile, public GlyphRequestor, IconRequestor {
 
     std::unordered_map<std::string, std::shared_ptr<Bucket>> symbolBuckets;
     std::unique_ptr<CollisionTile> collisionTile;
+
+    util::Throttler placementThrottler;
 };
 
 } // namespace mbgl

src/mbgl/util/throttler.cpp

@@ -0,0 +1,36 @@
+#include <mbgl/util/throttler.hpp>
+
+namespace mbgl {
+namespace util {
+
+Throttler::Throttler(Duration frequency_, std::function<void()>&& function_)
+    : frequency(frequency_)
+    , function(std::move(function_))
+    , pendingInvocation(false)
+    , lastInvocation(TimePoint::min())
+{}
+
+void Throttler::invoke() {
+    if (pendingInvocation) {
+        return;
+    }
+
+    Duration timeToNextInvocation = lastInvocation == TimePoint::min()
+        ? Duration::zero()
+        : (lastInvocation + frequency) - Clock::now();
+
+    if (timeToNextInvocation <= Duration::zero()) {
+        lastInvocation = Clock::now();
+        function();
+    } else {
+        pendingInvocation = true;
+        timer.start(timeToNextInvocation, Duration::zero(), [this]{
+            pendingInvocation = false;
+            lastInvocation = Clock::now();
+            function();
+        });
+    }
+}
+
+} // namespace util
+} // namespace mbgl

src/mbgl/util/throttler.hpp

@@ -0,0 +1,22 @@
+#include <mbgl/util/chrono.hpp>
+#include <mbgl/util/timer.hpp>
+
+namespace mbgl {
+namespace util {
+
+class Throttler {
+public:
+    Throttler(Duration frequency, std::function<void()>&& function);
+
+    void invoke();
+private:
+    Duration frequency;
+    std::function<void()> function;
+
+    Timer timer;
+    bool pendingInvocation;
+    TimePoint lastInvocation;
+};
+
+} // namespace util
+} // namespace mbgl