Temporarily memoize IPlugView::canResize()

This makes VST3 plugin resizing more responsive, because this function
would otherwise be constantly running in lockstep from the GUI thread.

CHANGELOG.md

@@ -6,6 +6,14 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic
 Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+### Added
+
+- Added a timed cache for the `IPluginView::canResize()` VST3 function, so value
+  will be remembered during an active resize. This makes resizing VST3 plugin
+  editor windows more responsive.
+
 ## [3.2.0] - 2021-05-03
 
 ### Added

src/common/logging/vst3.cpp

@@ -1400,9 +1400,14 @@ void Vst3Logger::log_response(bool is_host_vst, const Ack&) {
 }
 
 void Vst3Logger::log_response(bool is_host_vst,
-                              const UniversalTResult& result) {
-    log_response_base(is_host_vst,
-                      [&](auto& message) { message << result.string(); });
+                              const UniversalTResult& result,
+                              bool from_cache) {
+    log_response_base(is_host_vst, [&](auto& message) {
+        message << result.string();
+        if (from_cache) {
+            message << " (from cache)";
+        }
+    });
 }
 
 void Vst3Logger::log_response(

src/common/logging/vst3.h

@@ -242,7 +242,9 @@ class Vst3Logger {
                      const YaUnitHandler2::NotifyUnitByBusChange&);
 
     void log_response(bool is_host_vst, const Ack&);
-    void log_response(bool is_host_vst, const UniversalTResult&);
+    void log_response(bool is_host_vst,
+                      const UniversalTResult&,
+                      bool from_cache = false);
     void log_response(bool is_host_vst,
                       const Vst3PluginFactoryProxy::ConstructArgs&);
     void log_response(

src/plugin/bridges/vst3-impls/plug-view-proxy.cpp

@@ -244,8 +244,30 @@ Vst3PlugViewProxyImpl::setFrame(Steinberg::IPlugFrame* frame) {
 }
 
 tresult PLUGIN_API Vst3PlugViewProxyImpl::canResize() {
-    return send_mutually_recursive_message(
-        YaPlugView::CanResize{.owner_instance_id = owner_instance_id()});
+    const auto request =
+        YaPlugView::CanResize{.owner_instance_id = owner_instance_id()};
+
+    {
+        std::lock_guard lock(can_resize_cache_mutex);
+        if (const tresult* result = can_resize_cache.get_and_keep_alive(5)) {
+            const bool log_response = bridge.logger.log_request(true, request);
+            if (log_response) {
+                bridge.logger.log_response(
+                    true, YaPlugView::CanResize::Response(*result));
+            }
+
+            return *result;
+        }
+    }
+
+    const UniversalTResult result = send_mutually_recursive_message(request);
+
+    {
+        std::lock_guard lock(can_resize_cache_mutex);
+        can_resize_cache.set(result, 5);
+    }
+
+    return result;
 }
 
 tresult PLUGIN_API

src/plugin/bridges/vst3-impls/plug-view-proxy.h

@@ -301,4 +301,20 @@ class Vst3PlugViewProxyImpl : public Vst3PlugViewProxy {
      * This will be an `std::nullopt` if the hostdoes not support `IRunLoop`.
      */
     std::optional<RunLoopTasks> run_loop_tasks;
+
+    // Caches
+
+    /**
+     * During resizing the host will likely constantly ask the plugin if it can
+     * be freely resized. Even if it is technically possible, I'm not aware of
+     * any plugins that change from not being able arbitrarily resizeable to
+     * being able to be resized like this. The reason why we might want to cache
+     * `IPlugView::canResize()` is because this function has to be run on the
+     * GUI thread, just like `IPlugView::onSize()` and
+     * `IPlugView::checkSizeConstraint`. Everything running in lockstep makes
+     * resizing a lot laggier than they would have to be, so as a compromise
+     * we'll remember this value for the duration of the resize.
+     */
+    TimedValueCache<tresult> can_resize_cache;
+    std::mutex can_resize_cache_mutex;
 };