MessageManager: Add callSync counterpart to callAsync

modules/juce_events/messages/juce_MessageManager.cpp

@@ -161,47 +161,9 @@ bool MessageManager::runDispatchLoopUntil (int millisecondsToRunFor)
 #endif
 
 //==============================================================================
-class AsyncFunctionCallback final : public MessageManager::MessageBase
-{
-public:
-    AsyncFunctionCallback (MessageCallbackFunction* const f, void* const param)
-        : func (f), parameter (param)
-    {}
-
-    void messageCallback() override
-    {
-        result = (*func) (parameter);
-        finished.signal();
-    }
-
-    WaitableEvent finished;
-    std::atomic<void*> result { nullptr };
-
-private:
-    MessageCallbackFunction* const func;
-    void* const parameter;
-
-    JUCE_DECLARE_NON_COPYABLE (AsyncFunctionCallback)
-};
-
 void* MessageManager::callFunctionOnMessageThread (MessageCallbackFunction* func, void* parameter)
 {
-    if (isThisTheMessageThread())
-        return func (parameter);
-
-    // If this thread has the message manager locked, then this will deadlock!
-    jassert (! currentThreadHasLockedMessageManager());
-
-    const ReferenceCountedObjectPtr<AsyncFunctionCallback> message (new AsyncFunctionCallback (func, parameter));
-
-    if (message->post())
-    {
-        message->finished.wait();
-        return message->result.load();
-    }
-
-    jassertfalse; // the OS message queue failed to send the message!
-    return nullptr;
+    return callSync ([func, parameter] { return func (parameter); }).value_or (nullptr);
 }
 
 //==============================================================================

modules/juce_events/messages/juce_MessageManager.h

@@ -55,6 +55,11 @@ using MessageCallbackFunction = void* (void* userData);
 */
 class JUCE_API  MessageManager  final
 {
+    template <typename FunctionResult>
+    using CallSyncResult = std::conditional_t<std::is_same_v<FunctionResult, void>,
+                                              bool,
+                                              std::optional<FunctionResult>>;
+
 public:
     //==============================================================================
     /** Returns the global instance of the MessageManager. */
@@ -142,6 +147,47 @@ public:
     */
     void* callFunctionOnMessageThread (MessageCallbackFunction* callback, void* userData);
 
+    /** Similar to callFunctionOnMessageThread(), calls a function on the message thread,
+        blocking the current thread until a result is available.
+
+        Be careful not to cause any deadlocks with this! It's easy to do - e.g. if the caller
+        thread has a critical section locked, which an unrelated message callback then tries to lock
+        before the message thread gets round to processing this callback.
+
+        @param function     the function to call, which should have no parameters
+        @returns            if function() returns void, then callSync returns a boolean where
+                            'true' indicates that the function was called successfully, and 'false'
+                            indicates that the message could not be posted.
+                            if function() returns any other type 'T', then callSync returns
+                            std::optional<T>, where the optional value will be valid if the function
+                            was called successfully, or nullopt otherwise.
+    */
+    template <typename Function>
+    static auto callSync (Function&& function) -> CallSyncResult<decltype (function())>
+    {
+        using FinalResult = CallSyncResult<decltype (function())>;
+
+        if (MessageManager::getInstance()->isThisTheMessageThread())
+            return transformResult (std::forward<Function> (function));
+
+        std::promise<FinalResult> promise;
+        auto future = promise.get_future();
+
+        const auto sent = callAsync ([p = std::move (promise), fn = std::move (function)]() mutable
+        {
+            p.set_value (transformResult (std::move (fn)));
+        });
+
+        if (! sent)
+        {
+            // Failed to post message!
+            jassertfalse;
+            return {};
+        }
+
+        return future.get();
+    }
+
     /** Returns true if the caller-thread is the message thread. */
     bool isThisTheMessageThread() const noexcept;
 
@@ -366,6 +412,20 @@ private:
     Atomic<Thread::ThreadID> threadWithLock;
     mutable std::mutex messageThreadIdMutex;
 
+    template <typename Function>
+    static auto transformResult (Function&& f)
+    {
+        if constexpr (std::is_same_v<decltype (f()), void>)
+        {
+            f();
+            return true;
+        }
+        else
+        {
+            return f();
+        }
+    }
+
     static bool postMessageToSystemQueue (MessageBase*);
     static void* exitModalLoopCallback (void*);
     static void doPlatformSpecificInitialisation();