Make winrt::event clearable

strings/base_events.h

@@ -375,8 +375,8 @@ WINRT_EXPORT namespace winrt
         using delegate_type = Delegate;
 
         event() = default;
-        event(event<Delegate> const&) = delete;
-        event<Delegate>& operator =(event<Delegate> const&) = delete;
+        event(event const&) = delete;
+        event& operator =(event const&) = delete;
 
         explicit operator bool() const noexcept
         {
@@ -466,6 +466,24 @@ WINRT_EXPORT namespace winrt
             }
         }
 
+        void clear()
+        {
+            // Extends life of old targets array to release delegates outside of lock.
+            delegate_array temp_targets;
+
+            {
+                slim_lock_guard const change_guard(m_change);
+
+                if (!m_targets)
+                {
+                    return;
+                }
+
+                slim_lock_guard const swap_guard(m_swap);
+                temp_targets = std::exchange(m_targets, nullptr);
+            }
+        }
+
         template<typename...Arg>
         void operator()(Arg const&... args)
         {

test/test/event_clear.cpp

@@ -0,0 +1,35 @@
+#include "pch.h"
+
+using namespace winrt;
+using namespace Windows::Foundation;
+
+//
+// Checks that clear removes all event handlers
+//
+
+TEST_CASE("event_clear")
+{
+    event<TypedEventHandler<int, int>> event;
+    int counter{};
+
+    auto a = event.add([&](auto && ...)
+        {
+            counter += 1;
+        });
+
+    auto b = event.add([&](auto && ...)
+        {
+            counter += 10;
+        });
+
+    REQUIRE(counter == 0);
+    event(0, 0);
+    REQUIRE(counter == 11);
+
+    // Clear event
+    event.clear();
+
+    counter = 0;
+    event(0, 0);
+    REQUIRE(counter == 0);
+}

test/test/test.vcxproj

@@ -336,6 +336,7 @@
     <ClCompile Include="delegates.cpp" />
     <ClCompile Include="disconnected.cpp" />
     <ClCompile Include="enum.cpp" />
+    <ClCompile Include="event_clear.cpp" />
     <ClCompile Include="guid.cpp" />
     <ClCompile Include="hresult_class_not_registered.cpp" />
     <ClCompile Include="error_info.cpp" />