Create Vst3HostContextProxy from YaHostApplication

This is quite a huge refactor, but note everything is consistent (and
we're going to need one or two more of these `Vst3*Proxy` objects).
Right now nothing extends `IHostApplication`, but this way it will be
trivial to add support for more host context interfaces.

src/common/serialization/vst3/host-context-proxy.h

@@ -0,0 +1,104 @@
+// yabridge: a Wine VST bridge
+// Copyright (C) 2020  Robbert van der Helm
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+#pragma once
+
+#include "../common.h"
+#include "host-context/host-application.h"
+
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wnon-virtual-dtor"
+
+/**
+ * An abstract class that optionally implements all interfaces a `context`
+ * object passed to `IPluginBase::intialize()` or
+ * `IPluginFactory3::setHostContext()` might implement. This works exactly the
+ * same as `Vst3PluginProxy`, but instead of proxying for an object provided by
+ * the plugin we are proxying for the `FUnknown*` argument passed to plugin by
+ * the host. When we are proxying for a host context object passed to
+ * `IPluginBase::initialize()` we'll keep track of the object instance ID the
+ * actual context object belongs to.
+ */
+class Vst3HostContextProxy : public YaHostApplication {
+   public:
+    /**
+     * These are the arguments for constructing a
+     * `Vst3HostContextProxyImpl`.
+     */
+    struct ConstructArgs {
+        ConstructArgs();
+
+        /**
+         * Read from an existing object. We will try to mimic this object, so
+         * we'll support any interfaces this object also supports.
+         */
+        ConstructArgs(Steinberg::IPtr<FUnknown> object,
+                      std::optional<size_t> owner_instance_id);
+
+        /**
+         * The unique instance identifier of the proxy object instance this host
+         * context has been passed to and thus belongs to. If we are handling
+         * When handling `IPluginFactory::setHostContext()` this will be empty.
+         */
+        std::optional<native_size_t> owner_instance_id;
+
+        YaHostApplication::ConstructArgs host_application_args;
+
+        template <typename S>
+        void serialize(S& s) {
+            s.ext(owner_instance_id, bitsery::ext::StdOptional{},
+                  [](S& s, native_size_t& instance_id) {
+                      s.value8b(instance_id);
+                  });
+            s.object(host_application_args);
+        }
+    };
+
+    /**
+     * Instantiate this instance with arguments read from an actual host
+     * context.
+     *
+     * @note Since this is passed as part of `IPluginBase::intialize()` and
+     *   `IPluginFactory3::setHostContext()`, there are no direct `Construct` or
+     *   `Destruct` messages. This object's lifetime is bound to that of the
+     *   objects they are passed to. If those objects get dropped, then the host
+     *   contexts should also be dropped.
+     */
+    Vst3HostContextProxy(const ConstructArgs&& args);
+
+    /**
+     * The lifetime of this object should be bound to the object we created it
+     * for. When for instance the `Vst3PluginProxy` instance with id `n` gets
+     * dropped a corresponding `Vst3HostContextProxyImpl` should also be
+     * dropped.
+     */
+    virtual ~Vst3HostContextProxy() = 0;
+
+    DECLARE_FUNKNOWN_METHODS
+
+    /**
+     * Get the instance ID of the owner of this object, if this is not the
+     * global host context passed to the module's plugin factory.
+     */
+    inline std::optional<size_t> owner_instance_id() const {
+        return arguments.owner_instance_id;
+    }
+
+   private:
+    ConstructArgs arguments;
+};
+
+#pragma GCC diagnostic pop