filter/FilterInternal: split class Filter, add class PreparedFilter

For easier state management inside filter plugins.

src/filter/Observer.cxx

@@ -0,0 +1,115 @@
+/*
+ * Copyright 2003-2016 The Music Player Daemon Project
+ * http://www.musicpd.org
+ *
+ * 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 2 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, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#include "config.h"
+#include "Observer.hxx"
+#include "FilterInternal.hxx"
+#include "util/ConstBuffer.hxx"
+
+#include <assert.h>
+
+class FilterObserver::PreparedProxy final : public PreparedFilter {
+	FilterObserver &observer;
+
+	PreparedFilter *const prepared_filter;
+	Proxy *child = nullptr;
+
+public:
+	PreparedProxy(FilterObserver &_observer,
+		      PreparedFilter *_prepared_filter)
+		:observer(_observer),
+		 prepared_filter(_prepared_filter) {}
+
+	~PreparedProxy() {
+		assert(child == nullptr);
+		assert(observer.proxy == this);
+
+		observer.proxy = nullptr;
+	}
+
+	void Clear(gcc_unused Proxy *_child) {
+		assert(child == _child);
+		child = nullptr;
+	}
+
+	Filter *Get();
+
+	Filter *Open(AudioFormat &af, Error &error) override;
+};
+
+class FilterObserver::Proxy final : public Filter {
+	PreparedProxy &parent;
+
+	Filter *const filter;
+
+public:
+	Proxy(PreparedProxy &_parent, Filter *_filter)
+		:Filter(_filter->GetOutAudioFormat()),
+		 parent(_parent), filter(_filter) {}
+
+	~Proxy() {
+		parent.Clear(this);
+		delete filter;
+	}
+
+	Filter *Get() {
+		return filter;
+	}
+
+	ConstBuffer<void> FilterPCM(ConstBuffer<void> src,
+				    Error &error) override {
+		return filter->FilterPCM(src, error);
+	}
+};
+
+Filter *
+FilterObserver::PreparedProxy::Get()
+{
+	return child != nullptr
+		? child->Get()
+		: nullptr;
+}
+
+Filter *
+FilterObserver::PreparedProxy::Open(AudioFormat &af, Error &error)
+{
+	assert(child == nullptr);
+
+	Filter *f = prepared_filter->Open(af, error);
+	if (f == nullptr)
+		return f;
+
+	return child = new Proxy(*this, f);
+}
+
+PreparedFilter *
+FilterObserver::Set(PreparedFilter *pf)
+{
+	assert(proxy == nullptr);
+
+	return proxy = new PreparedProxy(*this, pf);
+}
+
+Filter *
+FilterObserver::Get()
+{
+	return proxy != nullptr
+		? proxy->Get()
+		: nullptr;
+}