mirror of
https://github.com/kyleneideck/BackgroundMusic
synced 2024-12-02 16:59:20 +00:00
81 lines
4 KiB
Objective-C
81 lines
4 KiB
Objective-C
// This file is part of Background Music.
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//
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// Background Music is free software: you can redistribute it and/or
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// modify it under the terms of the GNU General Public License as
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// published by the Free Software Foundation, either version 2 of the
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// License, or (at your option) any later version.
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//
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// Background Music is distributed in the hope that it will be useful,
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// but WITHOUT ANY WARRANTY; without even the implied warranty of
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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// GNU General Public License for more details.
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//
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// You should have received a copy of the GNU General Public License
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// along with Background Music. If not, see <http://www.gnu.org/licenses/>.
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//
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// main.m
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// BGMXPCHelper
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//
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// Copyright © 2016 Kyle Neideck
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//
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// BGMXPCHelper passes XPC messages between BGMDriver and BGMApp. So far it's only used for synchronization while starting IO.
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//
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// BGMApp and BGMDriver usually communicate by changing device properties and listening for notifications about those changes, which the
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// HAL sends. We use XPC, or plan to use it, for the few cases that notifications don't suit. For example, shared memory regions can be
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// sent via XPC. So we might be able to share a ring buffer between BGMDriver and BGMApp and avoid the latency and CPU overhead of
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// sending the audio data through an input stream. XPC should also let us safely send messages from real-time functions.
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//
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// Notifications would have probably worked fine for synchronizing BGMDriver and BGMApp while starting IO. But the Core Audio headers
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// don't specify which threads notifications are sent/received on, what (if anything) they can block on, or what can be blocked by their
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// handler callbacks. So using XPC instead makes it a bit easier to reason about the process and avoid potential deadlocks. (That said,
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// I ended up adding a huge amount of error handling and boilerplate code for XPC, so this way is probably more confusing overall.)
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//
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// CFMessagePort would be much simpler than XPC to use, but as far as I could tell there's no way to create a remote CFMessagePort for
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// a Mach port without looking the Mach port up using the bootstrap service. And because BGMXPCHelper and BGMApp have to use different
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// bootstrap namespaces, BGMXPCHelper wouldn't be able to see services vended by BGMApp. So BGMXPCHelper would be able to receive
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// messages from BGMApp and reply to them, but not send them.
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//
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// BGMXPCHelper has to run as a launchd daemon so the Mach service it vends will use the global bootstrap namespace. See Table 2 in
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// <https://developer.apple.com/library/mac/technotes/tn2083/_index.html>. This is because the coreaudiod process, where BGMDriver runs,
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// uses the global bootstrap namespace, which means it can only see services in that namespace. In practical terms, this just means we
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// have to run BGMXPCHelper by creating a launchd.plist file for it in /Library/LaunchDaemons and loading it with a command like
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// launchctl bootstrap system /Library/LaunchDaemons/com.bearisdriving.BGM.XPCHelper.plist
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//
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// Local Includes
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#import "BGMXPCProtocols.h"
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#import "BGMXPCListenerDelegate.h"
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// PublicUtility Includes
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#undef CoreAudio_ThreadStampMessages
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#define CoreAudio_ThreadStampMessages 0 // Requires C++
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#include "CADebugMacros.h"
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// System Includes
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#import <Foundation/Foundation.h>
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#pragma clang assume_nonnull begin
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int main(int argc, const char* __nullable argv[]) {
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#pragma unused (argc, argv)
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DebugMsg("BGMXPCHelper::main: Service starting up");
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// Set up the one NSXPCListener for this service. It will handle all incoming connections. This checks our service in with
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// the bootstrap service.
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NSXPCListener* listener = [[NSXPCListener alloc] initWithMachServiceName:kBGMXPCHelperMachServiceName];
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BGMXPCListenerDelegate* delegate = [BGMXPCListenerDelegate new];
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listener.delegate = delegate;
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// Start receiving requests.
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[listener resume];
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[[NSRunLoop currentRunLoop] run];
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return 0;
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}
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#pragma clang assume_nonnull end
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