This directory contains an example application that shows, in simplest possible implementation, how to play audio via the lowest-layer FIDL interfaces.
This example uses asynchronous interface calls, establishing an asynchronous message loop before calling into its MediaApp object. Once the MediaApp performs initial setup -- including establishing a callback that will be triggered later -- we run the loop. The loop accepts and dispatches incoming messages until instructed to quit (at which point loop.Run() returns, and this example app exits). The MediaApp knows when the last piece of audio has played, so it posts a ‘quit’ message to our loop. It does so using the closure that we provided it, in the MediaApp constructor.
Note: this example omits numerous important aspects of the Media interfaces, including the retrieval of supported media formats, setting the renderer gain, packet demand management, and nuanced treatment of timeline transforms.
Once the app object is created, we handle any command-line options, call
MediaApp::Run (which sets everything up and starts playback), then run the asynchronous message loop (which allows asynchronous callbacks from the system to flow to our app, so that playback can continue to the end). Once all of the audio has been played, the app posts the ‘quit’ task as detailed earlier, and the app subsequently exits.
Focusing on this example's media-specific setup and asynchronous tasks, the highest-level description of events (shown in
MediaApp::Run) is as follows:
Below is a more detailed account of the steps taken, and why each is necessary.
With the provided StartupContext, we obtain an Audio interface pointer to, and use that to obtain interface pointers to AudioRenderer. At that point we no longer need our Audio interface and can allow it to go out of scope (and hence be automatically closed).
We use the AudioRenderer interface to set playback format and start playback. If we so desired, we could also use AudioRenderer to set output gain for our audio stream (not shown in this example).
We must set_error_handler on each asynchronous interface before using it. These interfaces represent FIDL channels, which can close asynchronously. The system notifies us of closures via an error handler callback. In this example, if an unexpected channel closure occurs we log the problem and begin the shutdown process.
We populate an
AudioStreamType struct with the appropriate number of channels, sample rate, and sample format. This example uses 32-bit floating-point format, playing a 1-channel signal at 48 kHz.
In order to convey audio data to the renderer (and in turn the audio device), we must map a section of memory that will be shared cross-process to the renderer. In this example, we use 100 different “payload buffers” to keep our renderer supplied with audio data, continually reusing these buffers in sequence in ring-buffer fashion. For this reason, we want the buffers to be contiguous within a single large address range. To do this, we first create a Virtual Memory Object of the required size; we then map this address range to be writable and readable.
This example uses the VMO mapper object from FBL, to create and map this memory section, before instructing the audio renderer to use this section via the
Having mapped our shared memory section into our process (at
payload_buffer_.start()), we can now write audio data into it. In this simple example, we write our audio data only once during setup, but clients can write into mapped memory at any time before it is submitted to the system, or after the system returns it. This is described in more detail in the following section.
This example plays a sine wave signal, sending identical data to both left and right channels (as shown in the inner loop of
WriteAudioIntoBuffer). For each frame in our audio buffer we compute the appropriate sine value.
In order for the audio subsystem to use our buffer as a wraparound ring-buffer, we must subdivide it and continuously submit these pieces in sequence. This example uses 100 “payload buffers” that essentially represent each subsequent piece of the overall mapped buffer. Nonetheless, this is still a single contiguous memory range, so when writing our audio data we can think of it as a single large buffer. The audio subsystem guarantees that it will never write into your payload buffer (it maps the VMO that you have provided to it as read-only), so in a looped-signal scenario such as this one, audio buffers can be resubmitted without rewriting them.
As we prepare to begin playback, we provide an initial set of audio data to the renderer. We do this by supplying a set of media packets. Following that, as each packet returns, we submit a new packet that points to a portion of that same overall memory section. In this way, the example keeps the same number of media packets with the renderer at all times, each packet pointing to a different piece of the overall mapped buffer.
In this example, we do not specify presentation timestamps. This means that the audio subsystem should start playback immediately when ready, and play each buffer successively, without overlap or intervening gap. All media packets created in this example are identical except for one field:
payload_offset. This field (along with
payload_size) specifies the buffer location that this packet represents. Once we call
SendPacket, we should not access the associated payload memory until we receive a signal from the audio subsystem -- specifically the callback that we provided to
Once playback has begun, the sequence of [
SendPacket -> callback] is our primary mechanism for ongoing conversation with the audio subsystem. For this reason, most interactive clients will generate the next set of audio data within the
SendPacket callback itself, immediately before submitting it. In this example, data has been pre-written and payloads can simply be resubmitted without being rewritten.
This example plays a sine wave for one second. It knows the exact number of media packets needed, and submits exactly that number. Once the callback is received for that last packet, it begins an orderly shutdown (as there are no additional packets outstanding). An interactive application might do this same thing upon receiving a signal to stop, from its UI.
Once we tell the audio renderer to begin playback, we will almost immediately begin to receive
SendPacket callbacks. For this reason, we do not need to receive a separate callback when playback completes; this is why we call
PlayNoReply. The parameters to
PlayNoReply indicate that playback should begin as soon as the system can responsibly do so, using a media timestamp of the first packet that we provided (thus, 0 here). The callbacks (and subsequent sending of the next packets) will continue until we stop submitting them, or until we stop the system.
Even though we have not completed playback -- even though audio packets are still outstanding at the audio renderer, and others have not yet been sent -- we can nonetheless exit our
MediaApp::Run method at this point, returning to main.cc while our message loop dutifully waits until it receives each callback, and eventually the Quit task. Note: our thread must be a message loop thread, for FIDL interface calls (and callbacks) to function.
As stated above, once we receive a
SendPacket callback signaling that we have played the right number of packets, we then call Shutdown to close all remaining resources and exit our app.
There are actually two scenarios that can lead to our calling Shutdown.
Our Shutdown function unmaps our section of shared memory and closes (resets) our VMO object. It also posts a Quit task to our message loop thread, allowing it to exit its “dutiful” wait (and immediately thereafter, the app). Once the main function exits, our MediaApp object is destroyed, and at this point any remaining FIDL interface (
AudioRenderer, in case of normal shutdown) is released.