How to extend Audio playables?

I was playing with audio playables and clips. I came to these conclusions :

- To draw waveform on the clip, it must satisfy either that it must be a subclass of

AudioPlayableAsset

, so I must subclass from this, or one other criteria is that the asset must be able to

as AudioClip

as I see in the package code, but how could this be possible? (If it is

(AudioClip)

casting instead of

as

, I could write

explicit

operator to my playable asset.)
- To get live preview, the track must be a subclass of

AudioTrack

. (?) And

AudioTrack

create its mixer in internal

OnCreateClipPlayableGraph

so even with a subclass, I couldn't modify its course because it will create a mixer this way anyways. The only way to have my own

AudioTrack

is an empty subclass. (so it is just a rename, with different clip type or binding type check). I can't have an entirely original class that programmed to work like

AudioTrack

.
- It follows from that I must use

AudioTrack

, it's behaviour is that it is looking for

AudioClipProperties

which was set and get with

internal

methods (so my class could not emulate this). So in the end, a subclass of

AudioPlayableAsset

must somehow hack into inherited

AudioClipProperties

to change it's clip or volume rather than "providing" audio data, before it is passed on to the track to multiply its own volume, panning, etc. adding to it. I am able to use

AudioClipPlayable.SetClip

properly in my own

CreatePlayable

override that do

base

call then hack in a new clip, however the volume must be hacked in via reflection since

m_ClipProperties

is not

protected

. This is how it looks like in my subclass :

Code (CSharp):

1.         static FieldInfo hack = typeof(AudioPlayableAsset).GetField("m_ClipProperties", BindingFlags.Instance | BindingFlags.NonPublic);
2.         static Type acp = typeof(AudioPlayableAsset).Assembly.GetType("UnityEngine.Timeline.AudioClipProperties");
3.         static FieldInfo volumeField = acp.GetField("volume", BindingFlags.Instance | BindingFlags.Public);
4.  
5.         private void HackVolume()
6.         {
7.             var got = hack.GetValue(this);
8.             volumeField.SetValue(got, SummarizedVolume);
9.         }
10.  
11.  
12.         public override Playable CreatePlayable(PlayableGraph graph, GameObject owner)
13.         {
14.             //Somehow we have to hack in the clip property before the playable could be created
15.             //since it use the internal SetScriptInstance to pass on value and we have no chance after this.
16.             HackVolume();
17.    
18.             AudioClipPlayable p = (AudioClipPlayable)base.CreatePlayable(graph, owner);
19.             if (AudioObject != null)
20.             {
21.                 p.SetClip(AudioObject.AudioClip);
22.                 //Can't hack the internal SetVolume here, since the track would like to use it to blend clip prop with track volume.
23.             }
24.             return p;
25.         }

- There is no documentation of

AudioMixerPlayable

and

AudioPlayableBinding

. I have been guessing that it look for a binding of type

AudioSource

to use its properties and mixer channel, or use no mixer if no

AudioSource

bound. However it seems I didn't guessed all the criteria correctly as when I made a no-binding audio track I couldn't hear anything, while the official

AudioTrack

with

null

AudioSource

binding do produce sound in runtime.

What I want to do :

- Make a custom track binding-less and output audio like

AudioTrack

that has

null

AudioSource

binding, so I could see track's name instead of empty

AudioTrack

slot.
- Use Timeline tab to sequence a combination of sound effects by reusing little pieces of

AudioClip

, to create a new kind of serialized

PlayableAsset

that is subclassed from

TimelineAsset

. This may contains multiple unbounded tracks, which use the property of the 1st point so I could preview and design the sequence after putting it in a dummy

PlayableDirector

somewhere.
- Then, I want this asset to be able to live on "main"

TimelineAsset

as 1 unit, as a clip, and then be able to play it as if all the little audio clips are arranged on the timeline manually. I am fine with not seeing waveform of this one as it is an aggregation of audio, and also fine being not able to preview it at edit time. However it is impossible to design runtime behaviour without knowing the exact criteria that

AudioMixerPlayable

and

AudioClipPlayable

works together to output sound.

Also then, I found that subclassing from

TimelineAsset

with [CreateAssetMenu] :

- I could create a new kind of timeline asset in my object.
- Allows me to click on it in Project panel and arrange my audio around without preview
- Allows me to put that in a dummy

PlayableDirector

and preview it while design.
- But it could not become a clip, even if that track has a clip type of my subclass of

TimelineAsset

, and it has

ITimelineClipAsset

by inheritance, the mouse do not turn into green + sign when dragging the asset from Project panel to the track. Do I need something like Control Track, but instead "Timeline" track so it accepts the timeline asset and able to make a playable out of it?

If there is a solution that need to use 2019.1 new audio backend, it is fine. Thank you.