前言
前面分析了CarAudioDynamicRouting，我们继续对CarAudioService的init函数进行分析

        if (sUseCarAudioFocus) {
            // Configure our AudioPolicy to handle focus events.
            // This gives us the ability to decide which audio focus requests to accept and bypasses
            // the framework ducking logic.
            mFocusHandler = new CarZonesAudioFocus(mAudioManager,
                    mContext.getPackageManager(),
                    mCarAudioZones);
            builder.setAudioPolicyFocusListener(mFocusHandler);
            builder.setIsAudioFocusPolicy(true);
        }

        mAudioPolicy = builder.build();
        if (sUseCarAudioFocus) {
            // Connect the AudioPolicy and the focus listener
            mFocusHandler.setOwningPolicy(this, mAudioPolicy);
        }

sUseCarAudioFocus默认是true，如果我们不想在car上使用CarZoneAudioFocus这套逻辑，则可以改变这个值为false，如果为false，则AudioFocus的处理就会使用原生的逻辑。
1. CarZonesAudioFocus
首先看CarZonesAudioFocus的构造函数,这里的逻辑主要是围绕CarAudioZone 处理，为每个CarAudioZone 创建一个CarAudioFocus，下面我们会分析CarAudioFocus()

    CarZonesAudioFocus(AudioManager audioManager,
            PackageManager packageManager,
            @NonNull CarAudioZone[] carAudioZones) {
        //Create the zones here, the policy will be set setOwningPolicy,
        // which is called right after this constructor.

        Preconditions.checkNotNull(carAudioZones);
        Preconditions.checkArgument(carAudioZones.length != 0,
                "There must be a minimum of one audio zone");

        //Create focus for all the zones
        for (CarAudioZone audioZone : carAudioZones) {
            Log.d(CarLog.TAG_AUDIO,
                    "CarZonesAudioFocus adding new zone " + audioZone.getId());
            CarAudioFocus zoneFocusListener = new CarAudioFocus(audioManager, packageManager);
            mFocusZones.put(audioZone.getId(), zoneFocusListener);
        }
    }

2. CarAudioFocus
CarAudioFocus的构造函数如下，只是传了audioManager和packageManager

    CarAudioFocus(AudioManager audioManager, PackageManager packageManager) {
        mAudioManager = audioManager;
        mPackageManager = packageManager;
    }

最后我们会调用setAudioPolicyFocusListener把CarZonesAudioFocus作为参数设置为AudioPolicy，那么说明CarZonesAudioFocus应该是继承了AudioPolicy.AudioPolicyFocusListener，我们回到CarZonesAudioFocus的源码中去看

class CarZonesAudioFocus extends AudioPolicy.AudioPolicyFocusListener

通过源码来看，果真是这样，那么我们来看一下AudioPolicyFocusListener里面都有什么

    public static abstract class AudioPolicyFocusListener {
        public void onAudioFocusGrant(AudioFocusInfo afi, int requestResult) {}
        public void onAudioFocusLoss(AudioFocusInfo afi, boolean wasNotified) {}
        /**
         * Called whenever an application requests audio focus.
         * Only ever called if the {@link AudioPolicy} was built with
         * {@link AudioPolicy.Builder#setIsAudioFocusPolicy(boolean)} set to {@code true}.
         * @param afi information about the focus request and the requester
         * @param requestResult deprecated after the addition of
         *     {@link AudioManager#setFocusRequestResult(AudioFocusInfo, int, AudioPolicy)}
         *     in Android P, always equal to {@link #AUDIOFOCUS_REQUEST_GRANTED}.
         */
        public void onAudioFocusRequest(AudioFocusInfo afi, int requestResult) {}
        /**
         * Called whenever an application abandons audio focus.
         * Only ever called if the {@link AudioPolicy} was built with
         * {@link AudioPolicy.Builder#setIsAudioFocusPolicy(boolean)} set to {@code true}.
         * @param afi information about the focus request being abandoned and the original
         *     requester.
         */
        public void onAudioFocusAbandon(AudioFocusInfo afi) {}
    }

这里重点写了onAudioFocusRequest和onAudioFocusAbandon，这两个主要处理音源焦点用的。调用setIsAudioFocusPolicy(true)使自定义音源策略生效。最后mFocusHandler.setOwningPolicy(this, mAudioPolicy)，把AudioPolicy传给CarZonesAudioFocus，这样init函数就结束了。
总结
到这里，我们就分析完了Android10.0 CarAudioService中的init函数。
写的有点笼统，这里面有点绕，需要不断的去思考。