Android 10 音频焦点仲裁策略分析_android audio 仲裁

摘要：Android 9 的音频焦点仲裁策略基本上可以用一句话来概括：后来居上，电话最大。这种策略显然是不能满足音频焦点仲裁的复杂需求的，所以Google在Android 10 中做了大幅度的改进，其中最主要的就是引入了音频焦点判断矩阵，通过矩阵来仲裁后来者是否可以抢占当前焦点。

由于Android9的音频焦点策略基本不能满足项目需求，所以一般会引入外部焦点仲裁策略，不知道如何引入的可以参考这篇文章：自定义音频焦点策略的实现。既然要引入，何不引入Android 10 的音频焦点策略呢？

音频焦点仲裁策略分析

1.关键类及变量列表

2.申请焦点及仲裁过程详解

申请焦点的入口函数是：evaluateFocusRequest，接受一个AudioFocusInfo类型参数

新建两个boolean类型参数用来描述焦点申请者属性：
permanent：描述申请者是否申请永久获取焦点（即申请类型是否为AUDIOFOCUS_GAIN）
allowDucking：描述申请者是否申请暂时获取焦点且允许混音（即申请类型是否为AUDIOFOCUS_GAIN_TRANSIENT_MAY_DUCK）

        // Is this a request for premanant focus?
        // AUDIOFOCUS_GAIN_TRANSIENT_EXCLUSIVE -- Means Notifications should be denied
        // AUDIOFOCUS_GAIN_TRANSIENT -- Means current focus holders should get transient loss
        // AUDIOFOCUS_GAIN_TRANSIENT_MAY_DUCK -- Means other can duck (no loss message from us)
        // NOTE:  We expect that in practice it will be permanent for all media requests and
        //        transient for everything else, but that isn't currently an enforced requirement.
        final boolean permanent =
                (afi.getGainRequest() == AudioManager.AUDIOFOCUS_GAIN);
        //final boolean allowDucking =
        //        (afi.getGainRequest() == AudioManager.AUDIOFOCUS_GAIN_TRANSIENT_MAY_DUCK);
        // only interaction matrix can decide if ducking allowed
        final boolean allowDucking = 
        		(afi.getGainRequest() == AudioManager.AUDIOFOCUS_GAIN_TRANSIENT_MAY_DUCK);
1
2
3
4
5
6
7
8
9
10
11
12
13

将usage转化为Context：（什么是usage和Context？参考此处）

        // Convert from audio attributes "usage" to HAL level "context"
        final int requestedContext = mCarAudioService.getContextForUsage(
                afi.getAttributes().getUsage());
1
2
3

防止已经拥有或者暂时失去焦点的引用再次申请焦点，新建两个FocusEntry：
replacedCurrentEntry：当已经拥有焦点的应用再次申请焦点时，对这个变量赋值，如果申请成功则删除replacedCurrentEntry，用新的请申请代替
replacedBlockedEntry：当暂时失去焦点的应用再次申请焦点时，对这个变量赋值，如果申请成功则删除replacedBlockedEntry，用新的请申请代替

        // If we happen to find entries that this new request should replace, we'll store them here.
        // This happens when a client makes a second AF request on the same listener.
        // After we've granted audio focus to our current request, we'll abandon these requests.
        FocusEntry replacedCurrentEntry = null;
        FocusEntry replacedBlockedEntry = null;
1
2
3
4
5

接下来会遍历mFocusHolders：

首先判断如果当前申请者的requestedContext是NOTIFICATION并且mFocusHolders已经存在一个暂时独占焦点的申请者（AUDIOFOCUS_GAIN_TRANSIENT_EXCLUSIVE），则直接返回申请失败

            // If this request is for Notifications and a current focus holder has specified
            // AUDIOFOCUS_GAIN_TRANSIENT_EXCLUSIVE, then reject the request.
            // This matches the hardwired behavior in the default audio policy engine which apps
            // might expect (The interaction matrix doesn't have any provision for dealing with
            // override flags like this).
            if ((requestedContext == ContextNumber.NOTIFICATION) &&
                    (entry.mAfi.getGainRequest() ==
                            AudioManager.AUDIOFOCUS_GAIN_TRANSIENT_EXCLUSIVE)) {
   
                return AudioManager.AUDIOFOCUS_REQUEST_FAILED;
            }
1
2
3
4
5
6
7
8
9
10
11

接着判断是否是同一个应用申请的焦点，如果是同一个应用且申请的是同一个Context类型的焦点，则将目前mFocusHolders中的此应用的entry赋值给replacedCurrentEntry，之后如果焦点申请成功则会将这个entry删除并将新的加入，如果申请的是不同的Context类型，则直接返回申请失败

            // We don't allow sharing listeners (client IDs) between two concurrent requests
            // (because the app would have no way to know to which request a later event applied)
            if (afi.getClientId().equals(entry.mAfi.getClientId())) {
   
                if (entry.mAudioContext == requestedContext) {
   
                    // This is a request from a current focus holder.
                    // Abandon the previous request (without sending a LOSS notification to it),
                    // and don't check the interaction matrix for it.
                    Slog.i(TAG, "Replacing accepted request from same client");
                    replacedCurrentEntry = entry;
                    continue;
                } else {
   
                    // Trivially reject a request for a different USAGE
                    Slog.e(TAG, "Client " + entry.getClientId() + " has already requested focus "
                            + "for " + entry.mAfi.getAttributes().usageToString() + " - cannot "
                            + "request focus for " + afi.<
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17