AVsession Audio session

Audio Session

Audio Session Services   c语言接口

AVAudioSession Class Reference   object -c 接口

声音的协调者， 协调系统和应用之间的声音 处理。

根据不同的软件需求，设置不同的category

An  audio session  is the intermediary between your application and iOS for configuring audio behavior. Upon launch, your application automatically gets a  singleton  audio session. You configure it to express your application’s audio intentions.

Each audio session category specifies a particular pattern of “yes” and “no” for each of the following behaviors, as detailed in “Audio Session Categories”:

• Allows mixing:if yes, audio from other applications (such as the iPod) can continue playing when your application plays sound.

• Silenced by the Silent switch and by screen locking: if yes, your audio is silenced when the user moves the Silent switch to silent and when the screen locks. (On iPhone, this switch is called the Ring/Silent switch.)

• Supports audio input:if yes, application audio input, such as for recording, is allowed.

• Supports audio output: if yes, application audio output, such as for playback, is allowed.

An audio session comes with some default behavior. Specifically:

• Playback is enabled and recording is disabled.

• When the user moves the Silent switch (or Ring/Silent switch on iPhone) to the “silent” position, your audio is silenced.

• When the user presses the Sleep/Wake button to lock the screen, or when the Auto-Lock period expires, your audio is silenced.

• When your audio starts, other audio on the device—such as iPod audio that was already playing—is silenced.

AVAudioSessionCategorySoloAmbient  audio session category—the default category.

iOS has six audio session categories:

• Three for playback

• One for recording

• One that supports playback and recording—that need not occur simultaneously

• One for offline audio processing

AVAudioSessionCategoryAmbient ：

silenced by the Ring/Silent switch and when the screen locks.

audio from the iPod, Safari, and other built-in applications to play while your application is playing audio. 

AVAudioSessionCategorySoloAmbient    default category

 silenced when the user switches the Ring/Silent switch to the “silent” position and when the screen locks

silences other audio.

AVAudioSessionCategoryPlayback

Your audio plays even with the screen locked and with the Ring/Silent switch set to silent.

AVAudioSessionCategoryRecord

Use this category for recording audio. All playback on the phone—except for ringtones and Clock and Calendar alarms—is silenced.

AVAudioSessionCategoryPlayAndRecord

The input and output need not occur simultaneously, but can if needed. This is the category to use for audio chat applications.

AVAudioSessionCategoryAudioProcessing

Use this category when performing offline audio processing and not playing or recording.

 whether other audio is playing and then choose your category based on that. During launch—such as within the viewDidLoad method—check the value of thekAudioSessionProperty_OtherAudioIsPlaying property.

The following categories allow use of hardware-assisted audio encoding and decoding:

• AVAudioSessionCategorySoloAmbient or the equivalent kAudioSessionCategory_SoloAmbientSound

• AVAudioSessionCategoryPlayback or the equivalent kAudioSessionCategory_MediaPlayback

• AVAudioSessionCategoryPlayAndRecord or the equivalent kAudioSessionCategory_PlayAndRecord

• AVAudioSessionCategoryAudioProcessing or the equivalentkAudioSessionCategory_AudioProcessing

Fine-Tuning the Category

You can fine-tune an audio session category in a variety of ways. Depending on the category, you can:

• Allow other audio (such as from the iPod) to mix with yours when a category normally disallows it

• Change the audio output route from the receiver to the speaker

• Allow Bluetooth audio input

• Specify that other audio should reduce in volume (“duck”) when your audio plays

Audio Interruption Handling Techniques

There are two alternative approaches for responding to interruptions:

• ImplementObjective-C interruption delegate methodsprovided by the AV Foundation framework. In most cases, this approach is simpler and fits better with the rest of your application code.

• Write a C-based interruption callback function as declared in Audio Session Services. This option requires you to register your callback with the audio session by using an explicit audio session initialization call.

The AVAudioRecorder and AVAudioPlayback classes provide their own delegate methods for responding to audio interruptions, as listed here:

• audioPlayerBeginInterruption: Called when an audio session is interrupted during playback.

• audioPlayerEndInterruption: Called when a playback interruption ends.

• audioRecorderBeginInterruption: Called when the audio session is interrupted during a recording.

• audioRecorderEndInterruption: Called when a recording interruption ends.

Varieties of Audio Hardware Route Change

Handling audio hardware route changes

There are three parts to configuring your application to respond to route changes:

1. Implement methods to be invoked upon a route change.

2. Implement a property listener callback function that responds to route changes and uses the methods you implemented in step 1.

3. Register the callback function with the audio session.

Working with Movie Players

Configuring audio sessions when using a movie player

Desired behavior	Audio session configuration
Playing a movie silences all other audio	If your app does not itself play audio, do not configure an audio session. If your app does play audio, configure an audio session and set its mixability according to whether or not you want to mix with iPod and other audio. In either case, tell the movie player to use its own audio session: myMoviePlayer.useApplicationAudioSession = NO
Movie and application audio mix, but other audio, including iPod, is silenced	Configure an audio session using a nonmixable category. Take advantage of the movie player’s defaultuseApplicationAudioSession value of YES
All audio mixes	Configure an audio session using a mixable category configuration. Take advantage of the movie player’s defaultuseApplicationAudioSession value of YES

Audio Session Cookbook

Checking if Other Audio is Playing During App Launch

UInt32 otherAudioIsPlaying; // 1

UInt32 propertySize = sizeof (otherAudioIsPlaying);

AudioSessionGetProperty ( // 2

kAudioSessionProperty_OtherAudioIsPlaying,

&propertySize,

&otherAudioIsPlaying

);

if (otherAudioIsPlaying) { // 3

[[AVAudioSession sharedInstance]

setCategory: AVAudioSessionCategoryAmbient

error: nil];

} else {

[[AVAudioSession sharedInstance]

setCategory: AVAudioSessionCategorySoloAmbient

error: nil];

}

Modifying Playback Mixing Behavior

OSStatus propertySetError = 0;

UInt32 allowMixing = true;

propertySetError = AudioSessionSetProperty (

kAudioSessionProperty_OverrideCategoryMixWithOthers, // 1

sizeof (allowMixing), // 2

&allowMixing // 3

);

Redirecting Output Audio

Overriding the output audio route

UInt32 audioRouteOverride = kAudioSessionOverrideAudioRoute_Speaker; // 1

AudioSessionSetProperty (

kAudioSessionProperty_OverrideAudioRoute, // 2

sizeof (audioRouteOverride), // 3

&audioRouteOverride // 4

);

Supporting Bluetooth Audio Input

UInt32 allowBluetoothInput = 1;

AudioSessionSetProperty (

kAudioSessionProperty_OverrideCategoryEnableBluetoothInput,

sizeof (allowBluetoothInput),

&allowBluetoothInput

);

Responding to Audio Session Interruptions

Implementing AVAudioSessionDelegate interruption methods

- (void) beginInterruption {

if (playing) {

playing = NO;

interruptedWhilePlaying = YES;

[self updateUserInterface];

}

}

NSError *activationError = nil;

- (void) endInterruption {

if (interruptedWhilePlaying) {

[[AVAudioSession sharedInstance] setActive: YES error: &activationError];

[player play];

playing = YES;

interruptedWhilePlaying = NO;

[self updateUserInterface];

}

}

Determining Whether a Device Supports Recording

BOOL inputAvailable;

inputAvailable = [[AVAudioSession sharedInstance] inputIsAvailable];

Running Your App in the Simulator

#if TARGET_IPHONE_SIMULATOR

#warning *** Simulator mode: audio session code works only on a device

// Execute subset of code that works in the Simulator

#else

// Execute device-only code as well as the other code

#endif

Audio session category behavior

Category identifiers*	Silenced by the Ring/Silent switch and by screen locking	Allows audio from other applications	Allows audio input (recording) and output (playback)
AVAudioSessionCategoryAmbient kAudioSessionCategory_AmbientSound	Yes	Yes	Output only
AVAudioSessionCategorySoloAmbient kAudioSessionCategory_SoloAmbientSound	Yes	No	Output only
AVAudioSessionCategoryPlayback kAudioSessionCategory_MediaPlayback	No	No by default; yes by using override switch	Output only
AVAudioSessionCategoryRecord kAudioSessionCategory_RecordAudio	No (recording continues with the screen locked)	No	Input only
AVAudioSessionCategoryPlayAndRecord kAudioSessionCategory_PlayAndRecord	No	No by default; yes by using override switch	Input and output
AVAudioSessionCategoryAudioProcessing kAudioSessionCategory_AudioProcessing	–	No	No input and no output