AVSession, short for audio and video session, is also known as media session.
- Application developers can use the APIs provided by the **AVSession** module to connect their audio and video applications to the system's Media Controller.
- System developers can use the APIs provided by the **AVSession** module to display media information of system audio and video applications and carry out unified playback control.
You can implement the following features through the **AVSession** module:
1. Unified playback control entry
If there are multiple audio and video applications on the device, users need to switch to and access different applications to control media playback. With AVSession, a unified playback control entry of the system (such as Media Controller) is used for playback control of these audio and video applications. No more switching is required.
2. Better background application management
When an application running in the background automatically starts audio playback, it is difficult for users to locate the application. With AVSession, users can quickly find the application that plays the audio clip in Media Controller.
## Basic Concepts
- AVSession
A channel used for information exchange between applications and Media Controller. For AVSession, one end is the media application under control, and the other end is Media Controller. Through AVSession, an application can transfer the media playback information to Media Controller and receive control commands from Media Controller.
- AVSessionController
Object that controls media sessions and thereby controls the playback behavior of applications. Through AVSessionController, Media Controller can control the playback behavior of applications, obtain playback information, and send control commands. It can also monitor the playback state of applications to ensure synchronization of the media session information.
- Media Controller
Holder of AVSessionController. Through AVSessionController, Media Controller sends commands to control media playback of applications.
## Implementation Principle
The **AVSession** module provides two classes: **AVSession** and **AVSessionController**.
- Interaction between the application and Media Controller: First, an audio application creates an **AVSession** object and sets session information, including media metadata, launcher ability, and playback state information. Then, Media Controller creates an **AVSessionController** object to obtain session-related information and send the 'play' command to the audio application. Finally, the audio application responds to the command and updates the playback state.
- Distributed projection: When a connected device creates a local session, Media Controller or the audio application can select another device to be projected based on the device list, synchronize the local session to the remote device, and generate a controllable remote session. The remote session is controlled by sending control commands to the remote device's application through its AVSessionController.
## Constraints
- The playback information displayed in Media Controller is the media information proactively written by the media application to AVSession.
- Media Controller controls the playback of a media application based on the responses of the media application to control commands.
- AVSession can transmit media playback information and control commands. It does not display information or execute control commands.
- Do not develop Media Controller for common applications. For common audio and video applications running on OpenHarmony, the default control end is Media Controller, which is a system application. You do not need to carry out additional development for Media Controller.
- If you want to develop your own system running OpenHarmony, you can develop your own Media Controller.
- For better background management of audio and video applications, the **AVSession** module enforces background control for third-party applications. Only third-party applications that have accessed AVSession can play audio in the background. Otherwise, the system forcibly pauses the playback when a third-party application switches to the background.
The session service is abnormal, and the application cannot obtain a response from the session service. For example, the session service is not running or the communication with the session service fails.
**Possible Causes**
The session service is killed during session restart.
**Solution**
1. The system retries the operation automatically. If the error persists for 3 seconds or more, stop the operation on the session or controller.
2. Destroy the current session or session controller and re-create it. If the re-creation fails, stop the operation on the session.
## 6600102 Session Does Not Exist
**Error Message**
The session does not exist
**Description**
Parameters are set for or commands are sent to the session that does not exist.
**Possible Causes**
The session has been destroyed, and no session record exists on the server.
**Solution**
1. If the error occurs on the application, re-create the session. If the error occurs on Media Controller, stop sending query or control commands to the session.
2. If the error occurs on the session service, query the current session record and pass the correct session ID when creating the controller.
## 6600103 Session Controller Does Not Exist
**Error Message**
The session controller does not exist
**Description**
A control command or an event is sent to the controller that does not exist.
**Possible Causes**
The controller has been destroyed.
**Solution**
Query the session record and create the corresponding controller.
## 6600104 Remote Session Connection Failure
**Error Message**
The remote session connection failed
**Description**
The communication between the local session and the remote session fails.
**Possible Causes**
The communication between devices is interrupted.
**Solution**
Stop sending control commands to the session. Subscribe to output device changes, and resume the sending when the output device is changed.
## 6600105 Invalid Session Command
**Error Message**
Invalid session command
**Description**
The control command or event sent to the session is not supported.
**Possible Causes**
The session does not support this command.
**Solution**
Stop sending the command or event. Query the commands supported by the session, and send a command supported.
## 6600106 Session Not Activated
**Error Message**
The session not active
**Description**
A control command or event is sent to the session that is not activated.
**Possible Causes**
The session is in the inactive state.
**Solution**
Stop sending the command or event. Subscribe to the session activation status, and resume the sending when the session is activated.
## 6600107 Too Many Commands or Events
**Error Message**
Command or event overload
**Description**
The session client sends too many messages or commands to the server in a period of time, causing the server to be overloaded.
**Possible Causes**
The server is overloaded with messages or events.
**Solution**
Control the frequency of sending commands or events.
