Media listener андроид студио

Creating Custom Listeners

The «listener» or «observer» pattern is the most common tegy for creating asynchronous callbacks within Android development. Listeners are used for any type of asynchronous event in order to implement the code to run when an event occurs. We see this pattern with any type of I/O as well as for view events on screen. For example, here’s a common usage of the listener pattern to attach a click event to a button:

This listener is built-in but we can also create our own listeners and attach callbacks to the events they fire from other areas in our code. This is useful in a variety of cases including:

• Firing events from list items upwards to an activity from within an adapter
• Firing events from a fragment upwards to an activity.
• Firing async events from an abstraction (i.e networking library) to a parent handler.

In short, a listener is useful anytime a child object wants to emit events upwards to notify a parent object and allow that object to respond.

Listeners are a powerful mechanism for properly separating concerns in your code. One of the essential principles around writing maintainable code is to reduce coupling and complexity using proper encapsulation. Listeners are about ensuring that code is properly organized into the correct places. In particular, this table below offers guidelines about where different types of code should be called:

Type	Called By	Description
Intents	Activity	Intents should be created and executed within activities.
Networking	Activity , Fragment	Networking code should invoked in an activity or fragment.
FragmentManager	Activity	Fragment changes should be invoked by an activity.
Persistence	Activity , Fragment	Writing to disk should be invoked by activity or fragment.

While there are exceptions, generally speaking this table helps to provide guidelines as to when you need a listener to propagate an event to the appropriate owner. For example, if you are inside an adapter and you want to launch a new activity, this is best done by firing an event using a custom listener triggering the parent activity.

There are four steps to using a custom listener to manage callbacks in your code:

Define an interface as an event contract with methods that define events and arguments which are relevant event data.

Setup a listener member variable and setter in the child object which can be assigned an implementation of the interface.

Owner passes in a listener which implements the interface and handles the events from the child object.

Trigger events on the defined listener when the object wants to communicate events to it’s owner

The sample code below will demonstrate this process step-by-step.

First, we define an interface in the child object. This object can be a plain java object, an Adapter , a Fragment , or any object created by a «parent» object such as an activity which will handle the events that are triggered.

This involves creating an interface class and defining the events that will be fired up to the parent. We need to design the events as well as the data passed when the event is triggered.

With the interface defined, we need to setup a listener variable to store a particular implementation of the callbacks which will be defined by the owner.

In the child object, we need to define an instance variable for an implementation of the listener:

as well as a «setter» which allows the listener callbacks to be defined in the parent object:

This allows the parent object to pass in an implementation of the listener after this child object is created similar to how the button accepts the click listener.

Now that we have created the setters, the parent object can construct and set callbacks for the child object:

Here we’ve created the child object and passed in the callback implementation for the listener. These events can now be fired by the child object to pass the event to the parent as appropriate.

Now the child object should trigger events to the listener whenever appropriate and pass along the data to the parent object through the event. These events can be triggered when a user action is taken or when an asynchronous task such as networking or persistence is completed. For example, we will trigger the onDataLoaded(SomeData data) event once this network request comes back on the child:

Notice that we fire the listener event listener.onDataLoaded(data) when the asynchronous network request has completed. Whichever callback has been passed by the parent (shown in the previous step) will be fired.

The «listener pattern» is a very powerful Java pattern that can be used to emit events to a single parent in order to communicate important information asynchronously. This can be used to move complex logic out of adapters, create useful abstractions for your code or communicate from a fragment to your activities.

There are several different ways to pass a listener callback into the child object:

1. Pass the callback through a method call
2. Pass the callback through the constructor
3. Pass the callback through a lifecycle event

The code earlier demonstrated passing the callback through a method call like this:

which is defined in the child object as follows:

If the callback is critical to the object’s function, we can pass the callback directly into the constructor of the child object as an argument:

and then when we can create the child object from the parent we can do the following:

The third case is most common with fragments or other android components that need to communicate upward to a parent. In this case, we can leverage existing Android lifecycle events to get access to the listener. In the case below, a fragment being attached to an activity:

For the full details on this approach, check out the fragments guide.

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MediaPlayer overview

The Android multimedia framework includes support for playing variety of common media types, so that you can easily integrate audio, video and images into your applications. You can play audio or video from media files stored in your application’s resources (raw resources), from standalone files in the filesystem, or from a data stream arriving over a network connection, all using MediaPlayer APIs.

This document shows you how to use MediaPlayer to write a media-playing application that interacts with the user and the system in order to obtain good performance and a pleasant user experience. Alternatively, you might want to use ExoPlayer, which is a customizable open source library that supports high-performance features not available in MediaPlayer

Note: You can play back the audio data only to the standard output device. Currently, that is the mobile device speaker or a Bluetooth headset. You cannot play sound files in the conversation audio during a call.

The basics

The following classes are used to play sound and video in the Android framework:

MediaPlayer This class is the primary API for playing sound and video. AudioManager This class manages audio sources and audio output on a device.

Manifest declarations

Before starting development on your application using MediaPlayer, make sure your manifest has the appropriate declarations to allow use of related features.

• Internet Permission — If you are using MediaPlayer to stream network-based content, your application must request network access.
• Wake Lock Permission — If your player application needs to keep the screen from dimming or the processor from sleeping, or uses the MediaPlayer.setScreenOnWhilePlaying() or MediaPlayer.setWakeMode() methods, you must request this permission.

Using MediaPlayer

One of the most important components of the media framework is the MediaPlayer class. An object of this class can fetch, decode, and play both audio and video with minimal setup. It supports several different media sources such as:

• Local resources
• Internal URIs, such as one you might obtain from a Content Resolver
• External URLs (streaming)

For a list of media formats that Android supports, see the Supported Media Formats page.

Here is an example of how to play audio that’s available as a local raw resource (saved in your application’s res/raw/ directory):

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In this case, a «raw» resource is a file that the system does not try to parse in any particular way. However, the content of this resource should not be raw audio. It should be a properly encoded and formatted media file in one of the supported formats.

And here is how you might play from a URI available locally in the system (that you obtained through a Content Resolver, for instance):

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Playing from a remote URL via HTTP streaming looks like this:

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Note: If you’re passing a URL to stream an online media file, the file must be capable of progressive download.

Caution: You must either catch or pass IllegalArgumentException and IOException when using setDataSource() , because the file you are referencing might not exist.

Asynchronous preparation

Using MediaPlayer can be straightforward in principle. However, it’s important to keep in mind that a few more things are necessary to integrate it correctly with a typical Android application. For example, the call to prepare() can take a long time to execute, because it might involve fetching and decoding media data. So, as is the case with any method that may take long to execute, you should never call it from your application’s UI thread. Doing that causes the UI to hang until the method returns, which is a very bad user experience and can cause an ANR (Application Not Responding) error. Even if you expect your resource to load quickly, remember that anything that takes more than a tenth of a second to respond in the UI causes a noticeable pause and gives the user the impression that your application is slow.

To avoid hanging your UI thread, spawn another thread to prepare the MediaPlayer and notify the main thread when done. However, while you could write the threading logic yourself, this pattern is so common when using MediaPlayer that the framework supplies a convenient way to accomplish this task by using the prepareAsync() method. This method starts preparing the media in the background and returns immediately. When the media is done preparing, the onPrepared() method of the MediaPlayer.OnPreparedListener , configured through setOnPreparedListener() is called.

Managing state

Another aspect of a MediaPlayer that you should keep in mind is that it’s state-based. That is, the MediaPlayer has an internal state that you must always be aware of when writing your code, because certain operations are only valid when the player is in specific states. If you perform an operation while in the wrong state, the system may throw an exception or cause other undesirable behaviors.

The documentation in the MediaPlayer class shows a complete state diagram, that clarifies which methods move the MediaPlayer from one state to another. For example, when you create a new MediaPlayer , it is in the Idle state. At that point, you should initialize it by calling setDataSource() , bringing it to the Initialized state. After that, you have to prepare it using either the prepare() or prepareAsync() method. When the MediaPlayer is done preparing, it enters the Prepared state, which means you can call start() to make it play the media. At that point, as the diagram illustrates, you can move between the Started, Paused and PlaybackCompleted states by calling such methods as start() , pause() , and seekTo() , amongst others. When you call stop() , however, notice that you cannot call start() again until you prepare the MediaPlayer again.

Always keep the state diagram in mind when writing code that interacts with a MediaPlayer object, because calling its methods from the wrong state is a common cause of bugs.

Releasing the MediaPlayer

A MediaPlayer can consume valuable system resources. Therefore, you should always take extra precautions to make sure you are not hanging on to a MediaPlayer instance longer than necessary. When you are done with it, you should always call release() to make sure any system resources allocated to it are properly released. For example, if you are using a MediaPlayer and your activity receives a call to onStop() , you must release the MediaPlayer , because it makes little sense to hold on to it while your activity is not interacting with the user (unless you are playing media in the background, which is discussed in the next section). When your activity is resumed or restarted, of course, you need to create a new MediaPlayer and prepare it again before resuming playback.

Here’s how you should release and then nullify your MediaPlayer :

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As an example, consider the problems that could happen if you forgot to release the MediaPlayer when your activity is stopped, but create a new one when the activity starts again. As you may know, when the user changes the screen orientation (or changes the device configuration in another way), the system handles that by restarting the activity (by default), so you might quickly consume all of the system resources as the user rotates the device back and forth between portrait and landscape, because at each orientation change, you create a new MediaPlayer that you never release. (For more information about runtime restarts, see Handling Runtime Changes.)

You may be wondering what happens if you want to continue playing «background media» even when the user leaves your activity, much in the same way that the built-in Music application behaves. In this case, what you need is a MediaPlayer controlled by a Service, as discussed in the next section

Using MediaPlayer in a service

If you want your media to play in the background even when your application is not onscreen—that is, you want it to continue playing while the user is interacting with other applications—then you must start a Service and control the MediaPlayer instance from there. You need to embed the MediaPlayer in a MediaBrowserServiceCompat service and have it interact with a MediaBrowserCompat in another activity.

You should be careful about this client/server setup. There are expectations about how a player running in a background service interacts with the rest of the system. If your application does not fulfill those expectations, the user may have a bad experience. Read Building an Audio App for the full details.

This section describes special instructions for managing a MediaPlayer when it is implemented inside a service.

Running asynchronously

First of all, like an Activity , all work in a Service is done in a single thread by default—in fact, if you’re running an activity and a service from the same application, they use the same thread (the «main thread») by default. Therefore, services need to process incoming intents quickly and never perform lengthy computations when responding to them. If any heavy work or blocking calls are expected, you must do those tasks asynchronously: either from another thread you implement yourself, or using the framework’s many facilities for asynchronous processing.

For instance, when using a MediaPlayer from your main thread, you should call prepareAsync() rather than prepare() , and implement a MediaPlayer.OnPreparedListener in order to be notified when the preparation is complete and you can start playing. For example:

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Handling asynchronous errors

On synchronous operations, errors would normally be signaled with an exception or an error code, but whenever you use asynchronous resources, you should make sure your application is notified of errors appropriately. In the case of a MediaPlayer , you can accomplish this by implementing a MediaPlayer.OnErrorListener and setting it in your MediaPlayer instance:

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It’s important to remember that when an error occurs, the MediaPlayer moves to the Error state (see the documentation for the MediaPlayer class for the full state diagram) and you must reset it before you can use it again.

Using wake locks

When designing applications that play media in the background, the device may go to sleep while your service is running. Because the Android system tries to conserve battery while the device is sleeping, the system tries to shut off any of the phone’s features that are not necessary, including the CPU and the WiFi hardware. However, if your service is playing or streaming music, you want to prevent the system from interfering with your playback.

In order to ensure that your service continues to run under those conditions, you have to use «wake locks.» A wake lock is a way to signal to the system that your application is using some feature that should stay available even if the phone is idle.

Notice: You should always use wake locks sparingly and hold them only for as long as truly necessary, because they significantly reduce the battery life of the device.

To ensure that the CPU continues running while your MediaPlayer is playing, call the setWakeMode() method when initializing your MediaPlayer . Once you do, the MediaPlayer holds the specified lock while playing and releases the lock when paused or stopped:

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However, the wake lock acquired in this example guarantees only that the CPU remains awake. If you are streaming media over the network and you are using Wi-Fi, you probably want to hold a WifiLock as well, which you must acquire and release manually. So, when you start preparing the MediaPlayer with the remote URL, you should create and acquire the Wi-Fi lock. For example:

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When you pause or stop your media, or when you no longer need the network, you should release the lock:

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Performing cleanup

As mentioned earlier, a MediaPlayer object can consume a significant amount of system resources, so you should keep it only for as long as you need and call release() when you are done with it. It’s important to call this cleanup method explicitly rather than rely on system garbage collection because it might take some time before the garbage collector reclaims the MediaPlayer , as it’s only sensitive to memory needs and not to shortage of other media-related resources. So, in the case when you’re using a service, you should always override the onDestroy() method to make sure you are releasing the MediaPlayer :

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You should always look for other opportunities to release your MediaPlayer as well, apart from releasing it when being shut down. For example, if you expect not to be able to play media for an extended period of time (after losing audio focus, for example), you should definitely release your existing MediaPlayer and create it again later. On the other hand, if you only expect to stop playback for a very short time, you should probably hold on to your MediaPlayer to avoid the overhead of creating and preparing it again.

Digital Rights Management (DRM)

Starting with Android 8.0 (API level 26), MediaPlayer includes APIs that support the playback of DRM-protected material. They are similar to the low-level API provided by MediaDrm , but they operate at a higher level and do not expose the underlying extractor, drm, and crypto objects.

Although the MediaPlayer DRM API does not provide the full functionality of MediaDrm , it supports the most common use cases. The current implementation can handle the following content types:

• Widevine-protected local media files
• Widevine-protected remote/streaming media files

The following code snippet demonstrates how to use the new DRM MediaPlayer methods in a simple synchronous implementation.

To manage DRM-controlled media, you need to include the new methods alongside the usual flow of MediaPlayer calls, as shown below:

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Start by initializing the MediaPlayer object and setting its source using setDataSource() , as usual. Then, to use DRM, perform these steps:

1. If you want your app to perform custom configuration, define an OnDrmConfigHelper interface, and attach it to the player using setOnDrmConfigHelper() .
2. Call prepare() .
3. Call getDrmInfo() . If the source has DRM content, the method returns a non-null MediaPlayer.DrmInfo value.

1. Examine the map of available UUIDs and choose one.
2. Prepare the DRM configuration for the current source by calling prepareDrm() .
   • If you created and registered an OnDrmConfigHelper callback, it is called while prepareDrm() is executing. This lets you perform custom configuration of the DRM properties before opening the DRM session. The callback is called synchronously in the thread that called prepareDrm() . To access the DRM properties, call getDrmPropertyString() and setDrmPropertyString() . Avoid performing lengthy operations.
   • If the device has not yet been provisioned, prepareDrm() also accesses the provisioning server to provision the device. This can take a variable amount of time, depending on the network connectivity.
3. To get an opaque key request byte array to send to a license server, call )»>getKeyRequest() .
4. To inform the DRM engine about the key response received from the license server, call provideKeyResponse() . The result depends on the type of key request:
   • If the response is for an offline key request, the result is a key-set identifier. You can use this key-set identifier with restoreKeys() to restore the keys to a new session.
   • If the response is for a streaming or release request, the result is null.

Running prepareDrm() asynchronously

By default, prepareDrm() runs synchronously, blocking until preparation is finished. However, the very first DRM preparation on a new device may also require provisioning, which is handled internally by prepareDrm() , and may take some time to finish due to the network operation involved. You can avoid blocking on prepareDrm() by defining and setting a MediaPlayer.OnDrmPreparedListener .

When you set an OnDrmPreparedListener , prepareDrm() performs the provisioning (if needed) and preparation in the background. When provisioning and preparation have finished, the listener is called. You should not make any assumption about the calling sequence or the thread in which the listener runs (unless the listener is registered with a handler thread). The listener can be called before or after prepareDrm() returns.

Setting up DRM asynchronously

You can initialize the DRM asynchronously by creating and registering the MediaPlayer.OnDrmInfoListener for DRM preparation and the MediaPlayer.OnDrmPreparedListener to start the player. They work in conjunction with prepareAsync() , as shown below:

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Handling encrypted media

Starting with Android 8.0 (API level 26) MediaPlayer can also decrypt Common Encryption Scheme (CENC) and HLS sample-level encrypted media (METHOD=SAMPLE-AES) for the elementary stream types H.264, and AAC. Full-segment encrypted media (METHOD=AES-128) was previously supported.

Retrieving media from a ContentResolver

Another feature that may be useful in a media player application is the ability to retrieve music that the user has on the device. You can do that by querying the ContentResolver for external media:

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To use this with the MediaPlayer , you can do this:

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