重新想象 Windows 8 Store Apps (65) - 后台任务: 音乐的后台播放和控制

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[align=center]重新想象 Windows 8 Store Apps (65) - 后台任务: 音乐的后台播放和控制[/align]

作者：webabcd

介绍
重新想象 Windows 8 Store Apps 之 后台任务

音乐的后台播放和控制

示例
用于保存每首音乐的相关信息的对象
BackgroundTask/SongModel.cs

/*
* 用于保存每首音乐的相关信息
*/

using System;
using System.Threading.Tasks;
using Windows.Storage;

namespace XamlDemo.BackgroundTask
{
public class SongModel
{
/// <summary>
/// 音乐文件
/// </summary>
public StorageFile File;

/// <summary>
/// 艺术家
/// </summary>
public string Artist;

/// <summary>
/// 音乐名称
/// </summary>
public string Title;

public SongModel(StorageFile file)
{
File = file;
}

/// <summary>
/// 加载音乐的相关属性
/// </summary>
public async Task LoadMusicPropertiesAsync()
{
var properties = await this.File.Properties.GetMusicPropertiesAsync();

Artist = properties.Artist;
Title = properties.Title;
}
}
}

演示如何通过 MediaControl 实现音乐的后台播放和控制
BackgroundTask/MediaControlDemo.xaml

<Page
x:Class="XamlDemo.BackgroundTask.MediaControlDemo"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="using:XamlDemo.BackgroundTask"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d">

<Grid Background="Transparent">
<StackPanel Margin="120 0 0 0">

<TextBlock Name="lblMsg" FontSize="14.667" />

<Button Name="btnSelectFiles" Content="Select Files" Click="btnSelectFiles_Click" Margin="0 10 0 0" />
<Button Name="btnPlay" Content="Play" Click="btnPlay_Click" Margin="0 10 0 0" />

<!--
为了使音乐可以后台播放，需要将 MediaElement 的 AudioCategory 属性设置为 BackgroundCapableMedia
-->
<MediaElement Name="mediaElement" AudioCategory="BackgroundCapableMedia" AutoPlay="False"
MediaOpened="mediaElement_MediaOpened" MediaEnded="mediaElement_MediaEnded" CurrentStateChanged="mediaElement_CurrentStateChanged" />

</StackPanel>
</Grid>

</Page>

BackgroundTask/MediaControlDemo.cs

/*
* 演示如何通过 MediaControl 实现音乐的后台播放和控制
*
* 注：
* 1、需要在 Package.appxmanifest 中增加后台任务声明，并勾选“音频”
* 2、需要将 MediaElement 的 AudioCategory 属性设置为 BackgroundCapableMedia
*
* 另：
* 按下平板上的音量键或者多媒体键盘上的相关按键，会弹出后台音频的控制框，通过 MediaControl 来监听用户在此音频控制框上的操作
*/

using System;
using System.Collections.Generic;
using System.Threading.Tasks;
using Windows.Media;
using Windows.Storage;
using Windows.Storage.Pickers;
using Windows.Storage.Streams;
using Windows.UI.Core;
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Media;

namespace XamlDemo.BackgroundTask
{
public sealed partial class MediaControlDemo : Page
{
private List<SongModel> _playList = new List<SongModel>(); // 后台需要播放的音乐列表
private int _currentSongIndex = 0; // 当前播放的音乐

private bool _previousTrackPressedRegistered = false; // MediaControl 是否注册了 PreviousTrackPressed 事件
private bool _nextTrackPressedRegistered = false; // MediaControl 是否注册了 NextTrackPressed 事件

public MediaControlDemo()
{
this.InitializeComponent();

MediaControl.PlayPauseTogglePressed += MediaControl_PlayPauseTogglePressed; // 按下了 play/pause 键
MediaControl.PlayPressed += MediaControl_PlayPressed; // 按下了 play 键
MediaControl.PausePressed += MediaControl_PausePressed; // 按下了 pause 键
MediaControl.StopPressed += MediaControl_StopPressed; // 按下了 stop 键

MediaControl.PreviousTrackPressed += MediaControl_PreviousTrackPressed; // 按下了“上一首”键
MediaControl.NextTrackPressed += MediaControl_NextTrackPressed; // 按下了“下一首”键
MediaControl.RewindPressed += MediaControl_RewindPressed; // 按下了“快退”键
MediaControl.FastForwardPressed += MediaControl_FastForwardPressed; // 按下了“快进”键

MediaControl.SoundLevelChanged += MediaControl_SoundLevelChanged; // 音量级别发生了改变

// MediaControl.RecordPressed += MediaControl_RecordPressed;
// MediaControl.ChannelDownPressed += MediaControl_ChannelDownPressed;
// MediaControl.ChannelUpPressed += MediaControl_ChannelUpPressed;

// MediaControl.ArtistName; // 当前播放的音乐的艺术家
// MediaControl.TrackName; // 当前播放的音乐的名称
// MediaControl.AlbumArt; // 当前播放的音乐的专辑封面的路径
// MediaControl.IsPlaying; // 当前音乐是否正在播放
// MediaControl.SoundLevel; // 当前播放的音乐的音量级别（Muted, Low, Full）

_previousTrackPressedRegistered = true;
_nextTrackPressedRegistered = true;
}

// 按下了音频控制框上的 play/pause 键
async void MediaControl_PlayPauseTogglePressed(object sender, object e)
{
if (MediaControl.IsPlaying)
{
await Dispatcher.RunAsync(CoreDispatcherPriority.Normal, () =>
{
mediaElement.Pause();
});
await OutputMessage("Play/Pause Pressed - Pause");
}
else
{
await Dispatcher.RunAsync(CoreDispatcherPriority.Normal, () =>
{
mediaElement.Play();
});
await OutputMessage("Play/Pause Pressed - Play");
}
}

// 按下了音频控制框上的 play 键
async void MediaControl_PlayPressed(object sender, object e)
{
await Dispatcher.RunAsync(CoreDispatcherPriority.Normal, () =>
{
mediaElement.Play();
});
await OutputMessage("Play Pressed");
}

// 按下了音频控制框上的 pause 键
async void MediaControl_PausePressed(object sender, object e)
{
await Dispatcher.RunAsync(Windows.UI.Core.CoreDispatcherPriority.Normal, () =>
{
mediaElement.Pause();
});
await OutputMessage("Pause Pressed");
}

// 按下了音频控制框上的 stop 键
async void MediaControl_StopPressed(object sender, object e)
{
await Dispatcher.RunAsync(Windows.UI.Core.CoreDispatcherPriority.Normal, () =>
{
mediaElement.Stop();
});
await OutputMessage("Stop Pressed");
}

// 按下了音频控制框上的“上一首”键
async void MediaControl_PreviousTrackPressed(object sender, object e)
{
await OutputMessage("Previous Track Pressed");

if (_currentSongIndex > 0)
{
if (_currentSongIndex == (_playList.Count - 1))
{
if (!_nextTrackPressedRegistered)
{
MediaControl.NextTrackPressed += MediaControl_NextTrackPressed;
_nextTrackPressedRegistered = true;
}
}

_currentSongIndex--;

if (_currentSongIndex == 0)
{
MediaControl.PreviousTrackPressed -= MediaControl_PreviousTrackPressed;
_nextTrackPressedRegistered = false;
}

await SetCurrentPlayingAsync(_currentSongIndex);
}

}

// 按下了音频控制框上的“下一首”键
async void MediaControl_NextTrackPressed(object sender, object e)
{
await OutputMessage("Next Track Pressed");

if (_currentSongIndex < (_playList.Count - 1))
{
_currentSongIndex++;
await SetCurrentPlayingAsync(_currentSongIndex);

if (_currentSongIndex > 0)
{
if (!_previousTrackPressedRegistered)
{
MediaControl.PreviousTrackPressed += MediaControl_PreviousTrackPressed;
_previousTrackPressedRegistered = true;
}

}

if (_currentSongIndex == (_playList.Count - 1))
{
if (_nextTrackPressedRegistered)
{
MediaControl.NextTrackPressed -= MediaControl_NextTrackPressed;
_nextTrackPressedRegistered = false;
}
}
}
}

// 按下了音频控制框上的“快退”键，即长按“上一首”键
async void MediaControl_RewindPressed(object sender, object e)
{
await OutputMessage("Rewind Pressed");
}

// 按下了音频控制框上的“快进”键，即长按“下一首”键
async void MediaControl_FastForwardPressed(object sender, object e)
{
await OutputMessage("Fast Forward Pressed");
}

// 音量级别发生变化时（Muted, Low, Full）
async void MediaControl_SoundLevelChanged(object sender, object e)
{
await OutputMessage("Sound Level Changed");
}

// 创建一个需要在后台播放的音乐列表
private async void btnSelectFiles_Click(object sender, RoutedEventArgs e)
{
FileOpenPicker openPicker = new FileOpenPicker();
openPicker.ViewMode = PickerViewMode.List;
openPicker.SuggestedStartLocation = PickerLocationId.MusicLibrary;
openPicker.FileTypeFilter.Add(".mp3");
openPicker.FileTypeFilter.Add(".wma");
IReadOnlyList<StorageFile> files = await openPicker.PickMultipleFilesAsync();
if (files.Count > 0)
{
await CreatePlaylist(files);
await SetCurrentPlayingAsync(_currentSongIndex);
}
}

// 开始播放
private async void btnPlay_Click(object sender, RoutedEventArgs e)
{
if (MediaControl.IsPlaying)
{
await Dispatcher.RunAsync(CoreDispatcherPriority.Normal, () =>
{
mediaElement.Pause();
});
await OutputMessage("Play/Pause Pressed - Pause");
}
else
{
await Dispatcher.RunAsync(CoreDispatcherPriority.Normal, () =>
{
mediaElement.Play();
});
await OutputMessage("Play/Pause Pressed - Play");
}
}

// MediaElement 打开文件后，如果需要播放则立刻播放
private void mediaElement_MediaOpened(object sender, RoutedEventArgs e)
{
if (MediaControl.IsPlaying)
{
mediaElement.Play();
}
}

// 当前音乐播放完后，转到下一首
private async void mediaElement_MediaEnded(object sender, RoutedEventArgs e)
{
if (_currentSongIndex < _playList.Count - 1)
{
_currentSongIndex++;

await SetCurrentPlayingAsync(_currentSongIndex);

if (MediaControl.IsPlaying)
{
mediaElement.Play();
}
}
}

// 根据前台的操作，设置 MediaControl 的 IsPlaying 属性
private void mediaElement_CurrentStateChanged(object sender, RoutedEventArgs e)
{
if (mediaElement.CurrentState == MediaElementState.Playing)
{
MediaControl.IsPlaying = true;
btnPlay.Content = "Pause";
}
else
{
MediaControl.IsPlaying = false;
btnPlay.Content = "Play";
}
}

// 创建音乐列表
private async Task CreatePlaylist(IReadOnlyList<StorageFile> files)
{
if (_previousTrackPressedRegistered)
{
MediaControl.PreviousTrackPressed -= MediaControl_PreviousTrackPressed;
_previousTrackPressedRegistered = false;
}
if (_nextTrackPressedRegistered)
{
MediaControl.NextTrackPressed -= MediaControl_NextTrackPressed;
_nextTrackPressedRegistered = false;
}

_playList.Clear();
_currentSongIndex = 0;

if (files.Count > 0)
{
if (files.Count > 1)
{
MediaControl.NextTrackPressed += MediaControl_NextTrackPressed;
_nextTrackPressedRegistered = true;
}

foreach (StorageFile file in files)
{
SongModel song = new SongModel(file);
await song.LoadMusicPropertiesAsync();
_playList.Add(song);
}
}
}

// 指定当前需要播放的音乐
private async Task SetCurrentPlayingAsync(int playListIndex)
{
string errorMessage = null;
IRandomAccessStream stream = null;

try
{
stream = await _playList[playListIndex].File.OpenAsync(Windows.Storage.FileAccessMode.Read);
await Dispatcher.RunAsync(CoreDispatcherPriority.Normal, () =>
{
mediaElement.SetSource(stream, _playList[playListIndex].File.ContentType);
});
}
catch (Exception e)
{
errorMessage = e.Message;
}

if (errorMessage != null)
{
await OutputMessage(errorMessage);
}

MediaControl.ArtistName = _playList[playListIndex].Artist;
MediaControl.TrackName = _playList[playListIndex].Title;
}

// 输出信息
private async Task OutputMessage(string message)
{
await Dispatcher.RunAsync(CoreDispatcherPriority.Normal, () =>
{
lblMsg.Text = message + ": " + DateTime.Now.ToString("yyyy-MM-dd hh:mm:ss");
});
}
}
}

OK
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