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/*!

    \example phonon/qmusicplayer

    \title Music Player Example

    The Music Player Example shows how to use Phonon - the multimedia

    framework that comes with Qt - to create a simple music player.

    The player can play music files, and provides simple playback

    control, such as pausing, stopping, and resuming the music.

    \image musicplayer.png

    The player has a button group with the play, pause, and stop

    buttons familiar from most music players. The top-most slider

    controls the position in the media stream, and the bottom slider

    allows adjusting the sound volume. 

    The user can use a file dialog to add music files to a table,

    which displays meta information about the music - such as the

    title, album, and artist. Each row contains information about a

    single music file; to play it, the user selects that row and

    presses the play button. Also, when a row is selected, the files

    in the table are queued for playback.

    Phonon offers playback of sound using an available audio device,

    e.g., a sound card or an USB headset. For the implementation, we

    use two objects: a \l{Phonon::}{MediaObject}, which controls the

    playback, and an \l{Phonon::}{AudioOutput}, which can output the

    audio to a sound device. We will explain how they cooperate when

    we encounter them in the code. For a high-level introduction to

    Phonon, see its \l{Phonon Overview}{overview}.

    The API of Phonon is implemented through an intermediate

    technology on each supported platform: DirectShow, QuickTime, and

    GStreamer. The sound formats supported may therefore vary from

    system to system. We do not in this example try to determine which

    formats are supported, but let Phonon report an error if the user

    tries to play an unsupported sound file.

    Our player consists of one class, \c MainWindow, which both

    constructs the GUI and handles the playback. We will now go

    through the parts of its definition and implementation that

    concerns Phonon.

    \section1 MainWindow Class Definition

    Most of the API in \c MainWindow is private, as is often the case

    for classes that represent self-contained windows. We list Phonon

    objects and slots we connect to their signals; we take a closer

    look at them when we walk through the \c MainWindow

    implementation.

    \snippet examples/phonon/qmusicplayer/mainwindow.h 2

    We use the \l{Phonon::}{SeekSlider} to move the current playback

    position in the media stream, and the \l{Phonon::}{VolumeSlider}

    controls the sound volume. Both of these widgets come ready made

    with Phonon.  We use another \l{Phonon::}{MediaObject},

    metaInformationProvider, to get the meta information from the

    music files. More on this later.

    \snippet examples/phonon/qmusicplayer/mainwindow.h 1

    The \l{Phonon::}{MediaObject} informs us of the state of the playback and

    properties of the media it is playing back through a series of

    signals. We connect the signals we need to slots in \c MainWindow.

    The \c tableClicked() slot is connected to the table, so that we

    know when the user requests playback of a new music file, by

    clicking on the table.

    \section1 MainWindow Class Implementation

    The \c MainWindow class handles both the user interface and

    Phonon. We will now take a look at the code relevant for Phonon.

    The code required for setting up the GUI is explained elsewhere.

    We start with the constructor:

    \snippet examples/phonon/qmusicplayer/mainwindow.cpp 0

    We start by instantiating our media and audio output objects.

    As mentioned, the media object knows how to playback

    multimedia (in our case sound files) while the audio output

    can send it to a sound device.

    For the playback to work, the media and audio output objects need

    to get in contact with each other, so that the media object can

    send the sound to the audio output. Phonon is a graph based

    framework, i.e., its objects are nodes that can be connected by

    paths. Objects are connected using the \c createPath() function,

    which is part of the Phonon namespace.

    \snippet examples/phonon/qmusicplayer/mainwindow.cpp 1

    We also connect signals of the media object to slots in our \c

    MainWindow. We will examine them shortly.

    \snippet examples/phonon/qmusicplayer/mainwindow.cpp 2

    Finally, we call private helper functions to set up the GUI.

    The \c setupUi() function contains code for setting up the seek

    , and volume slider. We move on to \c setupUi():

    \snippet examples/phonon/qmusicplayer/mainwindow.cpp 3

    \dots

    \snippet examples/phonon/qmusicplayer/mainwindow.cpp 4

    After creating the widgets, they must be supplied with the

    \l{Phonon::}{MediaObject} and \l{Phonon::}{AudioOutput} objects

    they should control.  

    In the \c setupActions(), we connect the actions for the play,

    pause, and stop tool buttons, to slots of the media object.

    \snippet examples/phonon/qmusicplayer/mainwindow.cpp 5

    We move on to the slots of \c MainWindow, starting with \c

    addFiles(): 

    \snippet examples/phonon/qmusicplayer/mainwindow.cpp 6

    In the \c addFiles() slot, we add files selected by the user to

    the \c sources list. We then set the first source selected on the

    \c metaInformationProvider \l{Phonon::}{MediaObject}, which will

    send a state changed signal when the meta information is resolved;

    we have this signal connected to the \c metaStateChanged() slot.

    The media object informs us of state changes by sending the \c

    stateChanged() signal. The \c stateChanged() slot is connected

    to this signal.

    \snippet examples/phonon/qmusicplayer/mainwindow.cpp 9

    The \l{Phonon::MediaObject::}{errorString()} function gives a

    description of the error that is suitable for users of a Phonon

    application. The two values of the \l{Phonon::}{ErrorState} enum

    helps us determine whether it is possible to try to play the same

    file again.

    \snippet examples/phonon/qmusicplayer/mainwindow.cpp 10

    We update the GUI when the playback state changes, i.e., when it

    starts, pauses, stops, or resumes.

    The media object will report other state changes, as defined by the

    \l{Phonon::}{State} enum.

    The \c tick() slot is connected to a \l{Phonon::}{MediaObject} signal which is

    emitted when the playback position changes:

    \snippet examples/phonon/qmusicplayer/mainwindow.cpp 11

    The \c time is given in milliseconds.

    When the table is clicked on with the mouse, \c tableClick()

    is invoked:

    \snippet examples/phonon/qmusicplayer/mainwindow.cpp 12

    Since we stop the media object, we first check whether it is

    currently playing. \c row contains the row in the table that was

    clicked upon; the indices of \c sources follows the table, so we

    can simply use \c row to find the new source.

    \snippet examples/phonon/qmusicplayer/mainwindow.cpp 13

    When the media source changes, we simply need to select the

    corresponding row in the table.

    \snippet examples/phonon/qmusicplayer/mainwindow.cpp 14

    When \c metaStateChanged() is invoked, \c

    metaInformationProvider has resolved the meta data for its current

    source. A \l{Phonon::}{MediaObject} will do this before

    entering \l{Phonon::}{StoppedState}. Note that we could also

    have used the \l{Phonon::MediaObject::}{metaDataChanged()} signal for

    this purpose.

    Some of the meta data is then chosen to be displayed in the

    music table. A file might not contain the meta data requested,

    in which case an empty string is returned.

    \snippet examples/phonon/qmusicplayer/mainwindow.cpp 15

    If we have media sources in \c sources of which meta information

    is not resolved, we set a new source on the \c

    metaInformationProvider, which will invoke \c metaStateChanged()

    again.

    We move on to the \c aboutToFinish() slot:

    \snippet examples/phonon/qmusicplayer/mainwindow.cpp 16

    When a file is finished playing, the Music Player will move on and

    play the next file in the table. This slot is connected to the

    \l{Phonon::}{MediaObject}'s

    \l{Phonon::MediaObject::}{aboutToFinish()} signal, which is

    guaranteed to be emitted while there is still time to enqueue

    another file for playback.

    \section1 The main() function.

    Phonon requires that the application has a name; it is set with

    \l{QCoreApplication::}{setApplicationName()}. This is because

    D-Bus, which is used by Phonon on Linux systems, demands this.

    \snippet examples/phonon/qmusicplayer/main.cpp 1

*/