source: trunk/doc/src/examples/musicplayerexample.qdoc@ 1168

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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
*/