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/*! 
    \example widgets/tetrix
    \title Tetrix Example

    The Tetrix example is a Qt version of the classic Tetrix game.

    \image tetrix-example.png

    The object of the game is to stack pieces dropped from the top of the
    playing area so that they fill entire rows at the bottom of the playing area.

    When a row is filled, all the blocks on that row are removed, the player earns
    a number of points, and the pieces above are moved down to occupy that row.
    If more than one row is filled, the blocks on each row are removed, and the
    player earns extra points.

    The \gui{Left} cursor key moves the current piece one space to the left, the
    \gui{Right} cursor key moves it one space to the right, the \gui{Up} cursor
    key rotates the piece counter-clockwise by 90 degrees, and the \gui{Down}
    cursor key rotates the piece clockwise by 90 degrees.

    To avoid waiting for a piece to fall to the bottom of the board, press \gui{D}
    to immediately move the piece down by one row, or press the \gui{Space} key to
    drop it as close to the bottom of the board as possible.

    This example shows how a simple game can be created using only three classes:

    \list
    \o The \c TetrixWindow class is used to display the player's score, number of
       lives, and information about the next piece to appear.
    \o The \c TetrixBoard class contains the game logic, handles keyboard input, and
       displays the pieces on the playing area.
    \o The \c TetrixPiece class contains information about each piece.
    \endlist

    In this approach, the \c TetrixBoard class is the most complex class, since it
    handles the game logic and rendering. One benefit of this is that the
    \c TetrixWindow and \c TetrixPiece classes are very simple and contain only a
    minimum of code.

    \section1 TetrixWindow Class Definition

    The \c TetrixWindow class is used to display the game information and contains
    the playing area:

    \snippet examples/widgets/tetrix/tetrixwindow.h 0

    We use private member variables for the board, various display widgets, and
    buttons to allow the user to start a new game, pause the current game, and quit.

    Although the window inherits QWidget, the constructor does not provide an
    argument to allow a parent widget to be specified. This is because the window
    will always be used as a top-level widget.

    \section1 TetrixWindow Class Implementation

    The constructor sets up the user interface elements for the game:

    \snippet examples/widgets/tetrix/tetrixwindow.cpp 0

    We begin by constructing a \c TetrixBoard instance for the playing area and a
    label that shows the next piece to be dropped into the playing area; the label
    is initially empty.

    Three QLCDNumber objects are used to display the score, number of lives, and
    lines removed. These initially show default values, and will be filled in
    when a game begins:

    \snippet examples/widgets/tetrix/tetrixwindow.cpp 1

    Three buttons with shortcuts are constructed so that the user can start a
    new game, pause the current game, and quit the application:

    \snippet examples/widgets/tetrix/tetrixwindow.cpp 2
    \snippet examples/widgets/tetrix/tetrixwindow.cpp 3

    These buttons are configured so that they never receive the keyboard focus;
    we want the keyboard focus to remain with the \c TetrixBoard instance so that
    it receives all the keyboard events. Nonetheless, the buttons will still respond
    to \key{Alt} key shortcuts.

    We connect \l{QAbstractButton::}{clicked()} signals from the \gui{Start}
    and \gui{Pause} buttons to the board, and from the \gui{Quit} button to the
    application's \l{QApplication::}{quit()} slot.

    \snippet examples/widgets/tetrix/tetrixwindow.cpp 4
    \snippet examples/widgets/tetrix/tetrixwindow.cpp 5

    Signals from the board are also connected to the LCD widgets for the purpose of
    updating the score, number of lives, and lines removed from the playing area.

    We place the label, LCD widgets, and the board into a QGridLayout
    along with some labels that we create with the \c createLabel() convenience
    function:

    \snippet examples/widgets/tetrix/tetrixwindow.cpp 6

    Finally, we set the grid layout on the widget, give the window a title, and
    resize it to an appropriate size.

    The \c createLabel() convenience function simply creates a new label on the
    heap, gives it an appropriate alignment, and returns it to the caller:

    \snippet examples/widgets/tetrix/tetrixwindow.cpp 7

    Since each label will be used in the widget's layout, it will become a child
    of the \c TetrixWindow widget and, as a result, it will be deleted when the
    window is deleted.

    \section1 TetrixPiece Class Definition

    The \c TetrixPiece class holds information about a piece in the game's
    playing area, including its shape, position, and the range of positions it can
    occupy on the board:

    \snippet examples/widgets/tetrix/tetrixpiece.h 0

    Each shape contains four blocks, and these are defined by the \c coords private
    member variable. Additionally, each piece has a high-level description that is
    stored internally in the \c pieceShape variable.

    The constructor is written inline in the definition, and simply ensures that
    each piece is initially created with no shape. The \c shape() function simply
    returns the contents of the \c pieceShape variable, and the \c x() and \c y()
    functions return the x and y-coordinates of any given block in the shape.

    \section1 TetrixPiece Class Implementation

    The \c setRandomShape() function is used to select a random shape for a piece:

    \snippet examples/widgets/tetrix/tetrixpiece.cpp 0

    For convenience, it simply chooses a random shape from the \c TetrixShape enum
    and calls the \c setShape() function to perform the task of positioning the
    blocks.

    The \c setShape() function uses a look-up table of pieces to associate each
    shape with an array of block positions:

    \snippet examples/widgets/tetrix/tetrixpiece.cpp 1
    \snippet examples/widgets/tetrix/tetrixpiece.cpp 2

    These positions are read from the table into the piece's own array of positions,
    and the piece's internal shape information is updated to use the new shape.

    The \c x() and \c y() functions are implemented inline in the class definition,
    returning positions defined on a grid that extends horizontally and vertically
    with coordinates from -2 to 2. Although the predefined coordinates for each
    piece only vary horizontally from -1 to 1 and vertically from -1 to 2, each
    piece can be rotated by 90, 180, and 270 degrees.

    The \c minX() and \c maxX() functions return the minimum and maximum horizontal
    coordinates occupied by the blocks that make up the piece:

    \snippet examples/widgets/tetrix/tetrixpiece.cpp 3
    \snippet examples/widgets/tetrix/tetrixpiece.cpp 4

    Similarly, the \c minY() and \c maxY() functions return the minimum and maximum
    vertical coordinates occupied by the blocks:

    \snippet examples/widgets/tetrix/tetrixpiece.cpp 5
    \snippet examples/widgets/tetrix/tetrixpiece.cpp 6

    The \c rotatedLeft() function returns a new piece with the same shape as an
    existing piece, but rotated counter-clockwise by 90 degrees:

    \snippet examples/widgets/tetrix/tetrixpiece.cpp 7

    Similarly, the \c rotatedRight() function returns a new piece with the same
    shape as an existing piece, but rotated clockwise by 90 degrees:

    \snippet examples/widgets/tetrix/tetrixpiece.cpp 9

    These last two functions enable each piece to create rotated copies of itself.

    \section1 TetrixBoard Class Definition

    The \c TetrixBoard class inherits from QFrame and contains the game logic and display features:

    \snippet examples/widgets/tetrix/tetrixboard.h 0

    Apart from the \c setNextPieceLabel() function and the \c start() and \c pause()
    public slots, we only provide public functions to reimplement QWidget::sizeHint()
    and QWidget::minimumSizeHint(). The signals are used to communicate changes to
    the player's information to the \c TetrixWindow instance.

    The rest of the functionality is provided by reimplementations of protected event
    handlers and private functions:

    \snippet examples/widgets/tetrix/tetrixboard.h 1

    The board is composed of a fixed-size array whose elements correspond to
    spaces for individual blocks. Each element in the array contains a \c TetrixShape
    value corresponding to the type of shape that occupies that element.

    Each shape on the board will occupy four elements in the array, and these will
    all contain the enum value that corresponds to the type of the shape.

    We use a QBasicTimer to control the rate at which pieces fall toward the bottom
    of the playing area. This allows us to provide an implementation of
    \l{QObject::}{timerEvent()} that we can use to update the widget.

    \section1 TetrixBoard Class Implementation

    In the constructor, we customize the frame style of the widget, ensure that
    keyboard input will be received by the widget by using Qt::StrongFocus for the
    focus policy, and initialize the game state:

    \snippet examples/widgets/tetrix/tetrixboard.cpp 0

    The first (next) piece is also set up with a random shape.

    The \c setNextPieceLabel() function is used to pass in an externally-constructed
    label to the board, so that it can be shown alongside the playing area:

    \snippet examples/widgets/tetrix/tetrixboard.cpp 1

    We provide a reasonable size hint and minimum size hint for the board, based on
    the size of the space for each block in the playing area:

    \snippet examples/widgets/tetrix/tetrixboard.cpp 2
    \snippet examples/widgets/tetrix/tetrixboard.cpp 3

    By using a minimum size hint, we indicate to the layout in the parent widget
    that the board should not shrink below a minimum size.

    A new game is started when the \c start() slot is called. This resets the
    game's state, the player's score and level, and the contents of the board:

    \snippet examples/widgets/tetrix/tetrixboard.cpp 4

    We also emit signals to inform other components of these changes before creating
    a new piece that is ready to be dropped into the playing area. We start the
    timer that determines how often the piece drops down one row on the board.

    The \c pause() slot is used to temporarily stop the current game by stopping the
    internal timer:

    \snippet examples/widgets/tetrix/tetrixboard.cpp 5
    \snippet examples/widgets/tetrix/tetrixboard.cpp 6

    We perform checks to ensure that the game can only be paused if it is already
    running and not already paused.

    The \c paintEvent() function is straightforward to implement. We begin by
    calling the base class's implementation of \l{QWidget::}{paintEvent()} before
    constructing a QPainter for use on the board:

    \snippet examples/widgets/tetrix/tetrixboard.cpp 7

    Since the board is a subclass of QFrame, we obtain a QRect that covers the area
    \e inside the frame decoration before drawing our own content.

    If the game is paused, we want to hide the existing state of the board and
    show some text. We achieve this by painting text onto the widget and returning
    early from the function. The rest of the painting is performed after this point.

    The position of the top of the board is found by subtracting the total height
    of each space on the board from the bottom of the frame's internal rectangle.
    For each space on the board that is occupied by a piece, we call the
    \c drawSquare() function to draw a block at that position.

    \snippet examples/widgets/tetrix/tetrixboard.cpp 8
    \snippet examples/widgets/tetrix/tetrixboard.cpp 9

    Spaces that are not occupied by blocks are left blank.

    Unlike the existing pieces on the board, the current piece is drawn
    block-by-block at its current position:

    \snippet examples/widgets/tetrix/tetrixboard.cpp 10
    \snippet examples/widgets/tetrix/tetrixboard.cpp 11
    \snippet examples/widgets/tetrix/tetrixboard.cpp 12

    The \c keyPressEvent() handler is called whenever the player presses a key while
    the \c TetrixBoard widget has the keyboard focus.

    \snippet examples/widgets/tetrix/tetrixboard.cpp 13

    If there is no current game, the game is running but paused, or if there is no
    current shape to control, we simply pass on the event to the base class.

    We check whether the event is about any of the keys that the player uses to
    control the current piece and, if so, we call the relevant function to handle
    the input:

    \snippet examples/widgets/tetrix/tetrixboard.cpp 14

    In the case where the player presses a key that we are not interested in, we
    again pass on the event to the base class's implementation of
    \l{QWidget::}{keyPressEvent()}.

    The \c timerEvent() handler is called every time the class's QBasicTimer
    instance times out. We need to check that the event we receive corresponds to
    our timer. If it does, we can update the board:

    \snippet examples/widgets/tetrix/tetrixboard.cpp 15
    \snippet examples/widgets/tetrix/tetrixboard.cpp 16
    \snippet examples/widgets/tetrix/tetrixboard.cpp 17

    If a row (or line) has just been filled, we create a new piece and reset the
    timer; otherwise we move the current piece down by one row. We let the base
    class handle other timer events that we receive.

    The \c clearBoard() function simply fills the board with the
    \c TetrixShape::NoShape value:

    \snippet examples/widgets/tetrix/tetrixboard.cpp 18

    The \c dropDown() function moves the current piece down as far as possible on
    the board, either until it is touching the bottom of the playing area or it is
    stacked on top of another piece:

    \snippet examples/widgets/tetrix/tetrixboard.cpp 19
    \snippet examples/widgets/tetrix/tetrixboard.cpp 20

    The number of rows the piece has dropped is recorded and passed to the
    \c pieceDropped() function so that the player's score can be updated.

    The \c oneLineDown() function is used to move the current piece down by one row
    (line), either when the user presses the \gui{D} key or when the piece is
    scheduled to move:

    \snippet examples/widgets/tetrix/tetrixboard.cpp 21

    If the piece cannot drop down by one line, we call the \c pieceDropped() function
    with zero as the argument to indicate that it cannot fall any further, and that
    the player should receive no extra points for the fall.

    The \c pieceDropped() function itself is responsible for awarding points to the
    player for positioning the current piece, checking for full rows on the board
    and, if no lines have been removed, creating a new piece to replace the current
    one:

    \snippet examples/widgets/tetrix/tetrixboard.cpp 22
    \snippet examples/widgets/tetrix/tetrixboard.cpp 23

    We call \c removeFullLines() each time a piece has been dropped. This scans
    the board from bottom to top, looking for blank spaces on each row.

    \snippet examples/widgets/tetrix/tetrixboard.cpp 24
    \snippet examples/widgets/tetrix/tetrixboard.cpp 25
    \snippet examples/widgets/tetrix/tetrixboard.cpp 26
    \snippet examples/widgets/tetrix/tetrixboard.cpp 27

    If a row contains no blank spaces, the rows above it are copied down by one row
    to compress the stack of pieces, the top row on the board is cleared, and the
    number of full lines found is incremented.

    \snippet examples/widgets/tetrix/tetrixboard.cpp 28
    \snippet examples/widgets/tetrix/tetrixboard.cpp 29

    If some lines have been removed, the player's score and the total number of lines
    removed are updated. The \c linesRemoved() and \c scoreChanged() signals are
    emitted to send these new values to other widgets in the window.

    Additionally, we set the timer to elapse after half a second, set the
    \c isWaitingAfterLine flag to indicate that lines have been removed, unset
    the piece's shape to ensure that it is not drawn, and update the widget.
    The next time that the \c timerEvent() handler is called, a new piece will be
    created and the game will continue.

    The \c newPiece() function places the next available piece at the top of the
    board, and creates a new piece with a random shape:

    \snippet examples/widgets/tetrix/tetrixboard.cpp 30
    \snippet examples/widgets/tetrix/tetrixboard.cpp 31

    We place a new piece in the middle of the board at the top. The game is over if
    the piece can't move, so we unset its shape to prevent it from being drawn, stop
    the timer, and unset the \c isStarted flag.

    The \c showNextPiece() function updates the label that shows the next piece to
    be dropped:

    \snippet examples/widgets/tetrix/tetrixboard.cpp 32
    \snippet examples/widgets/tetrix/tetrixboard.cpp 33

    We draw the piece's component blocks onto a pixmap that is then set on the label.

    The \c tryMove() function is used to determine whether a piece can be positioned
    at the specified coordinates:

    \snippet examples/widgets/tetrix/tetrixboard.cpp 34

    We examine the spaces on the board that the piece needs to occupy and, if they
    are already occupied by other pieces, we return \c false to indicate that the
    move has failed.

    \snippet examples/widgets/tetrix/tetrixboard.cpp 35

    If the piece could be placed on the board at the desired location, we update the
    current piece and its position, update the widget, and return \c true to indicate
    success.

    The \c drawSquare() function draws the blocks (normally squares) that make up
    each piece using different colors for pieces with different shapes:

    \snippet examples/widgets/tetrix/tetrixboard.cpp 36

    We obtain the color to use from a look-up table that relates each shape to an
    RGB value, and use the painter provided to draw the block at the specified
    coordinates.
*/
