Detailed Description
The QCanvasItem class provides an abstract graphic object on a QCanvas.
A variety of QCanvasItem subclasses provide immediately usable
behaviour. This class is a pure abstract superclass providing the
behaviour that is shared among all the concrete canvas item classes.
QCanvasItem is not intended for direct subclassing. It is much easier
to subclass one of its subclasses, e.g. QCanvasPolygonalItem (the
commonest base class), QCanvasRectangle, QCanvasSprite, QCanvasEllipse
or QCanvasText.
Canvas items are added to a canvas by constructing them and passing the
canvas to the canvas item's constructor. An item can be moved to a
different canvas using setCanvas().
Items appear on the canvas after their show()
function has been called (or setVisible(TRUE)), and after update() has been called. The
canvas only shows items that are visible,
and then only if update() is called. If you created the canvas
without passing a width and height to the constructor you'll also need
to call resize(). Since the canvas
background defaults to white and canvas items default to white,
you may need to change colors to see your items.
A QCanvasItem object can be moved in the x(), y() and z() dimensions
using functions such as move(), moveBy(), setX(), setY() and setZ(). A
canvas item can be set in motion, `animated', using setAnimated() and
given a velocity in the x and y directions with setXVelocity() and
setYVelocity() -- the same effect can be achieved by calling
setVelocity(). Use the collidesWith() function to see if the canvas item
will collide on the next advance(1) and use collisions() to see what
collisions have occurred.
Use QCanvasSprite or your own subclass of QCanvasSprite to create canvas
items which are animated, i.e. which change over time.
The size of a canvas item is given by boundingRect(). Use
boundingRectAdvanced() to see what the size of the canvas item will be
after the next advance(1) call.
The rtti() function is used for identifying subclasses of QCanvasItem.
The canvas() function returns a pointer to the canvas which contains the
canvas item.
QCanvasItem provides the show() and isVisible() functions like those in
QWidget.
QCanvasItem also provides the setEnabled(), setActive() and
setSelected() functions; these functions set the relevant boolean and
cause a repaint but the boolean values they set are not used in
QCanvasItem itself. You can make use of these booleans in your subclasses.
By default, canvas items have no velocity, no size, and are not in
motion. The subclasses provided in Qt do not change these defaults
except where noted.
See also Graphics Classes and Image Processing Classes.
Member Type Documentation
QCanvasItem::RttiValues
This enum is used to name the different types of canvas item.
- QCanvasItem::Rtti_Item - Canvas item abstract base class
- QCanvasItem::Rtti_Ellipse
- QCanvasItem::Rtti_Line
- QCanvasItem::Rtti_Polygon
- QCanvasItem::Rtti_PolygonalItem
- QCanvasItem::Rtti_Rectangle
- QCanvasItem::Rtti_Spline
- QCanvasItem::Rtti_Sprite
- QCanvasItem::Rtti_Text
Member Function Documentation
QCanvasItem::QCanvasItem ( QCanvas * canvas )
Constructs a QCanvasItem on canvas canvas.
See also setCanvas().
QCanvasItem::~QCanvasItem () [virtual]
Destroys the QCanvasItem and removes it from its canvas.
bool QCanvasItem::active () const
This function is obsolete. It is provided to keep old source working. We strongly advise against using it in new code.
Use isActive() instead.
void QCanvasItem::advance ( int phase ) [virtual]
The default implementation moves the canvas item, if it is
animated(), by the preset velocity if phase is 1, and does
nothing if phase is 0.
Note that if you reimplement this function, the reimplementation
must not change the canvas in any way, for example it must not add
or remove items.
See also QCanvas::advance() and setVelocity().
Example: canvas/canvas.cpp.
Reimplemented in QCanvasSprite.
bool QCanvasItem::animated () const
Returns TRUE if the canvas item is in motion; otherwise returns
FALSE.
See also setVelocity() and setAnimated().
QRect QCanvasItem::boundingRect () const [pure virtual]
Returns the bounding rectangle in pixels that the canvas item covers.
See also boundingRectAdvanced().
Reimplemented in QCanvasSprite, QCanvasPolygonalItem, and QCanvasText.
QRect QCanvasItem::boundingRectAdvanced () const [virtual]
Returns the bounding rectangle of pixels that the canvas item will cover after advance(1) is called.
See also boundingRect().
QCanvas * QCanvasItem::canvas () const
Returns the canvas containing the canvas item.
bool QCanvasItem::collidesWith ( const QCanvasItem * other ) const [pure virtual]
Returns TRUE if the canvas item will collide with the other
item after they have moved by their current velocities;
otherwise returns FALSE.
See also collisions().
Example: canvas/canvas.cpp.
QCanvasItemList QCanvasItem::collisions ( bool exact ) const
Returns the list of canvas items that this canvas item has
collided with.
A collision is generally defined as occurring when the pixels of
one item draw on the pixels of another item, but not all
subclasses are so precise. Also, since pixel-wise collision
detection can be slow, this function works in either exact or
inexact mode, according to the exact parameter.
If exact is TRUE, the canvas items returned have been
accurately tested for collision with the canvas item.
If exact is FALSE, the canvas items returned are near the
canvas item. You can test the canvas items returned using
collidesWith() if any are interesting collision candidates. By
using this approach, you can ignore some canvas items for which
collisions are not relevant.
The returned list is a list of QCanvasItems, but often you will
need to cast the items to their subclass types. The safe way to do
this is to use rtti() before casting. This provides some of the
functionality of the standard C++ dynamic cast operation even on
compilers where dynamic casts are not available.
Note that a canvas item may be `on' a canvas, e.g. it was created
with the canvas as parameter, even though its coordinates place it
beyond the edge of the canvas's area. Collision detection only
works for canvas items which are wholly or partly within the
canvas's area.
Note that if items have a velocity (see setVelocity()), then
collision testing is done based on where the item will be when
it moves, not its current location. For example, a "ball" item
doesn't need to actually embed into a "wall" item before a
collision is detected. For items without velocity, plain
intersection is used.
void QCanvasItem::draw ( QPainter & painter ) [pure virtual]
This abstract virtual function draws the canvas item using painter.
Reimplemented in QCanvasSprite, QCanvasPolygonalItem, and QCanvasText.
bool QCanvasItem::enabled () const
This function is obsolete. It is provided to keep old source working. We strongly advise against using it in new code.
Use isEnabled() instead.
void QCanvasItem::hide ()
Shorthand for setVisible(FALSE).
bool QCanvasItem::isActive () const
Returns TRUE if the QCanvasItem is active; otherwise returns FALSE.
bool QCanvasItem::isEnabled () const
Returns TRUE if the QCanvasItem is enabled; otherwise returns FALSE.
bool QCanvasItem::isSelected () const
Returns TRUE if the canvas item is selected; otherwise returns FALSE.
bool QCanvasItem::isVisible () const
Returns TRUE if the canvas item is visible; otherwise returns
FALSE.
Note that in this context TRUE does not mean that the canvas
item is currently in a view, merely that if a view is showing the
area where the canvas item is positioned, and the item is not
obscured by items with higher z values, and the view is not
obscured by overlaying windows, it would be visible.
See also setVisible() and z().
void QCanvasItem::move ( double x, double y )
Moves the canvas item to the absolute position (x, y).
Example: canvas/canvas.cpp.
void QCanvasItem::moveBy ( double dx, double dy ) [virtual]
Moves the canvas item relative to its current position by (dx,
dy).
Example: canvas/canvas.cpp.
int QCanvasItem::rtti () const [virtual]
Returns 0 (QCanvasItem::Rtti_Item).
Make your derived classes return their own values for rtti(), so
that you can distinguish between objects returned by
QCanvas::at(). You should use values greater than 1000 to allow
for extensions to this class.
Overuse of this functionality can damage it's extensibility. For
example, once you have identified a base class of a QCanvasItem
found by QCanvas::at(), cast it to that type and call meaningful
methods rather than acting upon the object based on its rtti
value.
For example:
QCanvasItem* item;
// Find an item, e.g. with QCanvasItem::collisions().
...
if (item->rtti() == MySprite::RTTI ) {
MySprite* s = (MySprite*)item;
if (s->isDamagable()) s->loseHitPoints(1000);
if (s->isHot()) myself->loseHitPoints(1000);
...
}
Example: canvas/canvas.cpp.
Reimplemented in QCanvasSprite, QCanvasPolygonalItem, and QCanvasText.
bool QCanvasItem::selected () const
This function is obsolete. It is provided to keep old source working. We strongly advise against using it in new code.
Use isSelected() instead.
void QCanvasItem::setActive ( bool yes ) [virtual]
Sets the active flag of the item to yes. If this changes the
item's active state the item will be redrawn when
QCanvas::update() is next called.
The QCanvas, QCanvasItem and the Qt-supplied QCanvasItem
subclasses do not make use of this value. The setActive() function
is supplied because many applications need it, but it is up to you
how you use the isActive() value.
void QCanvasItem::setAnimated ( bool y ) [virtual]
Sets the canvas item to be in motion if y is TRUE, or not if y is FALSE. The speed and direction of the motion is set with
setVelocity(), or with setXVelocity() and setYVelocity().
See also advance() and QCanvas::advance().
void QCanvasItem::setCanvas ( QCanvas * c ) [virtual]
Sets the QCanvas upon which the canvas item is to be drawn to c.
See also canvas().
void QCanvasItem::setEnabled ( bool yes ) [virtual]
Sets the enabled flag of the item to yes. If this changes the
item's enabled state the item will be redrawn when
QCanvas::update() is next called.
The QCanvas, QCanvasItem and the Qt-supplied QCanvasItem
subclasses do not make use of this value. The setEnabled()
function is supplied because many applications need it, but it is
up to you how you use the isEnabled() value.
void QCanvasItem::setSelected ( bool yes ) [virtual]
Sets the selected flag of the item to yes. If this changes the
item's selected state the item will be redrawn when
QCanvas::update() is next called.
The QCanvas, QCanvasItem and the Qt-supplied QCanvasItem
subclasses do not make use of this value. The setSelected()
function is supplied because many applications need it, but it is
up to you how you use the isSelected() value.
void QCanvasItem::setVelocity ( double vx, double vy ) [virtual]
Sets the canvas item to be in motion, moving by vx and vy
pixels in the horizontal and vertical directions respectively.
See also advance(), setXVelocity(), and setYVelocity().
void QCanvasItem::setVisible ( bool yes ) [virtual]
Makes the canvas item visible if yes is TRUE, or invisible if
yes is FALSE. The change takes effect when QCanvas::update() is
next called.
void QCanvasItem::setX ( double x )
Moves the canvas item so that its x-position is x.
See also x() and move().
Example: chart/chartform_canvas.cpp.
void QCanvasItem::setXVelocity ( double vx )
Sets the horizontal component of the canvas item's velocity to vx.
See also setYVelocity() and setVelocity().
void QCanvasItem::setY ( double y )
Moves the canvas item so that its y-position is y.
See also y() and move().
Example: chart/chartform_canvas.cpp.
void QCanvasItem::setYVelocity ( double vy )
Sets the vertical component of the canvas item's velocity to vy.
See also setXVelocity() and setVelocity().
void QCanvasItem::setZ ( double z )
Sets the z index of the canvas item to z. Higher-z items
obscure (are in front of) lower-z items.
See also z() and move().
Examples: canvas/canvas.cpp and chart/chartform_canvas.cpp.
void QCanvasItem::show ()
Shorthand for setVisible(TRUE).
Examples: canvas/canvas.cpp and chart/chartform_canvas.cpp.
void QCanvasItem::update () [protected]
Call this function to repaint the canvas's changed chunks.
bool QCanvasItem::visible () const
This function is obsolete. It is provided to keep old source working. We strongly advise against using it in new code.
Use isVisible() instead.
double QCanvasItem::x () const
Returns the horizontal position of the canvas item. Note that
subclasses often have an origin other than the top-left corner.
Example: canvas/canvas.cpp.
double QCanvasItem::xVelocity () const
Returns the horizontal velocity component of the canvas item.
double QCanvasItem::y () const
Returns the vertical position of the canvas item. Note that
subclasses often have an origin other than the top-left corner.
Example: canvas/canvas.cpp.
double QCanvasItem::yVelocity () const
Returns the vertical velocity component of the canvas item.
double QCanvasItem::z () const
Returns the z index of the canvas item, which is used for visual
order: higher-z items obscure (are in front of) lower-z items.
This file is part of the Qt toolkit.
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