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# QCanvasView Class Reference
[canvas module]

The QCanvasView class provides an on-screen view of a QCanvas. More...

`#include <qcanvas.h>`

Inherits QScrollView.

List of all member functions.

## Public Members

- QCanvasView ( QWidget * parent = 0, const char * name = 0, WFlags f = 0 )
- QCanvasView ( QCanvas * canvas, QWidget * parent = 0, const char * name = 0, WFlags f = 0 )
- ~QCanvasView ()
- QCanvas * canvas () const
- void setCanvas ( QCanvas * canvas )
- const QWMatrix & worldMatrix () const
- const QWMatrix & inverseWorldMatrix () const
- bool setWorldMatrix ( const QWMatrix & wm )

## Protected Members

- virtual void drawContents ( QPainter * p, int cx, int cy, int cw, int ch )

## Detailed Description

The QCanvasView class provides an on-screen view of a QCanvas.

A QCanvasView is widget which provides a view of a QCanvas.

If you want users to be able to interact with a canvas view, subclass QCanvasView. You might then reimplement QScrollView::contentsMousePressEvent() for example:

        void MyCanvasView::contentsMousePressEvent( QMouseEvent* e )
        {
            QCanvasItemList l = canvas()->collisions(e->pos());
            for (QCanvasItemList::Iterator it=l.begin(); it!=l.end(); ++it) {
                if ( (*it)->rtti() == QCanvasRectangle::RTTI )
                    qDebug("A QCanvasRectangle lies somewhere at this point");
            }
        }

Set the canvas that the view shows with setCanvas() and retrieve the canvas which the view is showing with canvas().

A transformation matrix can be used to transform the view of the canvas in various ways, for example, zooming in or out or rotating. For example:

        QWMatrix wm;
        wm.scale( 2, 2 );   // Zooms in by 2 times
        wm.rotate( 90 );    // Rotates 90 degrees counter clockwise
                            // around the origin.
        wm.translate( 0, -canvas->height() );
                            // moves the canvas down so what was visible
                            // before is still visible.
        myCanvasView->setWorldMatrix( wm );

Use setWorldMatrix() to set the canvas view's world matrix: you must ensure that the world matrix is invertible. The current world matrix is retrievable with worldMatrix(), and its inversion is retrievable with inverseWorldMatrix().

Example:

The following code finds the part of the canvas that is visible in this view, i.e. the bounding rectangle of the view in canvas coordinates.

        QRect rc = QRect( myCanvasView->contentsX(), myCanvasView->contentsY(),
                          myCanvasView->visibleWidth(), myCanvasView->visibleHeight() );
        QRect canvasRect = myCanvasView->inverseWorldMatrix().mapRect(rc);

See also QWMatrix, QPainter::setWorldMatrix(), Graphics Classes and Image Processing Classes.

## Member Function Documentation

### QCanvasView::QCanvasView ( QWidget * parent = 0, const char * name = 0, WFlags f = 0 )

Constructs a QCanvasView with parent parent, and name name, using the widget flags f. The canvas view is not associated with a canvas, so you will need to call setCanvas() to display a canvas.

### QCanvasView::QCanvasView ( QCanvas * canvas, QWidget * parent = 0, const char * name = 0, WFlags f = 0 )

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

Constructs a QCanvasView which views canvas canvas, with parent parent, and name name, using the widget flags f.

### QCanvasView::~QCanvasView ()

Destroys the canvas view. The associated canvas is not deleted.

### QCanvas * QCanvasView::canvas () const

Returns a pointer to the canvas which the QCanvasView is currently showing.

### void QCanvasView::drawContents ( QPainter * p, int cx, int cy, int cw, int ch )`[virtual protected]`

Repaints part of the QCanvas that the canvas view is showing starting at cx by cy, with a width of cw and a height of ch using the painter p.

Reimplemented from QScrollView.

### const QWMatrix & QCanvasView::inverseWorldMatrix () const

Returns a reference to the inverse of the canvas view's current transformation matrix.

See also setWorldMatrix() and worldMatrix().

### void QCanvasView::setCanvas ( QCanvas * canvas )

Sets the canvas that the QCanvasView is showing to the canvas canvas.

### bool QCanvasView::setWorldMatrix ( const QWMatrix & wm )

Sets the transformation matrix of the QCanvasView to wm. The matrix must be invertible (i.e. if you create a world matrix that zooms out by 2 times, then the inverse of this matrix is one that will zoom in by 2 times).

When you use this, you should note that the performance of the QCanvasView will decrease considerably.

Returns FALSE if wm is not invertable; otherwise returns TRUE.

See also worldMatrix(), inverseWorldMatrix() and QWMatrix::isInvertible().

Example: canvas/canvas.cpp.

### const QWMatrix & QCanvasView::worldMatrix () const

Returns a reference to the canvas view's current transformation matrix.

See also setWorldMatrix() and inverseWorldMatrix().

Example: canvas/canvas.cpp.

This file is part of the Qt toolkit. Copyright © 1995-2002 Trolltech. All Rights Reserved.