QCanvasView Class

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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 )
- virtual QSize sizeHint () const

## 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, assuming no transformation matrix is set:

        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");
            }
        }

The canvas view shows canvas canvas(); this can be changed using setCanvas().

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 must to call setCanvas() to view 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.

Warning: When double buffering is enabled, drawContents() will not respect the current settings of the painter when setting up the painter for the double buffer (e.g., viewport() and window()). Also, be aware that QCanvas::update() bypasses drawContents(), which means any reimplementation of drawContents() is not called.

See also setDoubleBuffering().

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.

### QSize QCanvasView::sizeHint () const`[virtual protected]`

Suggests a size sufficient to view the entire canvas.

### 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.

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