QTextLayout Class

The QTextLayout class is used to lay out and render text. More...

Header: #include <QTextLayout>
qmake: QT += gui

Note: All functions in this class are reentrant.

Public Types

class FormatRange
enum CursorMode { SkipCharacters, SkipWords }

Public Functions

QTextLayout()
QTextLayout(const QString &text)
QTextLayout(const QString &text, const QFont &font, QPaintDevice *paintdevice = Q_NULLPTR)
~QTextLayout()
void beginLayout()
QRectF boundingRect() const
bool cacheEnabled() const
void clearFormats()
void clearLayout()
QTextLine createLine()
Qt::CursorMoveStyle cursorMoveStyle() const
void draw(QPainter *p, const QPointF &pos, const QVector<FormatRange> &selections = QVector<FormatRange> (), const QRectF &clip = QRectF()) const
void drawCursor(QPainter *painter, const QPointF &position, int cursorPosition, int width) const
void drawCursor(QPainter *painter, const QPointF &position, int cursorPosition) const
void endLayout()
QFont font() const
QVector<FormatRange> formats() const
QList<QGlyphRun> glyphRuns(int from = -1, int length = -1) const
bool isValidCursorPosition(int pos) const
int leftCursorPosition(int oldPos) const
QTextLine lineAt(int i) const
int lineCount() const
QTextLine lineForTextPosition(int pos) const
qreal maximumWidth() const
qreal minimumWidth() const
int nextCursorPosition(int oldPos, CursorMode mode = SkipCharacters) const
QPointF position() const
int preeditAreaPosition() const
QString preeditAreaText() const
int previousCursorPosition(int oldPos, CursorMode mode = SkipCharacters) const
int rightCursorPosition(int oldPos) const
void setCacheEnabled(bool enable)
void setCursorMoveStyle(Qt::CursorMoveStyle style)
void setFont(const QFont &font)
void setFormats(const QVector<FormatRange> &formats)
void setPosition(const QPointF &p)
void setPreeditArea(int position, const QString &text)
void setText(const QString &string)
void setTextOption(const QTextOption &option)
QString text() const
const QTextOption &textOption() const
bool operator!=(const FormatRange &lhs, const FormatRange &rhs)
bool operator==(const FormatRange &lhs, const FormatRange &rhs)

Detailed Description

The QTextLayout class is used to lay out and render text.

It offers many features expected from a modern text layout engine, including Unicode compliant rendering, line breaking and handling of cursor positioning. It can also produce and render device independent layout, something that is important for WYSIWYG applications.

The class has a rather low level API and unless you intend to implement your own text rendering for some specialized widget, you probably won't need to use it directly.

QTextLayout can be used with both plain and rich text.

QTextLayout can be used to create a sequence of QTextLine instances with given widths and can position them independently on the screen. Once the layout is done, these lines can be drawn on a paint device.

The text to be laid out can be provided in the constructor or set with setText().

The layout can be seen as a sequence of QTextLine objects; use createLine() to create a QTextLine instance, and lineAt() or lineForTextPosition() to retrieve created lines.

Here is a code snippet that demonstrates the layout phase:

int leading = fontMetrics.leading();
qreal height = 0;
textLayout.setCacheEnabled(true);
textLayout.beginLayout();
while (1) {
    QTextLine line = textLayout.createLine();
    if (!line.isValid())
        break;

    line.setLineWidth(lineWidth);
    height += leading;
    line.setPosition(QPointF(0, height));
    height += line.height();
}
textLayout.endLayout();

The text can then be rendered by calling the layout's draw() function:

QPainter painter(this);
textLayout.draw(&painter, QPoint(0, 0));

For a given position in the text you can find a valid cursor position with isValidCursorPosition(), nextCursorPosition(), and previousCursorPosition().

The QTextLayout itself can be positioned with setPosition(); it has a boundingRect(), and a minimumWidth() and a maximumWidth().

See also QStaticText.

Member Type Documentation

enum QTextLayout::CursorMode

ConstantValue
QTextLayout::SkipCharacters0
QTextLayout::SkipWords1

Member Function Documentation

QTextLayout::QTextLayout()

Constructs an empty text layout.

See also setText().

QTextLayout::QTextLayout(const QString &text)

Constructs a text layout to lay out the given text.

QTextLayout::QTextLayout(const QString &text, const QFont &font, QPaintDevice *paintdevice = Q_NULLPTR)

Constructs a text layout to lay out the given text with the specified font.

All the metric and layout calculations will be done in terms of the paint device, paintdevice. If paintdevice is 0 the calculations will be done in screen metrics.

QTextLayout::~QTextLayout()

Destructs the layout.

void QTextLayout::beginLayout()

Begins the layout process.

Warning: This will invalidate the layout, so all existing QTextLine objects that refer to the previous contents should now be discarded.

See also endLayout().

QRectF QTextLayout::boundingRect() const

The smallest rectangle that contains all the lines in the layout.

bool QTextLayout::cacheEnabled() const

Returns true if the complete layout information is cached; otherwise returns false.

See also setCacheEnabled().

void QTextLayout::clearFormats()

Clears the list of additional formats supported by the text layout.

This function was introduced in Qt 5.6.

See also formats() and setFormats().

void QTextLayout::clearLayout()

Clears the line information in the layout. After having called this function, lineCount() returns 0.

Warning: This will invalidate the layout, so all existing QTextLine objects that refer to the previous contents should now be discarded.

This function was introduced in Qt 4.4.

QTextLine QTextLayout::createLine()

Returns a new text line to be laid out if there is text to be inserted into the layout; otherwise returns an invalid text line.

The text layout creates a new line object that starts after the last line in the layout, or at the beginning if the layout is empty. The layout maintains an internal cursor, and each line is filled with text from the cursor position onwards when the QTextLine::setLineWidth() function is called.

Once QTextLine::setLineWidth() is called, a new line can be created and filled with text. Repeating this process will lay out the whole block of text contained in the QTextLayout. If there is no text left to be inserted into the layout, the QTextLine returned will not be valid (isValid() will return false).

Qt::CursorMoveStyle QTextLayout::cursorMoveStyle() const

The cursor movement style of this QTextLayout. The default is Qt::LogicalMoveStyle.

See also setCursorMoveStyle().

void QTextLayout::draw(QPainter *p, const QPointF &pos, const QVector<FormatRange> &selections = QVector<FormatRange> (), const QRectF &clip = QRectF()) const

Draws the whole layout on the painter p at the position specified by pos. The rendered layout includes the given selections and is clipped within the rectangle specified by clip.

void QTextLayout::drawCursor(QPainter *painter, const QPointF &position, int cursorPosition, int width) const

Draws a text cursor with the current pen and the specified width at the given position using the painter specified. The corresponding position within the text is specified by cursorPosition.

void QTextLayout::drawCursor(QPainter *painter, const QPointF &position, int cursorPosition) const

This is an overloaded function.

Draws a text cursor with the current pen at the given position using the painter specified. The corresponding position within the text is specified by cursorPosition.

void QTextLayout::endLayout()

Ends the layout process.

See also beginLayout().

QFont QTextLayout::font() const

Returns the current font that is used for the layout, or a default font if none is set.

See also setFont().

QVector<FormatRange> QTextLayout::formats() const

Returns the list of additional formats supported by the text layout.

This function was introduced in Qt 5.6.

See also setFormats() and clearFormats().

QList<QGlyphRun> QTextLayout::glyphRuns(int from = -1, int length = -1) const

Returns the glyph indexes and positions for all glyphs corresponding to the length characters starting at the position from in this QTextLayout. This is an expensive function, and should not be called in a time sensitive context.

If from is less than zero, then the glyph run will begin at the first character in the layout. If length is less than zero, it will span the entire string from the start position.

This function was introduced in Qt 4.8.

See also draw() and QPainter::drawGlyphRun().

bool QTextLayout::isValidCursorPosition(int pos) const

/ Returns true if position pos is a valid cursor position.

In a Unicode context some positions in the text are not valid cursor positions, because the position is inside a Unicode surrogate or a grapheme cluster.

A grapheme cluster is a sequence of two or more Unicode characters that form one indivisible entity on the screen. For example the latin character `Ä' can be represented in Unicode by two characters, `A' (0x41), and the combining diaresis (0x308). A text cursor can only validly be positioned before or after these two characters, never between them since that wouldn't make sense. In indic languages every syllable forms a grapheme cluster.

int QTextLayout::leftCursorPosition(int oldPos) const

Returns the cursor position to the left of oldPos, next to it. It's dependent on the visual position of characters, after bi-directional reordering.

See also rightCursorPosition() and previousCursorPosition().

QTextLine QTextLayout::lineAt(int i) const

Returns the i-th line of text in this text layout.

See also lineCount() and lineForTextPosition().

int QTextLayout::lineCount() const

Returns the number of lines in this text layout.

See also lineAt().

QTextLine QTextLayout::lineForTextPosition(int pos) const

Returns the line that contains the cursor position specified by pos.

See also isValidCursorPosition() and lineAt().

qreal QTextLayout::maximumWidth() const

The maximum width the layout could expand to; this is essentially the width of the entire text.

Warning: This function only returns a valid value after the layout has been done.

See also minimumWidth().

qreal QTextLayout::minimumWidth() const

The minimum width the layout needs. This is the width of the layout's smallest non-breakable substring.

Warning: This function only returns a valid value after the layout has been done.

See also maximumWidth().

int QTextLayout::nextCursorPosition(int oldPos, CursorMode mode = SkipCharacters) const

Returns the next valid cursor position after oldPos that respects the given cursor mode. Returns value of oldPos, if oldPos is not a valid cursor position.

See also isValidCursorPosition() and previousCursorPosition().

QPointF QTextLayout::position() const

The global position of the layout. This is independent of the bounding rectangle and of the layout process.

This function was introduced in Qt 4.2.

See also setPosition().

int QTextLayout::preeditAreaPosition() const

Returns the position of the area in the text layout that will be processed before editing occurs.

See also preeditAreaText().

QString QTextLayout::preeditAreaText() const

Returns the text that is inserted in the layout before editing occurs.

See also preeditAreaPosition().

int QTextLayout::previousCursorPosition(int oldPos, CursorMode mode = SkipCharacters) const

Returns the first valid cursor position before oldPos that respects the given cursor mode. Returns value of oldPos, if oldPos is not a valid cursor position.

See also isValidCursorPosition() and nextCursorPosition().

int QTextLayout::rightCursorPosition(int oldPos) const

Returns the cursor position to the right of oldPos, next to it. It's dependent on the visual position of characters, after bi-directional reordering.

See also leftCursorPosition() and nextCursorPosition().

void QTextLayout::setCacheEnabled(bool enable)

Enables caching of the complete layout information if enable is true; otherwise disables layout caching. Usually QTextLayout throws most of the layouting information away after a call to endLayout() to reduce memory consumption. If you however want to draw the laid out text directly afterwards enabling caching might speed up drawing significantly.

See also cacheEnabled().

void QTextLayout::setCursorMoveStyle(Qt::CursorMoveStyle style)

Sets the visual cursor movement style to the given style. If the QTextLayout is backed by a document, you can ignore this and use the option in QTextDocument, this option is for widgets like QLineEdit or custom widgets without a QTextDocument. Default value is Qt::LogicalMoveStyle.

See also cursorMoveStyle().

void QTextLayout::setFont(const QFont &font)

Sets the layout's font to the given font. The layout is invalidated and must be laid out again.

See also font().

void QTextLayout::setFormats(const QVector<FormatRange> &formats)

Sets the additional formats supported by the text layout to formats. The formats are applied with preedit area text in place.

This function was introduced in Qt 5.6.

See also formats() and clearFormats().

void QTextLayout::setPosition(const QPointF &p)

Moves the text layout to point p.

See also position().

void QTextLayout::setPreeditArea(int position, const QString &text)

Sets the position and text of the area in the layout that is processed before editing occurs. The layout is invalidated and must be laid out again.

See also preeditAreaPosition() and preeditAreaText().

void QTextLayout::setText(const QString &string)

Sets the layout's text to the given string. The layout is invalidated and must be laid out again.

Notice that when using this QTextLayout as part of a QTextDocument this method will have no effect.

See also text().

void QTextLayout::setTextOption(const QTextOption &option)

Sets the text option structure that controls the layout process to the given option.

See also textOption().

QString QTextLayout::text() const

Returns the layout's text.

See also setText().

const QTextOption &QTextLayout::textOption() const

Returns the current text option used to control the layout process.

See also setTextOption().

bool QTextLayout::operator!=(const FormatRange &lhs, const FormatRange &rhs)

Returns true if any of the start, length, or format fields in lhs and rhs contain different values respectively.

bool QTextLayout::operator==(const FormatRange &lhs, const FormatRange &rhs)

Returns true if the start, length, and format fields in lhs and rhs contain the same values respectively.

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