Class LineBreakMeasurer
LineBreakMeasurer class allows styled text to be
broken into lines (or segments) that fit within a particular visual
advance. This is useful for clients who wish to display a paragraph of
text that fits within a specific width, called the wrapping
width.
LineBreakMeasurer is constructed with an iterator over
styled text. The iterator's range should be a single paragraph in the
text.
LineBreakMeasurer maintains a position in the text for the
start of the next text segment. Initially, this position is the
start of text. Paragraphs are assigned an overall direction (either
left-to-right or right-to-left) according to the bidirectional
formatting rules. All segments obtained from a paragraph have the
same direction as the paragraph.
Segments of text are obtained by calling the method
nextLayout, which returns a TextLayout
representing the text that fits within the wrapping width.
The nextLayout method moves the current position
to the end of the layout returned from nextLayout.
LineBreakMeasurer implements the most commonly used
line-breaking policy: Every word that fits within the wrapping
width is placed on the line. If the first word does not fit, then all
of the characters that fit within the wrapping width are placed on the
line. At least one character is placed on each line.
The TextLayout instances returned by
LineBreakMeasurer treat tabs like 0-width spaces. Clients
who wish to obtain tab-delimited segments for positioning should use
the overload of nextLayout which takes a limiting offset
in the text.
The limiting offset should be the first character after the tab.
The TextLayout objects returned from this method end
at the limit provided (or before, if the text between the current
position and the limit won't fit entirely within the wrapping
width).
Clients who are laying out tab-delimited text need a slightly
different line-breaking policy after the first segment has been
placed on a line. Instead of fitting partial words in the
remaining space, they should place words which don't fit in the
remaining space entirely on the next line. This change of policy
can be requested in the overload of nextLayout which
takes a boolean parameter. If this parameter is
true, nextLayout returns
null if the first word won't fit in
the given space. See the tab sample below.
In general, if the text used to construct the
LineBreakMeasurer changes, a new
LineBreakMeasurer must be constructed to reflect
the change. (The old LineBreakMeasurer continues to
function properly, but it won't be aware of the text change.)
Nevertheless, if the text change is the insertion or deletion of a
single character, an existing LineBreakMeasurer can be
'updated' by calling insertChar or
deleteChar. Updating an existing
LineBreakMeasurer is much faster than creating a new one.
Clients who modify text based on user typing should take advantage
of these methods.
Examples:
Rendering a paragraph in a component
public void paint(Graphics graphics) { float dx = 0f, dy = 5f; Graphics2D g2d = (Graphics2D)graphics; FontRenderContext frc = g2d.getFontRenderContext(); AttributedString text = new AttributedString("....."); AttributedCharacterIterator paragraph = text.getIterator(); LineBreakMeasurer measurer = new LineBreakMeasurer(paragraph, frc); measurer.setPosition(paragraph.getBeginIndex()); float wrappingWidth = (float)getSize().width; while (measurer.getPosition() < paragraph.getEndIndex()) { TextLayout layout = measurer.nextLayout(wrappingWidth); dy += (layout.getAscent()); float dx = layout.isLeftToRight() ? 0 : (wrappingWidth - layout.getAdvance()); layout.draw(graphics, dx, dy); dy += layout.getDescent() + layout.getLeading(); } }
Rendering text with tabs. For simplicity, the overall text direction is assumed to be left-to-right
public void paint(Graphics graphics) { float leftMargin = 10, rightMargin = 310; float[] tabStops = { 100, 250 }; // assume styledText is an AttributedCharacterIterator, and the number // of tabs in styledText is tabCount int[] tabLocations = new int[tabCount+1]; int i = 0; for (char c = styledText.first(); c != styledText.DONE; c = styledText.next()) { if (c == '\t') { tabLocations[i++] = styledText.getIndex(); } } tabLocations[tabCount] = styledText.getEndIndex() - 1; // Now tabLocations has an entry for every tab's offset in // the text. For convenience, the last entry is tabLocations // is the offset of the last character in the text. LineBreakMeasurer measurer = new LineBreakMeasurer(styledText); int currentTab = 0; float verticalPos = 20; while (measurer.getPosition() < styledText.getEndIndex()) { // Lay out and draw each line. All segments on a line // must be computed before any drawing can occur, since // we must know the largest ascent on the line. // TextLayouts are computed and stored in a Vector; // their horizontal positions are stored in a parallel // Vector. // lineContainsText is true after first segment is drawn boolean lineContainsText = false; boolean lineComplete = false; float maxAscent = 0, maxDescent = 0; float horizontalPos = leftMargin; Vector layouts = new Vector(1); Vector penPositions = new Vector(1); while (!lineComplete) { float wrappingWidth = rightMargin - horizontalPos; TextLayout layout = measurer.nextLayout(wrappingWidth, tabLocations[currentTab]+1, lineContainsText); // layout can be null if lineContainsText is true if (layout != null) { layouts.addElement(layout); penPositions.addElement(Float.valueOf(horizontalPos)); horizontalPos += layout.getAdvance(); maxAscent = Math.max(maxAscent, layout.getAscent()); maxDescent = Math.max(maxDescent, layout.getDescent() + layout.getLeading()); } else { lineComplete = true; } lineContainsText = true; if (measurer.getPosition() == tabLocations[currentTab]+1) { currentTab++; } if (measurer.getPosition() == styledText.getEndIndex()) lineComplete = true; else if (horizontalPos >= tabStops[tabStops.length-1]) lineComplete = true; if (!lineComplete) { // move to next tab stop int j; for (j=0; horizontalPos >= tabStops[j]; j++) {} horizontalPos = tabStops[j]; } } verticalPos += maxAscent; Enumeration layoutEnum = layouts.elements(); Enumeration positionEnum = penPositions.elements(); // now iterate through layouts and draw them while (layoutEnum.hasMoreElements()) { TextLayout nextLayout = (TextLayout) layoutEnum.nextElement(); Float nextPosition = (Float) positionEnum.nextElement(); nextLayout.draw(graphics, nextPosition.floatValue(), verticalPos); } verticalPos += maxDescent; } }
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Constructor Summary
ConstructorsConstructorDescriptionConstructs aLineBreakMeasurerfor the specified text.LineBreakMeasurer(AttributedCharacterIterator text, BreakIterator breakIter, FontRenderContext frc) Constructs aLineBreakMeasurerfor the specified text. -
Method Summary
Modifier and TypeMethodDescriptionvoiddeleteChar(AttributedCharacterIterator newParagraph, int deletePos) Updates thisLineBreakMeasurerafter a single character is deleted from the text, and sets the current position to the beginning of the paragraph.intReturns the current position of thisLineBreakMeasurer.voidinsertChar(AttributedCharacterIterator newParagraph, int insertPos) Updates thisLineBreakMeasurerafter a single character is inserted into the text, and sets the current position to the beginning of the paragraph.nextLayout(float wrappingWidth) Returns the next layout, and updates the current position.nextLayout(float wrappingWidth, int offsetLimit, boolean requireNextWord) Returns the next layout, and updates the current position.intnextOffset(float wrappingWidth) Returns the position at the end of the next layout.intnextOffset(float wrappingWidth, int offsetLimit, boolean requireNextWord) Returns the position at the end of the next layout.voidsetPosition(int newPosition) Sets the current position of thisLineBreakMeasurer.
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Constructor Details
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LineBreakMeasurer
Constructs aLineBreakMeasurerfor the specified text.- Parameters:
text- the text for which thisLineBreakMeasurerproducesTextLayoutobjects; the text must contain at least one character; if the text available throughiterchanges, further calls to thisLineBreakMeasurerinstance are undefined (except, in some cases, wheninsertCharordeleteCharare invoked afterward - see below)frc- contains information about a graphics device which is needed to measure the text correctly; text measurements can vary slightly depending on the device resolution, and attributes such as antialiasing; this parameter does not specify a translation between theLineBreakMeasurerand user space- See Also:
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LineBreakMeasurer
public LineBreakMeasurer(AttributedCharacterIterator text, BreakIterator breakIter, FontRenderContext frc) Constructs aLineBreakMeasurerfor the specified text.- Parameters:
text- the text for which thisLineBreakMeasurerproducesTextLayoutobjects; the text must contain at least one character; if the text available throughiterchanges, further calls to thisLineBreakMeasurerinstance are undefined (except, in some cases, wheninsertCharordeleteCharare invoked afterward - see below)breakIter- theBreakIteratorwhich defines line breaksfrc- contains information about a graphics device which is needed to measure the text correctly; text measurements can vary slightly depending on the device resolution, and attributes such as antialiasing; this parameter does not specify a translation between theLineBreakMeasurerand user space- Throws:
IllegalArgumentException- if the text has less than one character- See Also:
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Method Details
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nextOffset
public int nextOffset(float wrappingWidth) Returns the position at the end of the next layout. Does NOT update the current position of thisLineBreakMeasurer.- Parameters:
wrappingWidth- the maximum visible advance permitted for the text in the next layout- Returns:
- an offset in the text representing the limit of the
next
TextLayout.
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nextOffset
public int nextOffset(float wrappingWidth, int offsetLimit, boolean requireNextWord) Returns the position at the end of the next layout. Does NOT update the current position of thisLineBreakMeasurer.- Parameters:
wrappingWidth- the maximum visible advance permitted for the text in the next layoutoffsetLimit- the first character that can not be included in the next layout, even if the text after the limit would fit within the wrapping width;offsetLimitmust be greater than the current positionrequireNextWord- iftrue, the current position that is returned if the entire next word does not fit withinwrappingWidth; iffalse, the offset returned is at least one greater than the current position- Returns:
- an offset in the text representing the limit of the
next
TextLayout
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nextLayout
Returns the next layout, and updates the current position.- Parameters:
wrappingWidth- the maximum visible advance permitted for the text in the next layout- Returns:
- a
TextLayout, beginning at the current position, which represents the next line fitting withinwrappingWidth
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nextLayout
Returns the next layout, and updates the current position.- Parameters:
wrappingWidth- the maximum visible advance permitted for the text in the next layoutoffsetLimit- the first character that can not be included in the next layout, even if the text after the limit would fit within the wrapping width;offsetLimitmust be greater than the current positionrequireNextWord- iftrue, and if the entire word at the current position does not fit within the wrapping width,nullis returned. Iffalse, a valid layout is returned that includes at least the character at the current position- Returns:
- a
TextLayout, beginning at the current position, that represents the next line fitting withinwrappingWidth. If the current position is at the end of the text used by thisLineBreakMeasurer,nullis returned
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getPosition
public int getPosition()Returns the current position of thisLineBreakMeasurer.- Returns:
- the current position of this
LineBreakMeasurer - See Also:
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setPosition
public void setPosition(int newPosition) Sets the current position of thisLineBreakMeasurer.- Parameters:
newPosition- the current position of thisLineBreakMeasurer; the position should be within the text used to construct thisLineBreakMeasurer(or in the text most recently passed toinsertCharordeleteChar- See Also:
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insertChar
Updates thisLineBreakMeasurerafter a single character is inserted into the text, and sets the current position to the beginning of the paragraph.- Parameters:
newParagraph- the text after the insertioninsertPos- the position in the text at which the character is inserted- Throws:
IndexOutOfBoundsException- ifinsertPosis less than the start ofnewParagraphor greater than or equal to the end ofnewParagraphNullPointerException- ifnewParagraphisnull- See Also:
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deleteChar
Updates thisLineBreakMeasurerafter a single character is deleted from the text, and sets the current position to the beginning of the paragraph.- Parameters:
newParagraph- the text after the deletiondeletePos- the position in the text at which the character is deleted- Throws:
IndexOutOfBoundsException- ifdeletePosis less than the start ofnewParagraphor greater than the end ofnewParagraphNullPointerException- ifnewParagraphisnull- See Also:
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