The
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) {
Point2D pen = new Point2D(10, 20);
Graphics2D g2d = (Graphics2D)graphics;
FontRenderContext frc = g2d.getFontRenderContext();
// let styledText be an AttributedCharacterIterator containing at least
// one character
LineBreakMeasurer measurer = new LineBreakMeasurer(styledText, frc);
float wrappingWidth = getSize().width - 15;
while (measurer.getPosition() < fStyledText.length()) {
TextLayout layout = measurer.nextLayout(wrappingWidth);
pen.y += (layout.getAscent());
float dx = layout.isLeftToRight() ?
0 : (wrappingWidth - layout.getAdvance());
layout.draw(graphics, pen.x + dx, pen.y);
pen.y += 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(new Float(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;
}
}