Module java.desktop
Package java.awt.font

Class LineBreakMeasurer

java.lang.Object
java.awt.font.LineBreakMeasurer
public final class LineBreakMeasurer extends Object
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) {

     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(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;
     }
 }
See Also:

  • Constructor Details

    • LineBreakMeasurer

      public LineBreakMeasurer(AttributedCharacterIterator text, FontRenderContext frc)
      Constructs a LineBreakMeasurer for the specified text.
      Parameters:
      text - the text for which this LineBreakMeasurer produces TextLayout objects; the text must contain at least one character; if the text available through iter changes, further calls to this LineBreakMeasurer instance are undefined (except, in some cases, when insertChar or deleteChar are 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 the LineBreakMeasurer and user space
      See Also:

    • LineBreakMeasurer

      public LineBreakMeasurer(AttributedCharacterIterator text, BreakIterator breakIter, FontRenderContext frc)
      Constructs a LineBreakMeasurer for the specified text.
      Parameters:
      text - the text for which this LineBreakMeasurer produces TextLayout objects; the text must contain at least one character; if the text available through iter changes, further calls to this LineBreakMeasurer instance are undefined (except, in some cases, when insertChar or deleteChar are invoked afterward - see below)
      breakIter - the BreakIterator which defines line breaks
      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 the LineBreakMeasurer and user space
      Throws:
      IllegalArgumentException - if the text has less than one character
      See Also:

  • Method Details

    • nextOffset

      public int nextOffset(float wrappingWidth)
      Returns the position at the end of the next layout. Does NOT update the current position of this LineBreakMeasurer.
      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.

    • 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 this LineBreakMeasurer.
      Parameters:
      wrappingWidth - the maximum visible advance permitted for the text in the next layout
      offsetLimit - 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; offsetLimit must be greater than the current position
      requireNextWord - if true, the current position that is returned if the entire next word does not fit within wrappingWidth; if false, 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

    • nextLayout

      public TextLayout nextLayout(float wrappingWidth)
      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 within wrappingWidth

    • nextLayout

      public TextLayout nextLayout(float wrappingWidth, int offsetLimit, boolean requireNextWord)
      Returns the next layout, and updates the current position.
      Parameters:
      wrappingWidth - the maximum visible advance permitted for the text in the next layout
      offsetLimit - 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; offsetLimit must be greater than the current position
      requireNextWord - if true, and if the entire word at the current position does not fit within the wrapping width, null is returned. If false, 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 within wrappingWidth. If the current position is at the end of the text used by this LineBreakMeasurer, null is returned

    • getPosition

      public int getPosition()
      Returns the current position of this LineBreakMeasurer.
      Returns:
      the current position of this LineBreakMeasurer
      See Also:

    • setPosition

      public void setPosition(int newPosition)
      Sets the current position of this LineBreakMeasurer.
      Parameters:
      newPosition - the current position of this LineBreakMeasurer; the position should be within the text used to construct this LineBreakMeasurer (or in the text most recently passed to insertChar or deleteChar
      See Also:

    • insertChar

      public void insertChar(AttributedCharacterIterator newParagraph, int insertPos)
      Updates this LineBreakMeasurer after 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 insertion
      insertPos - the position in the text at which the character is inserted
      Throws:
      IndexOutOfBoundsException - if insertPos is less than the start of newParagraph or greater than or equal to the end of newParagraph
      NullPointerException - if newParagraph is null
      See Also:

    • deleteChar

      public void deleteChar(AttributedCharacterIterator newParagraph, int deletePos)
      Updates this LineBreakMeasurer after 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 deletion
      deletePos - the position in the text at which the character is deleted
      Throws:
      IndexOutOfBoundsException - if deletePos is less than the start of newParagraph or greater than the end of newParagraph
      NullPointerException - if newParagraph is null
      See Also: