绘制多行文本

If you have a paragraph of styled text that you would like to fit within a specific width, you can use the LineBreakMeasurer class. This class enables styled text to be broken into lines so that they fit within a particular visual advance. Each line is returned as a TextLayout object, which represents unchangeable, styled character data. However, this class also enables access to layout information. The getAscent and getDescent methods of TextLayout return information about the font that is used to position the lines in the component. The text is stored as an AttributedCharacterIterator object so that the font and point size attributes can be stored with the text.

The following applet positions a paragraph of styled text within a component, using LineBreakMeasurer, TextLayout and AttributedCharacterIterator.

The complete code for this applet is in LineBreakSample.java.

The following code creates an iterator with the string vanGogh. The start and end of the iterator is retrieved and a new LineBreakMeasurer is created from the iterator.

    AttributedCharacterIterator paragraph = vanGogh.getIterator();
    paragraphStart = paragraph.getBeginIndex();
    paragraphEnd = paragraph.getEndIndex();
    FontRenderContext frc = g2d.getFontRenderContext();
    lineMeasurer = new LineBreakMeasurer(paragraph, frc);

The size of the window is used to determine where the line should break. Also a TextLayout object is created for each line in the paragraph.

// Set break width to width of Component.
float breakWidth = (float)getSize().width;
float drawPosY = 0;
// Set position to the index of the first
// character in the paragraph.
lineMeasurer.setPosition(paragraphStart);

// Get lines from until the entire paragraph
// has been displayed.
while (lineMeasurer.getPosition() < paragraphEnd) {

    TextLayout layout = lineMeasurer.nextLayout(breakWidth);

    // Compute pen x position. If the paragraph
    // is right-to-left we will align the
    // TextLayouts to the right edge of the panel.
    float drawPosX = layout.isLeftToRight()
        ? 0 : breakWidth - layout.getAdvance();

    // Move y-coordinate by the ascent of the
    // layout.
    drawPosY += layout.getAscent();

    // Draw the TextLayout at (drawPosX,drawPosY).
    layout.draw(g2d, drawPosX, drawPosY);

    // Move y-coordinate in preparation for next
    // layout.
    drawPosY += layout.getDescent() + layout.getLeading();
}

The TextLayout class is not frequently created directly by applications. However, this class is useful when applications need to work directly with text that has had styles (text attributes) applied at specific positions in text. For example, to draw a single word italicized in a paragraph, an application would need to perform measurements and set the font for each substring. If the text is bidirectional, this task is not so easy to do correctly. Creating a TextLayout object from an AttributedString object handles this problem for you. Consult the Java SE specification for more information about TextLayout.