Typesetters

The layout manager uses a helper object called a typesetter to lay out glyphs in line fragments. Typesetter objects are instantiated from a concrete subclass of NSTypesetter.

Working with other objects in the Cocoa text system, the typesetter creates line fragment rectangles, places glyphs within the line fragments, determines line breaks by word wrapping and hyphenation, and handles tab positioning. The typesetter also determines interline spacing, paragraph spacing, and the right-to-left positioning of bidirectional glyphs.

Filling Line Fragment Rectangles

The typesetter object generates line fragments by communicating with the text container, as described in “Line Fragment Generation.” The typesetter determines the suitable line fragment sizes and positions, which it returns in container coordinates.

After creating a line fragment rectangle, the typesetter determines the positions of glyphs within it, in response to the layoutGlyphsInLayoutManager:startingAtGlyphIndex:maxNumberOfLineFragments:nextGlyphIndex:message from the layout manager. The typesetter reports the glyph locations relative to the origin of their line fragment’s bounding rectangle. The typesetter fills the line fragment until it goes beyond the line fragment’s width. Then it creates a line break by wrapping text or hyphenating the last word. In this step, the typesetter performs glyph substitution, if necessary, and may add glyphs to the glyph stream. For example, the typesetter may substitute a ligature glyph for one or more single-character glyphs, or it may add a hyphen to the glyph stream.

NSTypesetter subclasses can control line breaking at word boundaries by overriding the shouldBreakLineByWordBeforeCharacterAtIndex: method. Similarly, subclasses can intervene in line breaking by hyphenation by overriding the shouldBreakLineByHyphenatingBeforeCharacterAtIndex: method.

Whenever the width of the laid-out line, divided by the width of the line rectangle, exceeds a hyphenation threshold maintained by the layout manager, the typesetter calls an internal hyphenator object which attempts to find hyphenation points in the last word in the line. If the hyphenator finds a good point, the typesetter inserts a hyphen glyph at the end of the line fragment rectangle.

Hyphenation is controlled by a threshold called the hyphenation factor, which is maintained by the layout manager. You can set the threshold using the NSLayoutManager method setHyphenationFactor:. the hyphenation factor is a float that ranges between 0.0 and 1.0. By default, its value is 0.0, meaning hyphenation is off. Setting the hyphenation factor to 1.0 causes the typesetter to attempt hyphenation always.

Typesetter Behaviors and Versions

The text system uses a shared, reentrant typesetter instance, made available by the NSLayoutManager method typesetter. The NSLayoutManager method setTypesetterBehavior: selects among the original default typesetter shipped with OS X prior to version 10.2, a typesetter encapsulating Apple Type Services (ATS) that shipped with OS X version 10.2, an enhanced version of the ATS-based typesetter that shipped with OS X version 10.3., and the typesetter behavior introduced in OS X version 10.4. The NSTypesetterBehavior enumeration defines the relevant constants.

The NSTypesetter subclass that implements the original typesetter behavior is NSSimpleHorizontalTypesetter, which is defined in the NSTypesetter.h header file. NSSimpleHorizontalTypesetter supports glyph layout with a left-to-right sweep and downward line movement only. NSSimpleHorizontalTypesetter is deprecated in OS X version 10.4 and later.

The typesetter behavior introduced in OS X version 10.2 is implemented by the NSATSTypesetter class, which is defined in the NSATSTypesetter.h header file. NSATSTypesetter provides enhanced line and character spacing accuracy and supports more languages, including bidirectional languages, than the original NSSimpleHorizontalTypesetter.

OS X version 10.3 introduced a new version of the NSATSTypesetter that declares public APIs for NSATSTypesetter and NSGlyphGenerator. These APIs open up the typesetter for use with a custom layout engine having a design different from the traditional Cocoa text system, as described in “Design of NSTypesetter.” In OS X version 10.4, these APIs moved to NSTypesetter.

Unless you require a specific behavior of an earlier typesetter version, you should use or subclass the latest version of NSATSTypesetter.

It is important to use the same typesetter behavior when both measuring and rendering text, to avoid differences in paragraph spacing, line spacing, and head indent handling. See “String Drawing and Typesetter Behaviors” for more information about typesetter behavior mismatches.

Design of NSTypesetter

In the Cocoa text system, the layout manager owns the typesetter and glyph generator as private objects and maintains an array of text containers, as described in “The Layout Manager.” The typesetter concept is tightly coupled with the layout manager and text container concepts. The typesetter’s responsibility is to fill the text containers in the array with glyphs supplied by the glyph generator. By default, NSATSTypesetter works in this way. However, NSTypesetter is designed to enable developers to decouple it from the other components of the Cocoa text system.

The design of NSTypesetter isolates the primitive, core typesetter from the rest of the Cocoa text system, as shown in Figure 1. NSTypesetter has a core typesetting engine, a layout phase interface, and a glyph storage interface layer that communicates with the text system and drives the layout engine. The core typesetting engine provides advanced typographic capabilities through a simplified API. The core typesetting engine lays out glyphs in an infinite horizontal line and knows nothing about text containers or text direction. The glyph storage interface layer calls out to the text system to generate line fragment rectangles and make sure they fit onto the page properly.

Figure 1  Design of NSTypesetter
Design of NSATSTypesetter

The API design for NSTypesetter has two primary goals. The first is to break the tie between the two classes and NSLayoutManager, allowing developers to tap deeply into Cocoa’s typographic capabilities without using NSLayoutManager. The second goal is to provide override points to allow developers to extend various aspects of the typesetting process. In addition, direct access to these classes makes it easier to port Carbon, Windows, or UNIX applications with their own layout engines to Cocoa.

NSTypesetter categorizes its methods as follows:

With its layered design, NSTypesetter can be instantiated and used in its standard configuration with the Cocoa text system or subclassed and adapted to work with another text system, even one that has entirely different concepts of how to perform page layout.