Core Text

Core Text has two major parts: a layout engine and font technology, each backed by its own collection of opaque types.

Core Text Layout Opaque Types

Core Text requires two objects whose opaque types are not native to it: an attributed string (CFAttributedStringRef) and a graphics path (CGPathRef). An attributed-string object encapsulates a string backing the displayed text and includes properties(or, “attributes”) that define stylistic aspects of the characters in the string—for example, font and color. The graphics path defines the shape of a frame of text, which is equivalent to a paragraph.

Core Text objects at runtime form a hierarchy that is reflective of the level of the text being processed (see Figure 8-1). At the top of this hierarchy is the framesetter object (CTFramesetterRef). With an attributed string and a graphics path as input, a framesetter generates one or more frames of text (CTFrameRef). As the text is laid out in a frame, the framesetter applies paragraph styles to it, including such attributes as alignment, tab stops, line spacing, indentation, and line-breaking mode.

To generate frames, the framesetter calls a typesetter object (CTTypesetterRef). The typesetter converts the characters in the attributed string to glyphs and fits those glyphs into the lines that fill a text frame. (A glyph is a graphic shape used to represent a character.) A line in a frame is represented by a CTLine object (CTLineRef). A CTFrame object contains an array of CTLineobjects.

A CTLine object, in turn, contains an array of glyph runs, represented by objects of the CTRunRef type. A glyph run is a series of consecutive glyphs that have the same attributes and direction. Although a typesetter object returns CTLine objects, it composes those lines from arrays of glyph runs.

Figure 8-1  Architecture of the Core Text layout engine

Using functions of the CTLine opaque type, you can draw a line of text from an attributed string without having to go through the CTFramesetter object. You simply position the origin of the text on the text baseline and request the line object to draw itself.

Core Text Font Opaque Types

Fonts are essential to text processing in Core Text. The typesetter object uses fonts (along with the source attributed string) to convert glyphs from characters and then position those glyphs relative to one another. A graphics context is central to fonts in Core Text. You can use graphics-context functions to set the current font and draw glyphs; or you can create a CTLine object from an attributed string and use its functions to draw into the graphics context. The Core Text font system handles Unicode fonts natively.

The font system includes objects of three opaque types: CTFont, CTFontDescriptor, and CTFontCollection:

• Font objects (CTFontRef) are initialized with a point size and specific characteristics (from a transformation matrix). You can query the font object for its character-to-glyph mapping, its encoding, glyph data, and metrics such as ascent, leading, and so on. Core Text also offers an automatic font-substitution mechanism called font cascading.

• Font descriptor objects (CTFontDescriptorRef) are typically used to create font objects. Instead of dealing with a complex transformation matrix, they allow you to specify a dictionary of font attributes that include such properties as PostScript name, font family and style, and traits (for example, bold or italic).

• Font collection objects (CTFontCollectionRef) are groups of font descriptors that provide services such as font enumeration and access to global and custom font collections.

It’s possible to convert UIFont objects to CTFont objects by calling CTFontCreateWithName, passing the font name and point size encapsulated by the UIFont object.

The Core Text Object Model

If you are a bit confused about the CTFramesetter and the CTFrame – that’s OK. Here I’ll make a short detour to explain how Core Text renders text content.

Here’s what the Core Text object model looks like:

You create a CTFramesetter reference and you provide it with NSAttributedString. At this point, an instance of CTTypesetter is automatically created for you, a class that manages your fonts. Next you use the CTFramesetter to create one or more frames in which you will be rendering text.

When you create a frame you tell it the subrange of text that will be rendered inside its rectangle. Core Text then automatically creates a CTLine for each line of text and (pay attention here) a CTRun for each piece of text with the same formatting.

As an example, Core Text would create a CTRun if you had several words in a row colored red, then another CTRun for the following plain text, then another CTRun for a bold sentence, etc. Again: very important – you don’t create CTRun instances, Core Text creates them for you based on the attributes of the supplied NSAttributedString.

Each of these CTRun objects can adopt different attributes, so you have fine control over kerning, ligatures, width, height and more.