Drawing Strings
There are three ways to draw text programmatically in Cocoa: using methods of NSString
or NSAttributedString
, using those of NSCell
, and using NSLayoutManager
directly. NSLayoutManager
is the most efficient.
Using the String-Drawing Convenience Methods
The NSString
class has two convenience methods for drawing string objects directly in an NSView
object: drawAtPoint:withAttributes:
and drawInRect:withAttributes:
. For strings that have multiple attributes associated with ranges and individual characters, you must use NSAttributedString
. You can draw a string (in a focused NSView
) with either the drawAtPoint:
or drawInRect:
method. These methods are designed for drawing small amounts of text or text that must be drawn rarely. They create and dispose of various supporting text objects, including NSLayoutManager
, every time you call them.
For repeated drawing of text, however, the string drawing convenience methods are not efficient because they do a lot of work behind the scenes. For example, to draw Unicode text, you must first convert the characters into glyphs, the elements of a font. Glyph generation is complicated because several characters may produce a single glyph and vice versa, depending on the context and other factors. In addition, the system does a lot of work setting up for glyph conversion, and the string drawing convenience methods do this work each time they draw a string. Using the layout manager directly provides significant performance improvements because it caches glyph layout and size information.
Drawing Text With NSCell
The NSCell
class also provides primitives for displaying and editing text. NSCell
text drawing methods are used by NSBrowser
and NSTableView
. Text drawing by NSCell
is more efficient than using the string convenience methods because it caches some information, such as the size of the text rectangle. So, for displaying the same text repeatedly, NSCell
works well, but for the most efficient display of an arbitrary text string, use NSLayoutManager
directly.
Drawing Text With NSLayoutManager
If you use the NSTextView
class to display text, either by dragging a text view object from the Interface Builder Data palette or creating a text view programmatically using the NSTextView
method initWithFrame:
method, Cocoa automatically creates an NSLayoutManager
instance to draw your text. If you create a text view using the NSTextView
initWithFrame:textContainer:
method, however, or if you need to draw text directly into a different type of NSView
object, you must create the NSLayoutManager
explicitly.
To use NSLayoutManager
to draw a text string directly into a view, you must create and initialize the three basic non-view components of the text system. First create an NSTextStorage
object to hold the string. Then create an NSTextContainer
object to describe the geometric area for the text. Then create the NSLayoutManager
object and hook the three objects together by adding the layout manager to the text storage object and adding the text container to the layout manager. The code in Listing 1, which could reside in the view’s initWithFrame:
method, illustrates this procedure.
Listing 1 Creating and configuring the non-view text objects
NSTextStorage *textStorage = [[NSTextStorage alloc] |
initWithString:@"This is the text string."]; |
NSLayoutManager *layoutManager = [[NSLayoutManager alloc] init]; |
NSTextContainer *textContainer = [[NSTextContainer alloc] init]; |
[layoutManager addTextContainer:textContainer]; |
[textContainer release]; |
[textStorage addLayoutManager:layoutManager]; |
[layoutManager release]; |
You can release the text container because the layout manager retains it, and you can release the layout manager because the text storage object retains it.
To draw glyphs directly in a view, you can use the NSLayoutManager
method drawGlyphsForGlyphRange:
in the view’s drawRect:
method. However, you must first convert the character range you want to draw into a glyph range. If you need to select a subrange of the text in the text storage object, you can use the glyphRangeForCharacterRange:actualCharacterRange:
method. If you want to draw the entire string in the text storage object, use the glyphRangeForTextContainer:
method as in Listing 2 (which uses the layoutManager
variable name from Listing 1).
Listing 2 Drawing glyphs directly in a view
NSRange glyphRange = [layoutManager |
glyphRangeForTextContainer:textContainer]; |
[self lockFocus]; |
[layoutManager drawGlyphsForGlyphRange: glyphRange atPoint: rect.origin]; |
[self unlockFocus]; |
String Drawing and Typesetter Behaviors
There are differences among Cocoa’s three ways to draw text with regard to typesetter behavior, which is described in Typesetter Behaviors and Versions. By default, the string-drawing convenience methods and NSCell
objects supplied by the Application Kit use NSTypesetterBehavior_10_2_WithCompatibility
, whereas NSLayoutManager
objects use NSTypesetterLatestBehavior
. 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.
In cases where you must measure text one way and render it another, set the typesetter behavior to match using the setTypesetterBehavior:
method defined by NSLayoutManager
and NSTypesetter
. For example, if you need to use an NSLayoutManager
object to measure text and convenience string drawing methods to draw it, change the layout manager’s typesetter behavior to NSTypesetterBehavior_10_2_WithCompatibility
.
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