NSLayoutManager object coordinates the layout and display of characters held in an
NSTextStorage object. It maps Unicode character codes to glyphs, sets the glyphs in a series of
NSTextContainer objects, and displays them in a series of
NSTextView objects. In addition to its core function of laying out text, an
NSLayoutManager object coordinates its
NSTextView objects, provides services to those text views to support
NSRulerView instances for editing paragraph styles, and handles the layout and display of text attributes not inherent in glyphs (such as underline or strikethrough). You can create a subclass of
NSLayoutManager to handle additional text attributes, whether inherent or not.
- iOS 7.0+
- macOS 10.0+
- tvOS 7.0+
NSLayoutManager provides the threshold for text antialiasing. It looks at the
AppleAntiAliasingThreshold default value. If the font size is smaller than or equal to this threshold size, the text is rendered aliased by
NSLayoutManager. In macOS, you can change the threshold value from the Appearance pane of System Preferences.
Thread Safety of NSLayoutManager
Generally speaking, a given layout manager (and associated objects) should not be used in more than one block, operation, or thread at a time. Most layout managers are used on the main thread, since it is the main thread on which their text views are displayed, and since background layout occurs on the main thread.
If you want to use a layout manager on a background thread, first make sure that text views associated with that layout manager (if any) are not displayed while the layout manager is being used on the background thread, and, second, turn off background layout for that layout manager while it is being used on the background thread. The most effective way to ensure that no text view is displayed, without knowing deep implementation, is just not to connect a text view to the layout manager.
Noncontiguous layout is an optional layout manager behavior. Previously, both glyph generation and layout were always performed, in order, from the beginning to the end of the document. When noncontiguous layout is turned on, however, the layout manager gains the option of performing glyph generation or layout for one portion of the document without having done so for previous sections. This can provide significant performance improvements for large documents.
Noncontiguous layout is not turned on automatically because direct clients of
NSLayoutManager typically have relied on the previous behavior—for example, by forcing layout for a given glyph range, and then assuming that previous glyphs would therefore be laid out. Clients who use
NSLayoutManager only indirectly—for example, those who use
NSTextView without directly calling the underlying layout manager—can usually turn on noncontiguous layout without difficulty. Clients using
NSLayoutManager directly need to examine their usage before turning on noncontiguous layout.
Enable noncontiguous layout using the
allowsNonContiguousLayout property. In addition, see the other methods in Causing Glyph Generation and Layout, many of which enable you to ensure that glyph generation and layout are performed for specified portions of the text. The behavior of a number of other layout manager methods is affected by the state of noncontiguous layout, as noted in the discussion sections of those method descriptions.