A template that describes available predicates and how to display them.
- macOS 10.5+
You can create instances of
NSPredicate programmatically or in Interface Builder. By default, a noncompound row template has three views: a popup (or static text field) on the left, a popup or static text field for operators, and either a popup or other view on the right. You can subclass
NSPredicate to create a row template with different numbers or types of views.
NSPredicate is a concrete class, but it has five primitive methods that are called by
NSPredicate implements all of these methods, but you can override them for custom templates. The primitive methods are used by an instance of
NSPredicate as follows.
First, an instance of
NSPredicate is created, and some row templates are set on it—either through a nib file or programmatically. The first thing predicate editor does is ask each of the templates for their views, using
After setting up the predicate editor, you typically send it a
object message to restore a saved predicate.
NSPredicate needs to determine which of its templates should display each predicate in the predicate tree. It does this by sending each of its row templates a
match message and choosing the one that returns the highest value.
After finding the best match for a predicate,
NSPredicate copies that template to get fresh views, inserts them into the proper row, and then sets the predicate on the template using
set. Within that method, the
NSPredicate object must set its views' values to represent that predicate.
NSPredicate next asks the template for the “displayable sub-predicates” of the predicate by sending a
displayable message. If a template represents a predicate in its entirety, or if the predicate has no subpredicates, it can return
nil for this. Otherwise, it should return a list of predicates to be made into sub-rows of that template's row. The whole process repeats for each sub-predicate.
At this point, the user sees the predicate that was saved. If the user then makes some changes to the views of the templates, this causes
NSPredicate to recompute its predicate by asking each of the templates to return the predicate represented by the new view values, passing in the subpredicates represented by the sub-rows (an empty array if there are none, or
nil if they aren't supported by that predicate type):