Implementing Text Management Support

Custom Cocoa views can provide varying levels of support for the text input management system. There are essentially three levels of support to choose from:

1. Override the keyDown: method.

2. Override keyDown: and use interpretKeyEvents: to support key bindings.

3. Also implement the full NSTextInput protocol.

In the first level of support, the keyDown: method recognizes a limited set of events and ignores others. This level of support is typical of games. (When overriding keyDown:, you must also override acceptsFirstResponder to make your custom view respond to key events, as described in Event Objects and Types.)

In the second level of support, you can override keyDown: and use the interpretKeyEvents: method to receive key-binding support without implementing the NSTextInput protocol. You then implement the standard key-binding methods that your view wants to support, such as moveForward: or deleteForward:. (The full list of key-binding methods can be found in NSResponder.h.)

If you are writing your own text view from scratch, you should use the third level of support and implement the NSTextInput protocol in addition to overriding keyDown: and using interpretKeyEvents:. NSTextView and its subclasses are the only classes provided in Cocoa that implement NSTextInput, and if your application needs more complex behavior than NSTextView can provide, as a word processor might, you may need to implement a text view from the ground up. To do this, you must subclass NSView and implement the NSTextInput protocol. A class implementing this protocol—by inheriting from NSTextView or another class, or by implementing the protocol directly—is called a text view.

If you are implementing NSTextInput, your view needs to manage marked text and communicate with the input server to support the text input management system. These tasks are described in the next two sections.

Managing Marked Text

One of the primary things that a text view must do to cooperate with an input server is to maintain a (possibly empty) range of marked text within its text storage. The text view should highlight text in this range in a distinctive way, and it should allow selection within the marked text. A text view must also maintain an insertion point, which is usually at the end of the marked text, but the user can place it within the marked text. The text view also maintains a (possibly empty) selection range within its text storage, and if there is any marked text, the selection must be entirely within the marked text.

A common example of marked text appears when a user enters a character with multiple keystrokes, such as “é”, in an NSTextView object. To enter this character, the user needs to type Option-E followed by the E key. After pressing Option-E, the accent mark appears in a highlighted box, indicating that the text is marked (not final). After the final E is pressed, the “é” character appears and the highlight disappears.

Communicating With the Input Server

An input server and a text view must cooperate so that the input server can implement its user interface. The bulk of the NSTextInput protocol comprises methods called by an input server to manipulate text within the text view for the input server’s user interface purposes.

A text view must also inform the current input manager when its marked text range changes or a mouse event happens. The text view accomplishes this by calling the markedTextSelectionChanged:client:, markedTextAbandoned:, and handleMouseEvent: methods with the current input manager.

If the text view receives a mouse event within its text area, and a marked text range exists, it must call the current input manager’s wantsToHandleMouseEvents. If wantsToHandleMouseEvents returns YES, it must forward those mouse events to the input manager’s handleMouseEvent: method. If it returns NO, it is up to the text view to determine what to do with the event.

If the input server returns NO to both wantsToHandleMouseEvents and handleMouseEvent:, and the text view decides to change selection within the marked range, it must notify the current input manager of the change with the markedTextSelectionChanged:client: method. In general, however, the input server is responsible for managing the selection within the marked text.

Also, the text view must call the current input manager’s markedTextAbandoned: method when the insertion point leaves the marked text or whenever it decides to clear the marked range.

The current input server generally uses all of the methods in the NSTextInput protocol except for the hasMarkedText method. The text view itself may call this method to determine whether there is currently marked text. NSTextView, for example, disables Copy in the Edit menu when hasMarkedText returns YES.