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Intercepting Key Events

This article explains how to catch key events received by a text view so that you can modify the result. It also explains the message sequence that occurs when a text view receives a key event.

You need to intercept key events, for example, if you want users to be able to insert a line-break character in a text field. By default, text fields hold only one line of text. Pressing either Enter or Return causes the text field to end editing and send its action message to its target, so you would need to modify the behavior.

You may also wish to intercept key events in a text view to do something different from simply entering characters in the text being displayed by the view, such as changing the contents of an in-memory buffer.

In both circumstances you need to deal with the text view object, which is obvious for the text view case but less so for a text field. Editing in a text field is performed by an NSTextView object, called the field editor, shared by all the text fields belonging to a window.

When a text view receives a key event, it sends the character input to the input manager for key binding and interpretation. In response, the input manager sends either insertText: or doCommandBySelector: to the text view, depending on whether the key event represents text to be inserted or a command to perform. (The message sequence invoked when a text view receives key events is described in more detail in “The Key-Input Message Sequence.”)

With the standard key bindings, an Enter or Return character causes the text view to receive doCommandBySelector: with a selector of insertNewline:, which can have one of two results. If the text view is not a field editor, the text view’s insertText: method inserts a line-break character. If the text view is a field editor, as when the user is editing a text field, the text view ends editing instead. You can cause a text view to behave in either way by calling setFieldEditor:.

Although you could alter the text view’s behavior by subclassing the text view and overriding insertText: and doCommandBySelector:, a better solution is to handle the event in the text view’s delegate. The delegate can take control over user changes to text by implementing the textView:shouldChangeTextInRange:replacementString: method.

To handle keystrokes that don’t insert text, the delegate can implement the textView:doCommandBySelector: method.

To distinguish between Enter and Return, for example, the delegate can test the selector passed with doCommandBySelector:. If it is @selector(insertNewline:), you can send currentEvent to NSApp to make sure the event is a key event and, if so, which key was pressed.

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