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Application Kit Reference: Java
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Inherits
from: NSObject
Package: com.apple.yellow.application
Class Description
NSResponder is an abstract class that forms the basis of event
and command processing in the Application Kit. The core classes-NSApplication, NSWindow,
and NSView-inherit from NSResponder, as must any class that handles
events. The responder model is built around three components: event messages,
action messages, and the responder chain. An event message is a message
corresponding directly to an input event, and includes as its sole argument
an NSEvent object describing the event; a mouse down or keypress,
for example. An action message is a higher-level message indicating
a command to be performed, which includes as an argument the object
requesting the action. Some examples of action messages are the
standard cut:, copy:,
and paste:.
The responder chain is a series of responder objects to which
an event or action message is applied. When a given responder object
doesn't handle a particular message, the message is passed to
its successor in the chain. This allows responder objects to delegate
responsibility to other, typically higher-level objects. The responder
chain is constructed automatically as described below, but you can
insert custom objects into it using the setNextResponder method and examine
it with nextResponder.
An application can contain any number of responder chains,
but only one is active at any given time. It begins with the first
responder in some NSWindow and proceeds to the NSWindow itself.
The first responder is typically the "selected" NSView within
the NSWindow, and its next responder is its containing NSView (also
called its superview), and so on up to the NSWindow itself. You
can safely inject other responders between NSViews, but you can't
add responders past the NSWindow. Nearly all event messages apply
to a single window's responder chain.
For action messages, a more elaborate responder chain is used,
constructed from the individual responder chains of two NSWindows
and the application object itself. The NSWindows are the key window,
whose responder chain gets first crack at action messages, and the
main window, which follows. The main window is sometimes identical
to the key window; the two are typically distinguished when an auxiliary
window or panel related to a primary window-such as a Find Panel-is
opened. In this case the primary window, which was the key window,
becomes the main window, and the Find Panel becomes key. The two
windows and the NSApplication object also give their delegates a
chance to handle action messages as though they were responders, even
though a delegate isn't formally in the responder chain (a nextResponder message
to a window or application object doesn't return the delegate).
Given all these components, then, the full responder chain comprises
these objects:
- The key window's first responder and successors,
including objects added with setNextResponder
- The key window itself
- The key window's delegate (which need not inherit from NSResponder)
- The main window's first responder and successors, including
objects added with setNextResponder
- The main window itself
- The main window's delegate (which need not inherit from
NSResponder)
- The application object,
NSApp
- The application object's delegate (which need not inherit
from NSResponder)
Selecting the First Responder
The first responder is typically chosen by the user, with
the mouse or keyboard. The mechanism by which one object loses its
first responder status and another gains it is public though, and
you can programmatically change the first responder if necessary.
The method that changes the first responder is NSWindow's makeFirstResponder.
An NSWindow's first responder is initially itself, though you
can set which object will be first responder when the NSWindow is
first placed on-screen using the setInitialFirstResponder method.
makeFirstResponder always asks the
current first responder if it is ready to resign its status, using resignFirstResponder.
If the current first responder returns false
when
sent this message, makeFirstResponder fails
and likewise returns false
. If the
current first responder returns true
then
the new one is sent a becomeFirstResponder message
to inform it that it can be the first responder. This object can
return false
to reject the assignment,
in which case the NSWindow itself becomes the first responder.
When an NSWindow that's the key window receives a mouse-down
event, it automatically tries to make first responder the NSView
under the event. It does so by asking the NSView whether it wants
to become first responder, using the acceptsFirstResponder method defined
by this class, with the mouse-down event as the argument. This method
normally returns false
; responder subclasses
that need to be first responder must override it to return true
.
This method is also used when the user changes the first responder
using the keyboard.
Normally a mouse-down event in a non-key window simply brings
the window forward and makes it key, and isn't sent to the NSView
over which it occurs. The NSView can claim an initial mouse-down,
however, by implementing acceptsFirstMouse to return true
.
The argument is the mouse-down event, which the NSView can examine
to determine whether it wants to receive the mouse event and potentially
become first responder.
An additional consideration for responders that manage selections
is of course to set the selection. An NSView that handles mouse
events should set this itself. However, objects can also define
methods for setting their selection that automatically make the
receiver first responder as well. NSTextField's selectText, for
example, does something quite like this.
Event and Action Messages in the Responder Chain
The main purpose of the responder chain is to route events
and action messages to an appropriate target. Event and action methods
are dispatched in different ways, by different methods. Nearly all
events enter an application from the Window Server, and are handled
automatically by NSApplication's sendEvent method. Action messages
are instigated by objects, who use NSApplication's sendActionToTargetFromSender method
to route them to their proper destinations.
NSApplication's sendEvent analyzes
the event and handles some things specially-key equivalents, for
example. Most events, however, it passes to the appropriate window
for dispatch up its responder chain using NSWindow's sendEvent method.
NSResponder's default implementations of all event methods simply
pass the message to the next responder, so if no object in the responder
chain does anything with the event it's simply lost. As mentioned before,
an NSView's next responder is nearly always its superview, so
if, for example, the NSView that receives a mouseDown message doesn't handle
it, its superview gets a chance, and so on up to the NSWindow. If
no object is found to handle the event, the last responder in the
chain invokes noResponderFor:, which
for a key-down event simply beeps. Event-handling objects (subclasses
of NSWindow and NSView) can override this method to perform additional
steps as needed.
Event messages form a well-known set, so NSResponder provides implementations
for all of them. Action messages, however, are defined by custom
classes and can't be predicted. For this reason they're dispatched
in different manner from events. To instigate an action message,
an object invokes NSApplication'ssendActionToTargetFromSender.
The first argument is the selector for the action method to invoke.
The second is the intended recipient of the message, often called
the target. The final argument is usually the object invoking sendActionToTargetFromSender,
thus indicating which object instigated the action message. If the
intended target isn't null
, the
action is simply sent directly to that object; this is called a
targeted action message. In the case of an untargeted action message,
where the target is null
, sendActionToTargetFromSender searches
the full responder chain for an object that implements the action
method specified. If it finds one, it sends the message to that
object with the instigator of the action message as the sole argument.
The receiver of the action message can then use the argument directly as
input or query it for additional information. You can find the recipient
of an untargeted action message without actually sending the message
using targetForAction.
A more general mechanism, which applies to the shorter form
of the responder chain, is provided by NSResponder's tryToPerform. This
method checks the receiver to see if it responds to the selector
provided, if so invoking the message. If not, it sends tryToPerform to
its next responder. NSWindow and NSApplication override this method
to include their delegates, but they don't link individual responder
chains in the way that NSApplication's sendActionToTargetFromSender does.
Similar to tryToPerform is doCommandBySelector:,
which takes a method selector and tries to find a responder that
implements it. If none is found, the method beeps.
WARNING
NSResponder declares a number of action messages, but doesn't
actually implement them. You should never send an action message
directly to a responder object of an unknown class. Always use NSApplication's sendActionToTargetFromSender,
NSResponder's tryToPerform or doCommandBySelector:,
or check that the target responds using the NSObject method respondsToSelector:.
Implementing Event and Action Methods
Implementing event methods is fairly straightforward. If your
subclass handles a particular event, it overrides the method- keyDown, for example-usurping
the implementation of its superclass. If your subclass needs to
handle particular events some of the time-only some typed characters,
perhaps-then it must override the event method to handle the cases
it's interested in and to invoke super
's
implementation otherwise. This allows a superclass to catch the
cases it's interested in, and ultimately allows the event to continue
on its way along the responder chain if it isn't handled. "Key
Events" below describes how to handle keyboard events in your
application. See the NSView class specification for information
on handling mouse events.
Action methods don't have default implementations, so responder
subclasses shouldn't blindly forward action messages to super
.
Passing of action messages is predicated merely on whether an object
responds to the method, unlike with the passing of event messages.
Of course, if you know that a superclass does in fact implement
the method, you can pass it on up from your subclass.
Key Events
Processing keyboard input is by far the most complex part
of event handling. The Application Kit goes to great lengths to
ease this process for you, and in fact handling the key events that
get to your custom objects is fairly straightforward. However, a
lot happens to those events on their way from the hardware to the responder
chain. The sections below attempt to explain how events are handled through
the operating system and the Application Kit, so you can understand what
your objects receive and don't receive.
The Path of a Key Event
Physical keyboard events must pass through the operating system
before becoming NSEvent objects in the Application Kit. Depending
on the operating system, some of these "raw" events might be
trapped before they ever become NSEvent objects. Reserved key combinations
are often handled in this way. Key events that arrive at the Application
Kit are processed by NSApplication's sendEvent method as indicated
before. The application object filters out key equivalents (also
known as "Command key events") and sends them out as described
under "Key Equivalents and Mnemonics" below. All other key events are
passed to the key window's sendEvent method.
The key window first checks the event to see if the Control
key is pressed. If it is, the window treats the event as a forced
control event, which is blocked from the responder chain and is
processed immediately as a potential mnemonic or keyboard interface
control event. If this doesn't apply, the event is passed to the window's
first responder in a keyDown message,
which is how your custom responders receive uninterpreted key events. "Keyboard
Input" describes how you can handle these events.
If no view object in the key window accepts the key event,
NSWindow's keyDown attempts to handle the key event itself. It
tries to interpret the key event as each of the following, in order,
beeping if it fails to match any of them to let the user know the
typing couldn't be processed:
- A mnemonic matching the character(s) typed, not
requiring the Alternate key to be pressed
- A key equivalent, not requiring the Command (or Control) key
to be pressed
- A keyboard interface control event
Key Equivalents and Mnemonics
A key equivalent is a character bound to some view in a window,
which causes that view to perform a specified action when the user
types that character, usually while pressing the Command key (the
Control key on Microsoft Windows). A mnemonic works similarly, using
the Alternate key as its cue to action. If both modifier keys are
pressed, the key event is interpreted only as a mnemonic. A key
equivalent or mnemonic must be a character that can be typed with
no modifier keys, or with Shift only. Each is sent down the view
hierarchy of a window instead of up the responder chain, but at
different times.
Key equivalents are dispatched by the NSApplication object's sendEvent method.
On the Mach operating system, this results in a performKeyEquivalent message being
sent to every NSWindow in the application until one of them returns true
.
On the Microsoft Windows operating system, it results in a performKeyEquivalent message
being sent to the menu of the key window, and of the main window
if the key window's menu doesn't handle it. This difference
in handling means that, among other things, NSWindow subclasses
shouldn't override performKeyEquivalent.
Also, objects other than menu items shouldn't be assigned key
equivalents; they should instead be assigned mnemonics. Key equivalents
sent to a window on Mach are passed down the view hierarchy through
NSView's abstract implementation of performKeyEquivalent,
which forwards the message to each of its subviews until one responds true
,
returning false
if none does.
Mnemonics, on the other hand, are dispatched by the key window.
If the user presses the Control key as well as the mnemonic's
key combination, NSWindow's sendEvent immediately treats that
event as a mnemonic to be performed, without sending the event up
the responder chain. If the user doesn't press the Control key,
the event passes through the window's responder chain, possibly
being handled by a responder, before arriving as a keyDown message to
the window. In either case, a mnemonic for a top-level menu on Microsoft Windows
is sent back to the operating system, and eventually results in
the Application Kit invoking a menu item's action. Any other mnemonic
is handled by sending a performMnemonic: message
down the window's view hierarchy, in the same manner as for a performKeyEquivalent message.
performKeyEquivalent takes
an NSEvent as its argument, while performMnemonic: takes an NSString
containing the uninterpreted characters of the key event. You should
extract the characters for a key equivalent using NSEvent's charactersIgnoringModifiers method. |
Keyboard Interface Control
Mnemonics are actually part of a more general means of controlling
the user interface via the keyboard. An NSWindow treats certain
key events specially, as commands to move control to a different
interface object, to simulate a mouse click on it, and so on. In
brief, pressing Tab moves control to the next object, whether a
button, a text field, or some other kind of control object. Shift-Tab moves
control to the previous one. Pressing Space simulates a mouse click
for many kinds of control objects, causing a push button to click,
a radio button to toggle its state, and so on. In selection lists,
pressing Space selects or deselects the highlighted item; the user
can also press Alternate or Shift to extend the selection, not affecting
other selected items. Some interface controls also accept arrow-key
input.
Each window can be assigned a default button, which is triggered
by the Return or Enter key. Also, in modal windows or panels the
user can press the Escape key to dismiss the window or panel. If
interface control moves to another button, the default button temporarily
loses this ability as the user's focus shifts to the button where
control resides. However, if control then moves to a different kind of
interface object, the default button resumes its normal ability.
The interface objects that are connected together within a
window make up the window's key view loop. You normally set up
the key view loop using Interface Builder, establishing connections
to each interface object's nextKeyView outlet. You
can also set the object that's initially selected when a window
is first opened by setting the window's initialFirstResponder outlet
in Interface Builder. If you do not set this outlet, the window
will set a key loop (not necessarily the same as the one you may
have specified!) and pick a default initial outlet for you. NSView
and NSWindow also define a number of methods for manipulating the
key view loop programmatically; see their class specifications for
more information.
Keyboard Input
A normal key event eventually makes its way to the responder
chain as a keyDown message,
which the receiver can handle in any way it sees fit. A text object
typically interprets the message as a request to insert text, while
a drawing object might only be interested in a few keys, such as
Delete and the arrow keys to delete and move selected items. The
receiver of a keyDown message can extract
the event's characters directly using NSEvent's characters or charactersIgnoringModifiers methods,
or it can pass the key event to the Application Kit's input manager
for interpretation according to the user's key bindings. Input
management allows key events to be interpreted as text not directly
available on the keyboard, such as Kanji and some accented characters, and
as commands that affect the content of the responder object handling
the event. See the NSInputManager and NSTextInput class and protocol specifications
for more information on input management and key binding.
To invoke the input manager, simply invoke NSResponder's interpretKeyEvents message
in your implementation of keyDown. This method
sends an NSArray of events to the input manager, which interprets
the events as text or commands and responds by sending insertText: or doCommandBySelector: to
your responder object. The section "Standard Action Methods for
Selecting and Editing" below describes the messages that might
be sent to your object.
Other Uses
The responder chain is utilized by two other mechanisms in
the Application Kit. In enabling and disabling a menu item, the
application object consults the full responder chain for an object
that implements the menu item's action method, as described in
the NSMenuActionResponder protocol specification. Similarly, the
Services facility passes validRequestorForTypes messages along
the full responder chain to check for objects that are eligible
for services offered by other applications. The Services validation
process is described fully in "Services" in OPENSTEP Programming
Topics.
Adopted
Protocols
- NSCoding
- - encodeWithCoder:
- - initWithCoder:
Method Types
- Changing the first responder
- acceptsFirstResponder
- becomeFirstResponder
- resignFirstResponder
- Setting the next responder
- setNextResponder
- nextResponder
- Event methods
- mouseDown
- mouseDragged
- mouseUp
- mouseMoved
- mouseEntered
- mouseExited:
- rightMouseDown
- rightMouseDragged
- rightMouseUp
- keyDown
- keyUp
- flagsChanged
- helpRequested
- Special key event methods
- interpretKeyEvents
- performKeyEquivalent
- performMnemonic:
- Clearing key events
- flushBufferedKeyEvents
- Action methods
- capitalizeWord:
- centerSelectionInVisibleArea:
- changeCaseOfLetter:
- complete:
- deleteBackward:
- deleteForward:
- deleteToBeginningOfLine:
- deleteToBeginningOfParagraph:
- deleteToEndOfLine:
- deleteToEndOfParagraph:
- deleteToMark:
- deleteWordBackward:
- deleteWordForward:
- indent:
- insertBacktab:
- insertNewline:
- insertNewlineIgnoringFieldEditor:
- insertParagraphSeparator:
- insertTab:
- insertTabIgnoringFieldEditor:
- insertText:
- lowercaseWord:
- moveBackward:
- moveBackwardAndModifySelection:
- moveDown:
- moveDownAndModifySelection:
- moveForward:
- moveForwardAndModifySelection:
- moveLeft:
- moveRight:
- moveToBeginningOfDocument:
- moveToBeginningOfLine:
- moveToBeginningOfParagraph:
- moveToEndOfDocument:
- moveToEndOfLine:
- moveToEndOfParagraph:
- moveUp:
- moveUpAndModifySelection:
- moveWordBackward:
- moveWordBackwardAndModifySelection:
- moveWordForward:
- moveWordForwardAndModifySelection:
- pageDown:
- pageUp:
- scrollLineDown:
- scrollLineUp:
- scrollPageDown:
- scrollPageUp:
- selectAll:
- selectLine:
- selectParagraph:
- selectToMark:
- selectWord:
- setMark:
- showContextHelp
- swapWithMark:
- transpose:
- transposeWords:
- uppercaseWord:
- yank:
- Dispatch methods
- doCommandBySelector:
- tryToPerform
- Terminating the responder
chain
- noResponderFor:
- Services menu updating
- validRequestorForTypes
- Setting the menu
- setMenu
- menu
- Setting the interface
style
- setInterfaceStyle
- interfaceStyle
Instance Methods
public boolean acceptsFirstResponder()
Overridden by subclasses to
return true
if the receiver can handle
key events and action messages sent up the responder chain. NSResponder's implementation
returns false
, indicating that by
default a responder object doesn't agree to become first responder.
Objects that aren't first responder can receive mouse event messages,
but no other event or action messages.See
Also: becomeFirstResponder, resignFirstResponder, - needsPanelToBecomeKey (NSView)
public boolean becomeFirstResponder()
Notifies the receiver that
it's about to become first responder in its NSWindow. NSResponder's
implementation returns true
, accepting
first responder status. Subclasses can override this method to update
state or perform some action such as highlighting the selection,
or to return false
, refusing first responder
status.Use NSWindow's makeFirstResponder:,
not this method, to make an object the first responder. Never invoke
this method directly.
See Also: resignFirstResponder, acceptsFirstResponder
public void flagsChanged(NSEvent theEvent)
Informs the receiver that the
user has pressed or released a modifier key (Shift, Control, and
so on). NSResponder's implementation simply
passes this message to the next responder.
public void flushBufferedKeyEvents()
Overridden by subclasses to
clear any unprocessed key events.
public void helpRequested(NSEvent theEvent)
Displays context-sensitive
help for the receiver if such exists, otherwise passes this message
to the next responder. NSWindow invokes this method automatically
when the user clicks for help. Subclasses need not override this method,
and application code shouldn't directly invoke it.See
Also: showContextHelp
public int interfaceStyle()
Returns the receiver's interface
style. interfaceStyle is
an abstract method in NSResponder and just returns NoInterfaceStyle
.
It is overridden in classes such as NSWindow and NSView to return
the interface style, such as MacintoshInterfaceStyle
or Windows95InterfaceStyle
.
A responder's style (if other than NoInterfaceStyle
)
overrides all other settings, such as those established by the defaults
system.See Also: setInterfaceStyle
public void interpretKeyEvents(NSArray eventArray)
Invoked by subclasses from
their keyDown method
to handle a series of key events. This method
sends the character input in eventArray to
the system input manager for interpretation as text to insert or
commands to perform. The input manager responds to the request by
sending insertText: and doCommandBySelector: messages
back to the invoker of this method. Subclasses shouldn't override
this method.See the NSInputManager and NSTextInput class and
protocol specifications for more information on input management.
public void keyDown(NSEvent theEvent)
Informs the receiver that the
user has pressed a key. The receiver can interpret theEvent itself,
or pass it to the system input manager using interpretKeyEvents. NSResponder's
implementation simply passes this message to the next responder.
public void keyUp(NSEvent theEvent)
Informs the receiver that the
user has released a key. NSResponder's implementation
simply passes this message to the next responder.
public NSMenu menu()
Returns the receiver's menu. For
NSApplication this is the same as the menu returned by its mainMenu method.See
Also: setMenu, - menuForEvent: (NSView) + defaultMenu (NSView)
public void mouseDown(NSEvent theEvent)
Informs the receiver that the
user has pressed the left mouse button. NSResponder's
implementation simply passes this message to the next responder.
public void mouseDragged(NSEvent theEvent)
Informs the receiver that the
user has moved the mouse with the left button pressed. NSResponder's
implementation simply passes this message to the next responder.
public void mouseEntered(NSEvent theEvent)
Informs the receiver that the
mouse has entered a tracking rectangle. NSResponder's
implementation simply passes this message to the next responder.
public void mouseExited(NSEvent theEvent)
Informs the receiver that the
mouse has exited a tracking rectangle. NSResponder's
implementation simply passes this message to the next responder.
public void mouseMoved(NSEvent theEvent)
Informs the receiver that the
mouse has moved. NSResponder's implementation
simply passes this message to the next responder.See
Also: - setAcceptsMouseMovedEvents: (NSWindow)
public void mouseUp(NSEvent theEvent)
Informs the receiver that the
user has released the left mouse button. NSResponder's
implementation simply passes this message to the next responder.
public NSResponder nextResponder()
Returns the receiver's next
responder, or null
if it has none.See
Also: setNextResponder, noResponderFor:
public void noResponderForSelector(NSSelector eventSelector)
Handles the case where an event
or action message falls off the end of the responder chain. NSResponder's
implementation beeps if eventSelector is keyDown.
public boolean performKeyEquivalent(NSEvent theEvent)
Overridden by subclasses to
handle a key equivalent. If the character code
or codes in theEvent match the receiver's
key equivalent, the receiver should respond to the event and return true
.
NSResponder's implementation does nothing and returns false
.
performKeyEquivalent takes an NSEvent
as its argument, while performMnemonic: takes
an NSString containing the uninterpreted characters of the key event.
You should extract the characters for a key equivalent using NSEvent's charactersIgnoringModifiers method. |
See Also: - performKeyEquivalent: (NSView) - performKeyEquivalent: (NSButton)
public boolean performMnemonic(String aString)
Overridden by subclasses to
handle a mnemonic. If the character code or codes in aString match
the receiver's mnemonic, the receiver should perform the mnemonic
and return true
. NSResponder's
implementation does nothing and returns false
.See
Also: - performMnemonic: (NSView)
public boolean resignFirstResponder()
Notifies the receiver that
it's been asked to relinquish its status as first responder in
its NSWindow. NSResponder's implementation returns true
, resigning
first responder status. Subclasses can override this method to update state
or perform some action such as unhighlighting the selection, or
to return false
, refusing to relinquish
first responder status.Use NSWindow's makeFirstResponder:,
not this method, to make an object the first responder. Never invoke
this method directly.
See Also: becomeFirstResponder, acceptsFirstResponder
public void rightMouseDown(NSEvent theEvent)
Informs the receiver that the
user has pressed the right mouse button. NSResponder's
implementation simply passes this message to the next responder.
public void rightMouseDragged(NSEvent theEvent)
Informs the receiver that the
user has moved the mouse with the right button pressed. NSResponder's
implementation simply passes this message to the next responder.
public void rightMouseUp(NSEvent theEvent)
Informs the receiver that the
user has released the right mouse button. NSResponder's
implementation simply passes this message to the next responder.
public void setInterfaceStyle(int interfaceStyle)
Sets the receiver's style
to the style specified by interfaceStyle,
such as MacintoshInterfaceStyle
or Windows95InterfaceStyle
. setInterfaceStyle: is
an abstract method in NSResponder, but is overridden in classes
such as NSWindow and NSView to actually set the interface style.
You should almost never need to invoke or override this method,
but if you do override it, your version should always invoke the
implementation in super
.See
Also: interfaceStyle
public void setMenu(NSMenu aMenu)
Sets the receiver's menu
to aMenu. For NSApplication
this is the same as the main menu, typically set using setMainMenu:.See
Also: menu
public void setNextResponder(NSResponder aResponder)
Sets the receiver's next
responder to aResponder.See
Also: nextResponder
public void showContextHelp(Object sender)
Implemented by subclasses to
invoke the host platform's help system, displaying information
relevant to the receiver and its current state.See
Also: helpRequested
public boolean tryToPerform(NSSelector anAction, Object anObject)
Attempts to perform the action
method indicated by anAction. The
method should take a single argument of type Object and
return void. If the receiver responds to anAction,
it invokes the method with anObject as
the argument and returns true
. If
the receiver doesn't respond, it sends this message to its next responder
with the same selector and object. Returns false
if
no responder is found that responds to anAction.See
Also: doCommandBySelector: - sendAction:to:from: (NSApplication)
public NSUndoManager undoManager()
<<Documentation Forthcoming>>
public Object validRequestorForTypes(String sendType, String returnType)
Overridden by subclasses to
determine what services are available. With each
event, and for each service in the Services menu, the application
object sends this message up the responder chain with the send and
return type for the service being checked. This method is therefore
invoked many times per event. If the receiver can place data of sendType on
the pasteboard and receive data of returnType,
it should return this; otherwise
it should return null
. NSResponder's
implementation simply forwards this message to the next responder,
ultimately returning null
.Either sendType or returnType-but
not both-may be empty. If sendType is empty,
the service doesn't require input from the application requesting
the service. If returnType is empty,
the service doesn't return data.
See "Services"
in OPENSTEP Programming Topics for more information.
See
Also: - registerServicesMenuSendTypes:returnTypes: (NSApplication), - writeSelectionToPasteboard:types: (NSServicesRequests, protocol), - readSelectionFromPasteboard: (NSServicesRequests protocol)
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