EOUndoManager

Inherits From:
NSObject

Conforms To: NSObject (NSObject)

Declared in: EOControl/EOUndoManager.h

EOUndoManager is a general purpose recorder of operations for undo and redo. You register an undo operation by specifying the object that's changing (or the owner of that object), along with a method to invoke to revert its state, and the arguments for that method. EOUndoManager automatically groups all operations within a single cycle of the run loop, so that performing an undo reverts all changes that occurred during the loop. Also, when performing undo an EOUndoManager saves the operations reverted so that you can redo the undos.

EOUndoManager normally creates undo groups automatically during the run loop. The first time it's asked to record an undo operation in the run loop, it creates a new group. Then, at the end of the loop, it closes the group. You can create additional, nested undo groups within these default groups using the beginUndoGrouping and endUndoGrouping methods. You can also turn off the default grouping behavior using setGroupsByEvent: .

Initially, both stacks are empty. Recording undo operations adds to the undo stack, but the redo stack remains empty until undo is performed. Performing undo causes the undo operations in the latest group to be applied to their objects. Since these operations cause changes to the objects' states, the objects presumably register new operations with the EOUndoManager, this time in the reverse direction from the original operations. Since the EOUndoManager is in the process of performing undo, it records these operations as redo operations on the redo stack. Consecutive undos add to the redo stack. Subsequent redo operations pull the operations off the redo stack and apply them to the objects.

The redo stack's contents last as long as undo and redo is performed. However, because applying a new change to objects invalidates the previous changes, as soon as a new undo operation is registered, the redo stack is cleared. This prevents redo from returning objects to an inappropriate prior state. You can check for the ability to undo and redo with the canUndo and canRedo methods.

Regardless of the type of operations recorded, a single instance of EOUndoManager typically belongs to a single document or container of objects, called the EOUndoManager's client. Each EOEditingContext in an application, for example, has its own private EOUndoManager. This keeps each pair of undo and redo stacks separate so that when an undo is performed, it applies to the focal document in the application (typically the one displayed in the key window). It also relieves the individual objects from having to know the identity of their EOUndoManager.

In order to use undo effectively, either the client must claim exclusive right to alter its undoable objects-in order to record undo operations for all changes-or the objects themselves must participate in recording their changes. The first case is exemplified by a text document that holds a private NSTextView, handling all text operations by registering undo operations and forwarding the change to the NSTextView. For the second case, the willChange method defined by Enterprise Objects Framework allows any object to notify observers that it's about to change. EOEditingContexts, being containers for enterprise objects, receive these change notifications and record undo operations (among many other things). Even in this case, interaction with the EOUndoManager is handled exclusively by the container object.

[undoManager registerUndoWithTarget:self selector:@selector(deleteObject:)

    arg:anObject];

An update might be recorded for undo like so:

NSDictionary *updateDict = [NSDictionary dictionaryWithObjectsAndKeys:anObject,

    @"object", [anObject snapshot], @"snapshot"];

			

[undoManager registerUndoWithTarget:self

    selector:@selector(revertUpdate:)

    arg:updateDict];

This fragment is likely to be executed as a result of anObject invoking the standard willChange method, which announces that the object's state is going to change. Since it hasn't changed yet, the state can be recorded for later undo. This fragment, then, registers the client (self ) to be sent a revertUpdate: message with the object and its old state when undo is performed. The old values are retrieved with a snapshot message. revertUpdate: can be implemented to pass the old state back to the object:

- (void)revertUpdate:(NSDictionary *)updateDict

{

    [[updateDict objectForKey:@"object"]

        udpateFromSnapshot:[updateDict objectForKey:@"snapshot"]];

    return;

}

Both snapshot and updateFromSnapshot: are methods added to NSObject by the Framework. See the NSObject Additions specification for more information.

[[myUndoManager prepareWithInvocationTarget:textObject]

    setFont:[textObject font] color:[textObject textColor]];

[textObject setFont:newFont color:newColor];

prepareWithInvocationTarget: records the argument as the target of the undo operation about to be established. Following this, you send the message that will revert the target's state-in this case, setFont:color: . Because EOUndoManager doesn't respond to this method, forwardInvocation: is invoked, which EOUndoManager implements to record the NSInvocation containing the target, selector, and all arguments. Performing undo later results in textObject being sent a setFont:color: message with the old values.

undo is intended for undoing top-level groups, and shouldn't be used for nested undo groups. If any unclosed, nested undo groups are on the stack when undo is invoked, it raises an exception. To undo nested groups, you must use explicitly close the group with an endUndoGrouping message, then use undoNestedGroup to undo it. Note also that if you turn off automatic grouping by event with setGroupsByEvent: , you must explicitly close the current undo group with endUndoGrouping before invoking either undo method.

To remedy this, the client must take care to clear undo operations for targets that are being deallocated. This typically occurs in one of three ways:

• The client is the exclusive owner of the EOUndoManager and the target of all undo operations. In this case the client can simply release the EOUndoManager in its dealloc method.

• The client shares the EOUndoManager with other clients. To handle this the client should send forgetAllWithTarget: to the EOUndoManager before releasing it in its dealloc method.

• The client registers objects other than itself for undo operations. Here either the client must watch for the other objects being deallocated in order to send forgetAllWithTarget: , or the other objects must do so themselves when deallocated (which requires that they have a reference to the EOUndoManager). This is likely to be needed with invocation-based undo.

Registering undo operations

- registerUndoWithTarget:selector:arg:

- prepareWithInvocationTarget:

- forwardInvocation:

Checking undo ability

- canUndo

- canRedo

Performing undo and redo

- undo

- undoNestedGroup

- redo

Limiting the undo stack

- setLevelsOfUndo:

- levelsOfUndo

Creating undo groups

- beginUndoGrouping

- endUndoGrouping

- setGroupsByEvent:

- groupsByEvent

Disabling undo

- disableUndoRegistration

- reenableUndoRegistration

Checking whether undo or redo is being performed

- isUndoing

- isRedoing

Clearing undo operations

beginUndoGrouping

- forgetAll

- forgetAllWithTarget:

This method posts an "UndoManagerCheckpointNotification".

canRedo

Returns YES if the receiver has any actions to redo, NO if it doesn't.

Because any undo operation registered clears the redo stack, this method posts an "UndoManagerCheckpointNotification" to allow clients to apply their pending operations before testing the redo stack.

See also: - canUndo , - redo

canUndo

Returns YES if the receiver has any actions to undo, NO if it doesn't. This does not mean that you can safely invoke undo or undoNestedGroup ; you may have to close open undo groups first.

See also: - canRedo , - endUndoGrouping , - registerUndoWithTarget:selector:arg:

disableUndoRegistration

Disables the recording of undo operations, whether by registerUndoWithTarget:selector:arg: or by invocation-based undo. This method can be invoked multiple times; reenableUndoRegistration must be invoked an equal number of times to actually re-enable undo registration.

endUndoGrouping

Marks the end of an undo group. All individual undo operations back to the matching beginUndoGrouping message are grouped together and reversed by a later undo or undoNestedGroup message. Undo groups can be nested, thus providing functionality similar to nested transactions. Raises an NSInternalInconsistencyException if there's no beginUndoGrouping message in effect.

This method posts an "UndoManagerCheckpointNotification".

forgetAll

Clears the undo and redo stacks and reenables the receiver.

See also: - levelsOfUndo ,- reenableUndoRegistration , - forgetAllWithTarget:

forgetAllWithTarget:

Clears the undo and redo stacks of all operations involving target as the recipient of the undo message. Doesn't re-enable the receiver if it's disabled. An object that shares an EOUndoManager with other clients should invoke this message in its implementation of dealloc .

See also: - reenableUndoRegistration , - forgetAll

forwardInvocation:

Overrides NSObject's implementation to record anInvocation as an undo operation. Also clears the redo stack. Raises an NSInternalInconsistencyException if prepareWithInvocationTarget: wasn't invoked before this method; this method then clears the prepared invocation target. See ""Invocation-Based Undo"" in the class description for more information.

Raises an NSInternalInconsistencyException if invoked when no undo group has been established using beginUndoGrouping . Undo groups are normally set by default, so you should rarely need to begin a top-level undo group explicitly.

See also: - undoNestedGroup , - registerUndoWithTarget:selector:arg: , - groupsByEvent

groupsByEvent

Returns YES if the receiver automatically creates undo groups around each pass of the run loop, NO if it doesn't. The default is YES.

See also: - beginUndoGrouping , - setGroupsByEvent:

isRedoing

Returns YES if the receiver is in the process of performing its redo method, NO otherwise.

See also: - isRedoing

isUndoing

Returns YES if the receiver is in the process of performing its undo or undoNestedGroup method, NO otherwise.

levelsOfUndo

Returns the maximum number of top-level undo groups the receiver will hold. When ending an undo group results in the number of groups exceeding this limit, the oldest groups are dropped from the stack. A limit of zero indicates no limit, so that old undo groups are never dropped. The default is zero.

See also: - endUndoGrouping , - isUndoing , - setLevelsOfUndo:

prepareWithInvocationTarget:

Prepares the receiver for invocation-based undo with target as the subject of the next undo operation and returns self . See ""Invocation-Based Undo"" in the class description for more information.

See also: - forwardInvocation:

redo

Performs the operations in the last group on the redo stack, if there are any, recording them on the undo stack as a single group.

This method posts an "UndoManagerCheckpointNotification".

See also: - redo , - registerUndoWithTarget:selector:arg:

reenableUndoRegistration

Balances a prior disableUndoRegistration message. Undo registration isn't actually re-enabled until a re-enable message balances the last disable message in effect. Raises an NSInternalInconsistencyException if invoked while no disableUndoRegistration message is in effect.

registerUndoWithTarget:selector:arg:

Records a single undo operation for target, so that when undo is performed it's sent aSelector with anObject as the sole argument. Also clears the redo stack. Doesn't retain target. See ""Simple Undo"" in the class description for more information.

Raises an NSInternalInconsistencyException if invoked when no undo group has been established using beginUndoGrouping . Undo groups are normally set by default, so you should rarely need to begin a top-level undo group explicitly.

See also: - undoNestedGroup , - forwardInvocation: , - groupsByEvent

setGroupsByEvent:

Sets whether the receiver automatically groups undo operations during the run loop. If flag is YES, the receiver creates undo groups around each pass through the run loop; if flag is NO it doesn't. The default is YES.

If you turn automatic grouping off, you must close groups explicitly before invoking either undo or undoNestedGroup .

See also: - groupsByEvent

setLevelsOfUndo:

Sets the maximum number of top-level undo groups the receiver will hold to anInt. When ending an undo group results in the number of groups exceeding this limit, the oldest groups are dropped from the stack. A limit of zero indicates no limit, so that old undo groups are never dropped. The default is zero.

If invoked with a limit below the prior limit, old undo groups are immediately dropped.

See also: - endUndoGrouping , - levelsOfUndo

undo

Closes the top-level undo group if necessary and invokes undoNestedGroup . Raises an NSInternalInconsistencyException if more than one undo group is open (that is, if the last group isn't at the top level).

This method posts an "UndoManagerCheckpointNotification".

See also: - endUndoGrouping , - groupsByEvent

undoNestedGroup

Performs the undo operations in the last undo group (whether top-level or nested), recording the operations on the redo stack as a single group. Raises an NSInternalInconsistencyException if any undo operations have been registered since the last endUndoGrouping message.

This method posts an "UndoManagerCheckpointNotification".

Notification

UndoManagerCheckpointNotification

Notification Object	The EOUndoManager
Userinfo	nil

See also: - undo