NSUndoManager: redo not working

Question

I am making a simple application that uses a NSSlider, which can be put to it's max or min value with two buttons.The undo manager shall track all changes and allow to undo/redo all changes made using these two buttons.
Here is the interface:

#import <Cocoa/Cocoa.h>

@interface AppDelegate : NSObject <NSApplicationDelegate>
{
@private
    NSUndoManager* undoManager;
}

@property (assign) IBOutlet NSWindow *window;
@property (weak) IBOutlet NSSlider *slider;


- (IBAction)putToMax:(id)sender;
- (IBAction)putToMin:(id)sender;
- (void) setSliderValue: (float) value;

@end

Implementation:

#import "AppDelegate.h"

@implementation AppDelegate

@synthesize window = _window;
@synthesize slider = _slider;

- (NSUndoManager*) windowWillReturnUndoManager: (NSWindow*) window
{
    return undoManager;
}

- (IBAction)putToMax:(id)sender 
{
    float value= [_slider floatValue];
    [ [undoManager prepareWithInvocationTarget: self] setSliderValue: value];
    if(![undoManager isUndoing])
        [undoManager setActionName: @"Put to Max"];
    NSLog(@"%f value added to the stack",value);
    [_slider setFloatValue: 100.0];
}

- (IBAction)putToMin:(id)sender 
{
    float value= [_slider floatValue];
    [ [undoManager prepareWithInvocationTarget: self] setSliderValue: value];
    if(![undoManager isUndoing])
        [undoManager setActionName: @"Put to Min"];
    NSLog(@"%f value added to the stack",value);
    [_slider setFloatValue: 0.0];
}

- (void) setSliderValue: (float) value
{
    [_slider setFloatValue: value];
}

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification
{
}

- (id) init
{
    self=[super init];
    if(self)
    {
        undoManager=[[NSUndoManager alloc]init];
    }
    return self;
}


@end

And a screenshot of the application:

The undo works fine, but I have problems with redo.

For example after launching the application:

• I click on the put to max button.
• Then menu Edit -> Undo put to max.

The slider goes back where it was.

But if I go to menu Edit -> Redo put to max, the slider doesn't go back to it's max position. And I don't understand why.

Answer 1

When the Undo system performs an undo action, it expects you to register the redo actions using the same code as for undo, (except that the NSUndoManager knows it's rewinding - but you should not care).

So add the proper NSUndoManager calls in -setSliderValue: