首先说明 ios 中 NSManagedObjectContext 默认的 undoManager是nil的，就是说 undo 和 redo 都是没用的。
但是 rollback函数和reset函数是不设置undoManager对象也能使用的！
看下rollback的说明
Removes everything from the undo stack, discards all insertions and deletions, and restores updated objects to their last committed values.
NSManagedObjectContext的undo和redo都是调用undoManager的undo和redo，在很多情况下，undo一组没有save的操作和rollback整个context是一样的效果---都把context恢复到了上次save后的状态。但是undoManager如果进行了更加复杂的设定，undo就仅仅是恢复一部分数据库操作，而不像rollback那么彻底了！
在 NSManagedObject 的awakeFromInsert函数中找到了以下说明：
If you create a managed object then perform undo operations to bring the managed object context to a state prior to the object’s creation, then perform redo operations to bring the managed object context back to a state after the object’s creation, awakeFromInsert is not invoked a second time.
这里说明了 context中的 undo的一个用处，就是取消一个插入对象！
为了理解 undomanager，贴一段简单的例子：
#import "ViewController.h"
@interface ViewController ()
{
NSUndoManager *undoManager;
float width;
}
@end
@implementation ViewController
- (void)viewDidLoad {
[super viewDidLoad];
// Do any additional setup after loading the view, typically from a nib.
undoManager = [[NSUndoManager alloc] init];
width = 1.0;
[self setMyWidth:2.0];
if([undoManager canUndo]){
NSLog(@"can undo");
}
[undoManager undo];
if([undoManager canUndo]){
NSLog(@"can undo");
}
[undoManager redo];
NSLog(@"width is %f",width);
}
- (void)setMyWidth:(float)newWidth{
if(undoManager.isUndoing){
NSLog(@"is undoing");
}
if(undoManager.isRedoing){
NSLog(@"is redoing ");
}
float currentWidth = width;
if ((newWidth != currentWidth)) {
[[undoManager prepareWithInvocationTarget:self] setMyWidth:currentWidth];
[undoManager setActionName:NSLocalizedString(@"Size Change", @"size undo")];
width = newWidth;
}
}
@end
这个例子里的undoManager的操作，coredata都已经实现了，我们不需要写
- (void)setMyWidth:(float)newWidth
这样的函数了。