ios Coredata 的 rollback undo 等事物处理函数

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。