Question

我有一个演示应用程序，用于在我正在处理的应用程序中显示可能的竞争条件。该应用程序显示2 UITextField：textField1和textField2。

以下条件适用：

textField1字符串应至少与textField2字符串一样长。
textField2字符串不应超过5个字符。

该应用程序使用MVVM模式和Reactive Cocoa。

应用程序应仅保存textField1和textField2的有效配置。

Entity.h

@interface Entity : NSManagedObject

@property (nonatomic, retain) NSString * name1;
@property (nonatomic, retain) NSString * name2;

@end

ViewController.h

@interface ViewController : UIViewController

@end

ViewController.m

#import "ViewController.h"
#import "ViewModel.h"

@interface ViewController ()

@property(nonatomic, weak) IBOutlet UITextField *textField1;
@property(nonatomic, weak) IBOutlet UITextField *textField2;

@property(nonatomic, strong) ViewModel *viewModel;

@end

@implementation ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.

    self.viewModel = [[ViewModel alloc] init];

    // Two-way binding between  textfield1 and self.viewModel.name1
    // We cannot use a simple RACChannelTo = RACChannelTo because Textfield.text doesn't fire KVO notification on each strokes but only when exit the textfield.
    RACChannelTerminal *name1ModelTerminal = RACChannelTo(self, viewModel.name1);
    RAC(self.textField1, text) = name1ModelTerminal;
    [self.textField1.rac_textSignal subscribe:name1ModelTerminal];

    // Two-way binding between  textfield2 and self.viewModel.name2
    // We cannot use a simple RACChannelTo = RACChannelTo because Textfield.text doesn't fire KVO notification on each strokes but only when exit the textfield.
    RACChannelTerminal *name2ModelTerminal = RACChannelTo(self, viewModel.name2);
    RAC(self.textField2, text) = name2ModelTerminal;
    [self.textField2.rac_textSignal subscribe:name2ModelTerminal];

    // Validation
    RAC(self.textField1, backgroundColor) = [self.viewModel.name1IsValidSignal map:^id(NSNumber *value) {
        if ([value boolValue]) {
            return [UIColor clearColor];
        } else {
            return [UIColor redColor];
        }
    }];

    RAC(self.textField2, backgroundColor) = [self.viewModel.name2IsValidSignal map:^id(NSNumber *value) {
        if ([value boolValue]) {
            return [UIColor clearColor];
        } else {
            return [UIColor redColor];
        }
    }];
}

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

@end

ViewModel.h

#import "RVMViewModel.h"

@interface ViewModel : RVMViewModel

@property(nonatomic, strong) NSString *name1;
@property(nonatomic, strong) NSString *name2;

@property(nonatomic, strong) RACSignal *name1IsValidSignal;
@property(nonatomic, strong) RACSignal *name2IsValidSignal;

@property(nonatomic, strong) RACSignal *isValidSignal;

@end

ViewModel.m

#import "ViewModel.h"
#import "Entity.h"

@interface ViewModel ()

@property(nonatomic, strong) Entity *model;
@property(nonatomic, strong) NSManagedObjectContext *managedObjectContext;

@end

@implementation ViewModel

-(instancetype)init {
    self = [super init];

    if (self) {
        [self initialize];
    }

    return self;
}

-(void)initialize {

    // setup Model
    self.managedObjectContext = [NSManagedObjectContext MR_defaultContext];

    self.model = [Entity MR_createInContext:self.managedObjectContext];

    // log
    [RACObserve(self, name1) subscribeNext:^(NSString *name) {
        NSLog(@"viewModel.name1 = %@", name);
    }];

    [RACObserve(self, name2) subscribeNext:^(NSString *name) {
        NSLog(@"viewModel.name2 = %@", name);
    }];

    [RACObserve(self, model.name1) subscribeNext:^(NSString *name) {
        NSLog(@"model.name1 = %@", name);
    }];

    [RACObserve(self, model.name2) subscribeNext:^(NSString *name) {
        NSLog(@"model.name2 = %@", name);
    }];

    // Validation
    @weakify(self);
    self.name1IsValidSignal = [RACSignal combineLatest:@[RACObserve(self, name1),
                                                         RACObserve(self, name2)]
                                                reduce:^id(NSString *name1, NSString *name2) {
                                                    return @([name1 length] >= [name2 length]);
    }];

    self.name2IsValidSignal = [RACObserve(self, name2) map:^id(NSString *value) {
        return @([value length] < 6);
    }];

    self.isValidSignal = [[RACSignal combineLatest:@[self.name1IsValidSignal, self.name2IsValidSignal]] and];

    // Initial value of Model -> ViewModel
    self.name1 = self.model.name1;
    self.name2 = self.model.name2;

    // Binding: ViewModel -> Model when name is valid

    RAC(self, model.name1) = [[self.name1IsValidSignal filter:^BOOL(id value) {
        return [value boolValue];
    }] map:^id(id value) {
        @strongify(self);

        return self.name1;
    }];

    RAC(self, model.name2) = [[self.name2IsValidSignal filter:^BOOL(id value) {
        return [value boolValue];
    }] map:^id(id value) {
        @strongify(self);

        return self.name2;
    }];

    // Save with Core Data when both name1IsValidSignal and name2IsValidSignal returns @YES and that there are some unsaved changes in the context
    // There is a possible race condition here when this subscription is called before the bindings from ViewModel -> Model is called.
    [[[RACSignal combineLatest:@[RACObserve(self.managedObjectContext, hasChanges), self.isValidSignal]] and] subscribeNext:^(id x) {

        if ([x boolValue]) {
            // Save with Core Data
            [self.managedObjectContext MR_saveToPersistentStoreWithCompletion:^(BOOL success, NSError *error) {
                // TODO: error management
                if (success || !error) {
                    NSLog(@"Success");
                    NSLog(@" ");
                } else {
                    NSLog(@"Problem when saving to Core Data");
                }

            }];
        }
    }];
}

@end

从我的角度来看，ViewModel.m中最后一个订阅中的Core Data可能会在模型更新之前发生。你如何防止这种情况发生？

修改我真正想要的是推送只通过验证的（name1和name2）配置的Core Data模型。

Xcode项目 https://github.com/guillaume-michel/ReactiveValidation

Answer 1

竞争条件可能是name2IsValidSignal在name1IsValidSignal为false之前为true。这意味着isValidSignal在变为false之前可以是真实的（这会触发保存）。一种解决方案是将验证逻辑组合到一个单元中：

    self.isValidSignal = [RACSignal combineLatest:@[RACObserve(self, name1),
                                                         RACObserve(self, name2)]
                                                reduce:^id(NSString *name1, NSString *name2) {
                                                    return @([name1 length] >= [name2 length] && [name2 length] < 6);
    }];

这应该确保只有在运行所有验证逻辑后才更新isValidSignal。

Answer 2

根据docs for -[NSManagedObjectContext hasChanges]，您不应该在收到KVO通知的同时向NSManagedObjectContext发送消息：

如果由于更改hasChanges的值而需要向上下文发送消息或更改其任何托管对象，则必须在调用堆栈展开后执行此操作（通常使用performSelector：withObject：afterDelay：或类似的方法）。

您可以通过明确传递到+[RACScheduler mainThreadScheduler]：

在ReactiveCocoa中完成此操作

[[[[RACSignal combineLatest:@[RACObserve(self.managedObjectContext, hasChanges),
                              self.isValidSignal]]
             and]
             deliverOn:RACScheduler.mainThreadScheduler]
             subscribeNext:^(id x) {
    // ...

使用Core Data在MVVM应用程序中使用ReactiveCocoa的可能竞争条件

2 个答案: