带有核心数据的 MVVM 应用程序中 ReactiveCocoa 的可能竞争条件

Question

我有一个演示应用程序来显示我正在开发的应用程序中可能存在的竞争情况。该应用显示 2 个 UITextField：textField1 和 textField2。

以下条件适用：

• textField1 字符串至少应该和 textField2 字符串一样长。
• textField2 字符串不应超过 5 个字符。

该应用使用 MVVM 模式和 Reactive Cocoa。

应用应该只保存 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

在我看来，在 ViewModel.m 中的最后一次订阅中保存到 Core Data 可能发生在模型更新之前。你如何防止这种情况发生？

编辑 我真正想要的是只推送通过验证的（name1 和 name2）的 Core Data 模型配置。

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

Answer 1

竞争条件可能是 name2IsValidSignal 可以在 name1IsValidSignal 为假之前为真。这意味着 isValidSignal 在变为假之前可以暂时为真（这将触发保存）。一种解决方案是将验证逻辑合并到一个单元中：

    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 isValidSignal 仅在所有验证逻辑运行后才更新。

Answer 2

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

如果您需要将消息发送到上下文或由于更改 hasChanges 的值而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）。

您可以在 ReactiveCocoa 中通过显式传递到 +[RACScheduler mainThreadScheduler] 来完成此操作：

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