pavelosipov/POSLens.m

## POSLens.m
//
//  POSLens.m
//  POSLens
//
//  Created by Pavel Osipov on 06/02/2018.
//  Copyright © 2018 Pavel Osipov. All rights reserved.
//

#import "POSLens.h"

#import "POSEphemeralValueStore.h"
#import "POSFileValueStore.h"
#import "POSKeychainValueStore.h"
#import "POSUserDefaultsValueStore.h"

#import "NSError+POSLens.h"

NS_ASSUME_NONNULL_BEGIN

typedef POSLensValue * _Nullable(^POSLensUpdateBlock)(POSLensValue * _Nullable oldValue, NSError **error);

@implementation POSLensValueUpdate

- (instancetype)initWithOldValue:(nullable POSLensValue *)oldValue
                     actualValue:(nullable POSLensValue *)actualValue {
    if (self = [super init]) {
        _oldValue = oldValue;
        _actualValue = actualValue;
    }
    return self;
}

- (BOOL)isEqual:(nullable POSLensValueUpdate *)other {
    if (self == other) {
        return YES;
    }
    if (![other isMemberOfClass:self.class]) {
        return NO;
    }
    return (POSObjectsAreEqual(_oldValue, other.oldValue) &&
            POSObjectsAreEqual(_actualValue, other.actualValue));
}

@end

#pragma mark -

@interface POSPropertyLens : POSMutableLens

@property (nonatomic, readonly) POSMutableLens<POSLensValue *> *parent;
@property (nonatomic, readonly) NSString *key;

- (instancetype)initWithParent:(POSMutableLens<POSLensValue *> *)parent
                  defaultValue:(nullable POSLensValue *)defaultValue
                           key:(NSString *)key NS_DESIGNATED_INITIALIZER;

@end

#pragma mark -

@interface POSLens ()
@property (nonatomic, readonly) NSString *keyPath;
@property (nonatomic, readonly, nullable) POSLensValue *defaultValue;
@end

@interface POSMutableLens ()

@property (nonatomic, readonly) RACSignal<POSLensValueUpdate<POSLensValue *> *> *recursiveValueUpdates;

- (BOOL)updateValueWithBlock:(POSLensUpdateBlock)updateBlock
           ignoreStoreErrors:(BOOL)ignoreStoreErrors
                       error:(NSError **)error;

@end

//
// Implementation for all interface methods of  provided by subclasses of POSLens and POSMutableLens.
// These classes exist because there is no template protocol concept in Objective-C language.
// POSLens library doesn't create direct instances of these classes.
//
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wincomplete-implementation"

@implementation POSLens

@dynamic keyPath;
@dynamic value;
@dynamic valueUpdates;

- (instancetype)init {
    return [self initWithDefaultValue:nil];
}

- (instancetype)initWithDefaultValue:(nullable POSLensValue *)defaultValue {
    if (self = [super init]) {
        _defaultValue = defaultValue;
    }
    return self;
}

- (instancetype)objectForKeyedSubscript:(NSString *)key {
    return [self lensForKey:key defaultValue:nil];
}

- (instancetype)lensForKey:(NSString *)key {
    return [self lensForKey:key defaultValue:nil];
}

@end

@implementation POSMutableLens
@dynamic recursiveValueUpdates;

- (POSMutableLens *)lensForKey:(NSString *)key defaultValue:(nullable POSLensValue *)defaultValue {
    POS_CHECK(key);
    return [[POSPropertyLens alloc] initWithParent:self defaultValue:defaultValue key:key];
}

- (instancetype)lensForKeyPath:(NSString *)keyPath {
    POS_CHECK(keyPath);
    POSMutableLens *lens = self;
    NSArray<NSString *> *keys = [keyPath componentsSeparatedByString:@"."];
    for (NSInteger i = 0, n = keys.count; i < n; ++i) {
        lens = [[POSPropertyLens alloc] initWithParent:lens defaultValue:nil key:keys[i]];
    }
    return lens;
}

- (RACSignal<POSLensValue *> *)valueUpdates {
    return [[self.recursiveValueUpdates
        map:^POSLensValue * _Nullable(POSLensValueUpdate<POSLensValue *> *update) {
            return update.actualValue;
        }]
        distinctUntilChanged];
}

- (RACSignal<POSLensValueUpdate<POSLensValue *> *> *)historicalValueUpdates {
    return [[self.recursiveValueUpdates
        skip:1]
        filter:^BOOL(POSLensValueUpdate<POSLensValue *> *update) {
            return !POSObjectsAreEqual(update.actualValue, update.oldValue);
        }];
}

- (void)forceUpdateValue:(nullable POSLensValue *)value {
    [self updateValueWithBlock:^id _Nullable(POSLensValue * _Nullable currentValue, NSError **error) {
        return value;
    } ignoreStoreErrors:YES error:nil];
}

- (BOOL)updateValue:(nullable POSLensValue *)value error:(NSError **)error {
    return [self updateValueWithBlock:^id _Nullable(POSLensValue * _Nullable currentValue, NSError **error) {
        return value;
    } ignoreStoreErrors:NO error:error];
}

- (void)forceUpdateValue:(nullable POSLensValue *)value atKey:(NSString *)key {
    [[self lensForKey:key] forceUpdateValue:value];
}

- (BOOL)updateValue:(nullable POSLensValue *)value atKey:(NSString *)key error:(NSError **)error {
    return [[self lensForKey:key] updateValue:value error:error];
}

- (void)forceUpdateValue:(nullable POSLensValue *)value atKeyPath:(NSString *)keyPath {
    [[self lensForKeyPath:keyPath] forceUpdateValue:value];
}

- (BOOL)updateValue:(nullable POSLensValue *)value atKeyPath:(NSString *)keyPath error:(NSError **)error {
    return [[self lensForKeyPath:keyPath] updateValue:value error:error];
}

- (void)forceUpdateValueWithBlock:(POSLensUpdateBlock)updateBlock {
    [self updateValueWithBlock:updateBlock ignoreStoreErrors:YES error:nil];
}

- (BOOL)updateValueWithBlock:(POSLensUpdateBlock)updateBlock error:(NSError **)error {
    return [self updateValueWithBlock:updateBlock ignoreStoreErrors:NO error:error];
}

- (void)forceUpdateValueAtKey:(NSString *)key
                    withBlock:(id _Nullable (^)(id _Nullable oldValue, NSError **error))block {
    [[self lensForKey:key] forceUpdateValueWithBlock:block];
}

- (BOOL)updateValueAtKey:(NSString *)key
               withBlock:(id _Nullable (^)(id _Nullable oldValue, NSError **error))block
                   error:(NSError **)error {
    return [[self lensForKey:key] updateValueWithBlock:block error:error];
}

- (void)forceUpdateValueAtKeyPath:(NSString *)key
                        withBlock:(id _Nullable (^)(id _Nullable oldValue, NSError **error))block {
    [[self lensForKeyPath:key] forceUpdateValueWithBlock:block];
}

- (BOOL)updateValueAtKeyPath:(NSString *)key
                   withBlock:(id _Nullable (^)(id _Nullable oldValue, NSError **error))block
                       error:(NSError **)error {
    return [[self lensForKeyPath:key] updateValueWithBlock:block error:error];
}

- (void)setObject:(nullable POSLensValue *)value forKeyedSubscript:(NSString *)keyPath {
    [[self lensForKeyPath:keyPath] forceUpdateValue:value];
}

- (BOOL)removeValue:(NSError **)error {
    return [self
        updateValueWithBlock:^POSLensValue * _Nullable(POSLensValue * _Nullable currentValue, NSError **error) {
            return nil;
        }
        ignoreStoreErrors:NO
        error:error];
}

- (void)removeValueAnyway {
    [self
        updateValueWithBlock:^POSLensValue * _Nullable(POSLensValue * _Nullable currentValue, NSError **error) {
            return nil;
        }
        ignoreStoreErrors:YES
        error:nil];
}

@end

#pragma clang diagnostic pop

#pragma mark -

@implementation POSPropertyLens

- (instancetype)initWithParent:(POSMutableLens<POSLensValue *> *)parent
                  defaultValue:(nullable POSLensValue *)defaultValue
                           key:(NSString *)key {
    POS_CHECK(parent);
    POS_CHECK(key);
    if (self = [super initWithDefaultValue:(defaultValue ?: [parent.defaultValue pos_valueForKey:key])]) {
        _parent = parent;
        _key = [key copy];
    }
    return self;
}

#pragma mark - POSLens

- (nullable id)value {
    id value = [_parent.value pos_valueForKey:_key];
    return value ?: self.defaultValue;
}

- (RACSignal<POSLensValueUpdate<POSLensValue *> *> *)recursiveValueUpdates {
    NSString *key = _key;
    POSLensValue *defaultValue = self.defaultValue;
    return [_parent.recursiveValueUpdates map:^id _Nullable(POSLensValueUpdate * _Nullable parentUpdate) {
        id oldValue = [parentUpdate.oldValue pos_valueForKey:key] ?: defaultValue;;
        id actualValue = [parentUpdate.actualValue pos_valueForKey:key] ?: defaultValue;
        return [[POSLensValueUpdate alloc] initWithOldValue:oldValue actualValue:actualValue];
    }];
}

- (NSString *)keyPath {
    return [_parent.keyPath stringByAppendingString:[NSString stringWithFormat:@".%@", _key]];
}

#pragma mark - POSMutableLens

- (BOOL)resetValue:(NSError **)error {
    return [_parent resetValue:error];
}

- (BOOL)updateValueWithBlock:(POSLensUpdateBlock)updateBlock
           ignoreStoreErrors:(BOOL)ignoreStoreErrors
                       error:(NSError **)error {
    POS_CHECK(updateBlock);
    @weakify(self);
    __auto_type parentUpdateBlock = ^POSLensValue * _Nullable(POSLensValue * _Nullable parentValue, NSError **error) {
        @strongify(self); // self is never nil because of synchronous nature of updateBlock.
        id currentValue = [parentValue pos_valueForKey:self->_key];
        id updatedValue = updateBlock(currentValue, error);
        if (*error != nil) {
            return parentValue;
        }
        if (updatedValue == currentValue || [updatedValue isEqual:currentValue]) {
            return parentValue;
        } else if (parentValue != nil) {
            return [parentValue pos_setValue:updatedValue forKey:self->_key];
        } else if (parentValue == nil && self.parent.defaultValue != nil) {
            return [self.parent.defaultValue pos_setValue:updatedValue forKey:self->_key];
        }
        POSAssignError(error, [NSError pos_lensErrorWithFormat:
                               @"Parent of property %@ has neither value or default value.", self.keyPath]);
        return parentValue;
    };
    return [_parent updateValueWithBlock:parentUpdateBlock ignoreStoreErrors:ignoreStoreErrors error:error];
}

@end

#pragma mark -

@interface POSRootLens : POSMutableLens

@property (nonatomic, readonly, nullable) id<POSLogger> logger;
@property (nonatomic, readonly) dispatch_queue_t syncQueue;
@property (nonatomic, readonly) id<POSValueStore> store;
@property (nonatomic, nullable) POSLensValue *currentValue;
@property (nonatomic, readonly) RACSubject<POSLensValueUpdate<POSLensValue *> *> *updatesSubject;

@end

@implementation POSRootLens

- (instancetype)initWithDefaultValue:(nullable POSLensValue *)defaultValue
                        currentValue:(nullable POSLensValue *)currentValue
                               store:(id<POSValueStore>)store
                              logger:(nullable id<POSLogger>)logger {
    POS_CHECK(store);
    if (self = [super initWithDefaultValue:defaultValue]) {
        _logger = logger;
        _syncQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
        _store = store;
        _currentValue = currentValue;
        _updatesSubject = [RACSubject subject];
    }
    return self;
}

#pragma mark - POSLens

- (nullable id)value {
    __block POSLensValue *value = nil;
    dispatch_sync(_syncQueue, ^{
        value = self->_currentValue ?: self.defaultValue;
    });
    return value;
}

- (RACSignal<POSLensValueUpdate<POSLensValue *> *> *)recursiveValueUpdates {
    return [[_updatesSubject
        takeUntil:self.rac_willDeallocSignal]
        startWith:[[POSLensValueUpdate alloc] initWithOldValue:nil actualValue:_currentValue]];
}

- (NSString *)keyPath {
    return @"root";
}

#pragma mark - POSMutableLens

- (BOOL)resetValue:(NSError **)error {
    __auto_type saveBlock = ^POSLensValue * _Nullable(POSLensValue * _Nullable value, BOOL *flush, NSError **error) {
        *flush = NO;
        return [self->_store loadValue:error];
    };
    return [self updateCurrentValueWithBlock:saveBlock ignoreStoreErrors:NO error:error];
}

- (BOOL)updateValueWithBlock:(POSLensUpdateBlock)block
           ignoreStoreErrors:(BOOL)ignoreStoreErrors
                       error:(NSError **)error {
    __auto_type updateBlock = ^POSLensValue * _Nullable(POSLensValue * _Nullable value, BOOL *flush, NSError **error) {
        return block(value, error);
    };
    return [self updateCurrentValueWithBlock:updateBlock ignoreStoreErrors:ignoreStoreErrors error:error];
}

- (BOOL)updateCurrentValueWithBlock:(POSLensValue *  _Nullable (^)(POSLensValue * _Nullable,
                                                                   BOOL *flush,
                                                                   NSError **error))updateBlock
                  ignoreStoreErrors:(BOOL)ignoreStoreErrors
                              error:(NSError **)error {
    POS_CHECK(updateBlock);
    __block BOOL flush = YES;
    __block BOOL updated = NO;
    __block NSError *updateError = nil;
    __block POSLensValue *updatingValue;
    __block POSLensValue *updatedValue;
    dispatch_barrier_sync(_syncQueue, ^{
        updatingValue = self->_currentValue;
        updatedValue = updateBlock(updatingValue, &flush, &updateError);
        updated = (updateError == nil && updatedValue != updatingValue && ![updatedValue isEqual:updatingValue]);
        if (updated) {
            BOOL saved = flush ? [self->_store saveValue:updatedValue error:&updateError] : YES;
            updated = saved || ignoreStoreErrors;
        }
        if (updated) {
            self.currentValue = updatedValue;
        }
    });
    if (error) {
        POSAssignError(error, updateError);
    } else if (updateError) {
        NSObject *failedValue = updatedValue ?: updatingValue;
        NSString *failedValueName = NSStringFromClass(failedValue.class);
        [_logger logError:@"Lens<%@>: Failed to update value: %@", failedValueName, updateError];
    }
    if (updated) {
        [_updatesSubject sendNext:[[POSLensValueUpdate alloc] initWithOldValue:updatingValue actualValue:updatedValue]];
    }
    return updateError == nil;
}

@end

#pragma mark -

@implementation POSLens (Factory)

+ (instancetype)lensWithValue:(nullable POSLensValue *)value {
    return [self
            lensWithDefaultValue:nil
            store:[[POSEphemeralValueStore alloc] initWithValue:value]
            logger:nil
            error:nil];
}

+ (nullable instancetype)lensWithDefaultValue:(nullable POSLensValue *)value
                                        store:(id<POSValueStore>)store
                                       logger:(nullable id<POSLogger>)logger
                                        error:(NSError **)error {
    NSError *loadError;
    POSLensValue *currentValue = [store loadValue:&loadError];
    if (loadError != nil) {
        [logger logError:@"Failed to create lens for %@ with default value %@: %@", store, value, loadError];
        POSAssignError(error, loadError);
        return nil;
    }
    return [[POSRootLens alloc] initWithDefaultValue:value currentValue:currentValue store:store logger:logger];
}

+ (nullable instancetype)lensWithDefaultValue:(nullable POSLensValue *)value
                                     filePath:(NSString *)filePath
                                       logger:(nullable id<POSLogger>)logger
                                        error:(NSError **)error {
    return [self
            lensWithDefaultValue:value
            store:[[POSFileValueStore alloc] initWithFilePath:filePath]
            logger:logger
            error:error];
}

+ (nullable instancetype)lensWithDefaultValue:(nullable POSLensValue *)value
                              keychainService:(NSString *)service
                                     valueKey:(NSString *)valueKey
                                       logger:(nullable id<POSLogger>)logger
                                        error:(NSError **)error {
    return [self
            lensWithDefaultValue:value
            store:[[POSKeychainValueStore alloc] initWithValueKey:valueKey service:service accessGroup:nil]
            logger:logger
            error:error];
}

+ (nullable instancetype)lensWithDefaultValue:(nullable POSLensValue *)value
                                 userDefaults:(NSUserDefaults *)userDefaults
                                     valueKey:(NSString *)valueKey
                                       logger:(nullable id<POSLogger>)logger
                                        error:(NSError **)error {
    return [self
            lensWithDefaultValue:value
            store:[[POSUserDefaultsValueStore alloc] initWithUserDefaults:userDefaults valueKey:valueKey]
            logger:logger
            error:error];
}

@end

NS_ASSUME_NONNULL_END
	//
	// POSLens.m
	// POSLens
	//
	// Created by Pavel Osipov on 06/02/2018.
	// Copyright © 2018 Pavel Osipov. All rights reserved.
	//

	#import "POSLens.h"

	#import "POSEphemeralValueStore.h"
	#import "POSFileValueStore.h"
	#import "POSKeychainValueStore.h"
	#import "POSUserDefaultsValueStore.h"

	#import "NSError+POSLens.h"

	NS_ASSUME_NONNULL_BEGIN

	typedef POSLensValue * _Nullable(^POSLensUpdateBlock)(POSLensValue * _Nullable oldValue, NSError **error);

	@implementation POSLensValueUpdate

	- (instancetype)initWithOldValue:(nullable POSLensValue *)oldValue
	actualValue:(nullable POSLensValue *)actualValue {
	if (self = [super init]) {
	_oldValue = oldValue;
	_actualValue = actualValue;
	}
	return self;
	}

	- (BOOL)isEqual:(nullable POSLensValueUpdate *)other {
	if (self == other) {
	return YES;
	}
	if (![other isMemberOfClass:self.class]) {
	return NO;
	}
	return (POSObjectsAreEqual(_oldValue, other.oldValue) &&
	POSObjectsAreEqual(_actualValue, other.actualValue));
	}

	@end

	#pragma mark -

	@interface POSPropertyLens : POSMutableLens

	@property (nonatomic, readonly) POSMutableLens<POSLensValue > parent;
	@property (nonatomic, readonly) NSString *key;

	- (instancetype)initWithParent:(POSMutableLens<POSLensValue > )parent
	defaultValue:(nullable POSLensValue *)defaultValue
	key:(NSString *)key NS_DESIGNATED_INITIALIZER;

	@end

	#pragma mark -

	@interface POSLens ()
	@property (nonatomic, readonly) NSString *keyPath;
	@property (nonatomic, readonly, nullable) POSLensValue *defaultValue;
	@end

	@interface POSMutableLens ()

	@property (nonatomic, readonly) RACSignal<POSLensValueUpdate<POSLensValue > > *recursiveValueUpdates;

	- (BOOL)updateValueWithBlock:(POSLensUpdateBlock)updateBlock
	ignoreStoreErrors:(BOOL)ignoreStoreErrors
	error:(NSError **)error;

	@end

	//
	// Implementation for all interface methods of provided by subclasses of POSLens and POSMutableLens.
	// These classes exist because there is no template protocol concept in Objective-C language.
	// POSLens library doesn't create direct instances of these classes.
	//
	#pragma clang diagnostic push
	#pragma clang diagnostic ignored "-Wincomplete-implementation"

	@implementation POSLens

	@dynamic keyPath;
	@dynamic value;
	@dynamic valueUpdates;

	- (instancetype)init {
	return [self initWithDefaultValue:nil];
	}

	- (instancetype)initWithDefaultValue:(nullable POSLensValue *)defaultValue {
	if (self = [super init]) {
	_defaultValue = defaultValue;
	}
	return self;
	}

	- (instancetype)objectForKeyedSubscript:(NSString *)key {
	return [self lensForKey:key defaultValue:nil];
	}

	- (instancetype)lensForKey:(NSString *)key {
	return [self lensForKey:key defaultValue:nil];
	}

	@end

	@implementation POSMutableLens
	@dynamic recursiveValueUpdates;

	- (POSMutableLens )lensForKey:(NSString )key defaultValue:(nullable POSLensValue *)defaultValue {
	POS_CHECK(key);
	return [[POSPropertyLens alloc] initWithParent:self defaultValue:defaultValue key:key];
	}

	- (instancetype)lensForKeyPath:(NSString *)keyPath {
	POS_CHECK(keyPath);
	POSMutableLens *lens = self;
	NSArray<NSString > keys = [keyPath componentsSeparatedByString:@"."];
	for (NSInteger i = 0, n = keys.count; i < n; ++i) {
	lens = [[POSPropertyLens alloc] initWithParent:lens defaultValue:nil key:keys[i]];
	}
	return lens;
	}

	- (RACSignal<POSLensValue > )valueUpdates {
	return [[self.recursiveValueUpdates
	map:^POSLensValue * _Nullable(POSLensValueUpdate<POSLensValue > update) {
	return update.actualValue;
	}]
	distinctUntilChanged];
	}

	- (RACSignal<POSLensValueUpdate<POSLensValue > > *)historicalValueUpdates {
	return [[self.recursiveValueUpdates
	skip:1]
	filter:^BOOL(POSLensValueUpdate<POSLensValue > update) {
	return !POSObjectsAreEqual(update.actualValue, update.oldValue);
	}];
	}

	- (void)forceUpdateValue:(nullable POSLensValue *)value {
	[self updateValueWithBlock:^id _Nullable(POSLensValue * _Nullable currentValue, NSError **error) {
	return value;
	} ignoreStoreErrors:YES error:nil];
	}

	- (BOOL)updateValue:(nullable POSLensValue )value error:(NSError *)error {
	return [self updateValueWithBlock:^id _Nullable(POSLensValue * _Nullable currentValue, NSError **error) {
	return value;
	} ignoreStoreErrors:NO error:error];
	}

	- (void)forceUpdateValue:(nullable POSLensValue )value atKey:(NSString )key {
	[[self lensForKey:key] forceUpdateValue:value];
	}

	- (BOOL)updateValue:(nullable POSLensValue )value atKey:(NSString )key error:(NSError **)error {
	return [[self lensForKey:key] updateValue:value error:error];
	}

	- (void)forceUpdateValue:(nullable POSLensValue )value atKeyPath:(NSString )keyPath {
	[[self lensForKeyPath:keyPath] forceUpdateValue:value];
	}

	- (BOOL)updateValue:(nullable POSLensValue )value atKeyPath:(NSString )keyPath error:(NSError **)error {
	return [[self lensForKeyPath:keyPath] updateValue:value error:error];
	}

	- (void)forceUpdateValueWithBlock:(POSLensUpdateBlock)updateBlock {
	[self updateValueWithBlock:updateBlock ignoreStoreErrors:YES error:nil];
	}

	- (BOOL)updateValueWithBlock:(POSLensUpdateBlock)updateBlock error:(NSError **)error {
	return [self updateValueWithBlock:updateBlock ignoreStoreErrors:NO error:error];
	}

	- (void)forceUpdateValueAtKey:(NSString *)key
	withBlock:(id _Nullable (^)(id _Nullable oldValue, NSError **error))block {
	[[self lensForKey:key] forceUpdateValueWithBlock:block];
	}

	- (BOOL)updateValueAtKey:(NSString *)key
	withBlock:(id _Nullable (^)(id _Nullable oldValue, NSError **error))block
	error:(NSError **)error {
	return [[self lensForKey:key] updateValueWithBlock:block error:error];
	}

	- (void)forceUpdateValueAtKeyPath:(NSString *)key
	withBlock:(id _Nullable (^)(id _Nullable oldValue, NSError **error))block {
	[[self lensForKeyPath:key] forceUpdateValueWithBlock:block];
	}

	- (BOOL)updateValueAtKeyPath:(NSString *)key
	withBlock:(id _Nullable (^)(id _Nullable oldValue, NSError **error))block
	error:(NSError **)error {
	return [[self lensForKeyPath:key] updateValueWithBlock:block error:error];
	}

	- (void)setObject:(nullable POSLensValue )value forKeyedSubscript:(NSString )keyPath {
	[[self lensForKeyPath:keyPath] forceUpdateValue:value];
	}

	- (BOOL)removeValue:(NSError **)error {
	return [self
	updateValueWithBlock:^POSLensValue * _Nullable(POSLensValue * _Nullable currentValue, NSError **error) {
	return nil;
	}
	ignoreStoreErrors:NO
	error:error];
	}

	- (void)removeValueAnyway {
	[self
	updateValueWithBlock:^POSLensValue * _Nullable(POSLensValue * _Nullable currentValue, NSError **error) {
	return nil;
	}
	ignoreStoreErrors:YES
	error:nil];
	}

	@end

	#pragma clang diagnostic pop

	#pragma mark -

	@implementation POSPropertyLens

	- (instancetype)initWithParent:(POSMutableLens<POSLensValue > )parent
	defaultValue:(nullable POSLensValue *)defaultValue
	key:(NSString *)key {
	POS_CHECK(parent);
	POS_CHECK(key);
	if (self = [super initWithDefaultValue:(defaultValue ?: [parent.defaultValue pos_valueForKey:key])]) {
	_parent = parent;
	_key = [key copy];
	}
	return self;
	}

	#pragma mark - POSLens

	- (nullable id)value {
	id value = [_parent.value pos_valueForKey:_key];
	return value ?: self.defaultValue;
	}

	- (RACSignal<POSLensValueUpdate<POSLensValue > > *)recursiveValueUpdates {
	NSString *key = _key;
	POSLensValue *defaultValue = self.defaultValue;
	return [_parent.recursiveValueUpdates map:^id _Nullable(POSLensValueUpdate * _Nullable parentUpdate) {
	id oldValue = [parentUpdate.oldValue pos_valueForKey:key] ?: defaultValue;;
	id actualValue = [parentUpdate.actualValue pos_valueForKey:key] ?: defaultValue;
	return [[POSLensValueUpdate alloc] initWithOldValue:oldValue actualValue:actualValue];
	}];
	}

	- (NSString *)keyPath {
	return [_parent.keyPath stringByAppendingString:[NSString stringWithFormat:@".%@", _key]];
	}

	#pragma mark - POSMutableLens

	- (BOOL)resetValue:(NSError **)error {
	return [_parent resetValue:error];
	}

	- (BOOL)updateValueWithBlock:(POSLensUpdateBlock)updateBlock
	ignoreStoreErrors:(BOOL)ignoreStoreErrors
	error:(NSError **)error {
	POS_CHECK(updateBlock);
	@weakify(self);
	__auto_type parentUpdateBlock = ^POSLensValue * _Nullable(POSLensValue * _Nullable parentValue, NSError **error) {
	@strongify(self); // self is never nil because of synchronous nature of updateBlock.
	id currentValue = [parentValue pos_valueForKey:self->_key];
	id updatedValue = updateBlock(currentValue, error);
	if (*error != nil) {
	return parentValue;
	}
	if (updatedValue == currentValue \|\| [updatedValue isEqual:currentValue]) {
	return parentValue;
	} else if (parentValue != nil) {
	return [parentValue pos_setValue:updatedValue forKey:self->_key];
	} else if (parentValue == nil && self.parent.defaultValue != nil) {
	return [self.parent.defaultValue pos_setValue:updatedValue forKey:self->_key];
	}
	POSAssignError(error, [NSError pos_lensErrorWithFormat:
	@"Parent of property %@ has neither value or default value.", self.keyPath]);
	return parentValue;
	};
	return [_parent updateValueWithBlock:parentUpdateBlock ignoreStoreErrors:ignoreStoreErrors error:error];
	}

	@end

	#pragma mark -

	@interface POSRootLens : POSMutableLens

	@property (nonatomic, readonly, nullable) id<POSLogger> logger;
	@property (nonatomic, readonly) dispatch_queue_t syncQueue;
	@property (nonatomic, readonly) id<POSValueStore> store;
	@property (nonatomic, nullable) POSLensValue *currentValue;
	@property (nonatomic, readonly) RACSubject<POSLensValueUpdate<POSLensValue > > *updatesSubject;

	@end

	@implementation POSRootLens

	- (instancetype)initWithDefaultValue:(nullable POSLensValue *)defaultValue
	currentValue:(nullable POSLensValue *)currentValue
	store:(id<POSValueStore>)store
	logger:(nullable id<POSLogger>)logger {
	POS_CHECK(store);
	if (self = [super initWithDefaultValue:defaultValue]) {
	_logger = logger;
	_syncQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
	_store = store;
	_currentValue = currentValue;
	_updatesSubject = [RACSubject subject];
	}
	return self;
	}

	#pragma mark - POSLens

	- (nullable id)value {
	__block POSLensValue *value = nil;
	dispatch_sync(_syncQueue, ^{
	value = self->_currentValue ?: self.defaultValue;
	});
	return value;
	}

	- (RACSignal<POSLensValueUpdate<POSLensValue > > *)recursiveValueUpdates {
	return [[_updatesSubject
	takeUntil:self.rac_willDeallocSignal]
	startWith:[[POSLensValueUpdate alloc] initWithOldValue:nil actualValue:_currentValue]];
	}

	- (NSString *)keyPath {
	return @"root";
	}

	#pragma mark - POSMutableLens

	- (BOOL)resetValue:(NSError **)error {
	__auto_type saveBlock = ^POSLensValue * _Nullable(POSLensValue * _Nullable value, BOOL flush, NSError *error) {
	*flush = NO;
	return [self->_store loadValue:error];
	};
	return [self updateCurrentValueWithBlock:saveBlock ignoreStoreErrors:NO error:error];
	}

	- (BOOL)updateValueWithBlock:(POSLensUpdateBlock)block
	ignoreStoreErrors:(BOOL)ignoreStoreErrors
	error:(NSError **)error {
	__auto_type updateBlock = ^POSLensValue * _Nullable(POSLensValue * _Nullable value, BOOL flush, NSError *error) {
	return block(value, error);
	};
	return [self updateCurrentValueWithBlock:updateBlock ignoreStoreErrors:ignoreStoreErrors error:error];
	}

	- (BOOL)updateCurrentValueWithBlock:(POSLensValue * _Nullable (^)(POSLensValue * _Nullable,
	BOOL *flush,
	NSError **error))updateBlock
	ignoreStoreErrors:(BOOL)ignoreStoreErrors
	error:(NSError **)error {
	POS_CHECK(updateBlock);
	__block BOOL flush = YES;
	__block BOOL updated = NO;
	__block NSError *updateError = nil;
	__block POSLensValue *updatingValue;
	__block POSLensValue *updatedValue;
	dispatch_barrier_sync(_syncQueue, ^{
	updatingValue = self->_currentValue;
	updatedValue = updateBlock(updatingValue, &flush, &updateError);
	updated = (updateError == nil && updatedValue != updatingValue && ![updatedValue isEqual:updatingValue]);
	if (updated) {
	BOOL saved = flush ? [self->_store saveValue:updatedValue error:&updateError] : YES;
	updated = saved \|\| ignoreStoreErrors;
	}
	if (updated) {
	self.currentValue = updatedValue;
	}
	});
	if (error) {
	POSAssignError(error, updateError);
	} else if (updateError) {
	NSObject *failedValue = updatedValue ?: updatingValue;
	NSString *failedValueName = NSStringFromClass(failedValue.class);
	[_logger logError:@"Lens<%@>: Failed to update value: %@", failedValueName, updateError];
	}
	if (updated) {
	[_updatesSubject sendNext:[[POSLensValueUpdate alloc] initWithOldValue:updatingValue actualValue:updatedValue]];
	}
	return updateError == nil;
	}

	@end

	#pragma mark -

	@implementation POSLens (Factory)

	+ (instancetype)lensWithValue:(nullable POSLensValue *)value {
	return [self
	lensWithDefaultValue:nil
	store:[[POSEphemeralValueStore alloc] initWithValue:value]
	logger:nil
	error:nil];
	}

	+ (nullable instancetype)lensWithDefaultValue:(nullable POSLensValue *)value
	store:(id<POSValueStore>)store
	logger:(nullable id<POSLogger>)logger
	error:(NSError **)error {
	NSError *loadError;
	POSLensValue *currentValue = [store loadValue:&loadError];
	if (loadError != nil) {
	[logger logError:@"Failed to create lens for %@ with default value %@: %@", store, value, loadError];
	POSAssignError(error, loadError);
	return nil;
	}
	return [[POSRootLens alloc] initWithDefaultValue:value currentValue:currentValue store:store logger:logger];
	}

	+ (nullable instancetype)lensWithDefaultValue:(nullable POSLensValue *)value
	filePath:(NSString *)filePath
	logger:(nullable id<POSLogger>)logger
	error:(NSError **)error {
	return [self
	lensWithDefaultValue:value
	store:[[POSFileValueStore alloc] initWithFilePath:filePath]
	logger:logger
	error:error];
	}

	+ (nullable instancetype)lensWithDefaultValue:(nullable POSLensValue *)value
	keychainService:(NSString *)service
	valueKey:(NSString *)valueKey
	logger:(nullable id<POSLogger>)logger
	error:(NSError **)error {
	return [self
	lensWithDefaultValue:value
	store:[[POSKeychainValueStore alloc] initWithValueKey:valueKey service:service accessGroup:nil]
	logger:logger
	error:error];
	}

	+ (nullable instancetype)lensWithDefaultValue:(nullable POSLensValue *)value
	userDefaults:(NSUserDefaults *)userDefaults
	valueKey:(NSString *)valueKey
	logger:(nullable id<POSLogger>)logger
	error:(NSError **)error {
	return [self
	lensWithDefaultValue:value
	store:[[POSUserDefaultsValueStore alloc] initWithUserDefaults:userDefaults valueKey:valueKey]
	logger:logger
	error:error];
	}

	@end

	NS_ASSUME_NONNULL_END