1. 初始化

[[YYCache alloc] initWithName:@""];

- (instancetype) init {
    NSLog(@"Use \"initWithName\" or \"initWithPath\" to create YYCache instance.");
    return [self initWithPath:@""];
}

- (instancetype)initWithName:(NSString *)name {
    if (name.length == 0) return nil;
    NSString *cacheFolder = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) firstObject];
    NSString *path = [cacheFolder stringByAppendingPathComponent:name];
    return [self initWithPath:path];
}

- (instancetype)initWithPath:(NSString *)path {
    if (path.length == 0) return nil;
    YYDiskCache *diskCache = [[YYDiskCache alloc] initWithPath:path];
    if (!diskCache) return nil;
    NSString *name = [path lastPathComponent];
    YYMemoryCache *memoryCache = [YYMemoryCache new];
    memoryCache.name = name;
    
    self = [super init];
    _name = name;
    _diskCache = diskCache;
    _memoryCache = memoryCache;
    return self;
}

+ (instancetype)cacheWithName:(NSString *)name {
    return [[self alloc] initWithName:name];
}

+ (instancetype)cacheWithPath:(NSString *)path {
    return [[self alloc] initWithPath:path];
}

1. YYCache提供静态方法和实例方法初始化，静态方法本质还是调用实例方法初始化。
2. 实例方法提供name和path初始化，name会构建路径使用path方法进行初始化
3. 初始化时有YYMemoryCache和YYDiskCache。
4. 使用name进行初始化时，每个name对应都会生成一个文件夹，数据库。

2. YYMemoryCache内存缓存

- (void)setObject:(id)object forKey:(id)key withCost:(NSUInteger)cost {
    if (!key) return;
    if (!object) {
        [self removeObjectForKey:key];
        return;
    }
    pthread_mutex_lock(&_lock);
    _YYLinkedMapNode *node = CFDictionaryGetValue(_lru->_dic, (__bridge const void *)(key));
    NSTimeInterval now = CACurrentMediaTime();
    if (node) {
        _lru->_totalCost -= node->_cost;
        _lru->_totalCost += cost;
        node->_cost = cost;
        node->_time = now;
        node->_value = object;
        [_lru bringNodeToHead:node];
    } else {
        node = [_YYLinkedMapNode new];
        node->_cost = cost;
        node->_time = now;
        node->_key = key;
        node->_value = object;
        [_lru insertNodeAtHead:node];
    }
    if (_lru->_totalCost > _costLimit) {
        dispatch_async(_queue, ^{
            [self trimToCost:_costLimit];
        });
    }
    if (_lru->_totalCount > _countLimit) {
        _YYLinkedMapNode *node = [_lru removeTailNode];
        if (_lru->_releaseAsynchronously) {
            dispatch_queue_t queue = _lru->_releaseOnMainThread ? dispatch_get_main_queue() : YYMemoryCacheGetReleaseQueue();
            dispatch_async(queue, ^{
                [node class]; //hold and release in queue
            });
        } else if (_lru->_releaseOnMainThread && !pthread_main_np()) {
            dispatch_async(dispatch_get_main_queue(), ^{
                [node class]; //hold and release in queue
            });
        }
    }
    pthread_mutex_unlock(&_lock);
}

从setObject:forKey:withCost看出YYMemoryCache的重要数据结构_YYLinkedMap

2.1 _YYLinkedMap

@interface _YYLinkedMap : NSObject {
    @package
    CFMutableDictionaryRef _dic; // do not set object directly
    NSUInteger _totalCost;
    NSUInteger _totalCount;
    _YYLinkedMapNode *_head; // MRU, do not change it directly
    _YYLinkedMapNode *_tail; // LRU, do not change it directly
    BOOL _releaseOnMainThread;
    BOOL _releaseAsynchronously;
}

- (void)insertNodeAtHead:(_YYLinkedMapNode *)node{
  ...
}
- (void)bringNodeToHead:(_YYLinkedMapNode *)node{
  ...
}

_YYLinkedMap是一个双向链接。
insertNodeAtHead如果是新数据时，会把node插入在链表头部
bringNodeToHead在设置缓存时，如果链表已经包含node节点，会移动节点到头部。
CFMutableDictionaryRef _dic，存储了key,model,方便快速查找。

2.2 setObject:forKey

- (void)setObject:(id)object forKey:(id)key {
    [self setObject:object forKey:key withCost:0];
}

- (void)setObject:(id)object forKey:(id)key withCost:(NSUInteger)cost {
    if (!key) return;
    if (!object) {
        [self removeObjectForKey:key];
        return;
    }
    pthread_mutex_lock(&_lock);
    _YYLinkedMapNode *node = CFDictionaryGetValue(_lru->_dic, (__bridge const void *)(key));
    NSTimeInterval now = CACurrentMediaTime();
    if (node) {
        _lru->_totalCost -= node->_cost;
        _lru->_totalCost += cost;
        node->_cost = cost;
        node->_time = now;
        node->_value = object;
        [_lru bringNodeToHead:node];
    } else {
        node = [_YYLinkedMapNode new];
        node->_cost = cost;
        node->_time = now;
        node->_key = key;
        node->_value = object;
        [_lru insertNodeAtHead:node];
    }
    if (_lru->_totalCost > _costLimit) {
        dispatch_async(_queue, ^{
            [self trimToCost:_costLimit];
        });
    }
    if (_lru->_totalCount > _countLimit) {
        _YYLinkedMapNode *node = [_lru removeTailNode];
        if (_lru->_releaseAsynchronously) {
            dispatch_queue_t queue = _lru->_releaseOnMainThread ? dispatch_get_main_queue() : YYMemoryCacheGetReleaseQueue();
            dispatch_async(queue, ^{
                [node class]; //hold and release in queue
            });
        } else if (_lru->_releaseOnMainThread && !pthread_main_np()) {
            dispatch_async(dispatch_get_main_queue(), ^{
                [node class]; //hold and release in queue
            });
        }
    }
    pthread_mutex_unlock(&_lock);
}

1. key不能为空，为空直接return
2. value不能为空，为空则需要移除对应的key
3. 获取缓存中的node

• 如果node为空,生成一个node节点，insertNodeAtHead插入到链表头部
• 如果node不为空,bringNodeToHead 修改时间并把node节点移动到链表头部
• 采用LRU算法，使用过即在最前面

4. trimToCost:移除超出Cost的节点，removeTailNode移除掉超出count节点

2.3 _trimToCost

- (void)_trimToCost:(NSUInteger)costLimit {
    BOOL finish = NO;
    pthread_mutex_lock(&_lock);
    if (costLimit == 0) {
        [_lru removeAll];
        finish = YES;
    } else if (_lru->_totalCost <= costLimit) {
        finish = YES;
    }
    pthread_mutex_unlock(&_lock);
    if (finish) return;
    
    NSMutableArray *holder = [NSMutableArray new];
    while (!finish) {
        if (pthread_mutex_trylock(&_lock) == 0) {
            if (_lru->_totalCost > costLimit) {
                _YYLinkedMapNode *node = [_lru removeTailNode];
                if (node) [holder addObject:node];
            } else {
                finish = YES;
            }
            pthread_mutex_unlock(&_lock);
        } else {
            usleep(10 * 1000); //10 ms
        }
    }
    if (holder.count) {
        dispatch_queue_t queue = _lru->_releaseOnMainThread ? dispatch_get_main_queue() : YYMemoryCacheGetReleaseQueue();
        dispatch_async(queue, ^{
            [holder count]; // release in queue
        });
    }
}

- (instancetype)init {
    self = super.init;
    ...
    _countLimit = NSUIntegerMax;
    _costLimit = NSUIntegerMax;
    _ageLimit = DBL_MAX;
    ...
}

1. costLimit = 0时，标识_costLimit已经初始化；_lru->_totalCost <= costLimit 在正常范围内
2. finish为NO,移除超出cost的节点；pthread_mutex_trylock判断是否有其他线程锁了，则10ms之后再试；否则移除removeTailNode

- (_YYLinkedMapNode *)removeTailNode {
    if (!_tail) return nil;
    _YYLinkedMapNode *tail = _tail;
    CFDictionaryRemoveValue(_dic, (__bridge const void *)(_tail->_key));
    _totalCost -= _tail->_cost;
    _totalCount--;
    if (_head == _tail) {
        _head = _tail = nil;
    } else {
        _tail = _tail->_prev;
        _tail->_next = nil;
    }
    return tail;
}

1. 移除_dic中的key,value
2. 尾部指针指向尾部倒数第二个指针，再把最后一个指针置nil

2.4 countLimit

if (_lru->_totalCount > _countLimit) {
        _YYLinkedMapNode *node = [_lru removeTailNode];
        if (_lru->_releaseAsynchronously) {
            dispatch_queue_t queue = _lru->_releaseOnMainThread ? dispatch_get_main_queue() : YYMemoryCacheGetReleaseQueue();
            dispatch_async(queue, ^{
                [node class]; //hold and release in queue
            });
        } else if (_lru->_releaseOnMainThread && !pthread_main_np()) {
            dispatch_async(dispatch_get_main_queue(), ^{
                [node class]; //hold and release in queue
            });
        }
    }

这一段只移除tailNode，而没有循环判断，countLimit是精确的，每次增加或修改了countLimit.

2.5 objectForKey:

- (id)objectForKey:(id)key {
    if (!key) return nil;
    pthread_mutex_lock(&_lock);
    _YYLinkedMapNode *node = CFDictionaryGetValue(_lru->_dic, (__bridge const void *)(key));
    if (node) {
        node->_time = CACurrentMediaTime();
        [_lru bringNodeToHead:node];
    }
    pthread_mutex_unlock(&_lock);
    return node ? node->_value : nil;
}

1. 使用时从字典获取node
2. 如果node不为空，更新时间，把节点移动到头部。
   符合LRU算法

2.6 内存管理

- (instancetype)init {
    self = super.init;
    pthread_mutex_init(&_lock, NULL);
    _lru = [_YYLinkedMap new];
    _queue = dispatch_queue_create("com.ibireme.cache.memory", DISPATCH_QUEUE_SERIAL);
    
    _countLimit = NSUIntegerMax;
    _costLimit = NSUIntegerMax;
    _ageLimit = DBL_MAX;
    _autoTrimInterval = 5.0;
    _shouldRemoveAllObjectsOnMemoryWarning = YES;
    _shouldRemoveAllObjectsWhenEnteringBackground = YES;
    
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(_appDidReceiveMemoryWarningNotification) name:UIApplicationDidReceiveMemoryWarningNotification object:nil];
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(_appDidEnterBackgroundNotification) name:UIApplicationDidEnterBackgroundNotification object:nil];
    
    [self _trimRecursively];
    return self;
}

2.6.1 清空链表数据

收到内存警告或进入后台时，清空缓存数据

2.6.2 _trimRecursively

每隔_autoTrimInterval时默认5s，检查Cost,Count,Age并清空过期数据

3. YYDiskCache持久化缓存

- (instancetype)initWithPath:(NSString *)path {
    return [self initWithPath:path inlineThreshold:1024 * 20]; // 20KB
}

- (instancetype)initWithPath:(NSString *)path
             inlineThreshold:(NSUInteger)threshold {
    self = [super init];
    if (!self) return nil;
    
    YYDiskCache *globalCache = _YYDiskCacheGetGlobal(path);
    if (globalCache) return globalCache;
    
    YYKVStorageType type;
    if (threshold == 0) {
        type = YYKVStorageTypeFile;
    } else if (threshold == NSUIntegerMax) {
        type = YYKVStorageTypeSQLite;
    } else {
        type = YYKVStorageTypeMixed;
    }
    
    YYKVStorage *kv = [[YYKVStorage alloc] initWithPath:path type:type];
    if (!kv) return nil;
    
    _kv = kv;
    _path = path;
    _lock = dispatch_semaphore_create(1);
    _queue = dispatch_queue_create("com.ibireme.cache.disk", DISPATCH_QUEUE_CONCURRENT);
    _inlineThreshold = threshold;
    _countLimit = NSUIntegerMax;
    _costLimit = NSUIntegerMax;
    _ageLimit = DBL_MAX;
    _freeDiskSpaceLimit = 0;
    _autoTrimInterval = 60;
    
    [self _trimRecursively];
    _YYDiskCacheSetGlobal(self);
    
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(_appWillBeTerminated) name:UIApplicationWillTerminateNotification object:nil];
    return self;
}

1. initWithPath默认大小20kb
2. 存储的核心类是YYKVStorage,存储分文件、数据库及混合存储。
3. YYKVStorage封装了数据库和文件的操作。

3.1 _trimRecursively

磁盘60s会检查一次，也可以手动设置

- (void)_trimRecursively {
    __weak typeof(self) _self = self;
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(_autoTrimInterval * NSEC_PER_SEC)), dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{
        __strong typeof(_self) self = _self;
        if (!self) return;
        [self _trimInBackground];
        [self _trimRecursively];
    });
}

- (void)_trimInBackground {
    __weak typeof(self) _self = self;
    dispatch_async(_queue, ^{
        __strong typeof(_self) self = _self;
        if (!self) return;
        Lock();
        [self _trimToCost:self.costLimit];
        [self _trimToCount:self.countLimit];
        [self _trimToAge:self.ageLimit];
        [self _trimToFreeDiskSpace:self.freeDiskSpaceLimit];
        Unlock();
    });
}

3.2 setObject:forKey

- (void)setObject:(id<NSCoding>)object forKey:(NSString *)key {
    if (!key) return;
    if (!object) {
        [self removeObjectForKey:key];
        return;
    }
    
    NSData *extendedData = [YYDiskCache getExtendedDataFromObject:object];
    NSData *value = nil;
    if (_customArchiveBlock) {
        value = _customArchiveBlock(object);
    } else {
        @try {
            value = [NSKeyedArchiver archivedDataWithRootObject:object];//转成NSData
        }
        @catch (NSException *exception) {
            // nothing to do...
        }
    }
    if (!value) return;
    NSString *filename = nil;
    if (_kv.type != YYKVStorageTypeSQLite) {
        if (value.length > _inlineThreshold) {
            filename = [self _filenameForKey:key];
        }
    }
    
    Lock();
    [_kv saveItemWithKey:key value:value filename:filename extendedData:extendedData];
    Unlock();
}

1. _customArchiveBlock自定义block,如果没有则使用系统方法object转data
2. saveItemWithKey保存key,value

- (BOOL)saveItemWithKey:(NSString *)key value:(NSData *)value filename:(NSString *)filename extendedData:(NSData *)extendedData {
    if (key.length == 0 || value.length == 0) return NO;
    if (_type == YYKVStorageTypeFile && filename.length == 0) {
        return NO;
    }
    
    if (filename.length) {
        if (![self _fileWriteWithName:filename data:value]) {
            return NO;
        }
        if (![self _dbSaveWithKey:key value:value fileName:filename extendedData:extendedData]) {
            [self _fileDeleteWithName:filename];
            return NO;
        }
        return YES;
    } else {
        if (_type != YYKVStorageTypeSQLite) {
            NSString *filename = [self _dbGetFilenameWithKey:key];
            if (filename) {
                [self _fileDeleteWithName:filename];
            }
        }
        return [self _dbSaveWithKey:key value:value fileName:nil extendedData:extendedData];
    }
}

1. filename不为空时，_fileWriteWithName写入文件
2. _dbSaveWithKey保存到数据库

3.3 objectForKey:

- (id<NSCoding>)objectForKey:(NSString *)key {
    if (!key) return nil;
    Lock();
    YYKVStorageItem *item = [_kv getItemForKey:key];
    Unlock();
    if (!item.value) return nil;
    
    id object = nil;
    if (_customUnarchiveBlock) {
        object = _customUnarchiveBlock(item.value);
    } else {
        @try {
            object = [NSKeyedUnarchiver unarchiveObjectWithData:item.value];//data转成对象
        }
        @catch (NSException *exception) {
            // nothing to do...
        }
    }
    if (object && item.extendedData) {
        [YYDiskCache setExtendedData:item.extendedData toObject:object];
    }
    return object;
}

- (YYKVStorageItem *)getItemForKey:(NSString *)key {
    if (key.length == 0) return nil;
    YYKVStorageItem *item = [self _dbGetItemWithKey:key excludeInlineData:NO];
    if (item) {
        [self _dbUpdateAccessTimeWithKey:key];//使用后需要更新时间
        if (item.filename) {
            item.value = [self _fileReadWithName:item.filename];
            if (!item.value) {
                [self _dbDeleteItemWithKey:key];
                item = nil;
            }
        }
    }
    return item;
}

总结:

1. YYCache很好的支持了对象的持久化问题，采用文件与数据库结合的方式进行存储。
2. 同时对象存储采用序列化的方式，解决了不同对象差异的存储问题。
3. 内存采用双链表实现LRU算法，磁盘存储采用最后更新时间的方式来实现LRU算法。

这里我有个小疑问:磁盘采用60s的方式进行轮询清理，每个YYCache对象都有60s,当有多个YYCache对象使用时，是否需要考虑下60s的长度问题。