本文分析YYDiskCache->YYKVStorage实现过程：

YYDiskCache对YYKVStorage一层封装，缓存方式：数据库+文件,下面先分析主要实现类YYKVStorage，再分析表层类YYDiskCache.

YYKVStorage.h方法结构图

YYKVStorage.h方法解释

#import <Foundation/Foundation.h>

NS_ASSUME_NONNULL_BEGIN

// 用YYKVStorageItem保存缓存相关参数
@interface YYKVStorageItem : NSObject
// 缓存键值
@property (nonatomic, strong) NSString *key;
// 缓存对象
@property (nonatomic, strong) NSData *value;
// 缓存文件名
@property (nullable, nonatomic, strong) NSString *filename;
// 缓存大小
@property (nonatomic) int size;
// 修改时间
@property (nonatomic) int modTime;
// 最后使用时间
@property (nonatomic) int accessTime;
// 拓展数据
@property (nullable, nonatomic, strong) NSData *extendedData;
@end


// 可以指定缓存类型
typedef NS_ENUM(NSUInteger, YYKVStorageType) {
    // 文件缓存(filename != null)
    YYKVStorageTypeFile = 0,
    // 数据库缓存
    YYKVStorageTypeSQLite = 1,
    // 如果filename != null，则value用文件缓存，缓存的其他参数用数据库缓存；如果filename == null,则用数据库缓存
    YYKVStorageTypeMixed = 2,
};

// 缓存操作实现
@interface YYKVStorage : NSObject

#pragma mark - Attribute
// 缓存路径
@property (nonatomic, readonly) NSString *path;
// 缓存方式
@property (nonatomic, readonly) YYKVStorageType type;
// 是否要打开错误日志
@property (nonatomic) BOOL errorLogsEnabled;

#pragma mark - Initializer
// 这两个方法不能使用，因为实例化对象时要有初始化path、type
- (instancetype)init UNAVAILABLE_ATTRIBUTE;
+ (instancetype)new UNAVAILABLE_ATTRIBUTE;

/**
 *  实例化对象
 *
 *  @param path 缓存路径
 *  @param type 缓存方式
 */
- (nullable instancetype)initWithPath:(NSString *)path type:(YYKVStorageType)type NS_DESIGNATED_INITIALIZER;


#pragma mark - Save Items
/**
 *  添加缓存
 *
 *  @param item 把缓存数据封装到YYKVStorageItem对象
 */
- (BOOL)saveItem:(YYKVStorageItem *)item;

/**
 *  添加缓存
 *
 *  @param key   缓存键值
 *  @param value 缓存对象
 */
- (BOOL)saveItemWithKey:(NSString *)key value:(NSData *)value;

/**
 *  添加缓存
 *
 *  @param key          缓存键值
 *  @param value        缓存对象
 *  @param filename     缓存文件名称
 *         filename     != null
 *                          则用文件缓存value，并把`key`,`filename`,`extendedData`写入数据库
 *         filename     == null
 *                          缓存方式type：YYKVStorageTypeFile 不进行缓存
 *                          缓存方式type：YYKVStorageTypeSQLite || YYKVStorageTypeMixed 数据库缓存
 *  @param extendedData 缓存拓展数据
 */
- (BOOL)saveItemWithKey:(NSString *)key
                  value:(NSData *)value
               filename:(nullable NSString *)filename
           extendedData:(nullable NSData *)extendedData;


#pragma mark - Remove Items

/**
 *  删除缓存
 */
- (BOOL)removeItemForKey:(NSString *)key;
- (BOOL)removeItemForKeys:(NSArray<NSString *> *)keys;

/**
 *  删除所有内存开销大于size的缓存
 */
- (BOOL)removeItemsLargerThanSize:(int)size;

/**
 *  删除所有时间比time小的缓存
 */
- (BOOL)removeItemsEarlierThanTime:(int)time;

/**
 *  减小缓存占的容量开销，使总缓存的容量开销值不大于maxSize(删除原则：LRU 最久未使用的缓存将先删除)
 */
- (BOOL)removeItemsToFitSize:(int)maxSize;

/**
 *  减小总缓存数量，使总缓存数量不大于maxCount(删除原则：LRU 最久未使用的缓存将先删除)
 */
- (BOOL)removeItemsToFitCount:(int)maxCount;

/**
 *  清空所有缓存
 */
- (BOOL)removeAllItems;
- (void)removeAllItemsWithProgressBlock:(nullable void(^)(int removedCount, int totalCount))progress
                               endBlock:(nullable void(^)(BOOL error))end;


#pragma mark - Get Items

/**
 *  读取缓存
 */
- (nullable YYKVStorageItem *)getItemForKey:(NSString *)key;
- (nullable YYKVStorageItem *)getItemInfoForKey:(NSString *)key;
- (nullable NSData *)getItemValueForKey:(NSString *)key;
- (nullable NSArray<YYKVStorageItem *> *)getItemForKeys:(NSArray<NSString *> *)keys;
- (nullable NSArray<YYKVStorageItem *> *)getItemInfoForKeys:(NSArray<NSString *> *)keys;
- (nullable NSDictionary<NSString *, NSData *> *)getItemValueForKeys:(NSArray<NSString *> *)keys;

#pragma mark - Get Storage Status

/**
 *  判断当前key是否有对应的缓存
 */
- (BOOL)itemExistsForKey:(NSString *)key;

/**
 *  获取缓存总数量
 */
- (int)getItemsCount;

/**
 *  获取缓存总内存开销
 */
- (int)getItemsSize;

@end

NS_ASSUME_NONNULL_END

YYKVStorage.m方法结构图

私有方法：

公开方法：

YYKVStorage.m方法实现

下面分别拎出添加、删除、查找各个主要的方法来讲解

1.添加

// 添加缓存
- (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) {
        //** `缓存方式为YYKVStorageTypeFile(文件缓存)`并且`未传缓存文件名`则不缓存(忽略) **
        return NO;
    }

    if (filename.length) {
        //** 存在文件名则用文件缓存，并把`key`,`filename`,`extendedData`写入数据库 **

        // 缓存数据写入文件
        if (![self _fileWriteWithName:filename data:value]) {
            return NO;
        }
        // 把`key`,`filename`,`extendedData`写入数据库,存在filenam,则不把value缓存进数据库
        if (![self _dbSaveWithKey:key value:value fileName:filename extendedData:extendedData]) {
            // 如果数据库操作失败，删除之前的文件缓存
            [self _fileDeleteWithName:filename];
            return NO;
        }
        return YES;
    } else {

        if (_type != YYKVStorageTypeSQLite) {
            // ** 缓存方式：非数据库 **

            // 根据缓存key查找缓存文件名
            NSString *filename = [self _dbGetFilenameWithKey:key];
            if (filename) {
                // 删除文件缓存
                [self _fileDeleteWithName:filename];
            }
        }
        // 把缓存写入数据库
        return [self _dbSaveWithKey:key value:value fileName:nil extendedData:extendedData];
    }
}

// 把data写入文件
- (BOOL)_fileWriteWithName:(NSString *)filename data:(NSData *)data {
    // 拼接文件路径
    NSString *path = [_dataPath stringByAppendingPathComponent:filename];
    // 写入文件
    return [data writeToFile:path atomically:NO];
}

// 写入数据库
- (BOOL)_dbSaveWithKey:(NSString *)key value:(NSData *)value fileName:(NSString *)fileName extendedData:(NSData *)extendedData {

    // 执行sql语句
    NSString *sql = @"insert or replace into manifest (key, filename, size, inline_data, modification_time, last_access_time, extended_data) values (?1, ?2, ?3, ?4, ?5, ?6, ?7);";
    // 所有sql执行前，都必须能run
    sqlite3_stmt *stmt = [self _dbPrepareStmt:sql];
    if (!stmt) return NO;

    // 时间
    int timestamp = (int)time(NULL);
    // 绑定参数值
    sqlite3_bind_text(stmt, 1, key.UTF8String, -1, NULL);
    sqlite3_bind_text(stmt, 2, fileName.UTF8String, -1, NULL);
    sqlite3_bind_int(stmt, 3, (int)value.length);
    if (fileName.length == 0) {
        // fileName为null时，缓存value
        sqlite3_bind_blob(stmt, 4, value.bytes, (int)value.length, 0);
    } else {
        // fileName不为null时，不缓存value
        sqlite3_bind_blob(stmt, 4, NULL, 0, 0);
    }
    sqlite3_bind_int(stmt, 5, timestamp);
    sqlite3_bind_int(stmt, 6, timestamp);
    sqlite3_bind_blob(stmt, 7, extendedData.bytes, (int)extendedData.length, 0);

    // 执行操作
    int result = sqlite3_step(stmt);
    if (result != SQLITE_DONE) {
        //** 未完成执行数据库 **

        // 输出错误logs
        if (_errorLogsEnabled) NSLog(@"%s line:%d sqlite insert error (%d): %s", __FUNCTION__, __LINE__, result, sqlite3_errmsg(_db));
        return NO;
    }
    return YES;
}

// 将sql编译成stmt
- (sqlite3_stmt *)_dbPrepareStmt:(NSString *)sql {
    if (![self _dbCheck] || sql.length == 0 || !_dbStmtCache) return NULL;
    // 从_dbStmtCache字典里取之前编译过sql的stmt(优化)
    sqlite3_stmt *stmt = (sqlite3_stmt *)CFDictionaryGetValue(_dbStmtCache, (__bridge const void *)(sql));
    if (!stmt) {
        // 将sql编译成stmt
        int result = sqlite3_prepare_v2(_db, sql.UTF8String, -1, &stmt, NULL);
        if (result != SQLITE_OK) {
            //** 未完成执行数据库 **

            // 输出错误logs
            if (_errorLogsEnabled) NSLog(@"%s line:%d sqlite stmt prepare error (%d): %s", __FUNCTION__, __LINE__, result, sqlite3_errmsg(_db));
            return NULL;
        }
        // 将新的stmt缓存到字典
        CFDictionarySetValue(_dbStmtCache, (__bridge const void *)(sql), stmt);
    } else {
        // 重置stmt状态
        sqlite3_reset(stmt);
    }
    return stmt;
}

// 删除文件
- (BOOL)_fileDeleteWithName:(NSString *)filename {
    NSString *path = [_dataPath stringByAppendingPathComponent:filename];
    return [[NSFileManager defaultManager] removeItemAtPath:path error:NULL];
}

// 从数据库查找文件名
- (NSString *)_dbGetFilenameWithKey:(NSString *)key {
    // 准备执行sql
    NSString *sql = @"select filename from manifest where key = ?1;";
    sqlite3_stmt *stmt = [self _dbPrepareStmt:sql];
    if (!stmt) return nil;
    // 绑定参数
    sqlite3_bind_text(stmt, 1, key.UTF8String, -1, NULL);
    // 执行操作
    int result = sqlite3_step(stmt);

    if (result == SQLITE_ROW) {
        //** 存在可读的row **

        // 取出stmt中的数据
        char *filename = (char *)sqlite3_column_text(stmt, 0);
        if (filename && *filename != 0) {
            // 转utf8 string
            return [NSString stringWithUTF8String:filename];
        }
    } else {
        if (result != SQLITE_DONE) {
            //** 未完成执行数据库 **

            // 输出错误logs
            if (_errorLogsEnabled) NSLog(@"%s line:%d sqlite query error (%d): %s", __FUNCTION__, __LINE__, result, sqlite3_errmsg(_db));
        }
    }
    return nil;
}

2.删除

// 删除缓存
- (BOOL)removeItemForKey:(NSString *)key {
    if (key.length == 0) return NO;

    // 判断缓存方式
    switch (_type) {
        case YYKVStorageTypeSQLite: {
            //** 数据库缓存 **

            // 删除数据库记录
            return [self _dbDeleteItemWithKey:key];
        } break;
        case YYKVStorageTypeFile:
        case YYKVStorageTypeMixed: {
            //** 数据库缓存 或 文件缓存 **

            // 查找缓存文件名
            NSString *filename = [self _dbGetFilenameWithKey:key];
            if (filename) {
                // 删除文件缓存
                [self _fileDeleteWithName:filename];
            }
            // 删除数据库记录
            return [self _dbDeleteItemWithKey:key];
        } break;
        default: return NO;
    }
}

// 删除数据库记录
- (BOOL)_dbDeleteItemWithKey:(NSString *)key {
    // 准备执行sql
    NSString *sql = @"delete from manifest where key = ?1;";
    sqlite3_stmt *stmt = [self _dbPrepareStmt:sql];
    if (!stmt) return NO;

    // 绑定参数
    sqlite3_bind_text(stmt, 1, key.UTF8String, -1, NULL);
    // 执行操作
    int result = sqlite3_step(stmt);
    if (result != SQLITE_DONE) {
        //** 未完成执行数据库 **

        // 输出错误logs
        if (_errorLogsEnabled) NSLog(@"%s line:%d db delete error (%d): %s", __FUNCTION__, __LINE__, result, sqlite3_errmsg(_db));
        return NO;
    }
    return YES;
}

3.查找

// 查找缓存
- (YYKVStorageItem *)getItemForKey:(NSString *)key {
    if (key.length == 0) return nil;
    // 数据库查询
    YYKVStorageItem *item = [self _dbGetItemWithKey:key excludeInlineData:NO];
    if (item) {
        //** 数据库存在记录 **

        // 更新操作时间
        [self _dbUpdateAccessTimeWithKey:key];
        if (item.filename) {
            //** 存在文件名 **

            // 读入文件
            item.value = [self _fileReadWithName:item.filename];
            if (!item.value) {
                //** 未找到文件 **

                // 删除数据库记录
                [self _dbDeleteItemWithKey:key];
                item = nil;
            }
        }
    }
    return item;
}

// 数据库查询
- (YYKVStorageItem *)_dbGetItemWithKey:(NSString *)key excludeInlineData:(BOOL)excludeInlineData {
    // 准备执行sql
    NSString *sql = excludeInlineData ? @"select key, filename, size, modification_time, last_access_time, extended_data from manifest where key = ?1;" : @"select key, filename, size, inline_data, modification_time, last_access_time, extended_data from manifest where key = ?1;";
    sqlite3_stmt *stmt = [self _dbPrepareStmt:sql];
    if (!stmt) return nil;

    // 绑定参数
    sqlite3_bind_text(stmt, 1, key.UTF8String, -1, NULL);

    YYKVStorageItem *item = nil;
    // 执行操作
    int result = sqlite3_step(stmt);
    if (result == SQLITE_ROW) {
        //** 存在可读的row **

        // 获取YYKVStorageItem
        item = [self _dbGetItemFromStmt:stmt excludeInlineData:excludeInlineData];
    } else {
        if (result != SQLITE_DONE) {
            //** 未完成执行数据库 **

            // 输出错误logs
            if (_errorLogsEnabled) NSLog(@"%s line:%d sqlite query error (%d): %s", __FUNCTION__, __LINE__, result, sqlite3_errmsg(_db));
        }
    }
    return item;
}

// 转换模型YYKVStorageItem
- (YYKVStorageItem *)_dbGetItemFromStmt:(sqlite3_stmt *)stmt excludeInlineData:(BOOL)excludeInlineData {
    int i = 0;
    // 取出数据
    char *key = (char *)sqlite3_column_text(stmt, i++);
    char *filename = (char *)sqlite3_column_text(stmt, i++);
    int size = sqlite3_column_int(stmt, i++);
    const void *inline_data = excludeInlineData ? NULL : sqlite3_column_blob(stmt, i);
    int inline_data_bytes = excludeInlineData ? 0 : sqlite3_column_bytes(stmt, i++);
    int modification_time = sqlite3_column_int(stmt, i++);
    int last_access_time = sqlite3_column_int(stmt, i++);
    const void *extended_data = sqlite3_column_blob(stmt, i);
    int extended_data_bytes = sqlite3_column_bytes(stmt, i++);

    // 赋值模型
    YYKVStorageItem *item = [YYKVStorageItem new];
    if (key) item.key = [NSString stringWithUTF8String:key];
    if (filename && *filename != 0) item.filename = [NSString stringWithUTF8String:filename];
    item.size = size;
    if (inline_data_bytes > 0 && inline_data) item.value = [NSData dataWithBytes:inline_data length:inline_data_bytes];
    item.modTime = modification_time;
    item.accessTime = last_access_time;
    if (extended_data_bytes > 0 && extended_data) item.extendedData = [NSData dataWithBytes:extended_data length:extended_data_bytes];
    return item;
}

YYDiskCache

1.初始化

- (instancetype)initWithPath:(NSString *)path
             inlineThreshold:(NSUInteger)threshold {
    self = [super init];
    if (!self) return nil;

    // 1.根据path先从缓存里面找YYDiskCache(未找到再去重新创建实例)
    YYDiskCache *globalCache = _YYDiskCacheGetGlobal(path);
    if (globalCache) return globalCache;

    // 2.重新创建实例

    // 2.1缓存方式
    YYKVStorageType type;
    if (threshold == 0) {
        type = YYKVStorageTypeFile;
    } else if (threshold == NSUIntegerMax) {
        type = YYKVStorageTypeSQLite;
    } else {
        type = YYKVStorageTypeMixed;
    }
    // 2.2实例化YYKVStorage对象(YYKVStorage上面已分析，YYDiskCache的缓存实现都在YKVStorage)
    YYKVStorage *kv = [[YYKVStorage alloc] initWithPath:path type:type];
    if (!kv) return nil;

    // 2.3初始化数据
    _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];
    // 2.4缓存self
    _YYDiskCacheSetGlobal(self);

    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(_appWillBeTerminated) name:UIApplicationWillTerminateNotification object:nil];
    return self;
}

// 获取已经缓存的YYDiskCache对象
static YYDiskCache *_YYDiskCacheGetGlobal(NSString *path) {
    if (path.length == 0) return nil;
    // 初始化字典(用来缓存YYDiskCache对象)与创建锁
    _YYDiskCacheInitGlobal();
    // 加锁
    dispatch_semaphore_wait(_globalInstancesLock, DISPATCH_TIME_FOREVER);
    id cache = [_globalInstances objectForKey:path];
    // 解锁
    dispatch_semaphore_signal(_globalInstancesLock);
    return cache;
}

// 初始化字典(用来缓存YYDiskCache对象)与创建锁
static void _YYDiskCacheInitGlobal() {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        // 创建一把锁
        _globalInstancesLock = dispatch_semaphore_create(1);
        // 初始化字典
        _globalInstances = [[NSMapTable alloc] initWithKeyOptions:NSPointerFunctionsStrongMemory valueOptions:NSPointerFunctionsWeakMemory capacity:0];
    });
}

// 保存新的YYDiskCache
static void _YYDiskCacheSetGlobal(YYDiskCache *cache) {
    if (cache.path.length == 0) return;
    _YYDiskCacheInitGlobal();
    dispatch_semaphore_wait(_globalInstancesLock, DISPATCH_TIME_FOREVER);
    [_globalInstances setObject:cache forKey:cache.path];
    dispatch_semaphore_signal(_globalInstancesLock);
}

2.添加缓存

// 添加缓存
- (void)setObject:(id<NSCoding>)object forKey:(NSString *)key {
    if (!key) return;
    if (!object) {
        //** 缓存对象为null **

        // 删除缓存
        [self removeObjectForKey:key];
        return;
    }

    NSData *extendedData = [YYDiskCache getExtendedDataFromObject:object];
    NSData *value = nil;
    // 你可以customArchiveBlock外部归档数据
    if (_customArchiveBlock) {
        value = _customArchiveBlock(object);
    } else {
        @try {
            // 归档数据
            value = [NSKeyedArchiver archivedDataWithRootObject:object];
        }
        @catch (NSException *exception) {
            // nothing to do...
        }
    }
    if (!value) return;
    NSString *filename = nil;
    if (_kv.type != YYKVStorageTypeSQLite) {
        // ** 缓存类型非YYKVStorageTypeSQLite **
        if (value.length > _inlineThreshold) {
            // ** 缓存对象大于_inlineThreshold值则用文件缓存 **

            // 生成文件名
            filename = [self _filenameForKey:key];
        }
    }
    // 加锁
    Lock();
    // 缓存数据(此方法上面讲过)
    [_kv saveItemWithKey:key value:value filename:filename extendedData:extendedData];
    // 解锁
    Unlock();
}

3.其他查找，删除都类似，就不一一分析了

YYCache源码分析(一)
YYCache源码分析(二)