這里會做一些源碼的解析，也會對使用較多的類或方法進行簡單的介紹！
目前為止sdwebimage已經更新到4.0.0 測試的第二個版本，我們拿到源碼來看看。
只要修改源碼的同志們記得吃藥，大的架構永遠不會變。
一. SDImageCache (sdwebimage緩存類)
我們先來看看SDImageCache.h文件中的內容
#import <Foundation/Foundation.h>
#import "SDWebImageCompat.h"
@class SDImageCacheConfig;
typedef NS_ENUM(NSInteger, SDImageCacheType) {
/**
* The image wasn't available the SDWebImage caches, but was downloaded from the web.
*/
//不用緩存
SDImageCacheTypeNone,
/**
* The image was obtained from the disk cache.
*/
//磁盤緩存
SDImageCacheTypeDisk,
/**
* The image was obtained from the memory cache.
*/
//內存緩存
SDImageCacheTypeMemory
};
typedef void(^SDCacheQueryCompletedBlock)(UIImage * _Nullable image, NSData * _Nullable data, SDImageCacheType cacheType);
typedef void(^SDWebImageCheckCacheCompletionBlock)(BOOL isInCache);
typedef void(^SDWebImageCalculateSizeBlock)(NSUInteger fileCount, NSUInteger totalSize);
/**
* SDImageCache maintains a memory cache and an optional disk cache. Disk cache write operations are performed
* asynchronous so it doesn’t add unnecessary latency to the UI.
* SDImageCache類用于操控內存緩存和可選磁盤緩存。 磁盤高速緩存寫入操作是異步執(zhí)行的，因此不會對UI增加不必要的延遲
*/
@interface SDImageCache : NSObject
#pragma mark - Properties
/**
* Cache Config object - storing all kind of settings
* 默認：解壓縮，但是會消耗很大內存，如果程序崩潰，設為NO
* 默認：禁用iCloud備份
* 默認：最長緩存時間 單位：秒 一周
* 默認：內存緩存
*/
@property (nonatomic, nonnull, readonly) SDImageCacheConfig *config;
/**
* The maximum "total cost" of the in-memory image cache. The cost function is the number of pixels held in memory.
* 內存中圖像緩存的最大“總成本”。 成本函數是存儲器中保存的像素數
*/
@property (assign, nonatomic) NSUInteger maxMemoryCost;
/**
* The maximum number of objects the cache should hold.
* 緩存應該保留的對象的最大數量
*/
@property (assign, nonatomic) NSUInteger maxMemoryCountLimit;
#pragma mark - Singleton and initialization
/**
* Returns global shared cache instance
*
* @return SDImageCache global instance
*/
//單例
+ (nonnull instancetype)sharedImageCache;
/**
* Init a new cache store with a specific namespace
*
* @param ns The namespace to use for this cache store
*/
//同下
- (nonnull instancetype)initWithNamespace:(nonnull NSString *)ns;
/**
* Init a new cache store with a specific namespace and directory
*
* @param ns The namespace to use for this cache store
* @param directory Directory to cache disk images in
*/
//初始化，執(zhí)行此方法會根據ns及directory生成緩存在磁盤中的路徑diskCachePath
- (nonnull instancetype)initWithNamespace:(nonnull NSString *)ns
diskCacheDirectory:(nonnull NSString *)directory NS_DESIGNATED_INITIALIZER;
#pragma mark - Cache paths
//同上，生成diskCachePath（兩者又是不同的，具體，看.m文件區(qū)別）
- (nullable NSString *)makeDiskCachePath:(nonnull NSString*)fullNamespace;
/**
* Add a read-only cache path to search for images pre-cached by SDImageCache
* Useful if you want to bundle pre-loaded images with your app
*
* @param path The path to use for this read-only cache path
*/
- (void)addReadOnlyCachePath:(nonnull NSString *)path;
#pragma mark - Store Ops
/**
* Asynchronously store an image into memory and disk cache at the given key.
*
* @param image The image to store
* @param key The unique image cache key, usually it's image absolute URL
* @param completionBlock A block executed after the operation is finished
*/
//根據key緩存圖片（通常情況下key是image absolute URL）
- (void)storeImage:(nullable UIImage *)image
forKey:(nullable NSString *)key
completion:(nullable SDWebImageNoParamsBlock)completionBlock;
/**
* Asynchronously store an image into memory and disk cache at the given key.
*
* @param image The image to store
* @param key The unique image cache key, usually it's image absolute URL
* @param toDisk Store the image to disk cache if YES
* @param completionBlock A block executed after the operation is finished
*/
//同上，指示是否寫入磁盤
- (void)storeImage:(nullable UIImage *)image
forKey:(nullable NSString *)key
toDisk:(BOOL)toDisk
completion:(nullable SDWebImageNoParamsBlock)completionBlock;
/**
* Asynchronously store an image into memory and disk cache at the given key.
*
* @param image The image to store
* @param imageData The image data as returned by the server, this representation will be used for disk storage
* instead of converting the given image object into a storable/compressed image format in order
* to save quality and CPU
* @param key The unique image cache key, usually it's image absolute URL
* @param toDisk Store the image to disk cache if YES
* @param completionBlock A block executed after the operation is finished
*/
//同上 imagedata服務器返回的未解壓的數據
- (void)storeImage:(nullable UIImage *)image
imageData:(nullable NSData *)imageData
forKey:(nullable NSString *)key
toDisk:(BOOL)toDisk
completion:(nullable SDWebImageNoParamsBlock)completionBlock;
/**
* Synchronously store image NSData into disk cache at the given key.
*
* @warning This method is synchronous, make sure to call it from the ioQueue
*
* @param imageData The image data to store
* @param key The unique image cache key, usually it's image absolute URL
*/
- (void)storeImageDataToDisk:(nullable NSData *)imageData forKey:(nullable NSString *)key;
#pragma mark - Query and Retrieve Ops
/**
* Async check if image exists in disk cache already (does not load the image)
*
* @param key the key describing the url
* @param completionBlock the block to be executed when the check is done.
* @note the completion block will be always executed on the main queue
*/
//在磁盤中查找是否有key對應的image對象，block中是一bool值
- (void)diskImageExistsWithKey:(nullable NSString *)key completion:(nullable SDWebImageCheckCacheCompletionBlock)completionBlock;
/**
* Operation that queries the cache asynchronously and call the completion when done.
*
* @param key The unique key used to store the wanted image
* @param doneBlock The completion block. Will not get called if the operation is cancelled
*
* @return a NSOperation instance containing the cache op
*/
//先在memory中查找，找不到去disk查找（sdwebimageManager的回調方法）
- (nullable NSOperation *)queryCacheOperationForKey:(nullable NSString *)key done:(nullable SDCacheQueryCompletedBlock)doneBlock;
/**
* Query the memory cache synchronously.
*
* @param key The unique key used to store the image
*/
//查找內存中key對應的圖片
- (nullable UIImage *)imageFromMemoryCacheForKey:(nullable NSString *)key;
/**
* Query the disk cache synchronously.
*
* @param key The unique key used to store the image
*/
//查找磁盤中key對應的圖片
- (nullable UIImage *)imageFromDiskCacheForKey:(nullable NSString *)key;
/**
* Query the cache (memory and or disk) synchronously after checking the memory cache.
*
* @param key The unique key used to store the image
*/
//這個注釋什么意思？after checking the memory cache？
- (nullable UIImage *)imageFromCacheForKey:(nullable NSString *)key;
#pragma mark - Remove Ops
/**
* Remove the image from memory and disk cache asynchronously
*
* @param key The unique image cache key
* @param completion A block that should be executed after the image has been removed (optional)
*/
//內存、磁盤都刪
- (void)removeImageForKey:(nullable NSString *)key withCompletion:(nullable SDWebImageNoParamsBlock)completion;
/**
* Remove the image from memory and optionally disk cache asynchronously
*
* @param key The unique image cache key
* @param fromDisk Also remove cache entry from disk if YES
* @param completion A block that should be executed after the image has been removed (optional)
*/
//可選磁盤中的文件是否刪除
- (void)removeImageForKey:(nullable NSString *)key fromDisk:(BOOL)fromDisk withCompletion:(nullable SDWebImageNoParamsBlock)completion;
#pragma mark - Cache clean Ops
/**
* Clear all memory cached images
*/
//清空內存
- (void)clearMemory;
/**
* Async clear all disk cached images. Non-blocking method - returns immediately.
* @param completion A block that should be executed after cache expiration completes (optional)
*/
//清理磁盤中的所有文件
- (void)clearDiskOnCompletion:(nullable SDWebImageNoParamsBlock)completion;
/**
* Async remove all expired cached image from disk. Non-blocking method - returns immediately.
* @param completionBlock A block that should be executed after cache expiration completes (optional)
*/
//清除過期文件
- (void)deleteOldFilesWithCompletionBlock:(nullable SDWebImageNoParamsBlock)completionBlock;
#pragma mark - Cache Info
/**
* Get the size used by the disk cache
*/
//獲取緩存文件總大小
- (NSUInteger)getSize;
/**
* Get the number of images in the disk cache
*/
//獲取緩存文件數量
- (NSUInteger)getDiskCount;
/**
* Asynchronously calculate the disk cache's size.
*/
//計算緩存總大小
- (void)calculateSizeWithCompletionBlock:(nullable SDWebImageCalculateSizeBlock)completionBlock;
#pragma mark - Cache Paths
/**
* Get the cache path for a certain key (needs the cache path root folder)
*
* @param key the key (can be obtained from url using cacheKeyForURL)
* @param path the cache path root folder
*
* @return the cache path
*/
//key經過md5之后生成一個字符串拼接在path后面生成完整路徑
- (nullable NSString *)cachePathForKey:(nullable NSString *)key inPath:(nonnull NSString *)path;
/**
* Get the default cache path for a certain key
*
* @param key the key (can be obtained from url using cacheKeyForURL)
*
* @return the default cache path
*/
//同上
- (nullable NSString *)defaultCachePathForKey:(nullable NSString *)key;
@end
下面，我們來看看SDImageCache.m中的方法實現
- (void)storeImage:(nullable UIImage *)image
imageData:(nullable NSData *)imageData
forKey:(nullable NSString *)key
toDisk:(BOOL)toDisk
completion:(nullable SDWebImageNoParamsBlock)completionBlock {
if (!image || !key) {
if (completionBlock) {
completionBlock();
}
return;
}
// if memory cache is enabled
if (self.config.shouldCacheImagesInMemory) {
NSUInteger cost = SDCacheCostForImage(image);
[self.memCache setObject:image forKey:key cost:cost];
}
if (toDisk) {
dispatch_async(self.ioQueue, ^{
NSData *data = imageData;
//image存在，但是其對應的NSData不存在，則重新生成data
if (!data && image) {
//判斷圖片類型，然后將圖片轉成data(詳見NSData+ImageContentType和UIImage+MultiFormat文件)
SDImageFormat imageFormatFromData = [NSData sd_imageFormatForImageData:data];
data = [image sd_imageDataAsFormat:imageFormatFromData];
}
//存儲到磁盤
[self storeImageDataToDisk:data forKey:key];
if (completionBlock) {
dispatch_async(dispatch_get_main_queue(), ^{
completionBlock();
});
}
});
} else {
if (completionBlock) {
completionBlock();
}
}
}
- (void)storeImageDataToDisk:(nullable NSData *)imageData forKey:(nullable NSString *)key {
if (!imageData || !key) {
return;
}
[self checkIfQueueIsIOQueue];
/*
* 獲取到需要存儲的data后，使用fileManager進行存儲
* 首先判斷disk cache的文件路徑是否存在，不存在的話就創(chuàng)建一個
* disk cache的文件路徑是存儲在_diskCachePath中的
*/
if (![_fileManager fileExistsAtPath:_diskCachePath]) {
[_fileManager createDirectoryAtPath:_diskCachePath withIntermediateDirectories:YES attributes:nil error:NULL];
}
// get cache Path for image key
/*
* 根據image的key(一般情況下理解為image的url)組合成最終的文件路徑
* 上面那個生成的文件路徑只是一個文件目錄，就跟/cache/images/img1.png和cache/images/的區(qū)別一樣
*/
NSString *cachePathForKey = [self defaultCachePathForKey:key];
// transform to NSUrl
/*
* 這個url可不是網絡端的url，而是file在系統(tǒng)路徑下的url
* 比如/foo/bar/baz --------> file:///foo/bar/baz
*/
NSURL *fileURL = [NSURL fileURLWithPath:cachePathForKey];
//根據存儲的路徑(cachePathForKey)和存儲的數據(data)將其存放到iOS的文件系統(tǒng)
[_fileManager createFileAtPath:cachePathForKey contents:imageData attributes:nil];
// disable iCloud backup（icould不備份）
if (self.config.shouldDisableiCloud) {
[fileURL setResourceValue:@YES forKey:NSURLIsExcludedFromBackupKey error:nil];
}
}
- (void)deleteOldFilesWithCompletionBlock:(nullable SDWebImageNoParamsBlock)completionBlock {
dispatch_async(self.ioQueue, ^{
/*
* 這兩個變量主要是為了下面生成NSDirectoryEnumerator準備的
* 一個是記錄遍歷的文件目錄，一個是記錄遍歷需要預先獲取文件的哪些屬性
*/
NSURL *diskCacheURL = [NSURL fileURLWithPath:self.diskCachePath isDirectory:YES];
NSArray<NSString *> *resourceKeys = @[NSURLIsDirectoryKey, NSURLContentModificationDateKey, NSURLTotalFileAllocatedSizeKey];
// This enumerator prefetches useful properties for our cache files.
/*
* 遞歸地遍歷diskCachePath這個文件夾中的所有目錄，此處不是直接使用diskCachePath，而是使用其生成的NSURL
* 此處使用includingPropertiesForKeys:resourceKeys，這樣每個file的resourceKeys對應的屬性也會在遍歷時預先獲取到
* NSDirectoryEnumerationSkipsHiddenFiles表示不遍歷隱藏文件
*/
NSDirectoryEnumerator *fileEnumerator = [_fileManager enumeratorAtURL:diskCacheURL
includingPropertiesForKeys:resourceKeys
options:NSDirectoryEnumerationSkipsHiddenFiles
errorHandler:NULL];
/*
* 獲取文件的過期時間，SDWebImage中默認是一個星期
* 不過這里雖然稱*expirationDate為過期時間，但是實質上并不是這樣
* 其實是這樣的，比如在2016/12/12/00:00:00最后一次修改文件，對應的過期時間應該是
* 2016/12/19/00:00:00，不過現在時間是2016/12/27/00:00:00，我先將當前時間減去1個星期，得到
* 2016/12/20/00:00:00，這個時間才是我們函數中的expirationDate
* 用這個expirationDate和最后一次修改時間modificationDate比較看誰更晚就行
*/
NSDate *expirationDate = [NSDate dateWithTimeIntervalSinceNow:-self.config.maxCacheAge];
//用來存儲對應文件的一些屬性，比如文件所需磁盤空間
NSMutableDictionary<NSURL *, NSDictionary<NSString *, id> *> *cacheFiles = [NSMutableDictionary dictionary];
//記錄當前已經使用的磁盤緩存大小
NSUInteger currentCacheSize = 0;
// 在緩存的目錄開始遍歷文件. 此次遍歷有兩個目的:
// 1. 移除過期的文件
// 2. 同時存儲每個文件的屬性（比如該file是否是文件夾、該file所需磁盤大小，修改時間）
NSMutableArray<NSURL *> *urlsToDelete = [[NSMutableArray alloc] init];
for (NSURL *fileURL in fileEnumerator) {
NSError *error;
NSDictionary<NSString *, id> *resourceValues = [fileURL resourceValuesForKeys:resourceKeys error:&error];
// 當前掃描的是目錄，就跳過
if (error || !resourceValues || [resourceValues[NSURLIsDirectoryKey] boolValue]) {
continue;
}
// 移除過期文件(這里判斷過期的方式：對比文件的最后一次修改日期和expirationDate誰更晚，如果expirationDate更晚，就認為該文件已經過期，具體解釋見上面)
NSDate *modificationDate = resourceValues[NSURLContentModificationDateKey];
if ([[modificationDate laterDate:expirationDate] isEqualToDate:expirationDate]) {
[urlsToDelete addObject:fileURL];
continue;
}
// 計算當前已經使用的cache大小,并將對應file的屬性存到cacheFiles中
NSNumber *totalAllocatedSize = resourceValues[NSURLTotalFileAllocatedSizeKey];
currentCacheSize += totalAllocatedSize.unsignedIntegerValue;
cacheFiles[fileURL] = resourceValues;
}
for (NSURL *fileURL in urlsToDelete) {
// 根據需要移除文件的url來移除對應file
[_fileManager removeItemAtURL:fileURL error:nil];
}
// 如果我們當前cache的大小已經超過了允許配置的緩存大小，那就刪除已經緩存的文件
// 刪除策略就是，首先刪除修改時間更早的緩存文件
if (self.config.maxCacheSize > 0 && currentCacheSize > self.config.maxCacheSize) {
// 直接將當前cache大小降到允許最大的cache大小的一般
const NSUInteger desiredCacheSize = self.config.maxCacheSize / 2;
// 根據文件修改時間來給所有緩存文件排序，按照修改時間越早越在前的規(guī)則排序
NSArray<NSURL *> *sortedFiles = [cacheFiles keysSortedByValueWithOptions:NSSortConcurrent
usingComparator:^NSComparisonResult(id obj1, id obj2) {
return [obj1[NSURLContentModificationDateKey] compare:obj2[NSURLContentModificationDateKey]];
}];
// 每次刪除file后，就計算此時的cache的大小.
//如果此時的cache大小已經降到期望的大小了，就停止刪除文件了
for (NSURL *fileURL in sortedFiles) {
if ([_fileManager removeItemAtURL:fileURL error:nil]) {
// 獲取該文件對應的屬性
NSDictionary<NSString *, id> *resourceValues = cacheFiles[fileURL];
// 根據resourceValues獲取該文件所需磁盤空間大小
NSNumber *totalAllocatedSize = resourceValues[NSURLTotalFileAllocatedSizeKey];
// 計算當前cache大小
currentCacheSize -= totalAllocatedSize.unsignedIntegerValue;
if (currentCacheSize < desiredCacheSize) {
break;
}
}
}
}
// 如果有completionBlock，就在主線程中調用
if (completionBlock) {
dispatch_async(dispatch_get_main_queue(), ^{
completionBlock();
});
}
});
}
- (nullable NSString *)cachedFileNameForKey:(nullable NSString *)key {
const char *str = key.UTF8String;
if (str == NULL) {
str = "";
}
/*
* 使用了MD5進行加密處理
* 開辟一個16字節(jié)（128位：md5加密出來就是128bit）的空間
*/
unsigned char r[CC_MD5_DIGEST_LENGTH];
/*
* 官方封裝好的加密方法
* 把str字符串轉換成了32位的16進制數列（這個過程不可逆轉） 存儲到了r這個空間中
*/
CC_MD5(str, (CC_LONG)strlen(str), r);
// 最終生成的文件名就是 "md5碼"+".文件類型"
NSString *filename = [NSString stringWithFormat:@"%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%@",
r[0], r[1], r[2], r[3], r[4], r[5], r[6], r[7], r[8], r[9], r[10],
r[11], r[12], r[13], r[14], r[15], [key.pathExtension isEqualToString:@""] ? @"" : [NSString stringWithFormat:@".%@", key.pathExtension]];
return filename;
}
