iOS-SDWebImage缓存机制
2019-02-23 本文已影响14人
路飞_Luck
序言
本文章中讲解的SDWebImage版本为4.4.4
一 存储
图片的存储主要是由类SDImageCache实现的，主要方法如下
- (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 = image.sd_memoryCost;
[self.memCache setObject:image forKey:key cost:cost];
}
if (toDisk) {
dispatch_async(self.ioQueue, ^{
@autoreleasepool {
NSData *data = imageData;
// 如果image存在，但是需要重新计算(recalculate)或者data为空
// 那就要根据image重新生成新的data
// 不过要是连image也为空的话，那就别存了
if (!data && image) {
// If we do not have any data to detect image format, check whether it contains alpha channel to use PNG or JPEG format
SDImageFormat format;
if (SDCGImageRefContainsAlpha(image.CGImage)) {
format = SDImageFormatPNG;
} else {
format = SDImageFormatJPEG;
}
data = [[SDWebImageCodersManager sharedInstance] encodedDataWithImage:image format:format];
}
[self _storeImageDataToDisk:data forKey:key];
}
if (completionBlock) {
dispatch_async(dispatch_get_main_queue(), ^{
completionBlock();
});
}
});
} else {
if (completionBlock) {
completionBlock();
}
}
}
- 存储图片
// Make sure to call form io queue by caller
- (void)_storeImageDataToDisk:(nullable NSData *)imageData forKey:(nullable NSString *)key {
if (!imageData || !key) {
return;
}
// 首先判断disk cache的文件路径是否存在，不存在的话就创建一个
// disk cache的文件路径是存储在_diskCachePath中的
if (![self.fileManager fileExistsAtPath:_diskCachePath]) {
[self.fileManager createDirectoryAtPath:_diskCachePath withIntermediateDirectories:YES attributes:nil error:NULL];
}
// get cache Path for image key
// 根据image的key(一般情况下理解为image的url)组合成最终的文件路径
NSString *cachePathForKey = [self defaultCachePathForKey:key];
// transform to NSUrl
NSURL *fileURL = [NSURL fileURLWithPath:cachePathForKey];
// 根据存储的路径(cachePathForKey)和存储的数据(data)将其存放到iOS的文件系统
[imageData writeToURL:fileURL options:self.config.diskCacheWritingOptions error:nil];
// disable iCloud backup
if (self.config.shouldDisableiCloud) {
[fileURL setResourceValue:@YES forKey:NSURLIsExcludedFromBackupKey error:nil];
}
}
二 取图片
- 内存缓存使用
SDImageCache的imageFromMemoryCacheForKey:取数据
- (nullable UIImage *)imageFromMemoryCacheForKey:(nullable NSString *)key {
return [self.memCache objectForKey:key];
}
- 从磁盘中读取数据
- (nullable UIImage *)imageFromDiskCacheForKey:(nullable NSString *)key {
UIImage *diskImage = [self diskImageForKey:key];
if (diskImage && self.config.shouldCacheImagesInMemory) {
NSUInteger cost = diskImage.sd_memoryCost;
[self.memCache setObject:diskImage forKey:key cost:cost];
}
return diskImage;
}
- 读取磁盘
- (nullable UIImage *)diskImageForKey:(nullable NSString *)key {
NSData *data = [self diskImageDataForKey:key];
return [self diskImageForKey:key data:data];
}
- (nullable UIImage *)diskImageForKey:(nullable NSString *)key data:(nullable NSData *)data {
return [self diskImageForKey:key data:data options:0];
}
- (nullable UIImage *)diskImageForKey:(nullable NSString *)key data:(nullable NSData *)data options:(SDImageCacheOptions)options {
if (data) {
UIImage *image = [[SDWebImageCodersManager sharedInstance] decodedImageWithData:data];
image = [self scaledImageForKey:key image:image];
if (self.config.shouldDecompressImages) {
BOOL shouldScaleDown = options & SDImageCacheScaleDownLargeImages;
image = [[SDWebImageCodersManager sharedInstance] decompressedImageWithImage:image data:&data options:@{SDWebImageCoderScaleDownLargeImagesKey: @(shouldScaleDown)}];
}
return image;
} else {
return nil;
}
}
三 删除图片
删除图片主要用到下面四个方法
-
removeImageForKeyfromDisk:withCompletion:异步地将image从缓存(内存缓存以及可选的磁盘缓存)中移除 -
clearMemory清楚内存缓存上的所有image -
clearDisk清除磁盘缓存上的所有image -
cleanDisk清除磁盘缓存上过期的image
主要由类SDImageCache实现
- (void)deleteOldFilesWithCompletionBlock:(nullable SDWebImageNoParamsBlock)completionBlock {
dispatch_async(self.ioQueue, ^{
// 记录遍历的文件目录
NSURL *diskCacheURL = [NSURL fileURLWithPath:self.diskCachePath isDirectory:YES];
// Compute content date key to be used for tests
NSURLResourceKey cacheContentDateKey = NSURLContentModificationDateKey;
switch (self.config.diskCacheExpireType) {
case SDImageCacheConfigExpireTypeAccessDate:
cacheContentDateKey = NSURLContentAccessDateKey;
break;
case SDImageCacheConfigExpireTypeModificationDate:
cacheContentDateKey = NSURLContentModificationDateKey;
break;
default:
break;
}
// 记录遍历需要预先获取文件的哪些属性
NSArray<NSString *> *resourceKeys = @[NSURLIsDirectoryKey, cacheContentDateKey, NSURLTotalFileAllocatedSizeKey];
// This enumerator prefetches useful properties for our cache files.
// 递归地遍历diskCachePath这个文件夹中的所有目录，此处不是直接使用diskCachePath，而是使用其生成的NSURL
// 此处使用includingPropertiesForKeys:resourceKeys，这样每个file的resourceKeys对应的属性也会在遍历时预先获取到
// NSDirectoryEnumerationSkipsHiddenFiles表示不遍历隐藏文件
NSDirectoryEnumerator *fileEnumerator = [self.fileManager enumeratorAtURL:diskCacheURL
includingPropertiesForKeys:resourceKeys
options:NSDirectoryEnumerationSkipsHiddenFiles
errorHandler:NULL];
// 获取文件的过期时间，SDWebImage中默认是一个星期
// 不过这里虽然称*expirationDate为过期时间，但是实质上并不是这样。
// 其实是这样的，比如在2015/12/12/00:00:00最后一次修改文件，对应的过期时间应该是
// 2015/12/19/00:00:00，不过现在时间是2015/12/27/00:00:00，我先将当前时间减去1个星期，得到
// 2015/12/20/00:00:00，这个时间才是我们函数中的expirationDate。
// 用这个expirationDate和最后一次修改时间modificationDate比较看谁更晚就行。
NSDate *expirationDate = [NSDate dateWithTimeIntervalSinceNow:-self.config.maxCacheAge];
// 用来存储对应文件的一些属性，比如文件所需磁盘空间
NSMutableDictionary<NSURL *, NSDictionary<NSString *, id> *> *cacheFiles = [NSMutableDictionary dictionary];
// 记录当前已经使用的磁盘缓存大小
NSUInteger currentCacheSize = 0;
// Enumerate all of the files in the cache directory. This loop has two purposes:
//
// 1. Removing files that are older than the expiration date.
// 2. Storing file attributes for the size-based cleanup pass.
NSMutableArray<NSURL *> *urlsToDelete = [[NSMutableArray alloc] init];
for (NSURL *fileURL in fileEnumerator) {
NSError *error;
NSDictionary<NSString *, id> *resourceValues = [fileURL resourceValuesForKeys:resourceKeys error:&error];
// Skip directories and errors. - 当前扫描的是目录，就跳过
if (error || !resourceValues || [resourceValues[NSURLIsDirectoryKey] boolValue]) {
continue;
}
// Remove files that are older than the expiration date;
// 移除过期文件 - 这里判断过期的方式：对比文件的最后一次修改日期和expirationDate谁更晚，如果expirationDate更晚，就认为该文件已经过期，具体解释见上面
NSDate *modifiedDate = resourceValues[cacheContentDateKey];
if ([[modifiedDate laterDate:expirationDate] isEqualToDate:expirationDate]) {
[urlsToDelete addObject:fileURL];
continue;
}
// Store a reference to this file and account for its total size.
// 计算当前已经使用的cache大小， - 并将对应file的属性存到cacheFiles中
NSNumber *totalAllocatedSize = resourceValues[NSURLTotalFileAllocatedSizeKey];
currentCacheSize += totalAllocatedSize.unsignedIntegerValue;
cacheFiles[fileURL] = resourceValues;
}
// 根据需要移除文件的url来移除对应file
for (NSURL *fileURL in urlsToDelete) {
[self.fileManager removeItemAtURL:fileURL error:nil];
}
// If our remaining disk cache exceeds a configured maximum size, perform a second
// size-based cleanup pass. We delete the oldest files first.
// 如果我们当前cache的大小已经超过了允许配置的缓存大小，那就删除已经缓存的文件。
// 删除策略就是，首先删除修改时间更早的缓存文件
if (self.config.maxCacheSize > 0 && currentCacheSize > self.config.maxCacheSize) {
// Target half of our maximum cache size for this cleanup pass.
// 直接将当前cache大小降到允许最大的cache大小的一半
const NSUInteger desiredCacheSize = self.config.maxCacheSize / 2;
// Sort the remaining cache files by their last modification time or last access time (oldest first).
// 根据文件修改时间来给所有缓存文件排序，按照修改时间越早越在前的规则排序
NSArray<NSURL *> *sortedFiles = [cacheFiles keysSortedByValueWithOptions:NSSortConcurrent
usingComparator:^NSComparisonResult(id obj1, id obj2) {
return [obj1[cacheContentDateKey] compare:obj2[cacheContentDateKey]];
}];
// Delete files until we fall below our desired cache size.
// 每次删除file后，就计算此时的cache的大小
for (NSURL *fileURL in sortedFiles) {
if ([self.fileManager removeItemAtURL:fileURL error:nil]) {
NSDictionary<NSString *, id> *resourceValues = cacheFiles[fileURL];
NSNumber *totalAllocatedSize = resourceValues[NSURLTotalFileAllocatedSizeKey];
currentCacheSize -= totalAllocatedSize.unsignedIntegerValue;
// 如果此时的cache大小已经降到期望的大小了，就停止删除文件了
if (currentCacheSize < desiredCacheSize) {
break;
}
}
}
}
if (completionBlock) {
dispatch_async(dispatch_get_main_queue(), ^{
completionBlock();
});
}
});
}
四 图片存储路径
- 磁盘存储图片路径
- (nullable NSString *)defaultCachePathForKey:(nullable NSString *)key {
return [self cachePathForKey:key inPath:self.diskCachePath];
}
// 将存储的文件路径和文件名绑定在一起，作为最终的存储路径
- (nullable NSString *)cachePathForKey:(nullable NSString *)key inPath:(nonnull NSString *)path {
NSString *filename = [self cachedFileNameForKey:key];
return [path stringByAppendingPathComponent:filename];
}
- (nullable NSString *)cachedFileNameForKey:(nullable NSString *)key {
const char *str = key.UTF8String;
if (str == NULL) {
str = "";
}
// 使用了MD5进行加密处理
// 开辟一个16字节（128位：md5加密出来就是128bit）的空间
unsigned char r[CC_MD5_DIGEST_LENGTH];
// 把str字符串转换成了32位的16进制数列（这个过程不可逆转） 存储到了r这个空间中
CC_MD5(str, (CC_LONG)strlen(str), r);
NSURL *keyURL = [NSURL URLWithString:key];
NSString *ext = keyURL ? keyURL.pathExtension : key.pathExtension;
// File system has file name length limit, we need to check if ext is too long, we don't add it to the filename
if (ext.length > SD_MAX_FILE_EXTENSION_LENGTH) {
ext = nil;
}
// 最终生成的文件名就是 "md5码"+".文件类型"
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], ext.length == 0 ? @"" : [NSString stringWithFormat:@".%@", ext]];
return filename;
}
- 解释
1.key为图片下载路径，例如 https://imgs3.taobao/banners/201812/82f6bcdeb55191b75f3371056f093388.jpg
2.self.diskCachePath为存储路径,例如模拟器路径
/Users/cs/Library/Developer/CoreSimulator/Devices/053EEA1A-F097-409F-A232-51E51A239304/data/Containers/Data/Application/69AA9017-C1CE-4FA4-8A81-253E6AB05CF9/Library/Caches/default/com.hackemist.SDWebImageCache.default
本文部分参考# SDWebImage缓存机制，非常感谢该作者。
