SDWebImage缓存策略

SDImageCache

SDImageCache,图片采用了二级缓存策略。图片缓存时,在内存有缓存,在磁盘中也有缓存。其中,内存缓存是通过NSCache实现的。

缓存步骤

  1. 将图片缓存在内存中

  2. 判断图片的格式是png或是jpeg,将图片转化为NSData数据

  3. 如果是在mac_os系统中,直接将图片转化为NSBitmapImageRep数据

  4. 获取图片的存储路径,其中图片的文件名是通过传入的key经过md5加密后获得的。

  5. 将图片存储磁盘中

- (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;
if (!data && image) {
SDImageFormat imageFormatFromData = [NSData sd_imageFormatForImageData:data];
data = [image sd_imageDataAsFormat:imageFormatFromData];
}
[self storeImageDataToDisk:data forKey:key];
if (completionBlock) {
dispatch_async(dispatch_get_main_queue(), ^{
completionBlock();
});
}
});
} else {
if (completionBlock) {
completionBlock();
}
}
}

获取图片步骤

  1. 先在内存缓存中寻找

  2. 如果内存中找不到图片,则在默认的磁盘根目录中寻找,如果还是找不到,再从自定义的只读目录路径中寻找

  3. 获取图片数据后,将图片数据从NSData转化为UIImage,转化是要根据图片的类型进行转化。

  4. 默认对图片进行解压缩,生成位图图片。

  5. 将位图图片返回。

//从缓存中读取图片
- (nullable UIImage *)imageFromCacheForKey:(nullable NSString *)key {
// First check the in-memory cache...
UIImage *image = [self imageFromMemoryCacheForKey:key];
if (image) {
return image;
}
// Second check the disk cache...
image = [self imageFromDiskCacheForKey:key];
return image;
}
//从内存中查找
- (nullable UIImage *)imageFromMemoryCacheForKey:(nullable NSString *)key {
return [self.memCache objectForKey:key];
}
//从磁盘中查找
- (nullable NSData *)diskImageDataBySearchingAllPathsForKey:(nullable NSString *)key {
NSString *defaultPath = [self defaultCachePathForKey:key];
NSData *data = [NSData dataWithContentsOfFile:defaultPath];
if (data) {
return data;
}
// fallback because of https://github.com/rs/SDWebImage/pull/976 that added the extension to the disk file name
// checking the key with and without the extension
data = [NSData dataWithContentsOfFile:defaultPath.stringByDeletingPathExtension];
if (data) {
return data;
}
NSArray<NSString *> *customPaths = [self.customPaths copy];
for (NSString *path in customPaths) {
NSString *filePath = [self cachePathForKey:key inPath:path];
NSData *imageData = [NSData dataWithContentsOfFile:filePath];
if (imageData) {
return imageData;
}
// fallback because of https://github.com/rs/SDWebImage/pull/976 that added the extension to the disk file name
// checking the key with and without the extension
imageData = [NSData dataWithContentsOfFile:filePath.stringByDeletingPathExtension];
if (imageData) {
return imageData;
}
}
return nil;
}

图片解压缩步骤

  1. 判断图片是否是动态图片,如果是,则不能解压缩

  2. 判断图片是否是透明的,如果是,则不能解压缩

  3. 判断图片的颜色空间模型是不是RGB,如果不是,不能解压缩

  4. 根据图片的大小创建一个上下文

  5. 将图片绘制在上下文中。

  6. 从上下文中读取一个不透明的位图图像,该图像就是解压缩后的图像

  7. 将位图图像返回

+ (BOOL)shouldDecodeImage:(nullable UIImage *)image {
// Prevent "CGBitmapContextCreateImage: invalid context 0x0" error
if (image == nil) {
return NO;
}
// do not decode animated images
if (image.images != nil) {
return NO;
}
CGImageRef imageRef = image.CGImage;
CGImageAlphaInfo alpha = CGImageGetAlphaInfo(imageRef);
BOOL anyAlpha = (alpha == kCGImageAlphaFirst ||
alpha == kCGImageAlphaLast ||
alpha == kCGImageAlphaPremultipliedFirst ||
alpha == kCGImageAlphaPremultipliedLast);
// do not decode images with alpha
if (anyAlpha) {
return NO;
}
return YES;
}
+ (nullable UIImage *)decodedImageWithImage:(nullable UIImage *)image {
if (![UIImage shouldDecodeImage:image]) {
return image;
}
// autorelease the bitmap context and all vars to help system to free memory when there are memory warning.
// on iOS7, do not forget to call [[SDImageCache sharedImageCache] clearMemory];
@autoreleasepool{
CGImageRef imageRef = image.CGImage;
CGColorSpaceRef colorspaceRef = [UIImage colorSpaceForImageRef:imageRef];
size_t width = CGImageGetWidth(imageRef);
size_t height = CGImageGetHeight(imageRef);
size_t bytesPerRow = kBytesPerPixel * width;
// kCGImageAlphaNone is not supported in CGBitmapContextCreate.
// Since the original image here has no alpha info, use kCGImageAlphaNoneSkipLast
// to create bitmap graphics contexts without alpha info.
CGContextRef context = CGBitmapContextCreate(NULL,
width,
height,
kBitsPerComponent,
bytesPerRow,
colorspaceRef,
kCGBitmapByteOrderDefault|kCGImageAlphaNoneSkipLast);
if (context == NULL) {
return image;
}
// Draw the image into the context and retrieve the new bitmap image without alpha
CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageRef);
CGImageRef imageRefWithoutAlpha = CGBitmapContextCreateImage(context);
UIImage *imageWithoutAlpha = [UIImage imageWithCGImage:imageRefWithoutAlpha
scale:image.scale
orientation:image.imageOrientation];
CGContextRelease(context);
CGImageRelease(imageRefWithoutAlpha);
return imageWithoutAlpha;
}
}

对图片进行缩放步骤

  1. 如果是普通图像,则直接进行缩放

  2. 如果是动态图像,则要对图像中的每一张图像都进行缩放

inline UIImage *SDScaledImageForKey(NSString * _Nullable key, UIImage * _Nullable image) {
if (!image) {
return nil;
}
#if SD_MAC
return image;
#elif SD_UIKIT || SD_WATCH
if ((image.images).count > 0) {
NSMutableArray<UIImage *> *scaledImages = [NSMutableArray array];
for (UIImage *tempImage in image.images) {
[scaledImages addObject:SDScaledImageForKey(key, tempImage)];
}
return [UIImage animatedImageWithImages:scaledImages duration:image.duration];
}
else {
#if SD_WATCH
if ([[WKInterfaceDevice currentDevice] respondsToSelector:@selector(screenScale)]) {
#elif SD_UIKIT
if ([[UIScreen mainScreen] respondsToSelector:@selector(scale)]) {
#endif
CGFloat scale = 1;
if (key.length >= 8) {
NSRange range = [key rangeOfString:@"@2x."];
if (range.location != NSNotFound) {
scale = 2.0;
}
range = [key rangeOfString:@"@3x."];
if (range.location != NSNotFound) {
scale = 3.0;
}
}
UIImage *scaledImage = [[UIImage alloc] initWithCGImage:image.CGImage scale:scale orientation:image.imageOrientation];
image = scaledImage;
}
return image;
}
#endif

磁盘缓存清理步骤

  1. 获取磁盘中图片的最后修改日期。(为了减少磁盘和内存数据交换,读取是并不将整个文件读入内存,仅仅将文件的一些属性读入内存中,包括最后修改日期,该文件是否为文件夹,文件的大小和对应文件的文件路径)

  2. 根据最后修改日期将图片进行分类,将那些已经存放超过最长存放时间的文件存储在删除数组,其他的文件信息存储在另一个字典中。并计算除去要删除的文件之外的所有文件大小

  3. 根据删除数组中的文件路径,将对应的文件删除。

  4. 判断剩下的文件大小是否超过用户现在的磁盘最大容量。

  5. 如果超过,则将剩余的文件进行安修改时间进行升序排列,然后删除修改时间最早的文件,直到甚剩余文件大小小于最大磁盘容量的一半。

- (void)clearMemory {
[self.memCache removeAllObjects];
}
- (void)clearDiskOnCompletion:(nullable SDWebImageNoParamsBlock)completion {
dispatch_async(self.ioQueue, ^{
[_fileManager removeItemAtPath:self.diskCachePath error:nil];
[_fileManager createDirectoryAtPath:self.diskCachePath
withIntermediateDirectories:YES
attributes:nil
error:NULL];
if (completion) {
dispatch_async(dispatch_get_main_queue(), ^{
completion();
});
}
});
}
- (void)deleteOldFilesWithCompletionBlock:(nullable SDWebImageNoParamsBlock)completionBlock {
dispatch_async(self.ioQueue, ^{
NSURL *diskCacheURL = [NSURL fileURLWithPath:self.diskCachePath isDirectory:YES];
NSArray<NSString *> *resourceKeys = @[NSURLIsDirectoryKey, NSURLContentModificationDateKey, NSURLTotalFileAllocatedSizeKey];
// This enumerator prefetches useful properties for our cache files.
NSDirectoryEnumerator *fileEnumerator = [_fileManager enumeratorAtURL:diskCacheURL
includingPropertiesForKeys:resourceKeys
options:NSDirectoryEnumerationSkipsHiddenFiles
errorHandler:NULL];
NSDate *expirationDate = [NSDate dateWithTimeIntervalSinceNow:-self.config.maxCacheAge];
NSMutableDictionary<NSURL *, NSDictionary<NSString *, id> *> *cacheFiles = [NSMutableDictionary dictionary];
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.
if (error || !resourceValues || [resourceValues[NSURLIsDirectoryKey] boolValue]) {
continue;
}
// Remove files that are older than the expiration date;
NSDate *modificationDate = resourceValues[NSURLContentModificationDateKey];
if ([[modificationDate laterDate:expirationDate] isEqualToDate:expirationDate]) {
[urlsToDelete addObject:fileURL];
continue;
}
// Store a reference to this file and account for its total size.
NSNumber *totalAllocatedSize = resourceValues[NSURLTotalFileAllocatedSizeKey];
currentCacheSize += totalAllocatedSize.unsignedIntegerValue;
cacheFiles[fileURL] = resourceValues;
}
for (NSURL *fileURL in urlsToDelete) {
[_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.
if (self.config.maxCacheSize > 0 && currentCacheSize > self.config.maxCacheSize) {
// Target half of our maximum cache size for this cleanup pass.
const NSUInteger desiredCacheSize = self.config.maxCacheSize / 2;
// Sort the remaining cache files by their last modification time (oldest first).
NSArray<NSURL *> *sortedFiles = [cacheFiles keysSortedByValueWithOptions:NSSortConcurrent
usingComparator:^NSComparisonResult(id obj1, id obj2) {
return [obj1[NSURLContentModificationDateKey] compare:obj2[NSURLContentModificationDateKey]];
}];
// Delete files until we fall below our desired cache size.
for (NSURL *fileURL in sortedFiles) {
if ([_fileManager removeItemAtURL:fileURL error:nil]) {
NSDictionary<NSString *, id> *resourceValues = cacheFiles[fileURL];
NSNumber *totalAllocatedSize = resourceValues[NSURLTotalFileAllocatedSizeKey];
currentCacheSize -= totalAllocatedSize.unsignedIntegerValue;
if (currentCacheSize < desiredCacheSize) {
break;
}
}
}
}
if (completionBlock) {
dispatch_async(dispatch_get_main_queue(), ^{
completionBlock();
});
}
});
}

图片缓存清理时机

  1. 当系统发出内存不足通知时,会将内存中的所有图片缓存都删除掉。

  2. 当程序进入后台时,会对磁盘的文件数据进行清理。

  3. 当收到程序关闭通知时,会对磁盘中的文件数据进行清理。

// Subscribe to app events
[[NSNotificationCenter defaultCenter] addObserver:self
selector:@selector(clearMemory)
name:UIApplicationDidReceiveMemoryWarningNotification
object:nil];
[[NSNotificationCenter defaultCenter] addObserver:self
selector:@selector(deleteOldFiles)
name:UIApplicationWillTerminateNotification
object:nil];
[[NSNotificationCenter defaultCenter] addObserver:self
selector:@selector(backgroundDeleteOldFiles)
name:UIApplicationDidEnterBackgroundNotification
object:nil];

如何判断imageData的图片类型。

根据imageData的第一个字节,可以判断其图片类型。

第一个字节

图片类型

0xFF

jpeg

0x89

png

0x47

gif

0x4D\0x49

tiff

0x52

将imageData的前12个字节转化为字符串,如果是RIFF前缀和WEBP后缀,则图片类型是webp``

+ (SDImageFormat)sd_imageFormatForImageData:(nullable NSData *)data {
if (!data) {
return SDImageFormatUndefined;
}
uint8_t c;
[data getBytes:&c length:1];
switch (c) {
case 0xFF:
return SDImageFormatJPEG;
case 0x89:
return SDImageFormatPNG;
case 0x47:
return SDImageFormatGIF;
case 0x49:
case 0x4D:
return SDImageFormatTIFF;
case 0x52:
// R as RIFF for WEBP
if (data.length < 12) {
return SDImageFormatUndefined;
}
NSString *testString = [[NSString alloc] initWithData:[data subdataWithRange:NSMakeRange(0, 12)] encoding:NSASCIIStringEncoding];
if ([testString hasPrefix:@"RIFF"] && [testString hasSuffix:@"WEBP"]) {
return SDImageFormatWebP;
}
}
return SDImageFormatUndefined;
}