IOS网络图片缓存详解

在开发移动应用的时候比如Android，IOS，因为手机流量、网速、内存等这些因素，当我们的移动应用是针对互联网，并要频繁访问网络的话，对网络优化这块就显得尤为重要了。

比如某个应用要经常显示网络图片，就不能每次显示图片都去网络上下载，那太耗费时间也太耗费流量，这时就要对网络图片进行缓存了，以下是我对IOS网络图片缓存的一些见解，有不足之处，欢迎大家指出来，一起探讨。

处理网络图片缓存步骤：

1、根据图片URL查找内存是否有这张图片，有则返回图片，没有则进入第二步

2、查找物理存储是否有这张图片，有则返回图片，没有则进入第三步

3、从网络上下载该图片，下载完后保存到内存和物理存储上，并返回该图片

注：因为URL包含特殊字符和长度不确定，要对URL进行MD5处理或其他处理

下面是针对以上步骤的代码讲解：

1、内存缓存图片处理

使用NSMutableDictionary存储图片UIImage，数组的Key为该图片的URL地址

//缓存图片到内存上

[memCache setObject:image forKey:key];

2、物理缓存图片处理

把图片保持到物理存储设备上，则直接使用NSFileManager，把URL作为文件名保存

3、网络图片下载处理

图片使用异步下载，下载完后把图片保持到NSMutableDictionary和物理存储上

以下是摘自SDWebImageleik网络图片缓存处理的一个类，有详细注释

.h文件

@interface SDImageCache : NSObject{    NSMutableDictionary *memCache;//内存缓存图片引用    NSString *diskCachePath;//物理缓存路径    NSOperationQueue *cacheInQueue, *cacheOutQueue;}+ (SDImageCache *)sharedImageCache;//保存图片- (void)storeImage:(UIImage *)image forKey:(NSString *)key;//保存图片，并选择是否保存到物理存储上- (void)storeImage:(UIImage *)image forKey:(NSString *)key toDisk:(BOOL)toDisk;//保存图片，可以选择把NSData数据保存到物理存储上- (void)storeImage:(UIImage *)image imageData:(NSData *)data forKey:(NSString *)key toDisk:(BOOL)toDisk;//通过key返回UIImage- (UIImage *)imageFromKey:(NSString *)key;//如果获取内存图片失败，是否可以在物理存储上查找- (UIImage *)imageFromKey:(NSString *)key fromDisk:(BOOL)fromDisk;- (void)queryDiskCacheForKey:(NSString *)key delegate:(id <SDImageCacheDelegate>)delegate userInfo:(NSDictionary *)info;//清除key索引的图片- (void)removeImageForKey:(NSString *)key;//清除内存图片- (void)clearMemory;//清除物理缓存- (void)clearDisk;//清除过期物理缓存- (void)cleanDisk;@end

.m文件

@implementation SDImageCache#pragma mark NSObject- (id)init{    if ((self = [super init]))    {        // Init the memory cache        memCache = [[NSMutableDictionary alloc] init];                // Init the disk cache        NSArray *paths = NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES);        diskCachePath = [[[paths objectAtIndex:0] stringByAppendingPathComponent:@"ImageCache"] retain];                if (![[NSFileManager defaultManager] fileExistsAtPath:diskCachePath])        {            [[NSFileManager defaultManager] createDirectoryAtPath:diskCachePath                                      withIntermediateDirectories:YES                                                       attributes:nil                                                            error:NULL];        }                // Init the operation queue        cacheInQueue = [[NSOperationQueue alloc] init];        cacheInQueue.maxConcurrentOperationCount = 1;        cacheOutQueue = [[NSOperationQueue alloc] init];        cacheOutQueue.maxConcurrentOperationCount = 1;        #if TARGET_OS_IPHONE        // Subscribe to app events        [[NSNotificationCenter defaultCenter] addObserver:self                                                 selector:@selector(clearMemory)                                                     name:UIApplicationDidReceiveMemoryWarningNotification                                                   object:nil];                [[NSNotificationCenter defaultCenter] addObserver:self                                                 selector:@selector(cleanDisk)                                                     name:UIApplicationWillTerminateNotification                                                   object:nil];        #if __IPHONE_OS_VERSION_MIN_REQUIRED >= __IPHONE_4_0        UIDevice *device = [UIDevice currentDevice];        if ([device respondsToSelector:@selector(isMultitaskingSupported)] && device.multitaskingSupported)        {            // When in background, clean memory in order to have less chance to be killed            [[NSNotificationCenter defaultCenter] addObserver:self                                                     selector:@selector(clearMemory)                                                         name:UIApplicationDidEnterBackgroundNotification                                                       object:nil];        }#endif#endif    }        return self;}- (void)dealloc{    [memCache release], memCache = nil;    [diskCachePath release], diskCachePath = nil;    [cacheInQueue release], cacheInQueue = nil;        [[NSNotificationCenter defaultCenter] removeObserver:self];        [super dealloc];}#pragma mark SDImageCache (class methods)+ (SDImageCache *)sharedImageCache{    if (instance == nil)    {        instance = [[SDImageCache alloc] init];    }        return instance;}#pragma mark SDImageCache (private)/* *创建指定图片key的路径 */- (NSString *)cachePathForKey:(NSString *)key{    const char *str = [key UTF8String];    unsigned char r[CC_MD5_DIGEST_LENGTH];    CC_MD5(str, (CC_LONG)strlen(str), r);    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]];        return [diskCachePath stringByAppendingPathComponent:filename];}/* *保存key和Data到物理存储 *keyAndData[0] ->key *keyAndData[1] ->Data */- (void)storeKeyWithDataToDisk:(NSArray *)keyAndData{    // Can't use defaultManager another thread    NSFileManager *fileManager = [[NSFileManager alloc] init];        NSString *key = [keyAndData objectAtIndex:0];    NSData *data = [keyAndData count] > 1 ? [keyAndData objectAtIndex:1] : nil;        //如果有数据，则保存到物理存储上    if (data)    {        [fileManager createFileAtPath:[self cachePathForKey:key] contents:data attributes:nil];    }    else    {        //如果没有data，则把UIImage转换为JPEG，并保存到物理存储上        // If no data representation given, convert the UIImage in JPEG and store it        // This trick is more CPU/memory intensive and doesn't preserve alpha channel        UIImage *image = [[self imageFromKey:key fromDisk:YES] retain]; // be thread safe with no lock        if (image)        {#if TARGET_OS_IPHONE            [fileManager createFileAtPath:[self cachePathForKey:key] contents:UIImageJPEGRepresentation(image, (CGFloat)1.0) attributes:nil];#else            NSArray*  representations  = [image representations];            NSData* jpegData = [NSBitmapImageRep representationOfImageRepsInArray: representations usingType: NSJPEGFileType properties:nil];            [fileManager createFileAtPath:[self cachePathForKey:key] contents:jpegData attributes:nil];#endif            [image release];        }    }        [fileManager release];}/* *查找图片委托 */- (void)notifyDelegate:(NSDictionary *)arguments{    NSString *key = [arguments objectForKey:@"key"];    id <SDImageCacheDelegate> delegate = [arguments objectForKey:@"delegate"];    NSDictionary *info = [arguments objectForKey:@"userInfo"];    UIImage *image = [arguments objectForKey:@"image"];        if (image)    {        [memCache setObject:image forKey:key];                if ([delegate respondsToSelector:@selector(imageCache:didFindImage:forKey:userInfo:)])        {            [delegate imageCache:self didFindImage:image forKey:key userInfo:info];        }    }    else    {        if ([delegate respondsToSelector:@selector(imageCache:didNotFindImageForKey:userInfo:)])        {            [delegate imageCache:self didNotFindImageForKey:key userInfo:info];        }    }}/* *查找物理缓存上的图片 */- (void)queryDiskCacheOperation:(NSDictionary *)arguments{    NSString *key = [arguments objectForKey:@"key"];    NSMutableDictionary *mutableArguments = [[arguments mutableCopy] autorelease];        UIImage *image = [[[UIImage alloc] initWithContentsOfFile:[self cachePathForKey:key]] autorelease];    if (image)    {#ifdef ENABLE_SDWEBIMAGE_DECODER        UIImage *decodedImage = [UIImage decodedImageWithImage:image];        if (decodedImage)        {            image = decodedImage;        }#endif        [mutableArguments setObject:image forKey:@"image"];    }        [self performSelectorOnMainThread:@selector(notifyDelegate:) withObject:mutableArguments waitUntilDone:NO];}#pragma mark ImageCache/* *缓存图片 * **/- (void)storeImage:(UIImage *)image imageData:(NSData *)data forKey:(NSString *)key toDisk:(BOOL)toDisk{    if (!image || !key)    {        return;    }        //缓存图片到内存上    [memCache setObject:image forKey:key];        //如果需要缓存到物理存储上，并data不为空，则把data缓存到物理存储上    if (toDisk)    {        if (!data) return;        NSArray *keyWithData;        if (data)        {            keyWithData = [NSArray arrayWithObjects:key, data, nil];        }        else        {            keyWithData = [NSArray arrayWithObjects:key, nil];        }        //后台线程缓存图片到物理存储上        [cacheInQueue addOperation:[[[NSInvocationOperation alloc] initWithTarget:self                                                                         selector:@selector(storeKeyWithDataToDisk:)                                                                           object:keyWithData] autorelease]];    }}/* *保存图片到内存上，不保存到物理存储上 */- (void)storeImage:(UIImage *)image forKey:(NSString *)key{    [self storeImage:image imageData:nil forKey:key toDisk:YES];}/* *保存图片到内存上，不保存到物理存储上 */- (void)storeImage:(UIImage *)image forKey:(NSString *)key toDisk:(BOOL)toDisk{    [self storeImage:image imageData:nil forKey:key toDisk:toDisk];}/* *通过key返回指定图片 */- (UIImage *)imageFromKey:(NSString *)key{    return [self imageFromKey:key fromDisk:YES];}/* *返回一张图像 *key：图像的key *fromDisk：如果内存中没有图片，是否在物理存储上查找 *return 返回查找到的图片，如果没有则返回nil */- (UIImage *)imageFromKey:(NSString *)key fromDisk:(BOOL)fromDisk{    if (key == nil)    {        return nil;    }        UIImage *image = [memCache objectForKey:key];        if (!image && fromDisk) //如果内存没有图片，并且可以在物理存储上查找，则返回物理存储上的图片    {        image = [[[UIImage alloc] initWithContentsOfFile:[self cachePathForKey:key]] autorelease];        if (image)        {            [memCache setObject:image forKey:key];        }    }        return image;}- (void)queryDiskCacheForKey:(NSString *)key delegate:(id <SDImageCacheDelegate>)delegate userInfo:(NSDictionary *)info{    if (!delegate)    {        return;    }        if (!key)    {        if ([delegate respondsToSelector:@selector(imageCache:didNotFindImageForKey:userInfo:)])        {            [delegate imageCache:self didNotFindImageForKey:key userInfo:info];        }        return;    }        // First check the in-memory cache...    UIImage *image = [memCache objectForKey:key];    if (image)    {        // ...notify delegate immediately, no need to go async        if ([delegate respondsToSelector:@selector(imageCache:didFindImage:forKey:userInfo:)])        {            [delegate imageCache:self didFindImage:image forKey:key userInfo:info];        }        return;    }        NSMutableDictionary *arguments = [NSMutableDictionary dictionaryWithCapacity:3];    [arguments setObject:key forKey:@"key"];    [arguments setObject:delegate forKey:@"delegate"];    if (info)    {        [arguments setObject:info forKey:@"userInfo"];    }    [cacheOutQueue addOperation:[[[NSInvocationOperation alloc] initWithTarget:self selector:@selector(queryDiskCacheOperation:) object:arguments] autorelease]];}/* *从内存和物理存储上移除指定图片 */- (void)removeImageForKey:(NSString *)key{    if (key == nil)    {        return;    }        [memCache removeObjectForKey:key];    [[NSFileManager defaultManager] removeItemAtPath:[self cachePathForKey:key] error:nil];}/* *清除内存缓存区的图片 */- (void)clearMemory{    [cacheInQueue cancelAllOperations]; // won't be able to complete    [memCache removeAllObjects];}/* *清除物理存储上的图片 */- (void)clearDisk{    [cacheInQueue cancelAllOperations];    [[NSFileManager defaultManager] removeItemAtPath:diskCachePath error:nil];    [[NSFileManager defaultManager] createDirectoryAtPath:diskCachePath                              withIntermediateDirectories:YES                                               attributes:nil                                                    error:NULL];}/* *清除过期缓存的图片 */- (void)cleanDisk{    NSDate *expirationDate = [NSDate dateWithTimeIntervalSinceNow:-cacheMaxCacheAge];    NSDirectoryEnumerator *fileEnumerator = [[NSFileManager defaultManager] enumeratorAtPath:diskCachePath];    for (NSString *fileName in fileEnumerator)    {        NSString *filePath = [diskCachePath stringByAppendingPathComponent:fileName];        NSDictionary *attrs = [[NSFileManager defaultManager] attributesOfItemAtPath:filePath error:nil];        if ([[[attrs fileModificationDate] laterDate:expirationDate] isEqualToDate:expirationDate])        {            [[NSFileManager defaultManager] removeItemAtPath:filePath error:nil];        }    }}@end

在网络图片缓存的处理上，有个比好的第三方类库“SDWebImage”，点击下载SDWebImage。

原文地址：http://blog.csdn.net/kqjob