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。