如何设置一个App的缓存机制

在手机应用程序开发中，为了减少与服务端的交互次数，加快用户的响应速度，一般都会在iOS设备中加一个缓存的机制，前面一篇文章介绍了iOS设备的内存缓存，这篇文章将设计一个本地缓存的机制。功能需求这个缓存机制满足下面这些功能。1、可以将数据缓存到本地磁盘。2、可以判断一个资源是否已经被缓存。如果已经被缓存，在请求相同的资源，先到本地磁盘搜索。3、可以判断文件缓存什么时候过期。这里为了简单起见这里，我们在请求url资源的时候，给每次请求的文件设定一个过期的时间。4、可以实现：如果文件已经被缓存，而且没有过期，这将本地的数据返回，否则重新请求url。5、可以实现：如果文件下载不成功或者下载没有完成，下次打开程序的时候，移除这些没有成功或者没有下载完成的文件。6、可以实现：同时请求或者下载多个资源。设计实现：1、设计一个CacheItem类，用来请求一个web连接，它的一个实例表示一个缓存项。这个CacheItem类，需要一个url创建一个NSURLConnection，去请求web资源。使用CacheItem类主要用来请求web资源。[plain] view plain copy/* ---------缓存项-------------- */@interface CacheItem : NSObject {@publicid<CacheItemDelegate> delegate;//web地址NSString *remoteURL;@private//是否正在下载BOOL isDownloading;//NSMutableData对象NSMutableData *connectionData;//NSURLConnection对象NSURLConnection *connection;}/* -------------------------- */@property (nonatomic, retain) id<CacheItemDelegate> delegate;@property (nonatomic, retain) NSString *remoteURL;@property (nonatomic, assign) BOOL isDownloading;@property (nonatomic, retain) NSMutableData *connectionData;@property (nonatomic, retain) NSURLConnection *connection;/* ----------开始下载方法----------- */- (BOOL) startDownloadingURL:(NSString *)paramRemoteURL;@end2、在NSURLConnection开始请求之前，调用CachedDownloadManager类，来搜索和管理本地的缓存文件。将缓存文件的情况保存到一个字典类中。这个字典设计如下：[plain] view plain copy{"http://www.cnn.com" = {DownloadEndDate = "2011-08-02 07:51:57 +0100";DownloadStartDate = "2011-08-02 07:51:55 +0100";ExpiresInSeconds = 20;ExpiryDate = "2011-08-02 07:52:17 +0100";LocalURL = "/var/mobile/Applications/ApplicationID/Documents/httpwww.cnn.com.cache";};"http://www.baidu.com" = {DownloadEndDate = "2011-08-02 07:51:49 +0100";DownloadStartDate = "2011-08-02 07:51:44 +0100";ExpiresInSeconds = 20;ExpiryDate = "2011-08-02 07:52:09 +0100";LocalURL = "/var/mobile/Applications/ApplicationID/Documents/httpwww.oreilly.com.cache";};}

上面这个字典里面嵌套了字典。里面那层字典表示一个缓存项的缓存信息：下载结束时间、下载开始时间、缓存有效时间、缓存过期时间、缓存到本地的路径。

下面看下CachedDownloadManager类。用它来实现和封装我们的缓存策略。[plain] view plain copy/* -----------CachedDownloadManager-------------- */@interface CachedDownloadManager : NSObject<CacheItemDelegate> {@publicid<CachedDownloadManagerDelegate> delegate;@private//记录缓存数据的字典NSMutableDictionary *cacheDictionary;//缓存的路径NSString *cacheDictionaryPath;}@property (nonatomic, assign)id<CachedDownloadManagerDelegate> delegate;@property (nonatomic, copy)NSMutableDictionary *cacheDictionary;@property (nonatomic, retain)NSString *cacheDictionaryPath;/* 保持缓存字典 */- (BOOL) saveCacheDictionary;/* 公有方法：下载 */- (BOOL) download:(NSString *)paramURLAsStringurlMustExpireInSeconds:(NSTimeInterval)paramURLMustExpireInSecondsupdateExpiryDateIfInCache:(BOOL)paramUpdateExpiryDateIfInCache;/* -------------------------- */@end从上面代码可以看出，这个管理缓存的类中，有一个缓存字典：cacheDictionary，用来表示所有资源的缓存情况；cacheDictionaryPath用来表示缓存的路径；saveCacheDictionary用来将缓存字典归档到本地文件中。download：urlMustExpireInSeconds：updateExpiryDateIfInCache是一个公共接口，通过传递url、缓存过期时间、是否更新缓存过期时间三个参数来方便的使用，实现我们的缓存策略。3、如果这个文件已经被下载，而且没有过期，则从本地获取文件的数据。如果文件已经过期，则重新下载。我们通过download：urlMustExpireInSeconds：updateExpiryDateIfInCache方法来实现，主要看这个方法的代码：[plain] view plain copy/* ---------下载-------------- */- (BOOL) download:(NSString *)paramURLAsStringurlMustExpireInSeconds:(NSTimeInterval)paramURLMustExpireInSecondsupdateExpiryDateIfInCache:(BOOL)paramUpdateExpiryDateIfInCache{BOOL result = NO;if (self.cacheDictionary == nil ||[paramURLAsString length] == 0){return(NO);}paramURLAsString = [paramURLAsString lowercaseString];//根据url，从字典中获取缓存项的相关数据NSMutableDictionary *itemDictionary =[self.cacheDictionary objectForKey:paramURLAsString];/* 使用下面这些变量帮助我们理解缓存逻辑 *///文件是否已经被缓存BOOL fileHasBeenCached = NO;//缓存是否过期BOOL cachedFileHasExpired = NO;//缓存文件是否存在BOOL cachedFileExists = NO;//缓存文件能否被加载BOOL cachedFileDataCanBeLoaded = NO;//缓存文件数据NSData *cachedFileData = nil;//缓存文件是否完全下载BOOL cachedFileIsFullyDownloaded = NO;//缓存文件是否已经下载BOOL cachedFileIsBeingDownloaded = NO;//过期时间NSDate *expiryDate = nil;//下载结束时间NSDate *downloadEndDate = nil;//下载开始时间NSDate *downloadStartDate = nil;//本地缓存路径NSString *localURL = nil;//有效时间NSNumber *expiresInSeconds = nil;NSDate *now = [NSDate date];if (itemDictionary != nil){fileHasBeenCached = YES;}//如果文件已经被缓存，则从缓存项相关数据中获取相关的值if (fileHasBeenCached == YES){expiryDate = [itemDictionaryobjectForKey:CachedKeyExpiryDate];downloadEndDate = [itemDictionaryobjectForKey:CachedKeyDownloadEndDate];downloadStartDate = [itemDictionaryobjectForKey:CachedKeyDownloadStartDate];localURL = [itemDictionaryobjectForKey:CachedKeyLocalURL];expiresInSeconds = [itemDictionaryobjectForKey:CachedKeyExpiresInSeconds];//如果下载开始和结束时间不为空，表示文件全部被下载if (downloadEndDate != nil &&downloadStartDate != nil){cachedFileIsFullyDownloaded = YES;}/* 如果expiresInSeconds不为空，downloadEndDate为空，表示文件已经正在下载 */if (expiresInSeconds != nil &&downloadEndDate == nil){cachedFileIsBeingDownloaded = YES;}/* 判断缓存是否过期 */if (expiryDate != nil &&[now timeIntervalSinceDate:expiryDate] > 0.0){cachedFileHasExpired = YES;}if (cachedFileHasExpired == NO){/* 如果缓存文件没有过期，加载缓存文件，并且更新过期时间 */NSFileManager *fileManager = [[NSFileManager alloc] init];if ([fileManager fileExistsAtPath:localURL] == YES){cachedFileExists = YES;cachedFileData = [NSData dataWithContentsOfFile:localURL];if (cachedFileData != nil){cachedFileDataCanBeLoaded = YES;} /* if (cachedFileData != nil){ */} /* if ([fileManager fileExistsAtPath:localURL] == YES){ */[fileManager release];/* 更新缓存时间 */if (paramUpdateExpiryDateIfInCache == YES){NSDate *newExpiryDate =[NSDate dateWithTimeIntervalSinceNow:paramURLMustExpireInSeconds];NSLog(@"Updating the expiry date from %@ to %@.",expiryDate,newExpiryDate);[itemDictionary setObject:newExpiryDateforKey:CachedKeyExpiryDate];NSNumber *expires =[NSNumber numberWithFloat:paramURLMustExpireInSeconds];[itemDictionary setObject:expiresforKey:CachedKeyExpiresInSeconds];}} /* if (cachedFileHasExpired == NO){ */}if (cachedFileIsBeingDownloaded == YES){NSLog(@"这个文件已经正在下载...");return(YES);}if (fileHasBeenCached == YES){if (cachedFileHasExpired == NO &&cachedFileExists == YES &&cachedFileDataCanBeLoaded == YES &&[cachedFileData length] > 0 &&cachedFileIsFullyDownloaded == YES){/* 如果文件有缓存而且没有过期 */NSLog(@"文件有缓存而且没有过期.");[self.delegatecachedDownloadManagerSucceeded:selfremoteURL:[NSURL URLWithString:paramURLAsString]localURL:[NSURL URLWithString:localURL]aboutToBeReleasedData:cachedFileDataisCachedData:YES];return(YES);} else {/* 如果文件没有被缓存，获取缓存失败 */NSLog(@"文件没有缓存.");[self.cacheDictionary removeObjectForKey:paramURLAsString];[self saveCacheDictionary];} /* if (cachedFileHasExpired == NO && */} /* if (fileHasBeenCached == YES){ *//* 去下载文件 */NSNumber *expires =[NSNumber numberWithFloat:paramURLMustExpireInSeconds];NSMutableDictionary *newDictionary =[[[NSMutableDictionary alloc] init] autorelease];[newDictionary setObject:expiresforKey:CachedKeyExpiresInSeconds];localURL = [paramURLAsStringstringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding];localURL = [localURL stringByReplacingOccurrencesOfString:@"://"withString:@""];localURL = [localURL stringByReplacingOccurrencesOfString:@"/"withString:@"{1}quot;];localURL = [localURL stringByAppendingPathExtension:@"cache"];NSString *documentsDirectory =[self documentsDirectoryWithTrailingSlash:NO];localURL = [documentsDirectorystringByAppendingPathComponent:localURL];[newDictionary setObject:localURLforKey:CachedKeyLocalURL];[newDictionary setObject:nowforKey:CachedKeyDownloadStartDate];[self.cacheDictionary setObject:newDictionaryforKey:paramURLAsString];[self saveCacheDictionary];CacheItem *item = [[[CacheItem alloc] init] autorelease];[item setDelegate:self];[item startDownloadingURL:paramURLAsString];return(result);}4、下面我们设计缓存项下载成功和失败的两个委托方法：[plain] view plain copy@protocol CacheItemDelegate <NSObject>//下载成功执行该方法- (void) cacheItemDelegateSucceeded:(CacheItem *)paramSenderwithRemoteURL:(NSURL *)paramRemoteURLwithAboutToBeReleasedData:(NSData *)paramAboutToBeReleasedData;//下载失败执行该方法- (void) cacheItemDelegateFailed:(CacheItem *)paramSenderremoteURL:(NSURL *)paramRemoteURLwithError:(NSError *)paramError;@end当我们下载成功的时候，修改缓存字典中的下载时间，表示已经下载完成，而且需要将请求的资源数据缓存到本地：[plain] view plain copy//缓存项的委托方法- (void) cacheItemDelegateSucceeded:(CacheItem *)paramSenderwithRemoteURL:(NSURL *)paramRemoteURLwithAboutToBeReleasedData:(NSData *)paramAboutToBeReleasedData{//从缓存字典中获取该缓存项的相关数据NSMutableDictionary *dictionary =[self.cacheDictionary objectForKey:[paramRemoteURL absoluteString]];//取当前时间NSDate *now = [NSDate date];//获取有效时间NSNumber *expiresInSeconds = [dictionaryobjectForKey:CachedKeyExpiresInSeconds];//转换成NSTimeIntervalNSTimeInterval expirySeconds = [expiresInSeconds floatValue];//修改字典中缓存项的下载结束时间[dictionary setObject:[NSDate date]forKey:CachedKeyDownloadEndDate];//修改字典中缓存项的缓存过期时间[dictionary setObject:[now dateByAddingTimeInterval:expirySeconds]forKey:CachedKeyExpiryDate];//保存缓存字典[self saveCacheDictionary];NSString *localURL = [dictionary objectForKey:CachedKeyLocalURL];/* 将下载的数据保持到磁盘 */if ([paramAboutToBeReleasedData writeToFile:localURLatomically:YES] == YES){NSLog(@"缓存文件到磁盘成功.");} else{NSLog(@"缓存文件到磁盘失败.");}//执行缓存管理的委托方法[self.delegatecachedDownloadManagerSucceeded:selfremoteURL:paramRemoteURLlocalURL:[NSURL URLWithString:localURL]aboutToBeReleasedData:paramAboutToBeReleasedDataisCachedData:NO];}如果下载失败我们需要从缓存字典中移除改缓存项：[plain] view plain copy//缓存项失败失败的委托方法- (void) cacheItemDelegateFailed:(CacheItem *)paramSenderremoteURL:(NSURL *)paramRemoteURLwithError:(NSError *)paramError{/* 从缓存字典中移除缓存项，并发送一个委托 */if (self.delegate != nil){NSMutableDictionary *dictionary =[self.cacheDictionaryobjectForKey:[paramRemoteURL absoluteString]];NSString *localURL = [dictionaryobjectForKey:CachedKeyLocalURL];[self.delegatecachedDownloadManagerFailed:selfremoteURL:paramRemoteURLlocalURL:[NSURL URLWithString:localURL]withError:paramError];}[self.cacheDictionaryremoveObjectForKey:[paramRemoteURL absoluteString]];}5、加载缓存字典的时候，我们可以将没有下载完成的文件移除：[plain] view plain copy//初始化缓存字典NSString *documentsDirectory =[self documentsDirectoryWithTrailingSlash:YES];//生产缓存字典的路径cacheDictionaryPath =[[documentsDirectorystringByAppendingString:@"CachedDownloads.dic"] retain];//创建一个NSFileManager实例NSFileManager *fileManager = [[NSFileManager alloc] init];//判断是否存在缓存字典的数据if ([fileManagerfileExistsAtPath:self.cacheDictionaryPath] == YES){NSLog(self.cacheDictionaryPath);//加载缓存字典中的数据NSMutableDictionary *dictionary =[[NSMutableDictionary alloc]initWithContentsOfFile:self.cacheDictionaryPath];cacheDictionary = [dictionary mutableCopy];[dictionary release];//移除没有下载完成的缓存数据[self removeCorruptedCachedItems];} else {//创建一个新的缓存字典NSMutableDictionary *dictionary =[[NSMutableDictionary alloc] init];cacheDictionary = [dictionary mutableCopy];[dictionary release];}

这样就基本上完成了我们需要的功能，下面看看我们如何使用我们设计的缓存功能。

例子场景：我们用一个UIWebView来显示stackoverflow这个网站，我们在这个网站的内容缓存到本地20秒，如果在20秒内用户去请求该网站，则从本地文件中获取内容，否则过了20秒，则重新获取数据，并缓存到本地。在界面上拖放一个button和一个webview控件，如下图。这样我们可以很方便使用前面定义好的类。我们在viewDidLoad 中实例化一个CachedDownloadManager，并设置它的委托为self。当下载完成的时候，执行CachedDownloadManager的下载成功的委托方法。

- (void)viewDidLoad {[super viewDidLoad];[self setTitle:@"本地缓存测试"];CachedDownloadManager *newManager = [[CachedDownloadManager alloc] init];self.downloadManager = newManager;[newManager release];[self.downloadManager setDelegate:self];}

在button的点击事件中加入下面代码，请求stackoverflow ：

  static NSString *url = @"http://stackoverflow.com";[self.downloadManager download:urlurlMustExpireInSeconds:20.0fupdateExpiryDateIfInCache:YES];

上面的代码表示将这个stackoverflow的缓存事件设置为20s，并且如果在20s内有相同的请求，则从本地获取stackoverflow的内容数据。updateExpiryDateIfInCache设置为yes表示：在此请求的时候，缓存时间又更新为20s，类似我们的session。如果设置成no，则第一次请求20s之后，该缓存就过期。请求完成之后会执行CachedDownloadManager的委托方法。我们将数据展示在uiwebview中，代码如下：

- (void) cachedDownloadManagerSucceeded:(CachedDownloadManager *)paramSenderremoteURL:(NSURL *)paramRemoteURLlocalURL:(NSURL *)paramLocalURLaboutToBeReleasedData:(NSData *)paramAboutToBeReleasedDataisCachedData:(BOOL)paramIsCachedData{[webview loadData:paramAboutToBeReleasedData MIMEType:@"text/html" textEncodingName:@"UTF-8" baseURL:[NSURL URLWithString:@"http://stackoverflow.com"]];}

这样我们就实现了20s的缓存。效果：第一次点击测试按钮：20s内点击按钮，程序就从本地获取数据，比较快速的就显示出该网页了。总结：本文通过代码和实例设计了一个iPhone应用程序本地缓存的方案。当然这个方案不是最好的，如果你有更好的思路，欢迎告诉我。