设计一个移动应用的本地缓存机制
From:http://www.2cto.com/kf/201108/98661.html
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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。
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功能需求
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这个缓存机制满足下面这些功能。
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1、可以将数据缓存到本地磁盘。
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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。
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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。
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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。
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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。
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6、可以实现：同时请求或者下载多个资源。
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设计实现：
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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。
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/* ---------缓存项-------------- */@interface CacheItem : NSObject {@public? id<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：
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{? "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。
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?? 下面看下CachedDownloadManager类。用它来实现和封装我们的缓存策略。
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/* -----------CachedDownloadManager-------------- */@interface CachedDownloadManager : NSObject??????????????????????????????????? <CacheItemDelegate> {@public? id<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 *)paramURLAsString?? urlMustExpireInSeconds:(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。
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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：
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- (BOOL)???????? download:(NSString *)paramURLAsString?? urlMustExpireInSeconds:(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 = [itemDictionary ??????????????????objectForKey:CachedKeyExpiryDate];??????? downloadEndDate = [itemDictionary?????????????????????? objectForKey:CachedKeyDownloadEndDate];??????? downloadStartDate = [itemDictionary???????????????????????? objectForKey:CachedKeyDownloadStartDate];??????? localURL = [itemDictionary??????????????? objectForKey:CachedKeyLocalURL];??????? expiresInSeconds = [itemDictionary??????????????????????? objectForKey: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:newExpiryDate?????????????????????????? forKey:CachedKeyExpiryDate];??????????????? NSNumber *expires =???????? [NSNumber numberWithFloat:paramURLMustExpireInSeconds];??????????????? [itemDictionary setObject:expires?????????????????????????? forKey: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.delegate? ??????cachedDownloadManagerSucceeded:self?????? remoteURL:[NSURL URLWithString:paramURLAsString]?????? localURL:[NSURL URLWithString:localURL]?????? aboutToBeReleasedData:cachedFileData?????? isCachedData: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*/4、下面我们设计缓存项下载成功和失败的两个委托方法：
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@protocol CacheItemDelegate <NSObject>//下载成功执行该方法- (void) cacheItemDelegateSucceeded? :(CacheItem *)paramSender? withRemoteURL:(NSURL *)paramRemoteURL? withAboutToBeReleasedData:(NSData *)paramAboutToBeReleasedData;//下载失败执行该方法- (void) cacheItemDelegateFailed? :(CacheItem *)paramSender? remoteURL:(NSURL *)paramRemoteURL? withError:(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：
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//缓存项的委托方法- (void) cacheItemDelegateSucceeded:(CacheItem *)paramSender???????? withRemoteURL:(NSURL *)paramRemoteURL??????? withAboutToBeReleasedData:(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 = [dictionary???????????????????????????????? objectForKey:CachedKeyExpiresInSeconds];? //转换成NSTimeInterval? NSTimeInterval 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:localURL?????????????????????????????????? atomically: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.delegate??? cachedDownloadManagerSucceeded:self?? remoteURL:paramRemoteURL?? localURL:[NSURL URLWithString:localURL]?? aboutToBeReleasedData:paramAboutToBeReleasedData?? isCachedData:NO];??? }?? 如果下载失败我们需要从缓存字典中移除改缓存项：
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//缓存项失败失败的委托方法- (void) cacheItemDelegateFailed:(CacheItem *)paramSender?????????????????????? remoteURL:(NSURL *)paramRemoteURL?????????????????????? withError:(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.cacheDictionary????? objectForKey:[paramRemoteURL absoluteString]];??????? NSString *localURL = [dictionary?????????????????????????? objectForKey:CachedKeyLocalURL];??????? [self.delegate???? cachedDownloadManagerFailed:self???? remoteURL:paramRemoteURL???? localURL:[NSURL URLWithString:localURL]???? withError:paramError];? }??? [self.cacheDictionary??? removeObjectForKey:[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：
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??? NSString *documentsDirectory =???? [self documentsDirectoryWithTrailingSlash:YES];??? //生产缓存字典的路径??? cacheDictionaryPath =???? [[documentsDirectory?????? stringByAppendingString:@"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 ([fileManager????????? fileExistsAtPath: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];????????? }??????? [fileManager 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。
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例子场景：
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??? 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。
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??? 在界面上拖放一个button和一个webview控件，如下图。
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总结：
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??? 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。
