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缓存机制
IOS开发缓存机制之—本地缓存机制分类: IOS开发技术系列专栏 2011-08-07 15:22 14288人阅读 评论(9) 收藏 举报iosdictionarydownload磁盘urlinterface 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。/* ---------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, assign) 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:{ "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。 /* -----------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。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:/* ---------下载-------------- */- (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 =http://www.mamicode.com/ 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 */ NSNumber *expires = [NSNumber numberWithFloat:paramURLMustExpireInSeconds]; NSMutableDictionary *newDictionary = [[[NSMutableDictionary alloc] init] autorelease]; [newDictionary setObject:expires forKey:CachedKeyExpiresInSeconds]; localURL = [paramURLAsString stringByAddingPercentEscapesUsingEncoding: NSUTF8StringEncoding]; localURL = [localURL stringByReplacingOccurrencesOfString:@"://" withString:@""]; localURL = [localURL stringByReplacingOccurrencesOfString:@"/" withString:@"{1}quot;]; localURL = [localURL stringByAppendingPathExtension:@"cache"]; NSString *documentsDirectory = [self documentsDirectoryWithTrailingSlash:NO]; localURL = [documentsDirectory stringByAppendingPathComponent:localURL]; [newDictionary setObject:localURL forKey:CachedKeyLocalURL]; [newDictionary setObject:now forKey:CachedKeyDownloadStartDate]; [self.cacheDictionary setObject:newDictionary forKey: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:@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://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://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: //初始化缓存字典 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]; } 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:url urlMustExpireInSeconds:20.0f updateExpiryDateIfInCache: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 *)paramSender remoteURL:(NSURL *)paramRemoteURL localURL:(NSURL *)paramLocalURL aboutToBeReleasedData:(NSData *)paramAboutToBeReleasedData isCachedData:(BOOL)paramIsCachedData{ [webview loadData:paramAboutToBeReleasedData MIMEType:@"text/html" textEncodingName:@"UTF-8" baseURL:[NSURL URLWithString:@"http://stackoverflow.com"]]; } 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。
缓存机制
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