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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