?在手機應用程序開發中，為了降低與服務端的交互次數，加快用戶的響應速度，一般都會在iOS設備中加一個緩存的機制，前面一篇文章介紹了iOS設備的內存緩存。這篇文章將設計一個本地緩存的機制。
功能需求
這個緩存機制滿足以下這些功能。
1、能夠將數據緩存到本地磁盤。
2、能夠推斷一個資源是否已經被緩存。假設已經被緩存。在請求同樣的資源。先到本地磁盤搜索。
3、能夠推斷文件緩存什么時候過期。這里為了簡單起見這里，我們在請求url資源的時候。給每次請求的文件設定一個過期的時間。
4、能夠實現：假設文件已經被緩存，并且沒有過期。這將本地的數據返回，否則又一次請求url。
5、能夠實現：假設文件下載不成功或者下載沒有完畢，下次打開程序的時候，移除這些沒有成功或者沒有下載完畢的文件。
6、能夠實現：同一時候請求或者下載多個資源。
設計實現：
1、設計一個CacheItem類，用來請求一個web連接，它的一個實例表示一個緩存項。
這個CacheItem類，須要一個url創建一個NSURLConnection，去請求web資源。
使用CacheItem類主要用來請求web資源。
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/*?---------緩存項--------------?*/?? ?? @interface?CacheItem?:?NSObject?{?? @public?? ??id<CacheItemDelegate>?delegate;?? ????//web地址?? ??NSString??????????????*remoteURL;?? @private?? ????//是否正在下載?? ??BOOL??????????????????isDownloading;?? ???????//NSMutableData對象?? ??NSMutableData?????????*connectionData;?? ????//NSURLConnection對象?? ??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;?? ?? /*?----------開始下載方法-----------?*/?? ?? -?(BOOL)?startDownloadingURL:(NSString?*)paramRemoteURL;?? ?? @end?? 2、在NSURLConnection開始請求之前，調用CachedDownloadManager類，來搜索和管理本地的緩存文件。將緩存文件的情況保存到一個字典類中。
這個字典設計例如以下：
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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";?? ??};?? }?? ?上面這個字典里面嵌套了字典。里面那層字典表示一個緩存項的緩存信息：下載結束時間、下載開始時間、緩存有效時間、緩存過期時間、緩存到本地的路徑。
?? 以下看下CachedDownloadManager類。
用它來實現和封裝我們的緩存策略。?
?
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/*?-----------CachedDownloadManager--------------?*/?? ?? @interface?CachedDownloadManager?:?NSObject??? ???????????????????????????????????<CacheItemDelegate>?{?? @public?? ??id<CachedDownloadManagerDelegate>??delegate;?? @private?? //記錄緩存數據的字典?? ??NSMutableDictionary????????????????*cacheDictionary;?? ???????????????????????????????????????//緩存的路徑?? ??NSString???????????????????????????*cacheDictionaryPath;?? }?? ?? ?? @property?(nonatomic,?assign)??? id<CachedDownloadManagerDelegate>?delegate;?? ?? @property?(nonatomic,?copy)??? NSMutableDictionary?*cacheDictionary;?? ?? @property?(nonatomic,?retain)??? NSString?*cacheDictionaryPath;?? ?? ?? /*?保持緩存字典?*/?? ?? -?(BOOL)?saveCacheDictionary;?? ?? /*?公有方法：下載?*/?? ?? -?(BOOL)?????????download:(NSString?*)paramURLAsString?? ???urlMustExpireInSeconds:(NSTimeInterval)paramURLMustExpireInSeconds?? updateExpiryDateIfInCache:(BOOL)paramUpdateExpiryDateIfInCache;?? ?? /*?--------------------------?*/?? ?? @end?? ?
?? 從上面代碼能夠看出，這個管理緩存的類中。有一個緩存字典：cacheDictionary，用來表示全部資源的緩存情況；cacheDictionaryPath用來表示緩存的路徑。saveCacheDictionary用來將緩存字典歸檔到本地文件里。
download：urlMustExpireInSeconds：updateExpiryDateIfInCache是一個公共接口，通過傳遞url、緩存過期時間、是否更新緩存過期時間三個參數來方便的使用，實現我們的緩存策略。
3、假設這個文件已經被下載，并且沒有過期。則從本地獲取文件的數據。假設文件已經過期。則又一次下載。
我們通過download：urlMustExpireInSeconds：updateExpiryDateIfInCache方法來實現，主要看這種方法的代碼：
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/*?---------下載--------------?*/?? ?? -?(BOOL)?????????download:(NSString?*)paramURLAsString?? ???urlMustExpireInSeconds:(NSTimeInterval)paramURLMustExpireInSeconds?? updateExpiryDateIfInCache:(BOOL)paramUpdateExpiryDateIfInCache{?? ???? ??BOOL?result?=?NO;?? ???? ??if?(self.cacheDictionary?==?nil?||?? ??????[paramURLAsString?length]?==?0){?? ????return(NO);?? ??}?? ???? ??paramURLAsString?=?[paramURLAsString?lowercaseString];?? ??//依據url。從字典中獲取緩存項的相關數據?? ??NSMutableDictionary?*itemDictionary?=??? ??[self.cacheDictionary?objectForKey:paramURLAsString];?? ???? ??/*?使用以下這些變量幫助我們理解緩存邏輯?*/?? ????//文件是否已經被緩存?? ??BOOL????fileHasBeenCached?=?NO;?? ????//緩存是否過期?? ??BOOL????cachedFileHasExpired?=?NO;?? ????//緩存文件是否存在?? ??BOOL????cachedFileExists?=?NO;?? ????//緩存文件是否能被載入?? ??BOOL????cachedFileDataCanBeLoaded?=?NO;?? ????//緩存文件數據?? ??NSData??*cachedFileData?=?nil;?? ????//緩存文件是否全然下載?? ??BOOL????cachedFileIsFullyDownloaded?=?NO;?? ????//緩存文件是否已經下載?? ??BOOL????cachedFileIsBeingDownloaded?=?NO;?? ??//過期時間?? ??NSDate????*expiryDate?=?nil;?? ????//下載結束時間?? ??NSDate????*downloadEndDate?=?nil;?? ????//下載開始時間?? ??NSDate????*downloadStartDate?=?nil;?? ????//本地緩存路徑?? ??NSString??*localURL?=?nil;?? ????//有效時間?? ??NSNumber??*expiresInSeconds?=?nil;?? ??NSDate????*now?=?[NSDate?date];?? ???? ??if?(itemDictionary?!=?nil){?? ????fileHasBeenCached?=?YES;?? ??}?? ??//假設文件已經被緩存。則從緩存項相關數據中獲取相關的值?? ??if?(fileHasBeenCached?==?YES){?? ?????? ????expiryDate?=?[itemDictionary??? ??????????????????objectForKey:CachedKeyExpiryDate];?? ?????? ????downloadEndDate?=?[itemDictionary?? ???????????????????????objectForKey:CachedKeyDownloadEndDate];?? ?????? ????downloadStartDate?=?[itemDictionary?? ?????????????????????????objectForKey:CachedKeyDownloadStartDate];?? ?????? ????localURL?=?[itemDictionary?? ????????????????objectForKey:CachedKeyLocalURL];?? ?????? ????expiresInSeconds?=?[itemDictionary?? ????????????????????????objectForKey:CachedKeyExpiresInSeconds];?? ????//假設下載開始和結束時間不為空，表示文件所有被下載?? ????if?(downloadEndDate?!=?nil?&&??? ????????downloadStartDate?!=?nil){?? ??????cachedFileIsFullyDownloaded?=?YES;?? ????}?? ?????? ????/*?假設expiresInSeconds不為空。downloadEndDate為空。表示文件已經正在下載?*/?? ????if?(expiresInSeconds?!=?nil?&&?? ????????downloadEndDate?==?nil){?? ??????cachedFileIsBeingDownloaded?=?YES;?? ????}?? ?????? ????/*?推斷緩存是否過期?*/?? ????if?(expiryDate?!=?nil?&&?? ????????[now?timeIntervalSinceDate:expiryDate]?>?0.0){?? ??????cachedFileHasExpired?=?YES;?? ????}?? ?????? ????if?(cachedFileHasExpired?==?NO){?? ??????/*?假設緩存文件沒有過期，載入緩存文件，而且更新過期時間?*/?? ??????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];?? ???????? ??????/*?更新緩存時間?*/?? ???????? ??????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(@"這個文件已經正在下載...");?? ????return(YES);?? ??}?? ???? ??if?(fileHasBeenCached?==?YES){?? ?????? ????if?(cachedFileHasExpired?==?NO?&&?? ????????cachedFileExists?==?YES?&&?? ????????cachedFileDataCanBeLoaded?==?YES?&&?? ????????[cachedFileData?length]?>?0?&&?? ????????cachedFileIsFullyDownloaded?==?YES){?? ???????? ??????/*?假設文件有緩存并且沒有過期?*/?? ???????? ??????NSLog(@"文件有緩存并且沒有過期.");?? ???????? ??????[self.delegate??? ???????cachedDownloadManagerSucceeded:self?? ???????remoteURL:[NSURL?URLWithString:paramURLAsString]?? ???????localURL:[NSURL?URLWithString:localURL]?? ???????aboutToBeReleasedData:cachedFileData?? ???????isCachedData:YES];?? ???????? ??????return(YES);?? ???????? ????}?else?{?? ??????/*?假設文件沒有被緩存。獲取緩存失敗?*/?? ??????NSLog(@"文件沒有緩存.");?? ??????[self.cacheDictionary?removeObjectForKey:paramURLAsString];?? ??????[self?saveCacheDictionary];?? ????}?/*?if?(cachedFileHasExpired?==?NO?&&?*/?? ?????? ??}?/*?if?(fileHasBeenCached?==?YES){?*/?? ???? ??/*?去下載文件?*/?? ???? ???? ??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、以下我們設計緩存項下載成功和失敗的兩個托付方法：
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@protocol?CacheItemDelegate?<NSObject>?? //下載成功運行該方法?? -?(void)?cacheItemDelegateSucceeded?? ??:(CacheItem?*)paramSender?? ??withRemoteURL:(NSURL?*)paramRemoteURL?? ??withAboutToBeReleasedData:(NSData?*)paramAboutToBeReleasedData;?? ?? //下載失敗運行該方法?? -?(void)?cacheItemDelegateFailed?? ??:(CacheItem?*)paramSender?? ??remoteURL:(NSURL?*)paramRemoteURL?? ??withError:(NSError?*)paramError;?? ?? @end??
?? 當我們下載成功的時候，改動緩存字典中的下載時間，表示已經下載完畢。并且須要將請求的資源數據緩存到本地：
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//緩存項的托付方法?? -?(void)?cacheItemDelegateSucceeded:(CacheItem?*)paramSender?? ?????????withRemoteURL:(NSURL?*)paramRemoteURL?? ????????withAboutToBeReleasedData:(NSData?*)paramAboutToBeReleasedData{?? ???? ??//從緩存字典中獲取該緩存項的相關數據?? ??NSMutableDictionary?*dictionary?=??? ??[self.cacheDictionary?objectForKey:[paramRemoteURL?absoluteString]];?? ??//取當前時間?? ??NSDate?*now?=?[NSDate?date];?? ??//獲取有效時間?? ??NSNumber?*expiresInSeconds?=?[dictionary??? ????????????????????????????????objectForKey:CachedKeyExpiresInSeconds];?? ??//轉換成NSTimeInterval?? ??NSTimeInterval?expirySeconds?=?[expiresInSeconds?floatValue];?? ??//改動字典中緩存項的下載結束時間?? ??[dictionary?setObject:[NSDate?date]?? ?????????????????forKey:CachedKeyDownloadEndDate];?? ??//改動字典中緩存項的緩存過期時間?? ??[dictionary?setObject:[now?dateByAddingTimeInterval:expirySeconds]?? ?????????????????forKey:CachedKeyExpiryDate];?? ??//保存緩存字典?? ??[self?saveCacheDictionary];?? ???? ??NSString?*localURL?=?[dictionary?objectForKey:CachedKeyLocalURL];?? ???? ??/*?將下載的數據保持到磁盤?*/?? ??if?([paramAboutToBeReleasedData?writeToFile:localURL?? ???????????????????????????????????atomically:YES]?==?YES){?? ????NSLog(@"緩存文件到磁盤成功.");?? ??}?else{?? ????NSLog(@"緩存文件到磁盤失敗.");?? ??}?? ??//運行緩存管理的托付方法?? ??[self.delegate??? ???cachedDownloadManagerSucceeded:self?? ???remoteURL:paramRemoteURL?? ???localURL:[NSURL?URLWithString:localURL]?? ???aboutToBeReleasedData:paramAboutToBeReleasedData?? ???isCachedData:NO];?? ???? ???? }?? 假設下載失敗我們須要從緩存字典中移除改緩存項：
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//緩存項失敗失敗的托付方法?? -?(void)?cacheItemDelegateFailed:(CacheItem?*)paramSender?? ???????????????????????remoteURL:(NSURL?*)paramRemoteURL?? ???????????????????????withError:(NSError?*)paramError{?? ???? ??/*?從緩存字典中移除緩存項，并發送一個托付?*/?? ???? ??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、載入緩存字典的時候，我們能夠將沒有下載完畢的文件移除：
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//初始化緩存字典?? ??NSString?*documentsDirectory?=??? ??[self?documentsDirectoryWithTrailingSlash:YES];?? ??//生產緩存字典的路徑?? ??cacheDictionaryPath?=??? ??[[documentsDirectory??? ????stringByAppendingString:@"CachedDownloads.dic"]?retain];?? ??//創建一個NSFileManager實例?? ??NSFileManager?*fileManager?=?[[NSFileManager?alloc]?init];?? ??//推斷是否存在緩存字典的數據?? ??if?([fileManager??? ???????fileExistsAtPath:self.cacheDictionaryPath]?==?YES){?? ??????NSLog(self.cacheDictionaryPath);?? ????//載入緩存字典中的數據?? ????NSMutableDictionary?*dictionary?=??? ????[[NSMutableDictionary?alloc]??? ?????initWithContentsOfFile:self.cacheDictionaryPath];?? ?????? ????cacheDictionary?=?[dictionary?mutableCopy];?? ?????? ????[dictionary?release];?? ?????? ????//移除沒有下載完畢的緩存數據?? ????[self?removeCorruptedCachedItems];?? ?????? ??}?else?{?? ????//創建一個新的緩存字典?? ????NSMutableDictionary?*dictionary?=??? ????[[NSMutableDictionary?alloc]?init];?? ?????? ????cacheDictionary?=?[dictionary?mutableCopy];?? ?????? ????[dictionary?release];?? ?????? ??}?? 這樣就基本上完畢了我們須要的功能。以下看看我們怎樣使用我們設計的緩存功能。 樣例場景：
??? 我們用一個UIWebView來顯示stackoverflow這個站點，我們在這個站點的內容緩存到本地20秒，假設在20秒內用戶去請求該站點，則從本地文件里獲取內容，否則過了20秒。則又一次獲取數據，并緩存到本地。
??? 在界面上拖放一個button和一個webview控件，例如以下圖。
?
??? 這樣我們能夠非常方便使用前面定義好的類。
我們在viewDidLoad 中實例化一個CachedDownloadManager，并設置它的托付為self。
當下載完畢的時候。運行CachedDownloadManager的下載成功的托付方法。
- (
void)viewDidLoad {[super viewDidLoad]; [self setTitle:
@"本地緩存測試"];CachedDownloadManager *newManager = [[CachedDownloadManager alloc] init];self.downloadManager = newManager;[newManager release];[self.downloadManager setDelegate:self];}
在button的點擊事件中增加以下代碼，請求stackoverflow ：
static NSString *url =
@"http://stackoverflow.com";[self.downloadManager download:urlurlMustExpireInSeconds:
20.0fupdateExpiryDateIfInCache:YES];
??? 上面的代碼表示將這個stackoverflow的緩存事件設置為20s。而且假設在20s內有同樣的請求，則從本地獲取stackoverflow的內容數據。updateExpiryDateIfInCache設置為yes表示：在此請求的時候。緩存時間又更新為20s。類似我們的session。假設設置成no。則第一次請求20s之后。該緩存就過期。
??? 請求完畢之后會運行CachedDownloadManager的托付方法。我們將數據展示在uiwebview中，代碼例如以下：
- (
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"]];
}
這樣我們就實現了20s的緩存。
效果：
第一次點擊測試button：
?
20s內點擊button，程序就從本地獲取數據。比較高速的就顯示出該網頁了。
?
總結：
??? 本文通過代碼和實例設計了一個iPhone應用程序本地緩存的方案。
當然這個方法不是最好的，假設你有更好的思路。歡迎告訴我。
總結
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