如何设置一个App的缓存机制

1. /* ---------缓存项-------------- */
2. @interface CacheItem : NSObject {
3. @public
4. id<CacheItemDelegate> delegate;
5. //web地址
6. NSString              *remoteURL;
7. @private
8. //是否正在下载
9. BOOL                  isDownloading;
10. //NSMutableData对象
11. NSMutableData         *connectionData;
12. //NSURLConnection对象
13. NSURLConnection       *connection;
14. }
15. /* -------------------------- */
16. @property (nonatomic, retain) id<CacheItemDelegate> delegate;
17. @property (nonatomic, retain) NSString  *remoteURL;
18. @property (nonatomic, assign) BOOL      isDownloading;
19. @property (nonatomic, retain) NSMutableData *connectionData;
20. @property (nonatomic, retain) NSURLConnection *connection;
21. /* ----------开始下载方法----------- */
22. - (BOOL) startDownloadingURL:(NSString *)paramRemoteURL;
23. @end

 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。

1. /* -----------CachedDownloadManager-------------- */
2. @interface CachedDownloadManager : NSObject
3. <CacheItemDelegate> {
4. @public
5. id<CachedDownloadManagerDelegate>  delegate;
6. @private
7. //记录缓存数据的字典
8. NSMutableDictionary                *cacheDictionary;
9. //缓存的路径
10. NSString                           *cacheDictionaryPath;
11. }
12. @property (nonatomic, assign)
13. id<CachedDownloadManagerDelegate> delegate;
14. @property (nonatomic, copy)
15. NSMutableDictionary *cacheDictionary;
16. @property (nonatomic, retain)
17. NSString *cacheDictionaryPath;
18. /* 保持缓存字典 */
19. - (BOOL) saveCacheDictionary;
20. /* 公有方法：下载 */
21. - (BOOL)         download:(NSString *)paramURLAsString
22. urlMustExpireInSeconds:(NSTimeInterval)paramURLMustExpireInSeconds
23. updateExpiryDateIfInCache:(BOOL)paramUpdateExpiryDateIfInCache;
24. /* -------------------------- */
25. @end

1. /* ---------下载-------------- */
2. - (BOOL)         download:(NSString *)paramURLAsString
3. urlMustExpireInSeconds:(NSTimeInterval)paramURLMustExpireInSeconds
4. updateExpiryDateIfInCache:(BOOL)paramUpdateExpiryDateIfInCache{
5. BOOL result = NO;
6. if (self.cacheDictionary == nil ||
7. [paramURLAsString length] == 0){
8. return(NO);
9. }
10. paramURLAsString = [paramURLAsString lowercaseString];
11. //根据url，从字典中获取缓存项的相关数据
12. NSMutableDictionary *itemDictionary =
13. [self.cacheDictionary objectForKey:paramURLAsString];
14. /* 使用下面这些变量帮助我们理解缓存逻辑 */
15. //文件是否已经被缓存
16. BOOL    fileHasBeenCached = NO;
17. //缓存是否过期
18. BOOL    cachedFileHasExpired = NO;
19. //缓存文件是否存在
20. BOOL    cachedFileExists = NO;
21. //缓存文件能否被加载
22. BOOL    cachedFileDataCanBeLoaded = NO;
23. //缓存文件数据
24. NSData  *cachedFileData = nil;
25. //缓存文件是否完全下载
26. BOOL    cachedFileIsFullyDownloaded = NO;
27. //缓存文件是否已经下载
28. BOOL    cachedFileIsBeingDownloaded = NO;
29. //过期时间
30. NSDate    *expiryDate = nil;
31. //下载结束时间
32. NSDate    *downloadEndDate = nil;
33. //下载开始时间
34. NSDate    *downloadStartDate = nil;
35. //本地缓存路径
36. NSString  *localURL = nil;
37. //有效时间
38. NSNumber  *expiresInSeconds = nil;
39. NSDate    *now = [NSDate date];
40. if (itemDictionary != nil){
41. fileHasBeenCached = YES;
42. }
43. //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
44. if (fileHasBeenCached == YES){
45. expiryDate = [itemDictionary
46. objectForKey:CachedKeyExpiryDate];
47. downloadEndDate = [itemDictionary
48. objectForKey:CachedKeyDownloadEndDate];
49. downloadStartDate = [itemDictionary
50. objectForKey:CachedKeyDownloadStartDate];
51. localURL = [itemDictionary
52. objectForKey:CachedKeyLocalURL];
53. expiresInSeconds = [itemDictionary
54. objectForKey:CachedKeyExpiresInSeconds];
55. //如果下载开始和结束时间不为空，表示文件全部被下载
56. if (downloadEndDate != nil &&
57. downloadStartDate != nil){
58. cachedFileIsFullyDownloaded = YES;
59. }
60. /* 如果expiresInSeconds不为空，downloadEndDate为空，表示文件已经正在下载 */
61. if (expiresInSeconds != nil &&
62. downloadEndDate == nil){
63. cachedFileIsBeingDownloaded = YES;
64. }
65. /* 判断缓存是否过期 */
66. if (expiryDate != nil &&
67. [now timeIntervalSinceDate:expiryDate] > 0.0){
68. cachedFileHasExpired = YES;
69. }
70. if (cachedFileHasExpired == NO){
71. /* 如果缓存文件没有过期，加载缓存文件，并且更新过期时间 */
72. NSFileManager *fileManager = [[NSFileManager alloc] init];
73. if ([fileManager fileExistsAtPath:localURL] == YES){
74. cachedFileExists = YES;
75. cachedFileData = [NSData dataWithContentsOfFile:localURL];
76. if (cachedFileData != nil){
77. cachedFileDataCanBeLoaded = YES;
78. } /* if (cachedFileData != nil){ */
79. } /* if ([fileManager fileExistsAtPath:localURL] == YES){ */
80. [fileManager release];
81. /* 更新缓存时间 */
82. if (paramUpdateExpiryDateIfInCache == YES){
83. NSDate *newExpiryDate =
84. [NSDate dateWithTimeIntervalSinceNow:
85. paramURLMustExpireInSeconds];
86. NSLog(@"Updating the expiry date from %@ to %@.",
87. expiryDate,
88. newExpiryDate);
89. [itemDictionary setObject:newExpiryDate
90. forKey:CachedKeyExpiryDate];
91. NSNumber *expires =
92. [NSNumber numberWithFloat:paramURLMustExpireInSeconds];
93. [itemDictionary setObject:expires
94. forKey:CachedKeyExpiresInSeconds];
95. }
96. } /* if (cachedFileHasExpired == NO){ */
97. }
98. if (cachedFileIsBeingDownloaded == YES){
99. NSLog(@"这个文件已经正在下载...");
100. return(YES);
101. }
102. if (fileHasBeenCached == YES){
103. if (cachedFileHasExpired == NO &&
104. cachedFileExists == YES &&
105. cachedFileDataCanBeLoaded == YES &&
106. [cachedFileData length] > 0 &&
107. cachedFileIsFullyDownloaded == YES){
108. /* 如果文件有缓存而且没有过期 */
109. NSLog(@"文件有缓存而且没有过期.");
110. [self.delegate
111. cachedDownloadManagerSucceeded:self
112. remoteURL:[NSURL URLWithString:paramURLAsString]
113. localURL:[NSURL URLWithString:localURL]
114. aboutToBeReleasedData:cachedFileData
115. isCachedData:YES];
116. return(YES);
117. } else {
118. /* 如果文件没有被缓存，获取缓存失败 */
119. NSLog(@"文件没有缓存.");
120. [self.cacheDictionary removeObjectForKey:paramURLAsString];
121. [self saveCacheDictionary];
122. } /* if (cachedFileHasExpired == NO && */
123. } /* if (fileHasBeenCached == YES){ */
124. /* 去下载文件 */
125. NSNumber *expires =
126. [NSNumber numberWithFloat:paramURLMustExpireInSeconds];
127. NSMutableDictionary *newDictionary =
128. [[[NSMutableDictionary alloc] init] autorelease];
129. [newDictionary setObject:expires
130. forKey:CachedKeyExpiresInSeconds];
131. localURL = [paramURLAsString
132. stringByAddingPercentEscapesUsingEncoding:
133. NSUTF8StringEncoding];
134. localURL = [localURL stringByReplacingOccurrencesOfString:@"://"
135. withString:@""];
136. localURL = [localURL stringByReplacingOccurrencesOfString:@"/"
137. withString:@"{1}quot;];
138. localURL = [localURL stringByAppendingPathExtension:@"cache"];
139. NSString *documentsDirectory =
140. [self documentsDirectoryWithTrailingSlash:NO];
141. localURL = [documentsDirectory
142. stringByAppendingPathComponent:localURL];
143. [newDictionary setObject:localURL
144. forKey:CachedKeyLocalURL];
145. [newDictionary setObject:now
146. forKey:CachedKeyDownloadStartDate];
147. [self.cacheDictionary setObject:newDictionary
148. forKey:paramURLAsString];
149. [self saveCacheDictionary];
150. CacheItem *item = [[[CacheItem alloc] init] autorelease];
151. [item setDelegate:self];
152. [item startDownloadingURL:paramURLAsString];
153. return(result);
154. }

1. @protocol CacheItemDelegate <NSObject>
2. //下载成功执行该方法
3. - (void) cacheItemDelegateSucceeded
4. :(CacheItem *)paramSender
5. withRemoteURL:(NSURL *)paramRemoteURL
6. withAboutToBeReleasedData:(NSData *)paramAboutToBeReleasedData;
7. //下载失败执行该方法
8. - (void) cacheItemDelegateFailed
9. :(CacheItem *)paramSender
10. remoteURL:(NSURL *)paramRemoteURL
11. withError:(NSError *)paramError;
12. @end

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。

- (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];}

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

- (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"]];
}