使用Three20框架會產生很多的緩存文件，緩存文件存放到應用程序的caches/Three20/文件夾下。時間久了就會占用很大的存儲空間，這時需要提供清空緩存的入口，如在設置界面：
在刪除之前我們需要顯示緩存的大小，提示一下當前緩存是否需要清空：
計算緩存大小的方法如下：
[cpp]
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void){
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES);
NSString *path = [[paths objectAtIndex:0] stringByAppendingPathComponent:@"/Three20/"];
NSFileManager *fileManager = [NSFileManager defaultManager];
if ([fileManager fileExistsAtPath:path]) {
NSEnumerator *childEnumber = [[fileManager subpathsOfDirectoryAtPath:path error:nil] objectEnumerator];
unsigned long long folderSize = 0;
NSString *fileName;
while ((fileName = [childEnumber nextObject]) != nil) {
NSString *childFilePath = [path stringByAppendingPathComponent:fileName];
folderSize += [fileManager attributesOfItemAtPath:childFilePath error:nil].fileSize;
}
dispatch_async(dispatch_get_main_queue(), ^(void){
clearCacheButton.text = [NSString stringWithFormat:@"清空圖片緩存（ %0.1f Mb）", (float)folderSize/1000/1000];
[self reloadTableCells];
});
}
});
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void){
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES);
NSString *path = [[paths objectAtIndex:0] stringByAppendingPathComponent:@"/Three20/"];
NSFileManager *fileManager = [NSFileManager defaultManager];
if ([fileManager fileExistsAtPath:path]) {
NSEnumerator *childEnumber = [[fileManager subpathsOfDirectoryAtPath:path error:nil] objectEnumerator];
unsigned long long folderSize = 0;
NSString *fileName;
while ((fileName = [childEnumber nextObject]) != nil) {
NSString *childFilePath = [path stringByAppendingPathComponent:fileName];
folderSize += [fileManager attributesOfItemAtPath:childFilePath error:nil].fileSize;
}
dispatch_async(dispatch_get_main_queue(), ^(void){
clearCacheButton.text = [NSString stringWithFormat:@"清空圖片緩存（ %0.1f Mb）", (float)folderSize/1000/1000];
[self reloadTableCells];
});
}
});
GCD的使用請參考：iOS多線程編程之Grand Central Dispatch(GCD)介紹和使用
主要是緩存文件過多時計算文件大小需要一定的時間。
清空緩存的方法：
[cpp]
[[TTURLCache sharedCache] removeAll:YES];
[[TTURLCache sharedCache] removeAll:YES];
