iphone 判断文件是否存在，获得图片资源

在iphone上使用NSFileManager判断文件是否存在时，有时总是返回NO.

原因可能是：NSFileManager没有初始化。

切换路径：

NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *documentsDirectory = [paths objectAtIndex:0];//获得需要的路径
NSLog(@"cur: %@",documentsDirectory);
//切换成当前目录
[fileManager changeCurrentDirectoryPath:[documentsDirectory stringByExpandingTildeInPath]];

浏览document下所有图片资源

#define DOCUMENTS_FOLDER [NSHomeDirectory() stringByAppendingPathComponent:@"Documents"]
NSArray *fileList = [[[NSFileManager defaultManager] directoryContentsAtPath:DOCUMENTS_FOLDER]
pathsMatchingExtensions:[NSArray arrayWithObject:@"png"]] ;

得到图片中的某一部分：

UIImage *image = [UIImage imageNamed:filename];
CGImageRef imageRef = image.CGImage;

CGRect rect = CGRectMake(origin.x, origin.y ,size.width, size.height);

CGImageRef imageRefRect = CGImageCreateWithImageInRect(imageRef, rect);

UIImage *imageRect = [[UIImage alloc] initWithCGImage:imageRefRect];