使用NSString + stringWithContentsOfFile:usedEncoding:error:

- (nullable NSString *)stringWithContentsOfFile:(NSString *)path
                                          encoding:(NSStringEncoding)enc
                                              error:(NSError **)error;

NSString *filePath = @"/path/to/file.txt";
NSString *content;
NSError *error;
content = [NSString stringWithContentsOfFile:filePath
                                     encoding:NSUTF8StringEncoding
                                         error:&error];
if (content == nil) {
    NSLog(@"Error reading file: %@", error.localizedDescription);
} else {
    NSLog(@"File content: %@", content);
}