I want to get the source code from .m file so I use this way:
NSString *path = [[NSBundle mainBundle] pathForResource:@"ViewController" ofType:@".m"];
NSString *content = [NSString stringWithContentsOfFile:path encoding:NSUTF8StringEncoding error:nil];
If you're copying the actual source file inside your application's bundle and including it, then sure, that'll work, although I don't know why you'd want to do that.
If you're wondering if you can decompile a binary file back to Objective-C code, then no, you can't do that—at least not simply. If you have good enough skill with assembly language, of course, you can read a disassembly of the code, figure out what the original Objective-C did, and use that knowledge to recreate it.