iOS 把NSLog打印到文件的方法

+ (void)redirectNSLogToDocumentFolder{
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory,NSUserDomainMask, YES);
    NSString *documentsDirectory = [paths objectAtIndex:0];
    NSString *fileName =[NSString stringWithFormat:@"%@.log",[NSDate date]];
    NSString *logFilePath = [documentsDirectory stringByAppendingPathComponent:fileName];
    freopen([logFilePath cStringUsingEncoding:NSASCIIStringEncoding],"a+",stderr);
}