Question

Can anyone explain for me what the purpose when we are doing like this

 #if

 // block of codes goes here

 #endif

Answer 1

They are preprocessor directives. Preprocessor directives are lines included in the code of our programs that are not program statements but directives for the preprocessor. These lines are always preceded by a hash sign (#). The preprocessor is executed before the actual compilation of code begins, therefore the preprocessor digests all these directives before any code is generated by the statements.

These preprocessor directives extend only across a single line of code. As soon as a newline character is found, the preprocessor directive is considered to end. No semicolon (;) is expected at the end of a preprocessor directive. The only way a preprocessor directive can extend through more than one line is by preceding the newline character at the end of the line by a backslash ().
For more detail refer Preprocessor directives and Using Objective-C Preprocessor Directives