Functions and usage of prefix. PCH

Hello world_prefix.pch: the extension. PCH indicates "precompiled header". This is a list of header files from the external framework used by your project. Xcode will compile these headers to files, which will reduce the time for you to compile the project when you select build or build and go. Usually, the header file is automatically included.

PCH: the system will include this file before compiling every CPP file. This saves the time for adding include, which is equivalent to accelerating compilation (to be obtained)

In addition, you can add macros here and use them throughout the project. Time saved