iOS development issues: adding libraries and copying other engineering files compile error issues

Add Library issues First:

Select page Check build Phases->link Binary with libraries + Add library

Then command+shift+k erase history's debug garbage.

Copying other project file compilation error issues:

When copying files into the project, if you do not pay attention to check "Add to Target", the runtime will be reported as follows similar error:
Undefined symbols for architecture i386:
 
"_objc_class_$_ Skpsmtpmessage ", referenced from:
  objc-class-ref in CONFIRMCONTROLLER.O
 
" _ Kskpsmtppartcontenttransferencodingkey ", referenced from:
  -[confirmcontroller SendEmail] in CONFIRMCONTROLLER.O
 
"_kskpsmtppartmessagekey", referenced from:
  -[confirmcontroller SendEmail] in confirmcontroller.o
 
"_ Kskpsmtppartcontenttypekey ", referenced from:
  -[confirmcontroller SendEmail] in CONFIRMCONTROLLER.O
 
LD: Symbol (s) not found for architecture i386
 
 
Workaround:
First click on the scheme (list on the left, click on the Schema)
Select target
Select page Sign build phases
Compile Source
If it's not expanded
, start looking for the plus sign, click the plus sign, and add the header file of the. m file that the target requires.
(if only one of the. h files are added)
That would be okay.