Profile: nigelp

Problem:
How can I populate a Post Title with the value of a Custom field when saving a post?

Solution:
You need Custom Code.

If you use CRED; you can make usage of the CRED API cred_save_data:
https://toolset.com/documentation/user-guides/cred-api/#csd

You would use WordPress API to

get_post_meta()

the Field's Values and update the

post_title

with it.
An example:
http://pastebin.com/2BTbVbcs
Keep in mind that Repeating Fields are arrays and therfore you need a complex

foreach()

on them, to get out the single values.

If you do not use CRED, you can use full WordPress API, it will just be the same as if you would use native WordPress fields.
Then you need to hook into the

wp_update_post()

https://developer.wordpress.org/reference/functions/wp_update_post/

The only to keep in mind here too is that a Types repeating Field is an array, and therefore you need to

foreach()

over it's stored values.
http://php.net/manual/de/control-structures.foreach.php