How to make a number field increment by one as per a certain criteria?

Question

On the Task object, I have to create a field "Barcode Number"( Bar_Code_Number__c), which starting from a specific value 68992001 and for every created task on any object increment to one, when the Task subject is " PTT Dispatch Task" only. For other task subject , it should be blank. Also, the users can also edit this field if they want. But the next task creation will follow the regular increment number ( and not from the one what the user has changed). Can you plz suggest how to achieve it. Thanks

I tried a workflow rule but did not succeeded.

As per the WF I created, the criteria as Task subject as " PTT Dispatch Task" , and formula as below, and first time i inputted the number above first number here. But i think this is not the correct way.

Answer 1

This is covered in many places:

IDEA - Provide conditional auto-numbering

Process Builder - How to Create a Conditional Auto Number with just Process Builder. This is kind of clever in a way - you link the subject Object via a lookup relationship to another object that has an auto-number field. Each time your subject object should get an incremented number, you insert the auto-number object, then take the autonumber field from the auto-number object and smash it back into your subject object.

Flow How to Create a Conditional Auto Number - same as above

APEX In an after update/insert trigger on your subject SObject, you can use (SELECT ... FOR UPDATE) on a custom object and increment a custom field Next_Sequential_Number__ by one. This gives you maximum flexibility if you need multiple numbering ranges, say, by business unit or product line.