Apex Trigger, Trigger Events, Trigger Context Variables in Salesforce.com

Trigger is an apex code which execute before and after a record is Inserted/Updated/Deleted from salesforce database. 

Trigger Syntax:
Trigger <trigger_name> on <Object_Name> (trigger Events) {
           
          // Logic here

}

Trigger Types:
1) Before Trigger
- It is used to perform the DML (Insert/Update/Delete) on same object and data validation functionality.
- It executes before data commit to database.

2) After Trigger
- It is used to perform the DML (Insert/Update/Delete) on cross object.
- It executes after data commit to database.

Note: In after trigger, we get "run time exception" whenever the user tries to modify any field record of the same object.

DML (Data Manipulation Language):

1) Insert
2) Update
3) Delete
4) Undelete


Trigger events:

1) for Insert
before insert
after insert

2) for Update
before update
after update

3) for Delete
before delete
after delete

4) for Undelete
after undelete


Trigger Context Variable:
Trigger context variable are used when you want to use different logic at different events. 

1) isInsert: Returns true if this trigger was fired due to an insert operation.

2) isUpdate: Returns true if this trigger was fired due to an update operation.

3) isDelete: Returns true if this trigger was fired due to a delete operation.

4) isBefore: Returns true if this trigger was fired before any record was saved.

5) isAfter: Returns true if this trigger was fired after all records were saved.

6) New: Returns a list of the new versions of the sObject records.

7) Old: Returns a list of the old versions of the sObject records.

8) NewMap: Returns a map of IDs to the new versions of the sObject records. (map<id, newRecords>)

9) OldMap: Returns a map of IDs to the old versions of the sObject records.(map<id, oldRecords>)

10) isExecuting: Returns true if the current context for the Apex code is a trigger.


11) Size: The total number of records in a trigger invocation, both old and new.

Note: All trigger context variables are written as Trigger.contextVariable such as Trigger.isInsert, etc...

Trigger Events using Context variables:

1) for Insert
before insert: Trigger.New
after insert: Trigger.New

2) for Update
before update: Trigger.New and Trigger.Old
after update: Trigger.New and Trigger.Old

3) for Delete
before delete: Trigger.Old
after delete: Trigger.Old

4) for Undelete
after undelete: Trigger.New