What Is a Trigger?
By the end of this lesson, you'll be able to:
- Explain what a trigger is and when it runs
- Recognize a trigger's basic syntax shape
- Distinguish a trigger from a regular Apex method call
Prerequisites: Module 23: "Project: Inventory System"
Code that runs itself
Every method written so far in this course has needed something to call it explicitly — library.checkOutBook(...), inventory.receiveShipment(...). A trigger is different: it's Apex code tied directly to an object, and Salesforce runs it automatically the moment a matching DML operation happens, with no explicit call anywhere.
The basic shape
trigger AccountTrigger on Account (before insert) {
System.debug('An Account is about to be inserted.');
}trigger AccountTrigger on Account names the trigger and the object it's attached to; (before insert) names when it runs — this trigger fires automatically every single time any code, anywhere, runs insert on an Account.
A real-world analogy: a motion-sensor light
A motion-sensor light doesn't need anyone to flip a switch — it reacts automatically the moment it detects movement. A trigger works the same way: it doesn't wait to be called, it reacts automatically the moment its specific event (like Account being inserted) actually happens, anywhere in the org.
Exercise
As a comment, explain the key difference between calling a method like library.checkOutBook(...) and a trigger running.
Show hint
Think about who initiates each one.
What Is a Trigger? Quiz
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Summary
A trigger is Apex code that runs automatically whenever a record is about to be (or has just been) inserted, updated, deleted, or undeleted — no one has to call it directly.