Salesforce Trigger context variables


Example

trigger ContactTrigger on Contact (before insert, after insert,
                                   before update, after update,
                                   before delete, after delete,
                                   after undelete) {
    /** Before or After trigger execution**/
    //Returns true if trigger is before
    System.debug('Trigger:Time:Before : ' + Trigger.isBefore);
    //Returns true if trigger is after
    System.debug('Trigger:Time:After  : ' + Trigger.isAfter);
    
    /**DML Operation trigger execution **/
    //Returns true if trigger is insert
    System.debug('Trigger:DML:Insert  : ' + Trigger.isInsert);
    //Returns true if trigger is update
    System.debug('Trigger:DML:Update  : ' + Trigger.isUpdate);
    //Returns true if trigger is delete
    System.debug('Trigger:DML:Delete  : ' + Trigger.isDelete);
    //Returns true if trigger is undelete
    System.debug('Trigger:DML:Undelete: ' + Trigger.isUndelete);
    
    /** Records on Trigger execution **/
    //Returns data in state before DML. Records are read only
    //Not available for Insert Operation
    //Format: List<sObject>
    List<Contact> old_contacts = Trigger.old;
    System.debug('Trigger:Data:Old    : ' + old_contacts);
    //Returns data in state before DML. Records are read only
    //Not available for Insert Operation
    //Format: Map<Id, sObject>
    Map<Id, Contact> old_contacts_map = Trigger.oldMap;
    System.debug('Trigger:Data:OldMap : ' + old_contacts_map);
    //Returns data in state after DML.
    //Allowed for modifications in before context only
    //Not available for Delete Operation
    //Format: List<sObject>
    List<Contact> new_contacts = Trigger.new;
    System.debug('Trigger:Data:New    : ' + new_contacts);
    //Returns data in after before DML. 
    //Allowed for modifications in before context only
    //Not available for InsertOperation
    //Format: Map<Id, sObject>
    Map<Id, Contact> new_contacts_map = Trigger.newMap;
    System.debug('Trigger:Data:NewMap : ' + new_contacts_map);
    
    /** Another context variables **/
    //Returns amount of record in DML for trigger execution
    System.debug('Trigger:Size        :' + Trigger.size);
    //Returns true if the current context for the Apex code 
    //is a trigger, not VF, web service or anonymous apex
    System.debug('Trigger:isExecuting :' + Trigger.isExecuting);

    //Simple example how to use above context variables 
    //for different scenarios in combination
    if (Trigger.isBefore && Trigger.isUpdate) {
        // actions for before update
    } else if (Trigger.isAfter) {
        if (Trigger.isUpdate) {
            // actions for after update
        } else if (Trigger.isInsert) {
            // actions for after insert
        }
    }
}