Why we use the Test.setCurrentPage(...) method in Salesforce and why should we in our apex controller unit test?

Hi Mohit,

The Test.setCurrentPage method sets the page context for your unit test method. Good example can be below code:-

public PageReference save() {
PageReference p = null;

if (this.qp == null || !'yyyy'.equals(this.qp)) {
p = Page.failure;
p.getParameters().put('error', 'noParam');
} else {
try {
Lead newlead = new Lead(LastName=this.lastName,
FirstName=this.firstName,
Company=this.company,
Email=this.email);
insert newlead;
} catch (Exception e) {
p = Page.failure;
p.getParameters().put('error', 'noInsert');
}
}

if (p == null) {
p = Page.success;
}

p.setRedirect(true);
return p;
}
The above code is taken from the VF guide

https://www.salesforce.com/docs/developer/pages/Content/pages_controller_error_handling.htm

Now there are multiple scenarios to assert the logic .One common use case is veryfying the URL after save action has happened .Unit test is below for above code

@isTest

public class thecontrollerTests {

public static testMethod void testMyController() {
PageReference pageRef = Page.success;//Observe how we set the Page here
Test.setCurrentPage(pageRef);//Applying page context here

thecontroller controller = new thecontroller();
String nextPage = controller.save().getUrl();

// Verify that page fails without parameters
System.assertEquals('/apex/failure?error=noParam', nextPage);

// Add parameters to page URL
ApexPages.currentPage().getParameters().put('qp', 'yyyy');//Observe how helpful it was to set the parameters in your page from Unit test

// Instantiate a new controller with all parameters in the page
controller = new thecontroller();
controller.setLastName('lastname');
controller.setFirstName('firstname');
controller.setCompany('acme');
controller.setEmail('firstlast@acme.com');
nextPage = controller.save().getUrl();

// Verify that the success page displays
System.assertEquals('/apex/success', nextPage);
Lead[] leads = [select id, email from lead where Company = 'acme'];
System.assertEquals('firstlast@acme.com', leads[0].email);
}
}

In short once you set context of the Page using method described, your controller is executing with parameters referred from the page context.

You basically indicate to your Unit test class the page you are testing your controller logic for. Your controller can be binded to multiple page and setting page context for unit test makes it crystal clear you are testing for specific page behaviour.