Adds MailtoURLRewriterObserver to MailtoURLRewriter

ios/chrome/browser/web/mailto_url_rewriter_unittest.mm

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import "ios/chrome/browser/web/mailto_url_rewriter.h"
 
 #import "ios/chrome/browser/web/mailto_handler.h"
 #import "ios/chrome/browser/web/mailto_handler_gmail.h"
 #import "ios/chrome/browser/web/mailto_handler_system_mail.h"
 #include "testing/gtest/include/gtest/gtest.h"
@@ skipping 32 matching lines @@
 
 @interface FakeMailtoHandlerGmailInstalled : MailtoHandlerGmail
 @end
 
 @implementation FakeMailtoHandlerGmailInstalled
 - (BOOL)isAvailable {
   return YES;
 }
 @end
 
+#pragma mark - Test Observer object
+
+@interface RewriterObserver : NSObject<MailtoURLRewriterObserver>
+@property(nonatomic, readonly) int changeCount;
+@end
+
+@implementation RewriterObserver
+@synthesize changeCount = _changeCount;
+- (void)rewriterDidChange:(MailtoURLRewriter*)rewriter {
+  ++_changeCount;
+}
+@end
+
 #pragma mark - MailtoURLRewriter private interfaces for testing.
 
 @interface MailtoURLRewriter ()
 + (void)resetDefaultHandlerIDForTesting;
 - (void)addMailtoApps:(NSArray<MailtoHandler*>*)handlerApps;
 @end
 
 #pragma mark - Unit Test Cases
 
 class MailtoURLRewriterTest : public PlatformTest {
  protected:
   void SetUp() override { [MailtoURLRewriter resetDefaultHandlerIDForTesting]; }
 };
 
 // Tests that a standard instance has the expected values.
 TEST_F(MailtoURLRewriterTest, TestStandardInstance) {
   MailtoURLRewriter* rewriter =
       [[MailtoURLRewriter alloc] initWithStandardHandlers];
   EXPECT_TRUE(rewriter);
+  EXPECT_GT([[rewriter defaultHandlerName] length], 0U);
   // ID for system Mail client app must not be an empty string.
   EXPECT_GT([[MailtoURLRewriter systemMailApp] length], 0U);
 
   NSArray<MailtoHandler*>* handlers = [rewriter defaultHandlers];
   EXPECT_GE([handlers count], 1U);
   for (MailtoHandler* handler in handlers) {
     ASSERT_TRUE(handler);
     NSString* appStoreID = [handler appStoreID];
     [rewriter setDefaultHandlerID:appStoreID];
     EXPECT_NSEQ(appStoreID, [rewriter defaultHandlerID]);
@@ skipping 26 matching lines @@
   ASSERT_TRUE([otherHandlerID length]);
   [rewriter setDefaultHandlerID:otherHandlerID];
 
   // Create a new MailtoURLRewriter object and verify that the current
   // default is the |otherHandlerID| set in the previous step.
   MailtoURLRewriter* rewriter2 = [[MailtoURLRewriter alloc] init];
   [rewriter2 addMailtoApps:@[ systemMailHandler, fakeGmailHandler ]];
   EXPECT_NSEQ(otherHandlerID, [rewriter2 defaultHandlerID]);
 }
 
+TEST_F(MailtoURLRewriterTest, TestChangeObserver) {
+  RewriterObserver* observer = [[RewriterObserver alloc] init];
+  ASSERT_EQ(0, [observer changeCount]);
+
+  // Sets up a MailtoURLRewriter object. The default handler is Gmail app
+  // because |fakeGmailHandler| reports that it is "installed".
+  MailtoURLRewriter* rewriter = [[MailtoURLRewriter alloc] init];
+  MailtoHandler* systemMailHandler = [[MailtoHandlerSystemMail alloc] init];
+  MailtoHandler* fakeGmailHandler =
+      [[FakeMailtoHandlerGmailInstalled alloc] init];
+  [rewriter addMailtoApps:@[ systemMailHandler, fakeGmailHandler ]];
+  EXPECT_NSEQ([fakeGmailHandler appStoreID], [rewriter defaultHandlerID]);
+  [rewriter setObserver:observer];
+
+  // Setting system Mail app as handler while current handler is Gmail should
+  // trigger the observer callback, incrementing count from 0 to 1.
+  [rewriter setDefaultHandlerID:[systemMailHandler appStoreID]];
+  EXPECT_EQ(1, [observer changeCount]);
+
+  // Setting system Mail app as handler while current handler is already
+  // system Mail app should not trigger the observer callback. Count remains
+  // at 1.
+  [rewriter setDefaultHandlerID:[systemMailHandler appStoreID]];
+  EXPECT_EQ(1, [observer changeCount]);
+
+  // Setting Gmail app as the default handler when current handler is system
+  // Mail app should trigger the observer callback. Count increases from 1 to 2.
+  [rewriter setDefaultHandlerID:[fakeGmailHandler appStoreID]];
+  EXPECT_EQ(2, [observer changeCount]);
+
+  // Deallocates the observer to test that there are no ill side-effects (e.g.
+  // crashes) when observer's lifetime is shorter than the rewriter.
+  observer = nil;
+  [rewriter setDefaultHandlerID:[systemMailHandler appStoreID]];
+}
+
 // Tests that a new user without Gmail app installed launches system Mail app.
 TEST_F(MailtoURLRewriterTest, TestNewUserNoGmail) {
   // Sets pre-condition for a user who did not have Chrome installed.
   NSUserDefaults* defaults = [NSUserDefaults standardUserDefaults];
   [defaults removeObjectForKey:kLegacyShouldAutoOpenKey];
   // A faked MailtoHandler for Gmail.
   MailtoHandler* fakeGmailHandler =
       [[FakeMailtoHandlerGmailNotInstalled alloc] init];
 
   // Sets up a MailtoURLRewriter for testing.
@@ skipping 77 matching lines @@
       [[FakeMailtoHandlerGmailInstalled alloc] init];
 
   // Sets up a MailtoURLRewriter for testing.
   MailtoURLRewriter* rewriter = [[MailtoURLRewriter alloc] init];
   MailtoHandler* systemMailHandler = [[MailtoHandlerSystemMail alloc] init];
   [rewriter addMailtoApps:@[ systemMailHandler, fakeGmailHandler ]];
 
   // Verify that MailtoURLRewriter will use Gmail app.
   EXPECT_NSEQ(kGmailAppStoreID, [rewriter defaultHandlerID]);
 }