Fix SNS event

docs/providers/aws/events/sns.md

@@ -50,6 +50,27 @@ functions:
       - sns: arn:xxx
 ```
 
+Or with intrinsic cloud formation function like `Fn::Join` or `Fn::GetAtt`,
+
+```yml
+functions:
+  dispatcher:
+    handler: dispatcher.dispatch
+    events:
+      - sns:
+          arn:
+            Fn::Join:
+              - ""
+              - - "arn:aws:sns:"
+                - Ref: "AWS::Region"
+                - ":"
+                - Ref: "AWS::AccountId"
+                - ":MyCustomTopic"
+          topicName: MyCustomTopic
+```
+
+**Note:** It is important to know that `topicArn` must contain the value given in the `topicName` property.
+
 ## Setting a display name
 
 This event definition ensures that the `aggregator` function gets called every time a message is sent to the

lib/plugins/aws/deploy/compile/events/sns/index.js

@@ -26,22 +26,55 @@ class AwsCompileSNSEvents {
             let displayName = '';
 
             if (typeof event.sns === 'object') {
-              ['topicName', 'displayName'].forEach((property) => {
-                if (typeof event.sns[property] === 'string') {
-                  return;
+              if (event.sns.arn) {
+                if (typeof event.sns.arn === 'object' ||
+                  typeof event.sns.arn === 'string') {
+                  if (event.sns.topicName && typeof event.sns.topicName === 'string') {
+                    topicArn = event.sns.arn;
+                    topicName = event.sns.topicName;
+                  } else {
+                    const errorMessage = [
+                      'Missing or invalid topicName property for sns event',
+                      ` in function ${functionName}`,
+                      ' The correct syntax is: sns: topic-name-or-arn',
+                      ' OR an object with:',
+                      '   - arn as object or string AND topicName as string OR',
+                      '   - topicName AND displayName strings.',
+                      ' Please check the docs for more info.',
+                    ].join('');
+                    throw new this.serverless.classes
+                      .Error(errorMessage);
+                  }
+                } else {
+                  const errorMessage = [
+                    'Invalid value type provided .arn property for sns event',
+                    ` in function ${functionName}`,
+                    ' The correct types are: object, string.',
+                    ' Please check the docs for more info.',
+                  ].join('');
+                  throw new this.serverless.classes
+                    .Error(errorMessage);
                 }
-                const errorMessage = [
-                  `Missing or invalid "${property}" property for sns event`,
-                  ` in function ${functionName}`,
-                  ' The correct syntax is: sns: topic-name-or-arn',
-                  ' OR an object with "topicName" AND "displayName" strings.',
-                  ' Please check the docs for more info.',
-                ].join('');
-                throw new this.serverless.classes
-                  .Error(errorMessage);
-              });
-              topicName = event.sns.topicName;
-              displayName = event.sns.displayName;
+              } else {
+                ['topicName', 'displayName'].forEach((property) => {
+                  if (typeof event.sns[property] === 'string') {
+                    return;
+                  }
+                  const errorMessage = [
+                    'Missing or invalid topicName property for sns event',
+                    ` in function ${functionName}`,
+                    ' The correct syntax is: sns: topic-name-or-arn',
+                    ' OR an object with:',
+                    '   - arn as object or string AND topicName as string OR',
+                    '   - topicName AND displayName strings.',
+                    ' Please check the docs for more info.',
+                  ].join('');
+                  throw new this.serverless.classes
+                    .Error(errorMessage);
+                });
+                displayName = event.sns.displayName;
+                topicName = event.sns.topicName;
+              }
             } else if (typeof event.sns === 'string') {
               if (event.sns.indexOf('arn:') === 0) {
                 topicArn = event.sns;

lib/plugins/aws/deploy/compile/events/sns/index.test.js

@@ -157,5 +157,49 @@ describe('AwsCompileSNSEvents', () => {
         .provider.compiledCloudFormationTemplate.Resources.FirstLambdaPermissionFooSNS.Type
       ).to.equal('AWS::Lambda::Permission');
     });
+
+
+    it('should raise error when only arn is present', () => {
+      awsCompileSNSEvents.serverless.service.functions = {
+        first: {
+          events: [
+            {
+              sns: {
+                arn: 'arn:aws:sns:region:accountid:bar',
+              },
+            },
+          ],
+        },
+      };
+
+      expect(() => { awsCompileSNSEvents.compileSNSEvents(); }).to.throw(Error);
+    });
+
+    it('should not create SNS topic when arn is provided', () => {
+      awsCompileSNSEvents.serverless.service.functions = {
+        first: {
+          events: [
+            {
+              sns: {
+                topicName: 'bar',
+                arn: 'arn:aws:sns:region:accountid:bar',
+              },
+            },
+          ],
+        },
+      };
+
+      awsCompileSNSEvents.compileSNSEvents();
+
+      expect(Object.keys(awsCompileSNSEvents.serverless.service
+        .provider.compiledCloudFormationTemplate.Resources)
+      ).to.have.length(2);
+      expect(awsCompileSNSEvents.serverless.service
+        .provider.compiledCloudFormationTemplate.Resources.FirstSnsSubscriptionBar.Type
+      ).to.equal('AWS::SNS::Subscription');
+      expect(awsCompileSNSEvents.serverless.service
+        .provider.compiledCloudFormationTemplate.Resources.FirstLambdaPermissionBarSNS.Type
+      ).to.equal('AWS::Lambda::Permission');
+    });
   });
 });