[composer][2/5]Implement pure RPC method's sample code

[summer-ji-eng]

SampleCode

 * try (SubscriptionAdminClient subscriptionAdminClient = SubscriptionAdminClient.create()) {
 *   ProjectSubscriptionName name = ProjectSubscriptionName.of("[PROJECT]", "[SUBSCRIPTION]");
 *   TopicName topic = TopicName.ofProjectTopicName("[PROJECT]", "[TOPIC]");
 *   PushConfig pushConfig = PushConfig.newBuilder().build();
 *   int ackDeadlineSeconds = 0;
 *   Subscription response = subscriptionAdminClient.createSubscription(name, topic, pushConfig, ackDeadlineSeconds);
 * }

[miraleung]

Left this comment in subsequent PRs. We should generally reuse the VariableExprs instead of recreating them multiple times. For instance, this could be a list of Exprs instead, which we would then map into AssignmentExprs -> Statements. Then we can pass the list of Exprs into the method invocation.

Same re-creation comment applies throughout.

[summer-ji-eng]

emm, if we use list of Exprs as body, it makes hard for insert a CommentStament, or breakstatement in the middle.
For example, when we need a comment in the middle

   * try (AutoscalingPolicyServiceClient autoscalingPolicyServiceClient = AutoscalingPolicyServiceClient.create()) {
   *   RegionName parent = RegionName.of("[PROJECT]", "[REGION]");
   *   ListAutoscalingPoliciesRequest request = ListAutoscalingPoliciesRequest.newBuilder()
   *     .setParent(parent.toString())
   *     .build();
   *   while (true) {
   *     ListAutoscalingPoliciesResponse response = autoscalingPolicyServiceClient.listAutoscalingPoliciesCallable().call(request);
   *     for (AutoscalingPolicy element : response.getPoliciesList()) {
   *       // doThingsWith(element);
   *     }
   *
   *     String nextPageToken = response.getNextPageToken();
   *     if (!Strings.isNullOrEmpty(nextPageToken)) {
   *       request = request.toBuilder().setPageToken(nextPageToken).build();
   *     } else {
   *       break;
   *     }
   *   }
   * }

For this situation, in the WhileStatement, the composing order is assignmentExpr, ForStatement, EmptyLineStatement, AssignmentExpr, IfStatement.
How do we handle the mixed situation?

[miraleung]

Keep a list of statements, and add to it iteratively. Example in pseudocode:

VariableExpr responseVarExpr; // Construct and assign value here.
Expr responseValueExpr; // Construct and assign value here.
AssignmentExpr responseAssignExpr = 
    AssignmentExpr.builder().setVariableExpr(responseVarExpr).setValueExpr(responseValueExpr).build();

List<Statement> whileBody = new ArrayList<>();
whileBody.add(ExprStatement.withExpr(responseAssignExpr));

// Construct for loop
whileBody.add(doThingsForStatement);
whileBody.add(EmptyLineStatement.create());

// etc.

[summer-ji-eng]

Gotcha, Thanks showing me example.

[miraleung]

Since this is no longer blocking the migration, let's add the unit tests as well.

[summer-ji-eng]

@miraleung could you give me feedback here. I am not sure. Do we need to check if the arguments pass in is empty?
Any possible for a methodSignatures contain an empty list List<List<MethodArgument>> ?

[summer-ji-eng]

Regarding to create variable expr once issue, for method arguments, what I did here is to create a Map<String, VariableExpr> for method arguments, where key is argument's name and value is its variable expr.
Then I fetch the values from map to make a List<Expr> and reused in the below session.
What do you think of this approach. Welcome any suggestion. Thanks.

[summer-ji-eng]

@miraleung Can you take another look at latest update. Mostly addressed your previous comment (thanks a lot).

Regarding to the string method signature refer to resource reference, I have created a bugizer cc you and @chingor13, I will follow up to get the feedback on default value.

[miraleung]

As already mentioned here, please remove resourceNames from this interface and have the class(es) that need it not inherit from ClassComposer. The TODO on me should go into those classes, not this one.

[summer-ji-eng]

Address comments and refactor in #573