Introduce support for emitting defaults for json responses

lib/src/CanEmitJsonDefaults.php

@@ -0,0 +1,12 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Twirp;
+
+interface CanEmitJsonDefaults
+{
+    public function setEmitJsonDefaults(bool $emitDefaults): void;
+
+    public function shouldEmitJsonDefaults(): bool;
+}

protoc-gen-twirp_php/templates/service/Server.php.tmpl

@@ -14,6 +14,7 @@ use Psr\Http\Message\ServerRequestInterface;
 use Psr\Http\Message\StreamFactoryInterface;
 use Psr\Http\Server\RequestHandlerInterface;
 use Twirp\BaseServerHooks;
+use Twirp\CanEmitJsonDefaults;
 use Twirp\Context;
 use Twirp\ErrorCode;
 use Twirp\ServerHooks;
@@ -23,7 +24,7 @@ use Twirp\ServerHooks;
  *
  * Generated from protobuf service <code>{{ .Service.Desc.FullName }}</code>
  */
-final class {{ .Service | phpServiceName .File }}Server implements RequestHandlerInterface
+final class {{ .Service | phpServiceName .File }}Server implements RequestHandlerInterface, CanEmitJsonDefaults
 {
     /**
      * A convenience constant that may identify URL paths.
@@ -60,12 +61,18 @@ final class {{ .Service | phpServiceName .File }}Server implements RequestHandle
      */
     private $prefix;
 
+    /**
+     * @var bool
+     */
+    private $emitDefaults = false;
+
     public function __construct(
         {{ .Service | phpServiceName .File }} $svc,
         ?ServerHooks $hook = null,
         ?ResponseFactoryInterface $responseFactory = null,
         ?StreamFactoryInterface $streamFactory = null,
-        string $prefix = '/twirp'
+        string $prefix = '/twirp',
+        bool $emitDefaults = false
     ) {
         if ($hook === null) {
             $hook = new BaseServerHooks();
@@ -84,6 +91,7 @@ final class {{ .Service | phpServiceName .File }}Server implements RequestHandle
         $this->responseFactory = $responseFactory;
         $this->streamFactory = $streamFactory;
         $this->prefix = rtrim($prefix, '/');
+        $this->setEmitJsonDefaults($emitDefaults);
     }
 
     /**
@@ -198,7 +206,11 @@ final class {{ .Service | phpServiceName .File }}Server implements RequestHandle
             return $this->writeError($ctx, $e);
         }
 
-        $data = $out->serializeToJsonString();
+        if ($this->shouldEmitJsonDefaults()) {
+            $data = $out->serializeToJsonString(\Google\Protobuf\PrintOptions::EMIT_DEFAULTS);
+        } else {
+            $data = $out->serializeToJsonString();
+        }
 
         $body = $this->streamFactory->createStream($data);
 
@@ -372,4 +384,28 @@ final class {{ .Service | phpServiceName .File }}Server implements RequestHandle
             // so anyway.
         }
     }
+
+    /**
+     * By default, protobuf will filter out default values from your return value.
+     * This changes that behavior to always emit default values, allowing for clearer
+     * responses for non-generated or customized clients.
+     * Only supported on versions greater than 4.33.0 of the protobuf package or extension.
+     * Will be a noop if on older versions.
+     *
+     * When false (default): `{myVal: 0, myStr: "hi"}` -> `{myStr: "hi"}`
+     * When true: `{myVal: 0, myStr: "hi"}` -> `{myVal: 0, myStr: "hi"}`
+     */
+    public function setEmitJsonDefaults(bool $emitDefaults): void
+    {
+        $this->emitDefaults = $emitDefaults;
+    }
+
+    public function shouldEmitJsonDefaults(): bool
+    {
+        if (!defined('Google\\Protobuf\\PrintOptions::EMIT_DEFAULTS')) {
+            return false;
+        }
+
+        return $this->emitDefaults;
+    }
 }