Added support for custom writer classes (closes #1088)

extras/tests/JsonSerializer/CustomWriter.cpp

@@ -0,0 +1,41 @@
+// ArduinoJson - arduinojson.org
+// Copyright Benoit Blanchon 2014-2019
+// MIT License
+
+#include <ArduinoJson.h>
+#include <catch.hpp>
+
+struct CustomWriter {
+  std::string str;
+
+  size_t write(uint8_t c) {
+    str.append(1, static_cast<char>(c));
+    return 1;
+  }
+
+  size_t write(const uint8_t *s, size_t n) {
+    str.append(reinterpret_cast<const char *>(s), n);
+    return n;
+  }
+};
+
+TEST_CASE("CustomWriter") {
+  DynamicJsonDocument doc(4096);
+  JsonArray array = doc.to<JsonArray>();
+  array.add(4);
+  array.add(2);
+
+  SECTION("serializeJson()") {
+    CustomWriter writer;
+    serializeJson(array, writer);
+
+    REQUIRE("[4,2]" == writer.str);
+  }
+
+  SECTION("serializeJsonPretty") {
+    CustomWriter writer;
+    serializeJsonPretty(array, writer);
+
+    REQUIRE("[\r\n  4,\r\n  2\r\n]" == writer.str);
+  }
+}