Refactor ReceiveMessageHandler

Introduce PlainTextWriter to improve indentation handling.

src/main/java/org/asamk/signal/PlainTextWriterImpl.java

@@ -0,0 +1,67 @@
+package org.asamk.signal;
+
+import org.slf4j.helpers.MessageFormatter;
+
+import java.io.BufferedWriter;
+import java.io.IOException;
+import java.io.OutputStream;
+import java.io.OutputStreamWriter;
+import java.io.Writer;
+
+public final class PlainTextWriterImpl implements PlainTextWriter {
+
+    private final Writer writer;
+
+    private PlainTextWriter indentedWriter;
+
+    public PlainTextWriterImpl(final OutputStream outputStream) {
+        this.writer = new BufferedWriter(new OutputStreamWriter(outputStream));
+    }
+
+    @Override
+    public void println(String format, Object... args) throws IOException {
+        final String message = MessageFormatter.arrayFormat(format, args).getMessage();
+
+        writer.write(message);
+        writer.write(System.lineSeparator());
+        writer.flush();
+    }
+
+    @Override
+    public PlainTextWriter indentedWriter() {
+        if (indentedWriter == null) {
+            indentedWriter = new IndentedPlainTextWriter(this, writer);
+        }
+        return indentedWriter;
+    }
+
+    private static final class IndentedPlainTextWriter implements PlainTextWriter {
+
+        private final static int INDENTATION = 2;
+
+        private final String spaces = " ".repeat(INDENTATION);
+        private final PlainTextWriter plainTextWriter;
+        private final Writer writer;
+
+        private PlainTextWriter indentedWriter;
+
+        private IndentedPlainTextWriter(final PlainTextWriter plainTextWriter, final Writer writer) {
+            this.plainTextWriter = plainTextWriter;
+            this.writer = writer;
+        }
+
+        @Override
+        public void println(final String format, final Object... args) throws IOException {
+            writer.write(spaces);
+            plainTextWriter.println(format, args);
+        }
+
+        @Override
+        public PlainTextWriter indentedWriter() {
+            if (indentedWriter == null) {
+                indentedWriter = new IndentedPlainTextWriter(this, writer);
+            }
+            return indentedWriter;
+        }
+    }
+}