Define CommandMessage pattern and implement first messages

app/src/main/kotlin/io/homeassistant/companion/android/frontend/externalbus/outgoing/CommandMessage.kt

@@ -0,0 +1,65 @@
+package io.homeassistant.companion.android.frontend.externalbus.outgoing
+
+import io.homeassistant.companion.android.frontend.externalbus.frontendExternalBusJson
+import kotlinx.serialization.SerialName
+import kotlinx.serialization.Serializable
+import kotlinx.serialization.json.JsonElement
+import kotlinx.serialization.json.encodeToJsonElement
+
+/**
+ * Outgoing command message sent to the Home Assistant frontend via the external bus.
+ *
+ * Command messages instruct the frontend to perform an action.
+ *
+ * The Home Assistant frontend uses `"type": "command"` for all commands, distinguishing them
+ * only by the `command` string field. This means kotlinx.serialization's sealed polymorphism
+ * cannot give each command its own subtype (all would need the same `@SerialName("command")`).
+ *
+ * Instead, this single private class handles serialization, and each command is exposed as a
+ * top-level factory: [NavigateToMessage] uses `operator fun invoke` for constructor-like syntax,
+ * and [ShowSidebarMessage] is a pre-built instance. Call sites read naturally:
+ * ```
+ * send(NavigateTo(path = "/dashboard", replace = true))
+ * send(ShowSidebar)
+ * ```
+ */
+@Serializable
+@SerialName("command")
+private data class CommandMessage(
+    override val id: Int? = null,
+    val command: String,
+    val payload: JsonElement? = null,
+) : OutgoingExternalBusMessage
+
+/**
+ * Creates a navigation command to navigate the frontend to the given path.
+ *
+ * When [replace] is `true`, the current history entry is replaced instead of
+ * pushing a new one (useful for resetting to the default dashboard).
+ *
+ * Requires Home Assistant 2025.6 or later. Callers must check the server version
+ * before sending this command.
+ *
+ * @see CommandMessage
+ */
+object NavigateToMessage {
+    operator fun invoke(path: String, replace: Boolean = false): OutgoingExternalBusMessage = CommandMessage(
+        command = "navigate",
+        payload = frontendExternalBusJson.encodeToJsonElement(
+            NavigatePayload(path = path, options = NavigateOptions(replace = replace)),
+        ),
+    )
+
+    @Serializable
+    private data class NavigatePayload(val path: String, val options: NavigateOptions = NavigateOptions())
+
+    @Serializable
+    private data class NavigateOptions(val replace: Boolean = false)
+}
+
+/**
+ * Command to toggle the frontend sidebar visibility.
+ *
+ * @see CommandMessage
+ */
+val ShowSidebarMessage: OutgoingExternalBusMessage = CommandMessage(command = "sidebar/show")

app/src/test/kotlin/io/homeassistant/companion/android/frontend/externalbus/outgoing/CommandMessageTest.kt

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+package io.homeassistant.companion.android.frontend.externalbus.outgoing
+
+import io.homeassistant.companion.android.frontend.externalbus.frontendExternalBusJson
+import io.homeassistant.companion.android.testing.unit.ConsoleLogExtension
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Test
+import org.junit.jupiter.api.extension.ExtendWith
+
+@ExtendWith(ConsoleLogExtension::class)
+class CommandMessageTest {
+
+    @Test
+    fun `Given NavigateToMessage with replace when serializing then produces correct JSON`() {
+        val message = NavigateToMessage(path = "/", replace = true)
+
+        val json = frontendExternalBusJson.encodeToString<OutgoingExternalBusMessage>(message)
+
+        assertEquals(
+            """{"type":"command","id":null,"command":"navigate","payload":{"path":"/","options":{"replace":true}}}""",
+            json,
+        )
+    }
+
+    @Test
+    fun `Given NavigateToMessage without replace when serializing then defaults replace to false`() {
+        val message = NavigateToMessage(path = "/lovelace/dashboard")
+
+        val json = frontendExternalBusJson.encodeToString<OutgoingExternalBusMessage>(message)
+
+        assertEquals(
+            """{"type":"command","id":null,"command":"navigate","payload":{"path":"/lovelace/dashboard","options":{"replace":false}}}""",
+            json,
+        )
+    }
+
+    @Test
+    fun `Given ShowSidebarMessage when serializing then produces correct JSON without payload`() {
+        val json = frontendExternalBusJson.encodeToString<OutgoingExternalBusMessage>(ShowSidebarMessage)
+
+        assertEquals(
+            """{"type":"command","id":null,"command":"sidebar/show","payload":null}""",
+            json,
+        )
+    }
+}