Merge pull request #78 from webxdc/hpk/realtime2

 `joinRealtimeChannel` method with webxdc realtime API

src-docs/SUMMARY.md

@@ -10,8 +10,8 @@
       - [sendToChat](./spec/sendToChat.md)
       - [importFiles](./spec/importFiles.md)
       - [selfAddr & selfName](./spec/selfAddr_and_selfName.md)
+      - [joinRealtimeChannel](./spec/joinRealtimeChannel.md)
   - [Messenger implementations](./spec/messenger.md)
-
 - [Shared Web Application state](./shared_state/README.md)
   - [Detecting conflicts](./shared_state/conflicts.md)
   - [Theory of Conflict-free Replicated Data Types (CRDTs)](./shared_state/crdts.md)

src-docs/spec/joinRealtimeChannel.md

@@ -0,0 +1,69 @@
+# joinRealtimeChannel (experimental)
+
+```js
+const realtimeChannel = window.webxdc.joinRealtimeChannel();
+```
+
+Setup and return the realtime channel for this app,
+with methods for listening and sending data as well as leaving the channel. 
+Per-app realtime channels are: 
+
+- **private**: no one outside the chat can participate in realtime channels. 
+
+- **isolated**: apps can not participate in realtime channels of other apps. 
+
+- **ephemeral**: any sent data will only be received by currently
+  connected peers but not by peers connecting later.
+
+Calling `joinRealtimeChannel` a second time without leaving the prior one
+will throw an error.
+
+## `realtimeChannel.setListener((data) => {})` 
+
+Start listening on the realtime channel using the specified callback. 
+The callback receives `Uint8Array` data items that were sent from connected peers. 
+Calling `setListener` a second time will replace the previous listener. 
+
+
+## `realtimeChannel.send(data)` 
+
+Send a `Uint8Array` data item to connected peers. 
+There is no guarantee anyone is receiving sent data
+because there might be no currently listening peers,
+or network connections fail. 
+It is up to the app to determine connectivity status with other peers
+by monitoring and triggering data messages. 
+
+
+## `realtimeChannel.leave()`
+
+Leave the realtime channel. 
+Afterwards the `realtimeChannel` is invalid and 
+can not be used anymore for sending or receiving data.
+You need to call `window.webxdc.joinRealtimeChannel()` again
+to re-join the per-app realtime channel. 
+
+## Example 
+
+```js
+const realtimeChannel = window.webxdc.joinRealtimeChannel();
+realtimeChannel.setListener((data) => {
+    console.log("Received realtime data: ", data);
+    const msg = new TextDecoder().decode(data);
+    console.log("decoded message: ", msg);
+})
+
+let numMsgs = 0
+const refreshIntervalId = setInterval(() => {
+    const myId = window.webxdc.selfAddr;
+    const data = new TextEncoder().encode(`[${numMsgs}] hello from ${myId}`);
+    numMsgs += 1
+    console.log("Sending message", data);
+    realtimeChannel.send(data);
+    if (numMsgs >= 100) {
+        realtimeChannel.leave();
+        clearInterval(refreshIntervalId);
+    }
+
+}, 1000)
+```