re-use live mode config - and improve docs on how to start liveMode (#1003)

* Use config.liveMode to turn on liveMode in the timer

* Discussion of `startLive(1500)` was incorrect as startLive is based on timestamps, not playback offsets.  Also reorder to introduce `addEvent` earlier

Co-authored-by: Eoghan Murray <[email protected]>

Author: Juice10

docs/recipes/live-mode.md

@@ -5,23 +5,28 @@ If you want to replay the events in a real-time way, you can use the live mode A
 When you are using rrweb's Replayer to do a real-time replay, you need to configure `liveMode: true` and call the `startLive` API to enable the live mode.
 
 ```js
-const replayer = new rrweb.Replayer([], {
+const ANY_OLD_EVENTS = [];
+const replayer = new rrweb.Replayer(ANY_OLD_EVENTS, {
   liveMode: true,
 });
-
-replayer.startLive(FIRST_EVENT.timestamp - BUFFER);
+replayer.startLive();
 ```
 
-When calling the `startLive` API, there is an optional parameter to set the baseline time. This is quite useful when you live scenario needs a buffer time.
-
-For example, you have an event recorded at timestamp 1500. Calling `startLive(1500)` will set the baseline time to 1500 and all the timing calculation will be based on this.
+Later when you receive new events (e.g. over websockets), you can add them using:
 
-But this may cause your replay to look laggy. Because data transportation needs time(such as the delay of the network). And some events have been throttled(such as mouse movements) which has a delay by default.
+```
+function onReceive(event) {
+  replayer.addEvent(event);
+}
+```
 
-So we can configure a smaller baseline time to the `startLive` API, like `startLive(500)`. This will let the replay always delay 1 second than the source. If the time of data transportation is not longer than 1 second, the user will not feel laggy.
+When calling the `startLive` API, there is an optional parameter to set the baseline time. By default, this is `Date.now()` so that events are applied as soon as they come in, however this may cause your replay to look laggy. Because data transportation needs time(such as the delay of the network). And some events have been throttled(such as mouse movements) which has a delay by default.
 
-When live mode is on, we can call `addEvent` API to add the latest events into the replayer:
+Here is how you introduce a buffer:
 
 ```js
-replayer.addEvent(NEW_EVENT);
+const BUFFER_MS = 1000;
+replayer.startLive(Date.now() - BUFFER_MS);
 ```
+
+This will let the replay always delay 1 second than the source. If the time of data transportation is not longer than 1 second, the user will not feel laggy.

packages/rrweb/src/replay/index.ts

@@ -262,7 +262,10 @@ export class Replayer {
       this.mirror.reset();
     });
 
-    const timer = new Timer([], config?.speed || defaultConfig.speed);
+    const timer = new Timer([], {
+      speed: this.config.speed,
+      liveMode: this.config.liveMode,
+    });
     this.service = createPlayerService(
       {
         events: events

packages/rrweb/src/replay/timer.ts

@@ -13,9 +13,16 @@ export class Timer {
   private raf: number | null = null;
   private liveMode: boolean;
 
-  constructor(actions: actionWithDelay[] = [], speed: number) {
+  constructor(
+    actions: actionWithDelay[] = [],
+    config: {
+      speed: number;
+      liveMode: boolean;
+    },
+  ) {
     this.actions = actions;
-    this.speed = speed;
+    this.speed = config.speed;
+    this.liveMode = config.liveMode;
   }
   /**
    * Add an action after the timer starts.