ui: Small EventSource additions (#5978)

1. EventSources now pass themselves thorugh to the run function as a
second argument. This enables the use of arrow functions along with the
EventSources API
`(configuration, source) => source.close()`. Order of arguments could
potentially be switched at a later date.
2. BlockingEventSources now let you pass an 'event' through at
instantation time. If you do this, the event will immediately be dispatched
once the EventSource is opened. The usecase for this is for 'unfreezing' cached
BlockingEvents. This makes it easier to provide a cache for
BlockingEventSources by caching its data rather than the entire
BlockingEventSource itself.

```
new BlockingEventSource(
  (config, source) => { /* something */ },
  {
    cursor: 1024, // this would also come from a cache
    currentEvent: getFromSomeSortOfCache(eventSourceId) //this is the new bit
  }
);

// more realistically

new BlockingEventSource(
  (config, source) => { return data.findSomething(slug, config) },
  getFromSomeSortOfCache(eventSourceId)
);

```

ui-v2/app/utils/dom/event-source/blocking.js

@@ -70,10 +70,11 @@ export default function(EventSource, backoff = create5xxBackoff()) {
    */
   return class extends EventSource {
     constructor(source, configuration = {}) {
+      const { currentEvent, ...config } = configuration;
       super(configuration => {
         const { createEvent, ...superConfiguration } = configuration;
         return source
-          .apply(this, [superConfiguration])
+          .apply(this, [superConfiguration, this])
           .catch(backoff)
           .then(result => {
             if (result instanceof Error) {
@@ -103,7 +104,19 @@ export default function(EventSource, backoff = create5xxBackoff()) {
             this.previousEvent = this.currentEvent;
             return throttledResolve(result);
           });
-      }, configuration);
+      }, config);
+      if (typeof currentEvent !== 'undefined') {
+        this.currentEvent = currentEvent;
+      }
+      // only on initialization
+      // if we already have an currentEvent set via configuration
+      // dispatch the event so things are populated immediately
+      this.addEventListener('open', e => {
+        const currentEvent = e.target.getCurrentEvent();
+        if (typeof currentEvent !== 'undefined') {
+          this.dispatchEvent(currentEvent);
+        }
+      });
     }
     // if we are having these props, at least make getters
     getCurrentEvent() {

ui-v2/app/utils/dom/event-source/callable.js

@@ -5,7 +5,7 @@ export const defaultRunner = function(target, configuration, isClosed) {
   }
   // TODO Consider wrapping this is a promise for none thenable returns
   return target.source
-    .bind(target)(configuration)
+    .bind(target)(configuration, target)
     .then(function(res) {
       return defaultRunner(target, configuration, isClosed);
     });
@@ -36,7 +36,7 @@ export default function(
       this.readyState = 2;
       this.source =
         typeof source !== 'function'
-          ? function(configuration) {
+          ? function(configuration, target) {
               this.close();
               return P.resolve();
             }
@@ -45,7 +45,7 @@ export default function(
       P.resolve()
         .then(() => {
           // if we are already closed, don't do anything
-          if (this.readyState !== 0) {
+          if (this.readyState > 1) {
             return;
           }
           this.readyState = 1; // open
@@ -68,13 +68,14 @@ export default function(
     close() {
       // additional readyState 3 = CLOSING
       switch (this.readyState) {
-        case 0: // CONNECTING
         case 2: // CLOSED
-          this.readyState = 2; // CLOSED
+          // it's already CLOSED , do nothing
           break;
         default:
           this.readyState = 3; // CLOSING
       }
+      // non-standard
+      return this;
     }
   };
 }