@uppy/transloadit: implement Server-sent event API

packages/@uppy/transloadit/src/Assembly.js

@@ -36,6 +36,8 @@ class TransloaditAssembly extends Emitter {
 
   #previousFetchStatusStillPending = false
 
+  #sse
+
   constructor (assembly, rateLimitedQueue) {
     super()
 
@@ -53,6 +55,7 @@ class TransloaditAssembly extends Emitter {
   }
 
   connect () {
+    this.#connectServerSentEvents()
     this.#connectSocket()
     this.#beginPolling()
   }
@@ -62,6 +65,64 @@ class TransloaditAssembly extends Emitter {
     this.close()
   }
 
+  #connectServerSentEvents () {
+    this.#sse = new EventSource(`${this.status.websocket_url}?assembly=${this.status.assembly_id}`)
+
+    this.#sse.addEventListener('open', () => {
+      // if server side events works, we don't need websockets anymore (it's just a fallback)
+      if (this.socket) {
+        this.socket.disconnect()
+        this.socket = null
+      }
+      clearInterval(this.pollInterval)
+      this.pollInterval = null
+    })
+
+    /*
+     * The event "message" is a special case, as it
+     * will capture events without an event field
+     * as well as events that have the specific type
+     * other event type.
+     */
+    this.#sse.addEventListener('message', (e) => {
+      if (e.data === 'assembly_finished') {
+        this.#onFinished()
+      }
+
+      if (e.data === 'assembly_uploading_finished') {
+        this.emit('executing')
+      }
+
+      if (e.data === 'assembly_upload_meta_data_extracted') {
+        this.emit('metadata')
+        this.#fetchStatus({ diff: false })
+      }
+    })
+
+    this.#sse.addEventListener('assembly_upload_finished', (e) => {
+      const file = e.data
+      this.emit('upload', file)
+      this.status.uploads.push(file)
+    })
+
+    this.#sse.addEventListener('assembly_result_finished', (e) => {
+      const [stepName, result] = JSON.parse(e.data)
+      this.emit('result', stepName, result)
+      ;(this.status.results[stepName] ??= []).push(result)
+    })
+
+    this.#sse.addEventListener('assembly_error', (e) => {
+      try {
+        const error = JSON.parse(e.data)
+        this.#onError(new Error(error.message, { error }))
+      } catch {
+        this.#onError(new Error(e.data))
+      }
+      // Refetch for updated status code
+      this.#fetchStatus({ diff: false })
+    })
+  }
+
   #connectSocket () {
     const parsed = parseUrl(this.status.websocket_url)
     const socket = io(parsed.origin, {
@@ -102,10 +163,7 @@ class TransloaditAssembly extends Emitter {
 
     socket.on('assembly_result_finished', (stepName, result) => {
       this.emit('result', stepName, result)
-      if (!this.status.results[stepName]) {
-        this.status.results[stepName] = []
-      }
-      this.status.results[stepName].push(result)
+      ;(this.status.results[stepName] ??= []).push(result)
     })
 
     socket.on('assembly_error', (status) => {
@@ -262,6 +320,10 @@ class TransloaditAssembly extends Emitter {
    */
   close () {
     this.closed = true
+    if (this.#sse) {
+      this.#sse.close()
+      this.#sse = null
+    }
     if (this.socket) {
       this.socket.disconnect()
       this.socket = null