From b6c638abbd2e60b6e0c68405567ed27346417850 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Nicol=C3=B2=20Ribaudo?= <nicolo.ribaudo@gmail.com>
Date: Sun, 6 Oct 2019 13:17:39 +0200
Subject: [PATCH] Add onFirstPause utility method

---
 README.md     |  5 +++++
 index.js      | 18 +++++++++++++++++-
 index.js.flow |  1 +
 3 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/README.md b/README.md
index f68ce1a..1f75c2c 100644
--- a/README.md
+++ b/README.md
@@ -194,3 +194,8 @@ const loadFiles = gensync(function* () {
   ]);
 });
 ```
+
+### gensync.onFirstPause(generator, callback)
+
+This method returns a gensync generator which calls `callback` when `generator`
+yields an async operation.
diff --git a/index.js b/index.js
index 399bc11..5fbcf3d 100644
--- a/index.js
+++ b/index.js
@@ -71,6 +71,16 @@ module.exports = Object.assign(
         }
       },
     }),
+    onFirstPause: buildOperation({
+      name: "onFirstPause",
+      arity: 2,
+      sync: function([item]) {
+        return evaluateSync(item);
+      },
+      async: function([item, cb], resolve, reject) {
+        evaluateAsync(item, resolve, reject, cb);
+      },
+    }),
   }
 );
 
@@ -247,7 +257,8 @@ function evaluateSync(gen) {
   return value;
 }
 
-function evaluateAsync(gen, resolve, reject) {
+function evaluateAsync(gen, resolve, reject, onFirstPause) {
+  let didFirstPause = false;
   (function step() {
     try {
       let value;
@@ -271,6 +282,11 @@ function evaluateAsync(gen, resolve, reject) {
         assertSuspend(out, gen);
 
         if (!didSyncResume) {
+          if (!didFirstPause && onFirstPause) {
+            didFirstPause = true;
+            onFirstPause();
+          } 
+
           // Callback wasn't called synchronously, so break out of the loop
           // and let it call 'step' later.
           return;
diff --git a/index.js.flow b/index.js.flow
index fa22e0b..9c124f8 100644
--- a/index.js.flow
+++ b/index.js.flow
@@ -29,4 +29,5 @@ declare module.exports: {
 
   all<Return>(Array<Handler<Return>>): Handler<Return[]>,
   race<Return>(Array<Handler<Return>>): Handler<Return>,
+  onFirstPause<Return>(Handler<Return>, cb: Function): Handler<Return>,
 };