Add resetOnSuccess option to retryBackoff operator

spec/retryBackoff-spec.ts

@@ -350,4 +350,52 @@ describe('retryBackoff operator', () => {
       expectSubscriptions(source.subscriptions).toBe(subs);
     });
   });
+
+  it('should reset the delay when resetOnSuccess is true', () => {
+    testScheduler.run(({ expectObservable, cold, expectSubscriptions }) => {
+      const source = cold('--1-2-3-#');
+      const subs = [
+        '                  ^-------!',
+        '                  ---------^-------!',
+        '                  ------------------^-------!',
+        '                  ---------------------------^-------!'
+        //                 interval always reset to 1 ^
+      ];
+      const unsub = '      -----------------------------------!';
+      const expected = '   --1-2-3----1-2-3----1-2-3----1-2-3--';
+
+      expectObservable(
+        source.pipe(
+          retryBackoff({
+            initialInterval: 1,
+            resetOnSuccess: true
+          })
+        ),
+        unsub
+      ).toBe(expected);
+      expectSubscriptions(source.subscriptions).toBe(subs);
+    });
+  });
+
+  it('should not reset the delay on consecutive errors when resetOnSuccess is true', () => {
+    testScheduler.run(({ expectObservable, cold, expectSubscriptions }) => {
+      const source = cold('--------#');
+      const unsub = '      -------------------------------------!';
+      const subs = [
+        '                  ^-------!                             ',
+        '                  ---------^-------!                    ',
+        '                  -------------------^-------!          ',
+        '                  -------------------------------^-----!'
+      ];
+      const expected = '   --------------------------------------';
+
+      const result = source.pipe(retryBackoff({
+        initialInterval: 1,
+        resetOnSuccess: true
+      }));
+
+      expectObservable(result, unsub).toBe(expected);
+      expectSubscriptions(source.subscriptions).toBe(subs);
+    });
+  });
 });

src/operators/retryBackoff.ts

@@ -1,7 +1,7 @@
 import { iif, Observable, throwError, timer } from 'rxjs';
-import { concatMap, retryWhen } from 'rxjs/operators';
+import { concatMap, retryWhen, tap } from 'rxjs/operators';
+import { exponentialBackoffDelay, getDelay } from '../utils';
 
-import { getDelay, exponentialBackoffDelay } from '../utils';
 
 export interface RetryBackoffConfig {
   // Initial interval. It will eventually go as high as maxInterval.
@@ -10,6 +10,9 @@ export interface RetryBackoffConfig {
   maxRetries?: number;
   // Maximum delay between retries.
   maxInterval?: number;
+  // When set to `true` every successful emission will reset the delay and the 
+  // error count.
+  resetOnSuccess?: boolean;
   // Conditional retry.
   shouldRetry?: (error: any) => boolean;
   backoffDelay?: (iteration: number, initialInterval: number) => number;
@@ -31,20 +34,29 @@ export function retryBackoff(
     maxRetries = Infinity,
     maxInterval = Infinity,
     shouldRetry = () => true,
+    resetOnSuccess = false,
     backoffDelay = exponentialBackoffDelay
   } = typeof config === 'number' ? { initialInterval: config } : config;
-  return <T>(source: Observable<T>) =>
-    source.pipe(
+  return <T>(source: Observable<T>) => {
+    let index = 0;
+    return source.pipe(
       retryWhen<T>(errors =>
         errors.pipe(
-          concatMap((error, i) =>
-            iif(
-              () => i < maxRetries && shouldRetry(error),
-              timer(getDelay(backoffDelay(i, initialInterval), maxInterval)),
+          concatMap(error => {
+            const attempt = index++;
+            return iif(
+              () => attempt < maxRetries && shouldRetry(error),
+              timer(getDelay(backoffDelay(attempt, initialInterval), maxInterval)),
               throwError(error)
             )
-          )
+          })
         )
-      )
+      ),
+      tap((_: T) => {
+        if (resetOnSuccess) {
+          index = 0;
+        }
+      })
     );
+  }
 }