add dispatcher class

Author: lilianammmatos

src/type/dispatcher.js

@@ -0,0 +1,106 @@
+// @flow strict
+
+import { $$asyncIterator } from 'iterall';
+
+import { type ObjMap } from '../jsutils/ObjMap';
+import { type Path, pathToArray } from '../jsutils/Path';
+import { type PromiseOrValue } from '../jsutils/PromiseOrValue';
+import isPromise from '../jsutils/isPromise';
+
+import { GraphQLError } from '../error/GraphQLError';
+
+export class Dispatcher {
+  _patches: Array<Promise<{| value: ExecutionPatchResult, done: boolean |}>>;
+
+  constructor() {
+    this._patches = [];
+  }
+
+  execute(
+    fn: () => PromiseOrValue<ObjMap<mixed>>,
+    errors: Array<GraphQLError>,
+  ) {
+    try {
+      return fn();
+    } catch (error) {
+      errors.push(error);
+      return null;
+    }
+  }
+
+  add(
+    label: string,
+    path: Path | void,
+    fn: () => PromiseOrValue<ObjMap<mixed>>,
+    errors: Array<GraphQLError>,
+  ) {
+    this._patches.push(
+      Promise.resolve(this.execute(fn, errors)).then(data => {
+        if (isPromise(data)) {
+          return data.then(undefined, error => ({
+            value: {
+              data: null,
+              path: pathToArray(path),
+              label,
+              errors: [error],
+            },
+            done: false,
+          }));
+        }
+        return {
+          value: {
+            data,
+            path: pathToArray(path),
+            label,
+            ...(errors && errors.length > 0 ? { errors } : {}),
+          },
+          done: false,
+        };
+      }),
+    );
+  }
+
+  get(): AsyncIterable<ExecutionPatchResult> | null {
+    if (this._patches.length === 0) {
+      return null;
+    }
+    const results = this._patches;
+
+    function race(promises) {
+      return new Promise(resolve => {
+        promises.forEach((promise, index) => {
+          promise.then(result => {
+            resolve({ result, index });
+          });
+        });
+      });
+    }
+
+    const getNext = promises => {
+      if (promises.length === 0) {
+        return Promise.resolve({ value: undefined, done: true });
+      }
+      return race(promises).then(({ result, index }) => {
+        promises.splice(index, 1);
+        return result;
+      });
+    };
+
+    return ({
+      next() {
+        return getNext(results);
+      },
+      [$$asyncIterator]() {
+        return this;
+      },
+    }: any);
+  }
+}
+
+export type ExecutionPatchResult = {
+  errors?: $ReadOnlyArray<GraphQLError>,
+  data?: ObjMap<mixed> | null,
+  path: $ReadOnlyArray<string | number>,
+  label: string,
+  ...
+};