support concurrent prepasses (#37)

* support concurrent prepasses

* remove unused import

* reset hook state before initialization

Author: JoviDeCroock

src/__tests__/concurrency.test.js

@@ -0,0 +1,91 @@
+import React, {
+  createContext,
+  useContext,
+  useState,
+  useMemo,
+  useRef,
+  useCallback
+} from 'react'
+import renderPrepass from '..'
+
+const CONCURRENCY = 2
+
+const Context = createContext({ test: 3, promise: null, resolved: false })
+
+function makeApp() {
+  return <App />
+}
+
+function App() {
+  const [state, setState] = useState(() => ({
+    test: Math.random(),
+    promise: null,
+    resolved: false
+  }))
+  const refresh = () =>
+    setState({ test: Math.random(), promise: null, resolved: false })
+
+  return (
+    <Context.Provider value={{ ...state, refresh }}>
+      <Outer />
+    </Context.Provider>
+  )
+}
+
+function Outer() {
+  useRef({
+    test: 1
+  })
+
+  const [, refresh] = useSuspenseHook()
+
+  useMemo(() => {
+    return { a: 1, b: 2 }
+  }, [])
+
+  return (
+    <div>
+      <button onClick={refresh}>Refresh</button>
+      <Inner />
+    </div>
+  )
+}
+
+function useSuspenseHook() {
+  const context = useContext(Context)
+
+  useRef({
+    test: 1
+  })
+
+  if (!context.resolved && !context.promise) {
+    context.promise = new Promise(resolve =>
+      setTimeout(resolve, Math.floor(30 + Math.random() * 50))
+    ).then(() => {
+      context.resolved = true
+      context.promise = null
+    })
+  }
+
+  if (context.promise) throw context.promise
+
+  return [true, context.refresh]
+}
+
+function Inner() {
+  const [state] = useState({ a: 3 })
+
+  useCallback(() => {
+    return state
+  }, [state])
+
+  return <h4>Inner</h4>
+}
+
+test('concurrency', () => {
+  return expect(
+    Promise.all(
+      new Array(CONCURRENCY).fill(0).map(() => renderPrepass(makeApp()))
+    )
+  ).resolves.not.toThrow()
+})

src/internals/dispatcher.js

@@ -113,6 +113,7 @@ export function renderWithHooks(
   props: any,
   refOrContext: any
 ): any {
+  workInProgressHook = null
   let children = Component(props, refOrContext)
 
   // NOTE: Excessive rerenders won't throw but will instead abort rendering