Deterministic updates

High priority updates typically require less work to render than
low priority ones. It's beneficial to flush those first, in their own
batch, before working on more expensive low priority ones. We do this
even if a high priority is scheduled after a low priority one.

However, we don't want this reordering of updates to affect the terminal
state. State should be deterministic: once all work has been flushed,
the final state should be the same regardless of how they were
scheduled.

To get both properties, we store updates on the queue in insertion
order instead of priority order (always append). Then, when processing
the queue, we skip over updates with insufficient priority. Instead of
removing updates from the queue right after processing them, we only
remove them if there are no unprocessed updates before it in the list.

This means that updates may be processed more than once.

As a bonus, the new implementation is simpler and requires less code.

package.json

@@ -60,6 +60,7 @@
     "glob-stream": "^6.1.0",
     "gzip-js": "~0.3.2",
     "gzip-size": "^3.0.0",
+    "jasmine-check": "^1.0.0-rc.0",
     "jest": "20.1.0-delta.1",
     "jest-config": "20.1.0-delta.1",
     "jest-jasmine2": "20.1.0-delta.1",

scripts/jest/test-framework-setup.js

@@ -68,4 +68,6 @@ if (process.env.REACT_CLASS_EQUIVALENCE_TEST) {
     return expectation;
   };
   global.expectDev = expectDev;
+
+  require('jasmine-check').install();
 }

src/renderers/dom/fiber/__tests__/ReactDOMFiberAsync-test.js

@@ -284,8 +284,8 @@ describe('ReactDOMFiberAsync', () => {
 
       // Flush the async updates
       jest.runAllTimers();
-      expect(container.textContent).toEqual('BCAD');
-      expect(ops).toEqual(['BC', 'BCAD']);
+      expect(container.textContent).toEqual('ABCD');
+      expect(ops).toEqual(['BC', 'ABCD']);
     });
   });
 });

src/renderers/noop/ReactNoopEntry.js

@@ -296,9 +296,18 @@ var ReactNoop = {
     const root = NoopRenderer.createContainer(container);
     roots.set(rootID, root);
     return {
+      render(children: any) {
+        const work = NoopRenderer.updateRoot(children, root, null);
+        work.then(() => work.commit());
+      },
       prerender(children: any) {
         return NoopRenderer.updateRoot(children, root, null);
       },
+      unmount() {
+        roots.delete(rootID);
+        const work = NoopRenderer.updateRoot(null, root, null);
+        work.then(() => work.commit());
+      },
       getChildren() {
         return ReactNoop.getChildren(rootID);
       },

src/renderers/shared/fiber/ReactFiberBeginWork.js

@@ -339,7 +339,6 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
         workInProgress,
         updateQueue,
         null,
-        prevState,
         null,
         renderExpirationTime,
       );
@@ -349,7 +348,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
         resetHydrationState();
         return bailoutOnAlreadyFinishedWork(current, workInProgress);
       }
-      const element = state.element;
+      const element = state !== null ? state.element : null;
       if (
         root.hydrate &&
         (current === null || current.child === null) &&

src/renderers/shared/fiber/ReactFiberClassComponent.js

@@ -398,7 +398,7 @@ module.exports = function(
     const unmaskedContext = getUnmaskedContext(workInProgress);
 
     instance.props = props;
-    instance.state = state;
+    instance.state = workInProgress.memoizedState = state;
     instance.refs = emptyObject;
     instance.context = getMaskedContext(workInProgress, unmaskedContext);
 
@@ -422,7 +422,6 @@ module.exports = function(
           workInProgress,
           updateQueue,
           instance,
-          state,
           props,
           renderExpirationTime,
         );
@@ -589,7 +588,6 @@ module.exports = function(
         workInProgress,
         workInProgress.updateQueue,
         instance,
-        oldState,
         newProps,
         renderExpirationTime,
       );

src/renderers/shared/fiber/ReactFiberCommitWork.js

@@ -29,12 +29,7 @@ var {
   clearCaughtError,
 } = require('ReactErrorUtils');
 
-var {
-  Placement,
-  Update,
-  Callback,
-  ContentReset,
-} = require('ReactTypeOfSideEffect');
+var {Placement, Update, ContentReset} = require('ReactTypeOfSideEffect');
 
 var invariant = require('fbjs/lib/invariant');
 
@@ -487,16 +482,26 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
     }
   }
 
-  function commitCallbacks(callbackList, context) {
-    for (let i = 0; i < callbackList.length; i++) {
-      const callback = callbackList[i];
+  function commitCallbacks(updateQueue, context) {
+    let callbackNode = updateQueue.firstCallback;
+    // Reset the callback list before calling them in case something throws.
+    updateQueue.firstCallback = updateQueue.lastCallback = null;
+
+    while (callbackNode !== null) {
+      const callback = callbackNode.callback;
+      // Remove this callback from the update object in case it's still part
+      // of the queue, so that we don't call it again.
+      callbackNode.callback = null;
       invariant(
         typeof callback === 'function',
         'Invalid argument passed as callback. Expected a function. Instead ' +
           'received: %s',
         callback,
       );
       callback.call(context);
+      const nextCallback = callbackNode.nextCallback;
+      callbackNode.nextCallback = null;
+      callbackNode = nextCallback;
     }
   }
 
@@ -529,31 +534,19 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
             }
           }
         }
-        if (
-          finishedWork.effectTag & Callback &&
-          finishedWork.updateQueue !== null
-        ) {
-          const updateQueue = finishedWork.updateQueue;
-          if (updateQueue.callbackList !== null) {
-            // Set the list to null to make sure they don't get called more than once.
-            const callbackList = updateQueue.callbackList;
-            updateQueue.callbackList = null;
-            commitCallbacks(callbackList, instance);
-          }
+        const updateQueue = finishedWork.updateQueue;
+        if (updateQueue !== null) {
+          commitCallbacks(updateQueue, instance);
         }
         return;
       }
       case HostRoot: {
         const updateQueue = finishedWork.updateQueue;
-        if (updateQueue !== null && updateQueue.callbackList !== null) {
-          // Set the list to null to make sure they don't get called more
-          // than once.
-          const callbackList = updateQueue.callbackList;
-          updateQueue.callbackList = null;
+        if (updateQueue !== null) {
           const instance = finishedWork.child !== null
             ? finishedWork.child.stateNode
             : null;
-          commitCallbacks(callbackList, instance);
+          commitCallbacks(updateQueue, instance);
         }
         return;
       }

src/renderers/shared/fiber/ReactFiberReconciler.js

@@ -286,43 +286,22 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
         callback,
       );
     }
-    const isTopLevelUnmount = nextState.element === null;
     const update = {
       priorityLevel,
       expirationTime,
       partialState: nextState,
       callback,
       isReplace: false,
       isForced: false,
-      isTopLevelUnmount,
+      nextCallback: null,
       next: null,
     };
-    const update2 = insertUpdateIntoFiber(current, update, currentTime);
-
-    if (isTopLevelUnmount) {
-      // TODO: Redesign the top-level mount/update/unmount API to avoid this
-      // special case.
-      const queue1 = current.updateQueue;
-      const queue2 = current.alternate !== null
-        ? current.alternate.updateQueue
-        : null;
-
-      // Drop all updates that are lower-priority, so that the tree is not
-      // remounted. We need to do this for both queues.
-      if (queue1 !== null && update.next !== null) {
-        update.next = null;
-        queue1.last = update;
-      }
-      if (queue2 !== null && update2 !== null && update2.next !== null) {
-        update2.next = null;
-        queue2.last = update;
-      }
-    }
+    insertUpdateIntoFiber(current, update, currentTime);
 
     if (isPrerender) {
       // Block the root from committing at this expiration time.
       if (root.topLevelBlockers === null) {
-        root.topLevelBlockers = createUpdateQueue();
+        root.topLevelBlockers = createUpdateQueue(null);
       }
       const block = {
         priorityLevel: null,
@@ -331,7 +310,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
         callback: null,
         isReplace: false,
         isForced: false,
-        isTopLevelUnmount: false,
+        nextCallback: null,
         next: null,
       };
       insertUpdateIntoQueue(root.topLevelBlockers, block, currentTime);
@@ -352,7 +331,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
     if (topLevelBlockers === null) {
       return;
     }
-    processUpdateQueue(topLevelBlockers, null, null, null, expirationTime);
+    processUpdateQueue(topLevelBlockers, null, null, expirationTime);
     expireWork(root, expirationTime);
   };
   WorkNode.prototype.then = function(callback) {
@@ -403,7 +382,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
 
       let completionCallbacks = container.completionCallbacks;
       if (completionCallbacks === null) {
-        completionCallbacks = createUpdateQueue();
+        completionCallbacks = createUpdateQueue(null);
       }
 
       return new WorkNode(container, expirationTime);

src/renderers/shared/fiber/ReactFiberScheduler.js

@@ -386,21 +386,23 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
     // the end of the current batch.
     const completionCallbacks = root.completionCallbacks;
     if (completionCallbacks !== null) {
-      processUpdateQueue(completionCallbacks, null, null, null, completedAt);
-      const callbackList = completionCallbacks.callbackList;
-      if (callbackList !== null) {
-        // Add new callbacks to list of completion callbacks
+      processUpdateQueue(completionCallbacks, null, null, completedAt);
+      // Add new callbacks to list of completion callbacks
+      let callbackNode = completionCallbacks.firstCallback;
+      completionCallbacks.firstCallback = completionCallbacks.lastCallback = null;
+      while (callbackNode !== null) {
+        const callback: () => mixed = (callbackNode.callback: any);
+        // Remove this callback from the update object in case it's still part
+        // of the queue, so that we don't call it again.
+        callbackNode.callback = null;
         if (rootCompletionCallbackList === null) {
-          rootCompletionCallbackList = callbackList;
+          rootCompletionCallbackList = [callback];
         } else {
-          for (let i = 0; i < callbackList.length; i++) {
-            rootCompletionCallbackList.push(callbackList[i]);
-          }
-        }
-        completionCallbacks.callbackList = null;
-        if (completionCallbacks.first === null) {
-          root.completionCallbacks = null;
+          rootCompletionCallbackList.push(callback);
         }
+        const nextCallback = callbackNode.nextCallback;
+        callbackNode.nextCallback = null;
+        callbackNode = nextCallback;
       }
     }
   }
@@ -1636,12 +1638,12 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
       callback,
       isReplace: false,
       isForced: false,
-      isTopLevelUnmount: false,
+      nextCallback: null,
       next: null,
     };
     const currentTime = recalculateCurrentTime();
     if (root.completionCallbacks === null) {
-      root.completionCallbacks = createUpdateQueue();
+      root.completionCallbacks = createUpdateQueue(null);
     }
     insertUpdateIntoQueue(root.completionCallbacks, update, currentTime);
     if (expirationTime === root.completedAt) {