build(broccoli): allow rebuild() to return DiffResult

Plugins may opt to return a DiffResult themselves, and avoid the
need to calculate a diff

Closes #2514

tools/broccoli/diffing-broccoli-plugin.ts

@@ -28,7 +28,7 @@ export function wrapDiffingPlugin(pluginClass): DiffingPluginWrapperFactory {
 
 
 export interface DiffingBroccoliPlugin {
-  rebuild(diff: (DiffResult | DiffResult[])): (Promise<any>| void);
+  rebuild(diff: (DiffResult | DiffResult[])): (Promise<DiffResult | void>| DiffResult | void);
   cleanup ? () : void;
 }
 
@@ -52,6 +52,8 @@ class DiffingPluginWrapper implements BroccoliTree {
   cachePath = null;
   outputPath = null;
 
+  private diffResult: DiffResult = null;
+
   constructor(private pluginClass, private wrappedPluginArguments) {
     if (Array.isArray(wrappedPluginArguments[0])) {
       this.inputTrees = this.stabilizeTrees(wrappedPluginArguments[0]);
@@ -62,33 +64,59 @@ class DiffingPluginWrapper implements BroccoliTree {
     this.description = this.pluginClass.name;
   }
 
-  private calculateDiff(firstRun: boolean): (DiffResult | DiffResult[]) {
-    // TODO(caitp): optionally log trees based on environment variable or
-    // command line option. It may be worth logging for trees where elapsed
-    // time exceeds some threshold, like 10ms.
-    if (this.treeDiffer) {
-      return this.treeDiffer.diffTree();
-    } else if (this.treeDiffers) {
-      return this.treeDiffers.map((treeDiffer) => treeDiffer.diffTree());
+  private getDiffResult(): (DiffResult | DiffResult[]) {
+    let returnOrCalculateDiffResult = (tree, index) => {
+      // returnOrCalculateDiffResult will do one of two things:
+      //
+      // If `this.diffResult` is null, calculate a DiffResult using TreeDiffer
+      // for the input tree.
+      //
+      // Otherwise, `this.diffResult` was produced from the output of the
+      // inputTree's rebuild() method, and can be used without being checked.
+      // Set `this.diffResult` to null and return the previously stored value.
+      if (!tree.diffResult) {
+        let differ = index === false ? this.treeDiffer : this.treeDiffers[index];
+        return differ.diffTree();
+      }
+      let diffResult = tree.diffResult;
+      tree.diffResult = null;
+      return diffResult;
+    };
+
+    if (this.inputTrees) {
+      return this.inputTrees.map(returnOrCalculateDiffResult);
+    } else if (this.inputTree) {
+      return returnOrCalculateDiffResult(this.inputTree, false);
     } else {
       throw new Error("Missing TreeDiffer");
     }
   }
 
+  private maybeStoreDiffResult(value: (DiffResult | void)) {
+    this.diffResult = value ? <DiffResult>(value) : null;
+  }
 
   rebuild() {
     try {
       let firstRun = !this.initialized;
       this.init();
 
-      let diffResult = this.calculateDiff(firstRun);
+      let diffResult = this.getDiffResult();
 
-      var rebuildPromise = this.wrappedPlugin.rebuild(diffResult);
+      let result = this.wrappedPlugin.rebuild(diffResult);
 
-      if (rebuildPromise) {
-        return (<Promise<any>>rebuildPromise).then(this.relinkOutputAndCachePaths.bind(this));
+      if (result) {
+        let resultPromise = <Promise<DiffResult | void>>(result);
+        if (resultPromise.then) {
+          // rebuild() -> Promise<>
+          return resultPromise.then((result: (DiffResult | void)) => {
+            this.maybeStoreDiffResult(result);
+            this.relinkOutputAndCachePaths();
+          });
+        }
       }
 
+      this.maybeStoreDiffResult(<(DiffResult | void)>(result));
       this.relinkOutputAndCachePaths();
     } catch (e) {
       e.message = `[${this.description}]: ${e.message}`;

tools/broccoli/tree-differ.ts

@@ -121,25 +121,28 @@ export class TreeDiffer {
 }
 
 
-export interface DiffResult {
-  addedPaths: string[];
-  changedPaths: string[];
-  removedPaths: string[];
-  log(verbose: boolean): void;
-  toString(): string;
-}
-
-
-class DirtyCheckingDiffResult {
-  public filesChecked: number = 0;
-  public directoriesChecked: number = 0;
+export class DiffResult {
   public addedPaths: string[] = [];
   public changedPaths: string[] = [];
   public removedPaths: string[] = [];
+
+  constructor(public label: string = '') {}
+
+  log(verbose: boolean): void {}
+
+  toString(): string {
+    // TODO(@caitp): more meaningful logging
+    return '';
+  }
+}
+
+class DirtyCheckingDiffResult extends DiffResult {
+  public filesChecked: number = 0;
+  public directoriesChecked: number = 0;
   public startTime: number = Date.now();
   public endTime: number = null;
 
-  constructor(public label: string, public directoryName: string) {}
+  constructor(label: string, public directoryName: string) { super(label); }
 
   toString() {
     return `${pad(this.label, 30)}, ${pad(this.endTime - this.startTime, 5)}ms, ` +