feat(HMR): apply changes in application styles at runtime

Expose `HmrContext` interface.
Apply changes in `app.css` instantly.
Avoid navigation on livesync when changes in `app.css` have been made.
Apply changes in `app.css` on back navigation.

tns-core-modules/ui/frame/frame.android.ts

@@ -17,6 +17,7 @@ import {
     _updateTransitions, _reverseTransitions, _clearEntry, _clearFragment, AnimationType
 } from "./fragment.transitions";
 
+import { loadCss } from "../styling/style-scope";
 import { profile } from "../../profiling";
 
 // TODO: Remove this and get it from global to decouple builder for angular
@@ -82,13 +83,24 @@ function getAttachListener(): android.view.View.OnAttachStateChangeListener {
     return attachStateChangeListener;
 }
 
-export function reloadPage(): void {
+export function reloadPage(context?: HmrContext): void {
     const activity = application.android.foregroundActivity;
     const callbacks: AndroidActivityCallbacks = activity[CALLBACKS];
     const rootView: View = callbacks.getRootView();
 
-    if (!rootView || !rootView._onLivesync()) {
-        callbacks.resetActivityContent(activity);
+    let executeLivesync = true;
+    // HMR has context, livesync does not
+    if (context) {
+        if (context.module === application.getCssFileName()) {
+            loadCss(context.module);
+            rootView._onCssStateChange();
+            executeLivesync = false;
+        }
+    }
+    if (executeLivesync) {
+        if (!rootView || !rootView._onLivesync()) {
+            callbacks.resetActivityContent(activity);
+        }
     }
 }
 
@@ -469,19 +481,19 @@ export class Frame extends FrameBase {
         switch (this.actionBarVisibility) {
             case "never":
                 return false;
-            
+
             case "always":
                 return true;
-            
+
             default:
                 if (page.actionBarHidden !== undefined) {
                     return !page.actionBarHidden;
                 }
-        
+
                 if (this._android && this._android.showActionBar !== undefined) {
                     return this._android.showActionBar;
                 }
-        
+
                 return true;
         }
     }
@@ -846,14 +858,14 @@ class FragmentCallbacksImplementation implements AndroidFragmentCallbacks {
         // parent while its supposed parent believes it properly removed its children; in order to "force" the child to 
         // lose its parent we temporarily add it to the parent, and then remove it (addViewInLayout doesn't trigger layout pass)
         const nativeView = page.nativeViewProtected;
-        if (nativeView != null) {	
-            const parentView = nativeView.getParent();	
+        if (nativeView != null) {
+            const parentView = nativeView.getParent();
             if (parentView instanceof android.view.ViewGroup) {
                 if (parentView.getChildCount() === 0) {
                     parentView.addViewInLayout(nativeView, -1, new org.nativescript.widgets.CommonLayoutParams());
                 }
 
-                parentView.removeView(nativeView);	
+                parentView.removeView(nativeView);
             }
         }
 

tns-core-modules/ui/page/page-common.ts

@@ -17,17 +17,17 @@ export class PageBase extends ContentView implements PageDefinition {
     public static navigatedToEvent = "navigatedTo";
     public static navigatingFromEvent = "navigatingFrom";
     public static navigatedFromEvent = "navigatedFrom";
-  
+
     private _navigationContext: any;
     private _actionBar: ActionBar;
 
     public _frame: Frame;
-    
+
     public actionBarHidden: boolean;
     public enableSwipeBackNavigation: boolean;
     public backgroundSpanUnderStatusBar: boolean;
     public hasActionBar: boolean;
-    
+
     get navigationContext(): any {
         return this._navigationContext;
     }
@@ -89,7 +89,7 @@ export class PageBase extends ContentView implements PageDefinition {
         const frame = this.parent;
         return frame instanceof Frame ? frame : undefined;
     }
-    
+
     private createNavigatedData(eventName: string, isBackNavigation: boolean): NavigatedData {
         return {
             eventName: eventName,
@@ -103,6 +103,10 @@ export class PageBase extends ContentView implements PageDefinition {
     public onNavigatingTo(context: any, isBackNavigation: boolean, bindingContext?: any) {
         this._navigationContext = context;
 
+        if (!this._cssState.isSelectorsLatestVersionApplied()) {
+            this._onCssStateChange();
+        }
+
         //https://github.com/NativeScript/NativeScript/issues/731
         if (!isBackNavigation && bindingContext !== undefined && bindingContext !== null) {
             this.bindingContext = bindingContext;

tns-core-modules/ui/styling/style-scope.d.ts

@@ -19,6 +19,11 @@ export class CssState {
      * Gets the static selectors that match the view and the dynamic selectors that may potentially match the view.
      */
     public changeMap: ChangeMap<ViewBase>;
+
+    /**
+     * Checks whether style scope and CSS state selectors are in sync.
+     */
+    public isSelectorsLatestVersionApplied(): boolean
 }
 
 export class StyleScope {
@@ -29,6 +34,9 @@ export class StyleScope {
     public static createSelectorsFromImports(tree: SyntaxTree, keyframes: Object): RuleSet[];
     public ensureSelectors(): number;
 
+    public isApplicationCssSelectorsLatestVersionApplied(): boolean;
+    public isLocalCssSelectorsLatestVersionApplied(): boolean;
+
     public applySelectors(view: ViewBase): void
     public query(options: Node): SelectorCore[];
 

tns-core-modules/ui/styling/style-scope.ts

@@ -271,7 +271,7 @@ export function removeTaggedAdditionalCSS(tag: String | Number): Boolean {
             changed = true;
         }
     }
-    if (changed) {  mergeCssSelectors(); }
+    if (changed) { mergeCssSelectors(); }
     return changed;
 }
 
@@ -307,7 +307,7 @@ function onLiveSync(args: applicationCommon.CssChangedEventData): void {
     loadCss(applicationCommon.getCssFileName());
 }
 
-const loadCss = profile(`"style-scope".loadCss`, (cssFile: string) => {
+export const loadCss = profile(`"style-scope".loadCss`, (cssFile: string) => {
     if (!cssFile) {
         return undefined;
     }
@@ -343,6 +343,7 @@ export class CssState {
     _appliedChangeMap: Readonly<ChangeMap<ViewBase>>;
     _appliedPropertyValues: Readonly<{}>;
     _appliedAnimations: ReadonlyArray<kam.KeyframeAnimation>;
+    _appliedSelectorsVersion: number;
 
     _match: SelectorsMatch<ViewBase>;
     _matchInvalid: boolean;
@@ -367,6 +368,15 @@ export class CssState {
         }
     }
 
+    public isSelectorsLatestVersionApplied(): boolean {
+        if (this._appliedSelectorsVersion && this.view._styleScope) {
+            this.view._styleScope.ensureSelectors();
+            return this.view._styleScope._getSelectorsVersion() === this._appliedSelectorsVersion;
+        } else {
+            return true;
+        }
+    }
+
     public onLoaded(): void {
         if (this._matchInvalid) {
             this.updateMatch();
@@ -381,6 +391,7 @@ export class CssState {
 
     @profile
     private updateMatch() {
+        this._appliedSelectorsVersion = this.view._styleScope._getSelectorsVersion();
         this._match = this.view._styleScope ? this.view._styleScope.matchSelectors(this.view) : CssState.emptyMatch;
         this._matchInvalid = false;
     }
@@ -597,8 +608,8 @@ export class StyleScope {
     }
 
     public ensureSelectors(): number {
-        if (this._applicationCssSelectorsAppliedVersion !== applicationCssSelectorVersion ||
-            this._localCssSelectorVersion !== this._localCssSelectorsAppliedVersion ||
+        if (!this.isApplicationCssSelectorsLatestVersionApplied() ||
+            !this.isLocalCssSelectorsLatestVersionApplied() ||
             !this._mergedCssSelectors) {
 
             this._createSelectors();
@@ -607,6 +618,14 @@ export class StyleScope {
         return this._getSelectorsVersion();
     }
 
+    public isApplicationCssSelectorsLatestVersionApplied(): boolean {
+        return this._applicationCssSelectorsAppliedVersion === applicationCssSelectorVersion;
+    }
+
+    public isLocalCssSelectorsLatestVersionApplied(): boolean {
+        return this._localCssSelectorsAppliedVersion === this._localCssSelectorVersion;
+    }
+
     @profile
     private _createSelectors() {
         let toMerge: RuleSet[][] = [];