feat(virtual-repeat): allow use of .call on virtual-repeat-next

Allow ‘virtual-repeat-next' to evaluate expressions via `.call` so that
the `getMore` function can be called from a scope different from the
`virtual-repeat` it’s contained in. Access context parameters via
`$scrollContext`, e.g. `virtual-repeat-next.call='getMore($scrollContext)'`.

Author: AStoker

README.md

@@ -94,7 +94,6 @@ If you are running the plugin in the `skeleton-naviagion` project, make sure to
   </div>
 </template>
 ```  
-
 ```javascript
 export class MyVirtualList {
     items = ['Foo', 'Bar', 'Baz'];
@@ -105,6 +104,26 @@ export class MyVirtualList {
     }
 }
 ```  
+
+Or to use an expression, use `.call` as shown below.
+```html
+<template>
+  <div virtual-repeat.for="item of items" virtual-repeat-next.call="getMore($scrollContext)">
+    ${$index} ${item}
+  </div>
+</template>
+```  
+```javascript
+export class MyVirtualList {
+    items = ['Foo', 'Bar', 'Baz'];
+    getMore(scrollContext) {
+        for(let i = 0; i < 100; ++i) {
+            this.items.push('item' + i);
+        }
+    }
+}
+```  
+
 The `virtual-repeat-next` attribute can accept a function, a promise, or a function that returns a promise.  
 The bound function will be called when the scroll container has reached a point where there are no more items to move into the DOM (i.e. when it reaches the end of a list, either from the top or the bottom).  
 There are three parameters that are passed to the function (`getMore(topIndex, isAtBottom, isAtTop)`) which helps determine the behavior or amount of items to get during scrolling.    

src/virtual-repeat.js

@@ -271,26 +271,43 @@ export class VirtualRepeat extends AbstractRepeater {
   _getMore(): void {
     if (this.isLastIndex || this._first === 0) {
       if (!this._calledGetMore) {
-        let getMoreFunc = this.view(0).firstChild.getAttribute('virtual-repeat-next');
-        if (!getMoreFunc) {
-          return;
-        }
-        let getMore = this.scope.overrideContext.bindingContext[getMoreFunc];
         let executeGetMore = () => {
           this._calledGetMore = true;
-          if (getMore instanceof Promise) {
-            return getMore.then(() => {
-              this._calledGetMore = false; //Reset for the next time
-            });
-          } else if (typeof getMore === 'function') {
-            let result = getMore.bind(this.scope.overrideContext.bindingContext)(this._first, this._bottomBufferHeight === 0, this._isAtTop);
-            if (!(result instanceof Promise)) {
-              this._calledGetMore = false; //Reset for the next time
-            } else {
-              return result.then(() => {
+          let func = (this.view(0) && this.view(0).firstChild.au) ? this.view(0).firstChild.au['virtual-repeat-next'].instruction.attributes['virtual-repeat-next'] : undefined;
+          let topIndex = this._first;
+          let isAtBottom = this._bottomBufferHeight === 0;
+          let isAtTop = this._isAtTop;
+          let scrollContext = {
+            topIndex: topIndex,
+            isAtBottom: isAtBottom,
+            isAtTop: isAtTop
+          };
+
+          this.scope.overrideContext.$scrollContext = scrollContext;
+
+          if (func === undefined) {
+            return null;
+          } else if (typeof func === 'string') {
+            let getMoreFuncName = this.view(0).firstChild.getAttribute('virtual-repeat-next');
+            let funcCall = this.scope.overrideContext.bindingContext[getMoreFuncName];
+
+            if (typeof funcCall === 'function') {
+              let result = funcCall.call(this.scope.overrideContext.bindingContext, topIndex, isAtBottom, isAtTop);
+              if (!(result instanceof Promise)) {
                 this._calledGetMore = false; //Reset for the next time
-              });
+              } else {
+                return result.then(() => {
+                  this._calledGetMore = false; //Reset for the next time
+                });
+              }
+            } else {
+              throw new Error("'virtual-repeat-next' must be a function or evaluate to one");
             }
+          } else if (func.sourceExpression) {
+            this._calledGetMore = false; //Reset for the next time
+            return func.sourceExpression.evaluate(this.scope);
+          } else {
+            throw new Error("'virtual-repeat-next' must be a function or evaluate to one");
           }
           return null;
         };