targeted updates of deep contextual bindings

compiler/generate/binding/index.js

@@ -1,4 +1,6 @@
 import deindent from '../utils/deindent.js';
+import isReference from '../utils/isReference.js';
+import flattenReference from '../utils/flattenReference.js';
 
 export default function createBinding ( node, name, attribute, current, initStatements, updateStatements, teardownStatements, allUsedContexts ) {
 	const parts = attribute.value.split( '.' );
@@ -20,8 +22,21 @@ export default function createBinding ( node, name, attribute, current, initStat
 	}
 
 	if ( contextual ) {
-		// TODO can we target only things that have changed?
-		// TODO computed values/observers that depend on this probably won't update...
+		// find the top-level property that this is a child of
+		let fragment = current;
+		let prop = parts[0];
+
+		do {
+			if ( fragment.expression && fragment.context === prop ) {
+				if ( !isReference( fragment.expression  ) ) {
+					// TODO this should happen in prior validation step
+					throw new Error( `${prop} is read-only, it cannot be bound` );
+				}
+
+				prop = flattenReference( fragment.expression ).name;
+			}
+		} while ( fragment = fragment.parent );
+
 		const listName = current.listNames[ parts[0] ];
 		const indexName = current.indexNames[ parts[0] ];
 
@@ -30,7 +45,7 @@ export default function createBinding ( node, name, attribute, current, initStat
 			var index = this.__svelte.${indexName};
 			list[index]${parts.slice( 1 ).map( part => `.${part}` ).join( '' )} = this.${attribute.name};
 
-			component.set({});
+			component.set({ ${prop}: component.get( '${prop}' ) });
 		`;
 	} else if ( deep ) {
 		setter = deindent`

compiler/generate/index.js

@@ -495,6 +495,8 @@ export default function generate ( parsed, template ) {
 						useAnchor: false,
 						name: renderer,
 						target: 'target',
+						expression: node.expression,
+						context: node.context,
 
 						contexts,
 						indexes,

test/compiler/binding-input-checkbox-deep-contextual/_config.js

@@ -0,0 +1,34 @@
+import * as assert from 'assert';
+
+export default {
+	data: {
+		items: [
+			{ description: 'one', completed: true },
+			{ description: 'two', completed: false },
+			{ description: 'three', completed: false }
+		]
+	},
+	html: `<div><input type="checkbox"><p>one</p></div><div><input type="checkbox"><p>two</p></div><div><input type="checkbox"><p>three</p></div><!--#each items-->\n\n<p>1 completed</p>`,
+	test ( component, target, window ) {
+		const inputs = [ ...target.querySelectorAll( 'input' ) ];
+
+		assert.ok( inputs[0].checked );
+		assert.ok( !inputs[1].checked );
+		assert.ok( !inputs[2].checked );
+
+		const event = new window.Event( 'change' );
+
+		inputs[1].checked = true;
+		inputs[1].dispatchEvent( event );
+
+		assert.equal( component.get( 'numCompleted' ), 2 );
+		assert.equal( target.innerHTML, `<div><input type="checkbox"><p>one</p></div><div><input type="checkbox"><p>two</p></div><div><input type="checkbox"><p>three</p></div><!--#each items-->\n\n<p>2 completed</p>` );
+
+		const items = component.get( 'items' );
+		items[2].completed = true;
+
+		component.set({ items });
+		assert.ok( inputs[2].checked );
+		assert.equal( target.innerHTML, `<div><input type="checkbox"><p>one</p></div><div><input type="checkbox"><p>two</p></div><div><input type="checkbox"><p>three</p></div><!--#each items-->\n\n<p>3 completed</p>` );
+	}
+};

test/compiler/binding-input-checkbox-deep-contextual/main.svelte

@@ -0,0 +1,15 @@
+{{#each items as item}}
+	<div><input type='checkbox' bind:checked='item.completed'><p>{{item.description}}</p></div>
+{{/each}}
+
+<p>{{numCompleted}} completed</p>
+
+<script>
+	export default {
+		computed: {
+			numCompleted ( items ) {
+				return items.reduce( ( total, item ) => total + ( item.completed ? 1 : 0 ), 0 );
+			}
+		}
+	};
+</script>