feat: add support for bind getter/setters

.changeset/slimy-donkeys-hang.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+feat: add support for bind getters/setters

documentation/docs/03-template-syntax/11-bind.md

@@ -12,10 +12,34 @@ The general syntax is `bind:property={expression}`, where `expression` is an _lv
 <input bind:value />
 ```
 
+
 Svelte creates an event listener that updates the bound value. If an element already has a listener for the same event, that listener will be fired before the bound value is updated.
 
 Most bindings are _two-way_, meaning that changes to the value will affect the element and vice versa. A few bindings are _readonly_, meaning that changing their value will have no effect on the element.
 
+## Function bindings
+
+You can also use `bind:property={get, set}`, where `get` and `set` are functions, allowing you to perform validation and transformation:
+
+```svelte
+<input bind:value={
+	() => value,
+	(v) => value = v.toLowerCase()}
+/>
+```
+
+In the case of readonly bindings like [dimension bindings](#Dimensions), the `get` value should be `null`:
+
+```svelte
+<div
+	bind:clientWidth={null, redraw}
+	bind:clientHeight={null, redraw}
+>...</div>
+```
+
+> [!NOTE]
+> Function bindings are available in Svelte 5.9.0 and newer.
+
 ## `<input bind:value>`
 
 A `bind:value` directive on an `<input>` element binds the input's `value` property:

documentation/docs/98-reference/.generated/compile-errors.md

@@ -78,10 +78,16 @@ Sequence expressions are not allowed as attribute/directive values in runes mode
 Attribute values containing `{...}` must be enclosed in quote marks, unless the value only contains the expression
 ```
 
+### bind_group_invalid_expression
+
+```
+`bind:group` can only bind to an Identifier or MemberExpression
+```
+
 ### bind_invalid_expression
 
 ```
-Can only bind to an Identifier or MemberExpression
+Can only bind to an Identifier or MemberExpression or a `{get, set}` pair
 ```
 
 ### bind_invalid_name
@@ -94,6 +100,12 @@ Can only bind to an Identifier or MemberExpression
 `bind:%name%` is not a valid binding. %explanation%
 ```
 
+### bind_invalid_parens
+
+```
+`bind:%name%={get, set}` must not have surrounding parentheses
+```
+
 ### bind_invalid_target
 
 ```

packages/svelte/messages/compile-errors/template.md

@@ -50,16 +50,24 @@
 
 > Attribute values containing `{...}` must be enclosed in quote marks, unless the value only contains the expression
 
+## bind_group_invalid_expression
+
+> `bind:group` can only bind to an Identifier or MemberExpression
+
 ## bind_invalid_expression
 
-> Can only bind to an Identifier or MemberExpression
+> Can only bind to an Identifier or MemberExpression or a `{get, set}` pair
 
 ## bind_invalid_name
 
 > `bind:%name%` is not a valid binding
 
 > `bind:%name%` is not a valid binding. %explanation%
 
+## bind_invalid_parens
+
+> `bind:%name%={get, set}` must not have surrounding parentheses
+
 ## bind_invalid_target
 
 > `bind:%name%` can only be used with %elements%

packages/svelte/src/compiler/errors.js

@@ -716,12 +716,21 @@ export function attribute_unquoted_sequence(node) {
 }
 
 /**
- * Can only bind to an Identifier or MemberExpression
+ * `bind:group` can only bind to an Identifier or MemberExpression
+ * @param {null | number | NodeLike} node
+ * @returns {never}
+ */
+export function bind_group_invalid_expression(node) {
+	e(node, "bind_group_invalid_expression", "`bind:group` can only bind to an Identifier or MemberExpression");
+}
+
+/**
+ * Can only bind to an Identifier or MemberExpression or a `{get, set}` pair
  * @param {null | number | NodeLike} node
  * @returns {never}
  */
 export function bind_invalid_expression(node) {
-	e(node, "bind_invalid_expression", "Can only bind to an Identifier or MemberExpression");
+	e(node, "bind_invalid_expression", "Can only bind to an Identifier or MemberExpression or a `{get, set}` pair");
 }
 
 /**
@@ -735,6 +744,16 @@ export function bind_invalid_name(node, name, explanation) {
 	e(node, "bind_invalid_name", explanation ? `\`bind:${name}\` is not a valid binding. ${explanation}` : `\`bind:${name}\` is not a valid binding`);
 }
 
+/**
+ * `bind:%name%={get, set}` must not have surrounding parentheses
+ * @param {null | number | NodeLike} node
+ * @param {string} name
+ * @returns {never}
+ */
+export function bind_invalid_parens(node, name) {
+	e(node, "bind_invalid_parens", `\`bind:${name}={get, set}\` must not have surrounding parentheses`);
+}
+
 /**
  * `bind:%name%` can only be used with %elements%
  * @param {null | number | NodeLike} node

packages/svelte/src/compiler/phases/2-analyze/visitors/BindDirective.js

@@ -17,102 +17,6 @@ import { is_content_editable_binding, is_svg } from '../../../../utils.js';
  * @param {Context} context
  */
 export function BindDirective(node, context) {
-	validate_no_const_assignment(node, node.expression, context.state.scope, true);
-
-	const assignee = node.expression;
-	const left = object(assignee);
-
-	if (left === null) {
-		e.bind_invalid_expression(node);
-	}
-
-	const binding = context.state.scope.get(left.name);
-
-	if (assignee.type === 'Identifier') {
-		// reassignment
-		if (
-			node.name !== 'this' && // bind:this also works for regular variables
-			(!binding ||
-				(binding.kind !== 'state' &&
-					binding.kind !== 'raw_state' &&
-					binding.kind !== 'prop' &&
-					binding.kind !== 'bindable_prop' &&
-					binding.kind !== 'each' &&
-					binding.kind !== 'store_sub' &&
-					!binding.updated)) // TODO wut?
-		) {
-			e.bind_invalid_value(node.expression);
-		}
-
-		if (context.state.analysis.runes && binding?.kind === 'each') {
-			e.each_item_invalid_assignment(node);
-		}
-
-		if (binding?.kind === 'snippet') {
-			e.snippet_parameter_assignment(node);
-		}
-	}
-
-	if (node.name === 'group') {
-		if (!binding) {
-			throw new Error('Cannot find declaration for bind:group');
-		}
-
-		// Traverse the path upwards and find all EachBlocks who are (indirectly) contributing to bind:group,
-		// i.e. one of their declarations is referenced in the binding. This allows group bindings to work
-		// correctly when referencing a variable declared in an EachBlock by using the index of the each block
-		// entries as keys.
-		const each_blocks = [];
-		const [keypath, expression_ids] = extract_all_identifiers_from_expression(node.expression);
-		let ids = expression_ids;
-
-		let i = context.path.length;
-		while (i--) {
-			const parent = context.path[i];
-
-			if (parent.type === 'EachBlock') {
-				const references = ids.filter((id) => parent.metadata.declarations.has(id.name));
-
-				if (references.length > 0) {
-					parent.metadata.contains_group_binding = true;
-
-					each_blocks.push(parent);
-					ids = ids.filter((id) => !references.includes(id));
-					ids.push(...extract_all_identifiers_from_expression(parent.expression)[1]);
-				}
-			}
-		}
-
-		// The identifiers that make up the binding expression form they key for the binding group.
-		// If the same identifiers in the same order are used in another bind:group, they will be in the same group.
-		// (there's an edge case where `bind:group={a[i]}` will be in a different group than `bind:group={a[j]}` even when i == j,
-		//  but this is a limitation of the current static analysis we do; it also never worked in Svelte 4)
-		const bindings = expression_ids.map((id) => context.state.scope.get(id.name));
-		let group_name;
-
-		outer: for (const [[key, b], group] of context.state.analysis.binding_groups) {
-			if (b.length !== bindings.length || key !== keypath) continue;
-			for (let i = 0; i < bindings.length; i++) {
-				if (bindings[i] !== b[i]) continue outer;
-			}
-			group_name = group;
-		}
-
-		if (!group_name) {
-			group_name = context.state.scope.root.unique('binding_group');
-			context.state.analysis.binding_groups.set([keypath, bindings], group_name);
-		}
-
-		node.metadata = {
-			binding_group_name: group_name,
-			parent_each_blocks: each_blocks
-		};
-	}
-
-	if (binding?.kind === 'each' && binding.metadata?.inside_rest) {
-		w.bind_invalid_each_rest(binding.node, binding.node.name);
-	}
-
 	const parent = context.path.at(-1);
 
 	if (
@@ -218,5 +122,123 @@ export function BindDirective(node, context) {
 		}
 	}
 
+	// When dealing with bind getters/setters skip the specific binding validation
+	// Group bindings aren't supported for getter/setters so we don't need to handle
+	// the metadata
+	if (node.expression.type === 'SequenceExpression') {
+		if (node.name === 'group') {
+			e.bind_group_invalid_expression(node);
+		}
+
+		let i = /** @type {number} */ (node.expression.start);
+		while (context.state.analysis.source[--i] !== '{') {
+			if (context.state.analysis.source[i] === '(') {
+				e.bind_invalid_parens(node, node.name);
+			}
+		}
+
+		if (node.expression.expressions.length !== 2) {
+			e.bind_invalid_expression(node);
+		}
+
+		return;
+	}
+
+	validate_no_const_assignment(node, node.expression, context.state.scope, true);
+
+	const assignee = node.expression;
+	const left = object(assignee);
+
+	if (left === null) {
+		e.bind_invalid_expression(node);
+	}
+
+	const binding = context.state.scope.get(left.name);
+
+	if (assignee.type === 'Identifier') {
+		// reassignment
+		if (
+			node.name !== 'this' && // bind:this also works for regular variables
+			(!binding ||
+				(binding.kind !== 'state' &&
+					binding.kind !== 'raw_state' &&
+					binding.kind !== 'prop' &&
+					binding.kind !== 'bindable_prop' &&
+					binding.kind !== 'each' &&
+					binding.kind !== 'store_sub' &&
+					!binding.updated)) // TODO wut?
+		) {
+			e.bind_invalid_value(node.expression);
+		}
+
+		if (context.state.analysis.runes && binding?.kind === 'each') {
+			e.each_item_invalid_assignment(node);
+		}
+
+		if (binding?.kind === 'snippet') {
+			e.snippet_parameter_assignment(node);
+		}
+	}
+
+	if (node.name === 'group') {
+		if (!binding) {
+			throw new Error('Cannot find declaration for bind:group');
+		}
+
+		// Traverse the path upwards and find all EachBlocks who are (indirectly) contributing to bind:group,
+		// i.e. one of their declarations is referenced in the binding. This allows group bindings to work
+		// correctly when referencing a variable declared in an EachBlock by using the index of the each block
+		// entries as keys.
+		const each_blocks = [];
+		const [keypath, expression_ids] = extract_all_identifiers_from_expression(node.expression);
+		let ids = expression_ids;
+
+		let i = context.path.length;
+		while (i--) {
+			const parent = context.path[i];
+
+			if (parent.type === 'EachBlock') {
+				const references = ids.filter((id) => parent.metadata.declarations.has(id.name));
+
+				if (references.length > 0) {
+					parent.metadata.contains_group_binding = true;
+
+					each_blocks.push(parent);
+					ids = ids.filter((id) => !references.includes(id));
+					ids.push(...extract_all_identifiers_from_expression(parent.expression)[1]);
+				}
+			}
+		}
+
+		// The identifiers that make up the binding expression form they key for the binding group.
+		// If the same identifiers in the same order are used in another bind:group, they will be in the same group.
+		// (there's an edge case where `bind:group={a[i]}` will be in a different group than `bind:group={a[j]}` even when i == j,
+		//  but this is a limitation of the current static analysis we do; it also never worked in Svelte 4)
+		const bindings = expression_ids.map((id) => context.state.scope.get(id.name));
+		let group_name;
+
+		outer: for (const [[key, b], group] of context.state.analysis.binding_groups) {
+			if (b.length !== bindings.length || key !== keypath) continue;
+			for (let i = 0; i < bindings.length; i++) {
+				if (bindings[i] !== b[i]) continue outer;
+			}
+			group_name = group;
+		}
+
+		if (!group_name) {
+			group_name = context.state.scope.root.unique('binding_group');
+			context.state.analysis.binding_groups.set([keypath, bindings], group_name);
+		}
+
+		node.metadata = {
+			binding_group_name: group_name,
+			parent_each_blocks: each_blocks
+		};
+	}
+
+	if (binding?.kind === 'each' && binding.metadata?.inside_rest) {
+		w.bind_invalid_each_rest(binding.node, binding.node.name);
+	}
+
 	context.next();
 }