hidden with transition (poc)

packages/svelte/src/compiler/phases/3-transform/client/visitors/RegularElement.js

@@ -545,6 +545,8 @@ function build_element_attribute_update_assignment(element, node_id, attribute,
 		update = b.stmt(b.call('$.set_value', node_id, value));
 	} else if (name === 'checked') {
 		update = b.stmt(b.call('$.set_checked', node_id, value));
+	} else if (name === 'hidden') {
+		update = b.stmt(b.call('$.set_hidden', node_id, value));
 	} else if (is_dom_property(name)) {
 		update = b.stmt(b.assignment('=', b.member(node_id, name), value));
 	} else {

packages/svelte/src/compiler/phases/3-transform/client/visitors/TransitionDirective.js

@@ -1,7 +1,12 @@
 /** @import { Expression } from 'estree' */
 /** @import { AST } from '#compiler' */
 /** @import { ComponentContext } from '../types' */
-import { TRANSITION_GLOBAL, TRANSITION_IN, TRANSITION_OUT } from '../../../../../constants.js';
+import {
+	TRANSITION_GLOBAL,
+	TRANSITION_HIDDEN,
+	TRANSITION_IN,
+	TRANSITION_OUT
+} from '../../../../../constants.js';
 import * as b from '../../../../utils/builders.js';
 import { parse_directive_name } from './shared/utils.js';
 
@@ -13,6 +18,9 @@ export function TransitionDirective(node, context) {
 	let flags = node.modifiers.includes('global') ? TRANSITION_GLOBAL : 0;
 	if (node.intro) flags |= TRANSITION_IN;
 	if (node.outro) flags |= TRANSITION_OUT;
+	if (node.modifiers.includes('hidden')) {
+		flags |= TRANSITION_HIDDEN;
+	}
 
 	const args = [
 		b.literal(flags),

packages/svelte/src/compiler/types/template.d.ts

@@ -251,7 +251,7 @@ export namespace AST {
 		name: string;
 		/** The 'y' in `transition:x={y}` */
 		expression: null | Expression;
-		modifiers: Array<'local' | 'global'>;
+		modifiers: Array<'local' | 'global' | 'hidden'>;
 		/** True if this is a `transition:` or `in:` directive */
 		intro: boolean;
 		/** True if this is a `transition:` or `out:` directive */

packages/svelte/src/constants.js

@@ -14,6 +14,7 @@ export const PROPS_IS_LAZY_INITIAL = 1 << 4;
 export const TRANSITION_IN = 1;
 export const TRANSITION_OUT = 1 << 1;
 export const TRANSITION_GLOBAL = 1 << 2;
+export const TRANSITION_HIDDEN = 1 << 3;
 
 export const TEMPLATE_FRAGMENT = 1;
 export const TEMPLATE_USE_IMPORT_NODE = 1 << 1;

packages/svelte/src/internal/client/dom/elements/attributes.js

@@ -74,6 +74,28 @@ export function set_checked(element, checked) {
 	element.checked = checked;
 }
 
+/**
+ * @param {Element & { hidden: boolean, __tm?: import('#client').TransitionManager}} element
+ * @param {boolean} hidden
+ */
+export function set_hidden(element, hidden) {
+	// @ts-expect-error
+	var attributes = (element.__attributes ??= {});
+
+	if (attributes.hidden === (attributes.hidden = !!hidden)) return;
+
+	if (element.__tm) {
+		if (hidden) {
+			element.__tm.out(() => (element.hidden = true));
+		} else {
+			element.hidden = false;
+			element.__tm.in();
+		}
+	} else {
+		element.hidden = hidden;
+	}
+}
+
 /**
  * @param {Element} element
  * @param {string} attribute

packages/svelte/src/internal/client/dom/elements/transitions.js

@@ -5,7 +5,12 @@ import { active_effect, untrack } from '../../runtime.js';
 import { loop } from '../../loop.js';
 import { should_intro } from '../../render.js';
 import { current_each_item } from '../blocks/each.js';
-import { TRANSITION_GLOBAL, TRANSITION_IN, TRANSITION_OUT } from '../../../../constants.js';
+import {
+	TRANSITION_GLOBAL,
+	TRANSITION_HIDDEN,
+	TRANSITION_IN,
+	TRANSITION_OUT
+} from '../../../../constants.js';
 import { BLOCK_EFFECT, EFFECT_RAN, EFFECT_TRANSPARENT } from '../../constants.js';
 import { queue_micro_task } from '../task.js';
 
@@ -169,6 +174,7 @@ export function transition(flags, element, get_fn, get_params) {
 	var is_outro = (flags & TRANSITION_OUT) !== 0;
 	var is_both = is_intro && is_outro;
 	var is_global = (flags & TRANSITION_GLOBAL) !== 0;
+	var is_hidden = (flags & TRANSITION_HIDDEN) !== 0;
 
 	/** @type {'in' | 'out' | 'both'} */
 	var direction = is_both ? 'both' : is_intro ? 'in' : 'out';
@@ -243,6 +249,12 @@ export function transition(flags, element, get_fn, get_params) {
 		}
 	};
 
+	if (is_hidden) {
+		// @ts-expect-error
+		element.__tm = transition;
+		return;
+	}
+
 	var e = /** @type {Effect} */ (active_effect);
 
 	(e.transitions ??= []).push(transition);

packages/svelte/src/internal/client/index.js

@@ -33,7 +33,8 @@ export {
 	set_xlink_attribute,
 	handle_lazy_img,
 	set_value,
-	set_checked
+	set_checked,
+	set_hidden
 } from './dom/elements/attributes.js';
 export { set_class, set_svg_class, set_mathml_class, toggle_class } from './dom/elements/class.js';
 export { apply, event, delegate, replay_events } from './dom/elements/events.js';

packages/svelte/types/index.d.ts

@@ -1060,7 +1060,7 @@ declare module 'svelte/compiler' {
 			name: string;
 			/** The 'y' in `transition:x={y}` */
 			expression: null | Expression;
-			modifiers: Array<'local' | 'global'>;
+			modifiers: Array<'local' | 'global' | 'hidden'>;
 			/** True if this is a `transition:` or `in:` directive */
 			intro: boolean;
 			/** True if this is a `transition:` or `out:` directive */