support once modifier on component events, fail on others (#2987)

* support once modifier on component events, fail on others. fixes #2654

* appease the gods of typescript

src/compiler/compile/nodes/InlineComponent.ts

@@ -100,6 +100,17 @@ export default class InlineComponent extends Node {
 			this.scope = scope;
 		}
 
+		this.handlers.forEach(handler => {
+			handler.modifiers.forEach(modifier => {
+				if (modifier !== 'once') {
+					component.error(handler, {
+						code: 'invalid-event-modifier',
+						message: `Event modifiers other than 'once' can only be used on DOM elements`
+					});
+				}
+			});
+		});
+
 		this.children = map_children(component, this, this.scope, info.children);
 	}
 }

src/compiler/compile/render-dom/wrappers/InlineComponent/index.ts

@@ -363,7 +363,9 @@ export default class InlineComponentWrapper extends Wrapper {
 		});
 
 		const munged_handlers = this.node.handlers.map(handler => {
-			const snippet = handler.render(block);
+			let snippet = handler.render(block);
+			if (handler.modifiers.has('once')) snippet = `@once(${snippet})`;
+
 			return `${name}.$on("${handler.name}", ${snippet});`;
 		});
 

src/runtime/internal/utils.ts

@@ -81,6 +81,15 @@ export function exclude_internal_props(props) {
 	return result;
 }
 
+export function once(fn) {
+	let ran = false;
+	return function(this: any, ...args) {
+		if (ran) return;
+		ran = true;
+		fn.call(this, ...args);
+	}
+}
+
 const is_client = typeof window !== 'undefined';
 
 export let now: () => number = is_client

test/runtime/samples/component-event-handler-modifier-once/Button.svelte

@@ -0,0 +1 @@
+<button on:click><slot></slot></button>

test/runtime/samples/component-event-handler-modifier-once/_config.js

@@ -0,0 +1,16 @@
+export default {
+	html: `
+		<button>0</button>
+	`,
+
+	async test({ assert, component, target, window }) {
+		const button = target.querySelector('button');
+		const event = new window.MouseEvent('click');
+
+		await button.dispatchEvent(event);
+		assert.equal(component.count, 1);
+
+		await button.dispatchEvent(event);
+		assert.equal(component.count, 1);
+	}
+};

test/runtime/samples/component-event-handler-modifier-once/main.svelte

@@ -0,0 +1,6 @@
+<script>
+	import Button from './Button.svelte';
+	export let count = 0;
+</script>
+
+<Button on:click|once="{() => count += 1}">{count}</Button>

test/validator/samples/component-event-modifiers-invalid/errors.json

@@ -0,0 +1,15 @@
+[{
+	"message": "Event modifiers other than 'once' can only be used on DOM elements",
+	"code": "invalid-event-modifier",
+	"start": {
+		"line": 6,
+		"column": 8,
+		"character": 93
+	},
+	"end": {
+		"line": 6,
+		"column": 40,
+		"character": 125
+	},
+	"pos": 93
+}]

test/validator/samples/component-event-modifiers-invalid/input.svelte

@@ -0,0 +1,6 @@
+<script>
+	import Button from './Button.svelte';
+	const doThat = () => {};
+</script>
+
+<Button on:click|preventDefault={doThat}></Button>