add preventable and reversible event helper system

- added isRawError helper
- added catchErrorString helper to convert any error to a string
- added checkGraphResponse to throw if error is present or data missing
- ErrorMessage now supports wrapping multiple errors ergonomically

src/lib/Combobox.svelte

@@ -29,10 +29,11 @@
 	};
 
 	/** returns option label, falling back to value or 'Undefined Option' if no option provided */
-	const getLabel = (opt: ComboboxOption | undefined): string =>
+	const getLabel = (opt: ComboboxOption | undefined | null): string =>
 		opt ? (opt.label ?? opt.value) : 'Undefined Option';
 	/** returns option preview, falling back to getLabel if missing */
-	const getPreview = (opt: ComboboxOption | undefined): string => opt?.preview ?? getLabel(opt);
+	const getPreview = (opt: ComboboxOption | undefined | null): string =>
+		opt?.preview ?? getLabel(opt);
 </script>
 
 <script lang="ts">
@@ -78,7 +79,7 @@
 		stateless?: boolean;
 
 		/** Bindable value of the combobox, the currently selected option */
-		value?: ComboboxOption;
+		value?: ComboboxOption | null;
 		/** Array of ComboboxOptions for the picker */
 		options: ComboboxOption[];
 		/**
@@ -271,7 +272,7 @@
 	});
 
 	/** currently highlighted option, updated by keyboard navigation or defaults to first item */
-	let highlighted = $derived.by((): ComboboxOption | undefined => {
+	let highlighted = $derived.by((): ComboboxOption | undefined | null => {
 		if (!searching) return undefined; // otherwise, the first item is highlighted on first open
 		if (filteredItems.length === 0) return undefined;
 		if (value !== undefined && filteredItems.find((v) => v.value === value?.value)) return value;

src/lib/Dialog.svelte

@@ -126,7 +126,7 @@
 		/** Sets bottom alignment of controls (default: end) */
 		controlsAlign?: 'start' | 'center' | 'end';
 		/** Top-right close control */
-		close?: Snippet | Omit<DialogControlButton, 'label'> | null;
+		close?: Snippet<[state: DialogState]> | Omit<DialogControlButton, 'label'> | null;
 		/**
 		 * Callback when the dialog is opened
 		 * @deprecated use onopenchange instead and check the open parameter
@@ -319,7 +319,7 @@
 				<!-- Close Button -->
 				{#if close !== null}
 					{#if typeof close === 'function'}
-						{@render close()}
+						{@render close(getState())}
 					{:else}
 						{@render dialogCloseButton(getState(), close)}
 					{/if}

src/lib/error.ts

@@ -7,21 +7,105 @@ export interface GraphError {
 }
 
 /** RawError is an error that can be converted to a string by ErrorMessage */
-export type RawError = Error | string | GraphError[];
+export type RawError = ErrorMessage | Error | string | GraphError[];
 
+/**
+ * Type guard to check if an error is a RawError
+ * @param error The error to check
+ * @returns true if the error is a RawError, false otherwise
+ */
+export const isRawError = (error: unknown): error is RawError => {
+	return (
+		error instanceof ErrorMessage ||
+		error instanceof Error ||
+		typeof error === 'string' ||
+		(Array.isArray(error) &&
+			error.every(
+				(e) => typeof e.message === 'string' && (e.path === undefined || Array.isArray(e.path))
+			))
+	);
+};
+
+/**
+ * Converts any error (including GraphQL errors, standard Errors, strings, and
+ * ErrorMessages) into a consistent string format for display.
+ * @param error The error to convert to a string.
+ * @returns A string representation of the error, suitable for display to users.
+ */
+export const catchErrorString = (error: unknown): string => {
+	if (error instanceof ErrorMessage) {
+		return error.toString();
+	} else if (isRawError(error)) {
+		return new ErrorMessage(error).toString();
+	} else {
+		return String(error);
+	}
+};
+
+/**
+ * Checks a typical response from a GraphQL server for error and missing data,
+ * throwing an appropriate ErrorMessage if an error is found or if no data is
+ * returned.
+ * @param resourceName A human-readable name for the resource being fetched,
+ * used in error messages.
+ * @param response The response from the GraphQL server, which may contain an
+ * 'errors' array and a 'data' field.
+ * @throws An ErrorMessage if the response contains errors or if the data field
+ * is missing (undefined or null).
+ */
+export const checkGraphResponse = (
+	resourceName: string,
+	response: { errors?: GraphError[] | null; data?: unknown }
+): void => {
+	if (response.errors && response.errors.length > 0) {
+		throw new ErrorMessage(`Error fetching ${resourceName}`, response.errors);
+	}
+	if (!response.data) {
+		throw new ErrorMessage(`No data returned for ${resourceName}`);
+	}
+};
+
+/**
+ * A class that represents an error message, which can be constructed from various types of raw errors
+ * and provides methods to convert those errors into a consistent format (lines of text) for display.
+ * It also supports wrapping existing error messages with additional context.
+ */
 export class ErrorMessage {
 	private _lines: string[] = [];
 
 	/**
-	 * Converts a RawError to an array of lines and stores it for later access,
+	 * Always creates a new ErrorMessage instance, even if there are no errors.
-	 * or initializes without any errors if the input is null or undefined.
+	 * @param errors The raw errors to convert and store, or null/undefined for no error.
-	 * @param raw The raw error to convert and store, or null/undefined for no error.
+	 * @throws If any of the raw errors are of an unsupported type.
-	 * @throws If the raw error is of an unsupported type.
 	 */
-	constructor(raw: RawError | null | undefined) {
+	constructor(...errors: (RawError | null | undefined)[]) {
-		if (raw) {
+		if (errors.length === 0) return;
-			this._lines = ErrorMessage.rawErrorToLines(raw);
+		this._lines = errors.flatMap((e) => ErrorMessage.rawErrorToLines(e));
-		}
+	}
+
+	/**
+	 * Creates a new ErrorMessage only if the provided errors are not null or
+	 * undefined. If no errors are provided, returns null.
+	 */
+	static from(...errors: (RawError | null | undefined)[]): ErrorMessage | null {
+		if (errors.length === 0) return null;
+		return new ErrorMessage(...errors);
+	}
+
+	/**
+	 * Wraps this ErrorMessage inside another error, nesting the original error.
+	 * @param errors The raw errors to wrap around this error, or null/undefined for no additional error.
+	 * @returns A new ErrorMessage instance that includes the original error and any new errors.
+	 */
+	wrap(...errors: (RawError | null | undefined)[]): ErrorMessage {
+		if (errors.length === 0) return this;
+		const newLines = errors.flatMap((e) => ErrorMessage.rawErrorToLines(e));
+		return new ErrorMessage(...newLines, ...this._lines);
+	}
+
+	/** returns true if there are any error lines */
+	hasError(): boolean {
+		return this._lines.length > 0;
 	}
 
 	/** returns the stored lines */
@@ -37,28 +121,14 @@ export class ErrorMessage {
 		return this._lines.join('<br />');
 	}
 
-	/** returns true if there are any error lines */
-	hasError(): boolean {
-		return this._lines.length > 0;
-	}
-
-	/** adds a new line to the error message */
-	addLine(line: string): void {
-		this._lines.push(line);
-	}
-
-	/** optionally returns a new ErrorMessage only if the RawError is not empty */
-	static from(raw: RawError | null | undefined): ErrorMessage | null {
-		if (!raw) return null;
-		return new ErrorMessage(raw);
-	}
-
 	/** converts a RawError to an array of lines */
 	static rawErrorToLines(raw: RawError | null | undefined): string[] {
-		if (!raw) return ['No error'];
+		if (!raw) return [];
 
 		let errorLines: string[];
-		if (typeof raw === 'string') {
+		if (raw instanceof ErrorMessage) {
+			errorLines = raw.lines;
+		} else if (typeof raw === 'string') {
 			errorLines = [raw];
 		} else if (raw instanceof Error) {
 			errorLines = [raw.message];

src/lib/index.ts

@@ -31,6 +31,7 @@ export { default as Label } from './Label.svelte';
 export { default as Link, rewriteHref } from './Link.svelte';
 export { default as PhoneInput } from './PhoneInput.svelte';
 export { default as PinInput } from './PinInput.svelte';
+export { type Preventable, checkPreventer, type Reversible, checkReversible } from './prevent';
 export { default as RadioGroup } from './RadioGroup.svelte';
 export { default as ScrollBox } from './ScrollBox.svelte';
 export { default as Spinner } from './Spinner.svelte';
@@ -67,7 +68,14 @@ export {
 	ToolbarGroup,
 	Toolbar
 } from './Toolbar';
-export { type GraphError, type RawError, ErrorMessage } from './error';
+export {
+	type GraphError,
+	type RawError,
+	isRawError,
+	catchErrorString,
+	checkGraphResponse,
+	ErrorMessage
+} from './error';
 export {
 	NavigationItem,
 	NavigationManager,

src/lib/prevent.ts

@@ -0,0 +1,63 @@
+/**
+ * A type for a function that can be prevented from executing its default behavior.
+ * See checkPreventer for easy usage of this type.
+ */
+export type Preventable<T> = (ev: T & { prevent: () => void }) => Promise<void> | void;
+
+/**
+ * Calls the provided function with a preventer object, and if the preventer is not
+ * prevented, calls the callback function.
+ * @param fn The function to call with the preventer.
+ * @param ev The event object to pass to the function.
+ * @param cb The callback function to call if the preventer is not prevented.
+ * @returns A promise that resolves to true if the action was not prevented, or false if it was prevented.
+ */
+export const checkPreventer = async <T>(
+	fn: Preventable<T>,
+	ev: T,
+	cb?: () => void
+): Promise<boolean> => {
+	let prevented = false;
+	const preventer = {
+		prevent: () => {
+			prevented = true;
+		}
+	};
+	await fn({ ...ev, ...preventer });
+	if (!prevented && cb) {
+		cb();
+	}
+	return !prevented;
+};
+
+/**
+ * A type for a function that can be reversed after executing its behavior.
+ * See checkReversible for easy usage of this type.
+ */
+export type Reversible<T> = (ev: T & { reverse: () => void }) => Promise<void> | void;
+
+/**
+ * Calls the provided function with a reverser object, and if the reverser is
+ * reversed, calls the callback function.
+ * @param fn The function to call with the reverser.
+ * @param ev The event object to pass to the function.
+ * @param cb The callback function to call if the reverser is reversed.
+ * @returns A promise that resolves to true if the action was not reversed, or false if it was reversed.
+ */
+export const checkReversible = async <T>(
+	fn: Reversible<T>,
+	ev: T,
+	cb?: () => void
+): Promise<boolean> => {
+	let reversed = false;
+	const reverser = {
+		reverse: () => {
+			reversed = true;
+		}
+	};
+	await fn({ ...ev, ...reverser });
+	if (reversed && cb) {
+		cb();
+	}
+	return !reversed;
+};