Add swww support (#94)

Co-authored-by: Sindre Sorhus <[email protected]>

source/linux/background-managers/index.js

@@ -11,3 +11,4 @@ export * as qdbus from './qdbus.js';
 export * as setroot from './setroot.js';
 export * as xfconfquery from './xfconf-query.js';
 export * as swaybg from './swaybg.js';
+export * as swww from './swww.js';

source/linux/background-managers/swww.js

@@ -0,0 +1,28 @@
+import {commandExists, execFile} from '../util.js';
+
+export async function isAvailable() {
+	return commandExists('swww');
+}
+
+async function initialize() {
+	try {
+		await execFile('swww', ['init']);
+	} catch (error) {
+		if (error.stderr.includes('There seems to already be another instance running')) {
+			return;
+		}
+
+		throw new Error(`Failed to set image for swww: ${error.stderr}`);
+	}
+}
+
+export async function get() {
+	await initialize();
+	const {stdout: query} = await execFile('swww', ['query']);
+	return query.slice(query.indexOf('/'), query.indexOf('\n'));
+}
+
+export async function set(imagePath) {
+	await initialize();
+	await execFile('swww', ['img', imagePath]);
+}