feat: add mock functions for testing and SSG

.changes/mock-functions.md

@@ -0,0 +1,5 @@
+---
+"tauri": minor
+---
+
+Provide functions to mock IPC calls during testing and static site generation.

tooling/api/src/mocks.ts

@@ -0,0 +1,33 @@
+interface IPCMessage {
+  cmd: string,
+  callback: number,
+  error: number,
+  [key: string]: unknown
+}
+
+export function mockIPC(cb: (cmd: string, args: Record<string, unknown>) => any): void {
+  if ("__TAURI_IPC__" in window)
+    throw new Error("window.__TAURI_IPC__ is already defined");
+
+  window.__TAURI_IPC__ = async ({ cmd, callback, error, ...args }: IPCMessage) => {
+    try {
+      // @ts-expect-error The function key is dynamic and therefore not typed
+      // eslint-disable-next-line @typescript-eslint/no-unsafe-call
+      window[`_${callback}`](await cb(cmd, args));
+    } catch (err) {
+      // @ts-expect-error The function key is dynamic and therefore not typed
+      // eslint-disable-next-line @typescript-eslint/no-unsafe-call
+      window[`_${error}`](err);
+    }
+  };
+}
+
+export function mockWindows(current: string, ...additionalWindows: string[]): void {
+  if ("__TAURI_METADATA__" in window)
+    throw new Error("window.__TAURI_METADATA__ is already defined");
+
+  window.__TAURI_METADATA__ = {
+    __windows: [current, ...additionalWindows].map((label) => ({ label })),
+    __currentWindow: { label: current },
+  };
+}