Activate android emulator window (#3345)

* Activate android emulator window

* Update Android.ts

packages/xdl/src/Android.ts

@@ -1,5 +1,6 @@
 import { ExpoConfig, getConfig, readExpRcAsync } from '@expo/config';
 import { AndroidConfig } from '@expo/config-plugins';
+import * as osascript from '@expo/osascript';
 import spawnAsync from '@expo/spawn-async';
 import chalk from 'chalk';
 import child_process, { execFileSync } from 'child_process';
@@ -551,8 +552,45 @@ async function _openUrlAsync({
   return openProject;
 }
 
+function getUnixPID(port: number | string) {
+  return execFileSync('lsof', [`-i:${port}`, '-P', '-t', '-sTCP:LISTEN'], {
+    encoding: 'utf8',
+    stdio: ['pipe', 'pipe', 'ignore'],
+  })
+    .split('\n')[0]
+    .trim();
+}
+
+export async function activateEmulatorWindowAsync(device: Pick<Device, 'type' | 'pid'>) {
+  if (
+    // only mac is supported for now.
+    process.platform !== 'darwin' ||
+    // can only focus emulators
+    device.type !== 'emulator'
+  ) {
+    return;
+  }
+
+  // Google Emulator ID: `emulator-5554` -> `5554`
+  const androidPid = device.pid!.match(/-(\d+)/)?.[1];
+  if (!androidPid) {
+    return;
+  }
+  // Unix PID
+  const pid = getUnixPID(androidPid);
+
+  try {
+    await osascript.execAsync(`
+  tell application "System Events"
+    set frontmost of the first process whose unix id is ${pid} to true
+  end tell`);
+  } catch {
+    // noop -- this feature is very specific and subject to failure.
+  }
+}
+
 export async function openAppAsync(
-  device: Pick<Device, 'pid'>,
+  device: Pick<Device, 'pid' | 'type'>,
   {
     packageName,
     mainActivity,
@@ -583,6 +621,8 @@ export async function openAppAsync(
     throw new Error(openProject.substring(openProject.indexOf('Error: ')));
   }
 
+  await activateEmulatorWindowAsync(device);
+
   return openProject;
 }
 
@@ -635,6 +675,9 @@ async function openUrlAsync({
     if (!bootedDevice) {
       return;
     }
+
+    await activateEmulatorWindowAsync(bootedDevice);
+
     device = bootedDevice;
 
     let installedExpo = false;