adds log-ios and log-android cli commands

Summary:
solves first part of #8186
Closes https://github.com/facebook/react-native/pull/8352

Differential Revision: D3475905

Pulled By: mkonicek

fbshipit-source-id: 99f6aa15b92ba3192fa0b812d369bd7020469573

local-cli/logIOS/logIOS.js

@@ -0,0 +1,77 @@
+'use strict';
+
+const chalk = require('chalk');
+const child_process = require('child_process');
+const os = require('os');
+const path = require('path');
+const Promise = require('promise');
+
+/**
+ * Starts iOS device syslog tail
+ */
+function logIOS() {
+  return new Promise((resolve, reject) => {
+    _logIOS(resolve, reject);
+  });
+}
+
+function _logIOS(resolve, reject) {
+  let rawDevices;
+
+  try {
+    rawDevices = child_process.execFileSync(
+      'xcrun', ['simctl', 'list', 'devices', '--json'], {encoding: 'utf8'}
+    );
+  } catch (e) {
+    console.log(chalk.red(
+      'xcrun invocation failed. Please check that Xcode is installed.'
+    ));
+    reject(e);
+    return;
+  }
+
+  const { devices } = JSON.parse(rawDevices);
+
+  const device = _findAvailableDevice(devices);
+  if (device === undefined) {
+    console.log(chalk.red(
+      'No active iOS device found'
+    ));
+    reject();
+  }
+
+  tailDeviceLogs(device.udid, reject);
+}
+
+function _findAvailableDevice(devices) {
+  for (const key of Object.keys(devices)) {
+    for (const device of devices[key]) {
+      if (device.availability === '(available)' && device.state === 'Booted') {
+        return device;
+      }
+    }
+  }
+}
+
+function tailDeviceLogs(udid, reject) {
+  const logDir = path.join(
+    os.homedir(),
+    'Library',
+    'Logs',
+    'CoreSimulator',
+    udid,
+    'asl',
+  );
+
+  const log =
+    child_process.spawnSync('syslog', ['-w', '-F', 'std', '-d', logDir], {stdio: 'inherit'});
+
+  if (log.error !== null) {
+    console.log(chalk.red(
+      'syslog invocation failed.'
+    ));
+    reject(log.error);
+  }
+}
+
+module.exports = logIOS;