diff --git a/tools/run_on_as_app.py b/tools/run_on_as_app.py
new file mode 100755
index 0000000..25385dd
--- /dev/null
+++ b/tools/run_on_as_app.py
@@ -0,0 +1,131 @@
+#!/usr/bin/env python
+# Copyright (c) 2018, the R8 project authors. Please see the AUTHORS file
+# for details. All rights reserved. Use of this source code is governed by a
+# BSD-style license that can be found in the LICENSE file.
+
+import apk_utils
+import os
+import optparse
+import subprocess
+import sys
+import utils
+
+SHRINKERS = ['r8', 'r8full', 'proguard']
+
+APPS = {
+  # 'app-name': {
+  #     'git_repo': ...
+  #     'app_module': ... (default app)
+  #     'archives_base_name': ... (default same as app_module)
+  #     'flavor': ... (default no flavor)
+  # },
+  'tachiyomi': {
+      'git_repo': 'https://github.com/sgjesse/tachiyomi.git',
+      'flavor': 'standard',
+  },
+  # This does not build yet.
+  'muzei': {
+      'git_repo': 'https://github.com/sgjesse/muzei.git',
+      'app_module': 'main',
+      'archives_base_name': 'muzei',
+  },
+}
+
+def GitClone(git_url):
+  return subprocess.check_output(['git', 'clone', git_url]).strip()
+
+def GitPull():
+  return subprocess.check_output(['git', 'pull']).strip()
+
+def GitCheckout(file):
+  return subprocess.check_output(['git', 'checkout', file]).strip()
+
+def ParseOptions(argv):
+  result = optparse.OptionParser()
+  result.add_option('--app',
+                    help='What app to run on',
+                    choices=APPS.keys())
+  return result.parse_args(argv)
+
+def main(argv):
+  (options, args) = ParseOptions(argv)
+
+  # Common environment setup.
+  user_home = os.path.expanduser('~')
+  android_home = os.path.join(user_home, 'Android', 'Sdk')
+  android_build_tools_version = '28.0.3'
+  android_build_tools = os.path.join(
+      android_home, 'build-tools', android_build_tools_version)
+
+  # App setup.
+  app = options.app
+  if not app:
+    raise Exception(
+        "You need to specify '--app={}'".format('|'.join(APPS.keys())))
+  config = APPS.get(app)
+  git_repo = config['git_repo']
+  app_module = config.get('app_module', 'app')
+  archives_base_name = config.get(' archives_base_name', app_module)
+  flavor = config.get('flavor')
+
+  # Checkout and build in the build directory.
+  working_dir = utils.BUILD
+  checkout_dir = os.path.join(working_dir, app)
+
+  if not os.path.exists(checkout_dir):
+    with utils.ChangedWorkingDirectory(working_dir):
+      GitClone(git_repo)
+  else:
+    with utils.ChangedWorkingDirectory(checkout_dir):
+      GitPull()
+
+  with utils.ChangedWorkingDirectory(checkout_dir):
+    for shrinker in SHRINKERS:
+
+      # Ensure that gradle.properties are not modified before modifying it to
+      # select shrinker.
+      GitCheckout('gradle.properties')
+      with open("gradle.properties", "a") as gradle_properties:
+        if shrinker == 'r8full':
+          gradle_properties.write('\nandroid.enableR8.fullMode=true\n')
+        if shrinker == 'proguard':
+          gradle_properties.write('\nandroid.enableR8=false\n')
+
+      out = os.path.join(checkout_dir, 'out', shrinker)
+      if not os.path.exists(out):
+        os.makedirs(out)
+
+      env = os.environ.copy()
+      env['ANDROID_HOME'] = android_home
+      releaseTarget = app_module + ':' + 'assembleRelease'
+      subprocess.check_call(
+          ['./gradlew', '--no-daemon', 'clean', releaseTarget], env=env)
+
+      unsigned_apk_name = (archives_base_name
+          + (('-' + flavor) if flavor else '')
+          + '-release-unsigned.apk')
+      signed_apk_name = archives_base_name + '-release.apk'
+
+      build_output_apks = os.path.join(app_module, 'build', 'outputs', 'apk')
+      if flavor:
+        unsigned_apk = os.path.join(
+            build_output_apks, flavor, 'release', unsigned_apk_name)
+      else:
+        unsigned_apk = os.path.join(
+            build_output_apks, 'release', unsigned_apk_name)
+
+      signed_apk = os.path.join(out, signed_apk_name)
+
+      keystore = 'app.keystore'
+      keystore_password = 'android'
+      apk_utils.sign_with_apksigner(
+          android_build_tools,
+          unsigned_apk,
+          signed_apk,
+          keystore,
+          keystore_password)
+
+    GitCheckout('gradle.properties')
+
+if __name__ == '__main__':
+  sys.exit(main(sys.argv[1:]))