How to delete app cache for all apps in Android M?

Is there an option to delete the cache of all apps or certain apps in Android M?

A third-party app cannot delete the cache of another app in Android 6.0+. The protection level of Manifest.permission.CLEAR_APP_CACHE changed from "dangerous" to "signature|privileged" or "system|signature" in Android 6.0+. Now, only apps signed with the firmware's key can hold this permission.

Is there really no way to delete app cache on devices with Android M?

Unless the app is installed as a system app or you have root access, there is no way to delete app cache on Android 6.0+.

How does the Settings app handle it?

Android is, of course, open source. Lets look at the code. In AppStorageSettings.java lines 172 - 178 we find:

if (v == mClearCacheButton) {
    // Lazy initialization of observer
    if (mClearCacheObserver == null) {
        mClearCacheObserver = new ClearCacheObserver();
    }
    mPm.deleteApplicationCacheFiles(mPackageName, mClearCacheObserver);
}

So, the Settings app is using the hidden method PackageManager#deleteApplicationCacheFiles(String, IPackageDataObserver). It can do this because it holds the system level permission "android.permission.CLEAR_APP_USER_DATA" (a permission a third-party app cannot hold).

External Cache

However, cleaning of external cache is still supported.

This is still possible on Android 6.0+. I haven't looked at the source code for the app you mentioned but I would assume all you need to do is request the WRITE_EXTERNAL_STORAGE permission, get all installed packages using PackageManager, get the app's external cache directory, and delete the directory.

Root Access

Of course, if you have root access you can delete another app's cache. Here is a quick example of using root access to delete all app cache. You can use Chainfire's libsuperuser to run commands in a root shell:

PackageManager pm = getPackageManager();
List<ApplicationInfo> installedApplications = pm.getInstalledApplications(0);
for (ApplicationInfo applicationInfo : installedApplications) {
  try {
    Context packageContext = createPackageContext(applicationInfo.packageName, 0);
    List<File> directories = new ArrayList<>();
    directories.add(packageContext.getCacheDir());
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) {
      Collections.addAll(directories, packageContext.getExternalCacheDirs());
    } else {
      directories.add(packageContext.getExternalCacheDir());
    }

    StringBuilder command = new StringBuilder("rm -rf");
    for (File directory : directories) {
      command.append(" \"" + directory.getAbsolutePath() + "\"");
    }

    Shell.SU.run(command.toString());
  } catch (PackageManager.NameNotFoundException wtf) {
  }
}