How to lock WhatsApp's chat on android.

How to lock any particular chat of WhatsApp on android.
Requirement
1)A rooted device.
2)xposed installer.

1)Grant root access and install xposed installer.

2)open xposed installer and download an module called WhatsAppx,
after downloading put mark on the box against WhatsAppx(under module section)and reboot your device to activate it.

3)Open WhatsAppx you will see one lock icon.put mark under the lock icon and press the back button of your phone,a dialog box appears click yes.

4)Now,open WhatsApp and click on the chat which you want to lock you will see one lock icon at the top left side of the screen.

5)Click that icon and draw the lock pattern two times and click on continue button

 You have successfully locked the chat.
To unlocked the chat simply click on the top left icon again.