Enable and Disable text input field in Flex 4

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Flex

I am developing an application in Flex 4.6 and AIR 3.9 Here I have two TextInput fields to get userName and Password and then a login Button.

Simple I want this functionality:
Initially Both Login button and Password text field are disabled. Only User name is enabled.
when someone clicks in the username text field then password should be enabled.
when both username and password are filled with some data then Login button is enabled.

Means, pasword field should be enabled when some data is written in user name field.
And Login button is enabled iff username and password fields both are filled with some data.

Any help? Suggestions??