Programming your own lock screen

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http://forum.xda-developers.com – Hello, I would like to know which method you need to use to access the unlock screen and replace it (something like SlideUnlock or Widget Locker). I want to design an app which could work on any phone even those without a custom ROM. This app would then be overriding the default Lock Screen. You probably need to create a service but I can't seem to find a source code or a good tutorial on it. (HowTos)