Simple Fullscreen Clock Screensaver
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Open Source means free as "free speech" but not as in "free beer". At first glance, it may not look like it, but even this "tiny" clock app takes a lot of time to maintain because of the quirks of different Android versions and devices in combination with auto text sizing and the DreamService implementation (FireTV!!!).
Works only up to FireOS 7.x. On FireOS 8 and newer, it currently no longer seems to be possible to set a custom screensaver.
Amazon FireOS (on FireTV devices) does currently not officially allow changing the system screensaver to another (3rd-party) app. While nobody understands this decision Amazon made, there is a workaround possible using the Android developer tools, which works up to FireOS 7. Without this workaround, the clock can still be started on FireTV devices like a normal app.
Enable Debugging on your FireTV.
All details (including how to install ADB on your computer) are described here.
Execute the following command:
# execute this command to set this app as screensaver
# please note: after that, the FireTV settings app still shows the Amazon screensaver, but the underlying Android system will now start the clock instead
adb shell settings put secure screensaver_components systems.sieber.fsclock/systems.sieber.fsclock.FullscreenDream
# if you want to restore the Amazon default screen saver, execute the following command
adb shell settings put secure screensaver_components com.amazon.ftv.screensaver/.app.services.ScreensaverService