How do I make a not saved notifier appear?

So, I have created an Image Markup app. Now, I want it to give a notifier with buttons if the image is not saved by clicking the save button. Like, it will have the message "File not saved. Are you sure?" and it will have a "Save Image", "Don't Save Image" and "Cancel" button. How do I do this? Please also share the blocks.

When and how is the Notifier going to be called?

It should be called if the exit button is pressed but before it, the save button is not pressed and the image is not saved.

(below) This is probably the best way of saving a file:

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Look in the Screen1 blocks for an event that fires when the Android Back button is pressed.
That's another place to check global saved.