I am sorry to tell you that the file maybe lost. It's best to 'save' the file before putting the computer to sleep.
I too have lost a file I was working on on my iPad... I am new and thought that when you close a file that it asks you to save and then click yes. As it is an ipad version it did not ask me 'where' to save as most programs would give you that option. I assumed that it was saved somewhere mysteriously on the iPad itself... Saving files in SketchBook Pro takes additional care over working on a desktop, you cannot trust the application to favorably save or store files in remote locations like iCloud or DropBox, which is where you must be sure you have saved to when working on any version of SketchBook, desktop or mobile device. I am sorry and know the pain of losing a irreplaceable file... I still work very carefully as I said, it takes extra care when working in SketchBook Pro.
For this reason I you suggest to implement a save button in the Android (and IOS Version?).
Cause if I am not mistaken, you can only save if your opening another image from your gallery or if your're creating an new image. Isn't that the case?
So probably a save button would be useful, cause I don't want to go to my gallery or create a new image before pushing my standby button.
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