Screen Pixel Densities
In activity_main.xml, you specified attribute values in terms of dp units.
Now it is time to learn what they are.
Sometimes you need to specify values for view attributes in terms of specific sizes (usually in pixels, but sometimes points, millimeters, or inches). You see this most commonly with attributes for text size, margins, and padding. Text size is the pixel height of the text on the device’s screen. Margins specify the distances between views, and padding specifies the distance between a view’s outside edges and its content.
As you saw in the section called Adding an Icon, Android automatically scales images to different screen pixel densities using density-qualified drawable folders (such as drawable-xhdpi ...
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