How to make a building game in canvas?

Question

Android

There was a task: there is a panel of available blocks (should be scrolled as ScrollView/ListView).
From the Blocks panel, you can drag and drop different blocks onto the canvas. The block "magnetizes" to another if brought close enough. What to read so as not to step on a rake during implementation? Haven't worked with canvas before.
I see the main difficulties in magnetism and dragging the block from the scrolling panel to the main canvas.