    toolView.focus()

Focus the tool within its containing `toolsView`.

When a tool is focused, all other tools within the `toolsView` are hidden and the tool's opacity is changed according to the tool's `focusOpacity` option. Only one tool can be focused at a time; the focus passes to the last tool within a tools view on which the `focus` function was called.

The effect of this function can be reversed by the `toolView.blur` function.