Mob.SizeClass (mob v0.9.13)

The window's size class, {horizontal, vertical}, each :compact or :regular.

Every screen socket carries it as assigns.size_class. The framework sets it before mount/3 and keeps it current; a screen never asks for it.

Size classes are the layout signal, not orientation or raw dimensions. An iPad in Split View is a phone-shaped window on a tablet; a foldable flips class when it opens. Typical values:

Window	Size class
iPhone, portrait	`{:compact, :regular}`
Large iPhone (Plus, Pro Max, XR/11-class, 414pt wide or more), landscape	`{:regular, :compact}`
Smaller iPhones, landscape	`{:compact, :compact}`
iPad full screen, either orientation	`{:regular, :regular}`
iPad Slide Over, narrow Split View	`{:compact, :regular}`
Android phone, portrait	`{:compact, :regular}`
Android phone, landscape	`{:regular, :compact}`
Android tablet / unfolded foldable	`{:regular, :regular}`

Where the value comes from

iOS — the window's traitCollection.horizontalSizeClass and .verticalSizeClass, exactly as UIKit reports them.
Android — Android has no OS size class, so it is derived from the activity's Configuration.screenWidthDp / screenHeightDp with the Material window-size-class breakpoints: horizontal is :compact below 600dp wide, vertical is :compact below 480dp tall; Material's "medium" and "expanded" both map to :regular. These are the numbers Android's own w600dp / h480dp resource qualifiers select on. See decisions/2026-10-01-size-class-in-socket-assigns.md for why this and not Compose's WindowSizeClass.

Changes

When the window's class changes — rotation, an iPad Split View / Slide Over / Stage Manager resize, an Android multi-window resize, a foldable opening — every live screen gets the new value in assigns.size_class and then handle_info({:mob_size_class_changed, new}, socket). By the time that callback runs the assign already holds new; a screen that only reads assigns.size_class in render/1 needs no clause at all, and one without a matching clause is not crashed by it.

Before the platform can answer

On iOS the BEAM can start before a window exists (a prewarmed or background launch). Screens mounted then hold placeholder/0; the root view reports the real value as soon as it appears, which arrives as an ordinary change. Mob.ScreenCase.mount_screen/4 uses the same placeholder unless told otherwise.

Summary

Types

class()

One axis of a size class.

t()

{horizontal, vertical}.

Functions

placeholder()

The value a screen holds when the platform cannot answer yet, and the default under Mob.ScreenCase: {:compact, :regular}, a portrait phone.

valid?(arg1)

True for a well-formed size class.