Add Focusable mixin and FocusContext

Focusable objects have ::focus and ::blur methods and a ::focused
property. Focusable objects can be assigned a ::focusContext, and the
::focused property will only be true for at most one object with the
same context.

spec/focusable-spec.coffee

@@ -0,0 +1,36 @@
+{Model} = require 'theorist'
+Focusable = require '../src/focusable'
+FocusContext = require '../src/focus-context'
+
+describe "Focusable mixin", ->
+  it "ensures that only a single model is focused for a given focus manager", ->
+    class Item extends Model
+      Focusable.includeInto(this)
+
+    focusContext = new FocusContext
+    item1 = new Item({focusContext})
+    item2 = new Item({focusContext})
+    item3 = new Item({focusContext})
+
+    expect(focusContext.focusedObject).toBe null
+    expect(item1.focused).toBe false
+    expect(item2.focused).toBe false
+    expect(item3.focused).toBe false
+
+    item1.focus()
+    expect(focusContext.focusedObject).toBe item1
+    expect(item1.focused).toBe true
+    expect(item2.focused).toBe false
+    expect(item3.focused).toBe false
+
+    item2.focus()
+    expect(focusContext.focusedObject).toBe item2
+    expect(item1.focused).toBe false
+    expect(item2.focused).toBe true
+    expect(item3.focused).toBe false
+
+    item2.blur()
+    expect(focusContext.focusedObject).toBe null
+    expect(item1.focused).toBe false
+    expect(item2.focused).toBe false
+    expect(item3.focused).toBe false

src/focus-context.coffee

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+{Model} = require 'theorist'
+
+module.exports =
+class FocusContext extends Model
+  @property 'focusedObject', null

src/focusable.coffee

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+Mixin = require 'mixto'
+
+module.exports =
+class Focusable extends Mixin
+  @included: ->
+    @property 'focusContext'
+    @behavior 'focused', ->
+      @$focusContext
+        .flatMapLatest((context) -> context?.$focusedObject)
+        .map((focusedObject) => focusedObject is this)
+
+  focus: ->
+    throw new Error("Object must be assigned a focusContext to be focus") unless @focusContext
+    @focusContext.focusedObject = this
+
+  blur: ->
+    throw new Error("Object must be assigned a focusContext to be blurred") unless @focusContext
+    @focusContext.focusedObject = null if @focused