What's "tools:context" in Android layout files?

This is the activity the tools UI editor uses to render your layout preview. It is documented here:

This attribute declares which activity this layout is associated with by default. This enables features in the editor or layout preview that require knowledge of the activity, such as what the layout theme should be in the preview and where to insert onClick handlers when you make those from a quickfix

That attribute is basically the persistence for the "Associated Activity" selection above the layout. At runtime, a layout is always associated with an activity. It can of course be associated with more than one, but at least one. In the tool, we need to know about this mapping (which at runtime happens in the other direction; an activity can call setContentView(layout) to display a layout) in order to drive certain features.

Right now, we're using it for one thing only: Picking the right theme to show for a layout (since the manifest file can register themes to use for an activity, and once we know the activity associated with the layout, we can pick the right theme to show for the layout). In the future, we'll use this to drive additional features - such as rendering the action bar (which is associated with the activity), a place to add onClick handlers, etc.

The reason this is a tools: namespace attribute is that this is only a designtime mapping for use by the tool. The layout itself can be used by multiple activities/fragments etc. We just want to give you a way to pick a designtime binding such that we can for example show the right theme; you can change it at any time, just like you can change our listview and fragment bindings, etc.

(Here's the full changeset which has more details on this)

And yeah, the link Nikolay listed above shows how the new configuration chooser looks and works

One more thing: The "tools" namespace is special. The android packaging tool knows to ignore it, so none of those attributes will be packaged into the APK. We're using it for extra metadata in the layout. It's also where for example the attributes to suppress lint warnings are stored -- as tools:ignore.

According to the Android Tools Project Site:

tools:context

This attribute is typically set on the root element in a layout XML file, and records which activity the layout is associated with (at designtime, since obviously a layout can be used by more than one layout). This will for example be used by the layout editor to guess a default theme, since themes are defined in the Manifest and are associated with activities, not layouts. You can use the same dot prefix as in manifests to just specify the activity class without the full application package name as a prefix.

<android.support.v7.widget.GridLayout    xmlns:android="http://schemas.android.com/apk/res/android"        xmlns:tools="http://schemas.android.com/tools"    tools:context=".MainActivity">  

Used by: Layout editors in Studio & Eclipse, Lint