What’s “tools:context” in Android layout files?

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Starting with a recent new version of ADT, I’ve noticed this new attribute on the layout XML files, for example:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    tools:context=".MainActivity" />

What is “tools:context” used for?

How does it even know the exact path to the activity that is written there? Does it look at the package of the app, inside the manifest?

Is it limited to classes that extend Context or only activities? Is it usable for ListView items etc.?

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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

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