What's "Tools:Context" in Android Layout Files

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

What is the real purpose of tools:context in Android XML

onClick will not work for any View until we specify tools:context

There is nothing in that documentation that makes that claim.

Everything is working fine without tools:context

Correct. That is because that is nothing in that documentation that states that onClick() will only work if you have tools:context. It does say that for quick-fixes, you need tools:context for the tools to know where to add the onClick() method.

also read from the stackoverflow that it used to choose proper theme for the layout

This too is incorrect.

What is the real purpose of tools:context in Android XML

It provides context for development tools, as to where this layout will be used, so that the tools can provide better assistance to developers, such as:

• more accurate rendering of previews, taking the hosting activity and its theme into account
• more intelligent assistants, such as the aforementioned quick-fixes

tools:context is completely optional. Development tools can and do work without it. Those tools may provide somewhat degraded assistance to you, but that is your choice.

tools:context — or any attribute in the tools namespace — will have no impact at runtime. I have not checked, but I would hope that they even get stripped out when packaging the APK, as they have no use at runtime.

what is reason for bringing tools:context in parent layout?

tools support's attribute that enable design-time features like tools:context =".Mainactivity" would tell which layout the current xml belongs to. If you want to show a placeholder of your ui with sample data that is available only at design time - say display a list with each row having a image - you could use the available tools attribute and see how the list view looks at design time without having to actually run the app. Its only for design and it does not affect when you run the app.

Some more info on the same https://developer.android.com/studio/write/tool-attributes.

I want to know what is usage and will it cause any problem in child
layouts if I remove this line?

Should be fine. Nothing to worry.

How to use tools:context to associtate Fragment files?

Have you tried it doing like this:

Let's say Relative layout is your parent layout in your fragment's layout:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
            xmlns:tools="http://schemas.android.com/tools"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            tools:context=".Folder.Fragment"  >

Now this .xml file will be associated with your .Fragment class which is a fragment.

Android - What´s the meaning of context on layouts?

tools:context provides the context(i.e which activity the layout is associated with) which is used by the visual layout editor in Android Studio to render your layout. You can read more about it here.

Does tools:context affect apk?

it does not, this attr just helps rendering View in preview tab in Android Studio

Used by: Lint, Android Studio layout editor

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...

from DOC

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