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

Question

I am learning Android and wondering why I cannot find the real attribute target tools:contextin the XML layout file.

I read here https://developer.android.com/studio/write/tool-attributes.html#toolscontext under the image that onClickwon't work for any view until we specify tools:context.

I have tried and am wondering if it works without any. tools:context I also read from stackoverflow that it used to select a suitable theme for the layout. But it works great for me without using tools:context, then what is the true purpose of this?

XML layout:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/activity_main"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    >

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:onClick="CallIt"
        />
</RelativeLayout>

Primary activity:

package com.something.testingwith42;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }

    public void CallIt(View view){
        Toast.makeText(this, "Checking", Toast.LENGTH_LONG).show();
    }
}

Everything is working fine tools:context

Answer 1

onClick , : context

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: context

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stackoverflow,

.

: Android XML

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tools:context . . , .

tools:context — tools — . , , APK, .