android - How does findViewById work?

Question

package com.example.dell.helloworld;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
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 showToast(View view)
    {
        Toast t= new Toast(this);
        LayoutInflater inflater=getLayoutInflater();
        View v=inflater.inflate(R.layout.toast, (ViewGroup) findViewById(R.id.toastViewGroup));
        t.setView(v);
        t.show();


    }
}

From the android developer site findViewById searches for child views with give id.

In the above code, who is the parent view, whose children are being searched for given id?

Answer 1

The root view in an Activity is determined by setContentView(int layoutId) or setContentView(View rootView).

In your case, it is

setContentView(R.layout.activity_main);

Therefore, any call you make to findViewById will lookup the id from activity_main.xml.

If it is unable to find the id that you have specified, it will return null.

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It is worth mentioning that that you aren't calling that method and this is typically how a Toast is made.

Toast.makeText(getApplicationContext(), "Hello toast!", Toast.LENGTH_SHORT).show();