How to assign event handlers in a Widget Tab application?

Question

Java

Hello. I am very young in mobile development. I'm trying to deal with Tabbed Activity. Each tab is divided into a class that is responsible for it and for the xml representation. For example, the QRScaner.java file is responsible for the "Scanner" tab:

package com.savelitomak.qrreaderprototype;

import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

public class QRScaner extends Fragment {

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.qr_scaner, container, false);
        return rootView;
    }
}

And scanner.xml

<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"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:paddingBottom="@dimen/activity_vertical_margin"
    tools:context="com.savelitomak.qrreaderprototype.MainActivity$PlaceholderFragment">

    <TextView
        android:id="@+id/section_label"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
    <Button
            android:text="@string/button_scan"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_centerVertical="true"
            android:layout_centerHorizontal="true"
            android:id="@+id/button"
            android:background="@color/colorPrimary"
            android:textColor="@android:color/white"/>

</RelativeLayout>

MainActivity.java:

package com.savelitomak.qrreaderprototype;

import android.support.design.widget.TabLayout;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.view.ViewPager;
import android.os.Bundle;

public class MainActivity extends AppCompatActivity {

    private SectionsPagerAdapter mSectionsPagerAdapter;

    private ViewPager mViewPager;

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

        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());

        mViewPager = (ViewPager) findViewById(R.id.container);
        mViewPager.setAdapter(mSectionsPagerAdapter);

        TabLayout tabLayout = (TabLayout) findViewById(R.id.tabs);
        tabLayout.setupWithViewPager(mViewPager);
      
    }

    public class SectionsPagerAdapter extends FragmentPagerAdapter {

        public SectionsPagerAdapter(FragmentManager fm) {
            super(fm);
        }

        @Override
        public Fragment getItem(int position) {
            switch (position) {
                case 0:
                    QRScaner qrScaner = new QRScaner();
                    return qrScaner;
                case 1:
                    RawList rawList = new RawList();
                    return rawList;
            }
            return null;
        }

        @Override
        public int getCount() {
            return 2;
        }

        @Override
        public CharSequence getPageTitle(int position) {
            switch (position) {
                case 0:
                    return "Scanner";
                case 1:
                    return "Sounds";
            }
            return null;
        }
    }
}

(I apologize for the huge amount of code)
In general, the problem is that I cannot put an onClick handler on the R.id.button_scan button, I tried to do this in MainActivity -> onCreate (the application, in turn, crashed immediately after launch)
Do not tell me what is my problem?

Answer 1

A simple solution - look for a button inside the created fragment and process the click there.
More confusing, if you need to catch a click in the activity:
- write implement OnClickListener in the activity
- throw it into the fragment as an object of the OnClickListener class
- find the button in the fragment and hang it with the passed listener as a handler