Why is my floating action button not responding to clicks? - android-fragments

Question

I have two floating action buttons in a fragment that is being created by a ViewPager. My intention is for the FABs to do the same thing that can be done in the ViewPager, so that the user can have a choice in terms of navigation. My problem is that the FABs do not respond to clicks initially, so if I click the left FAB, and then click the right FAB, the ViewPager will flip to the left instead of flipping to the right. What am I doing wrong?
Here is my code:
activity_view_pager:
<?xml version="1.0" encoding="utf-8"?>
<androidx.viewpager.widget.ViewPager
xmlns:android="http://schemas.android.com/apk/res/android"
android:id="#+id/view_pager"
android:layout_width="match_parent"
android:layout_height="match_parent" />
fragment_test:
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent">
<TextView
android:id="#+id/textView"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_marginTop="50dp"
android:text="Title"
app:layout_constraintTop_toTopOf="parent" />
<TextView
android:id="#+id/a"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_marginTop="100dp"
app:layout_constraintTop_toTopOf="parent" />
<TextView
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_marginTop="200dp"
android:text="Description"
app:layout_constraintTop_toTopOf="parent" />
<TextView
android:id="#+id/b"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_marginTop="250dp"
app:layout_constraintTop_toTopOf="parent" />
<TextView
android:id="#+id/c"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_marginTop="300dp"
app:layout_constraintTop_toTopOf="parent" />
<com.google.android.material.floatingactionbutton.FloatingActionButton
android:id="#+id/left_btn"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginStart="50dp"
android:layout_marginTop="450dp"
android:src="#drawable/ic_left_arrow"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
<com.google.android.material.floatingactionbutton.FloatingActionButton
android:id="#+id/right_btn"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginTop="450dp"
android:layout_marginEnd="50dp"
android:src="#drawable/ic_right_arrow"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintTop_toTopOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>
ViewPagerActivity:
package com.fl4me.android.exampleapplication;
import static com.fl4me.android.exampleapplication.TestAdapter.EXTRA_ID;
import android.os.Bundle;
import androidx.appcompat.app.AppCompatActivity;
import androidx.fragment.app.Fragment;
import androidx.fragment.app.FragmentManager;
import androidx.fragment.app.FragmentStatePagerAdapter;
import androidx.viewpager.widget.ViewPager;
public class ViewPagerActivity extends AppCompatActivity {
public static int index;
public static ViewPager viewPager;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_view_pager);
index = (int) getIntent().getSerializableExtra(EXTRA_ID);
viewPager = (ViewPager) findViewById(R.id.view_pager);
FragmentManager fragmentManager = getSupportFragmentManager();
viewPager.setAdapter(new FragmentStatePagerAdapter(fragmentManager) {
#Override
public Fragment getItem(int position) {
return TestFragment.newInstance(position);
}
#Override
public int getCount() {
return 5;
}
});
for(int i = 0; i < Test.testData.length; i++) {
if(i == index) {
viewPager.setCurrentItem(i);
break;
}
}
}
}
TestFragment:
package com.fl4me.android.exampleapplication;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
import androidx.fragment.app.Fragment;
import androidx.viewpager.widget.ViewPager;
import com.google.android.material.floatingactionbutton.FloatingActionButton;
public class TestFragment extends Fragment {
private static final String ARG_ID = "Id";
private int Id;
private String a;
private TextView aTV;
private String b;
private TextView bTV;
private int c;
private TextView cTV;
private FloatingActionButton leftBtn;
private FloatingActionButton rightBtn;
public static TestFragment newInstance(int Id) {
Bundle args = new Bundle();
args.putSerializable(ARG_ID, Id);
TestFragment fragment = new TestFragment();
fragment.setArguments(args);
return fragment;
}
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.fragment_test, container, false);
Id = getArguments().getInt(ARG_ID);
a = Test.testData[Id].getA();
b = Test.testData[Id].getB();
c = Test.testData[Id].getC();
aTV = (TextView) view.findViewById(R.id.a);
aTV.setText(a);
bTV = (TextView) view.findViewById(R.id.b);
bTV.setText(b);
cTV = (TextView) view.findViewById(R.id.c);
cTV.setText(String.valueOf(c));
leftBtn = view.findViewById(R.id.left_btn);
rightBtn = view.findViewById(R.id.right_btn);
leftBtn.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View view) {
ViewPagerActivity.viewPager.setCurrentItem(ViewPagerActivity.index--);
}
});
rightBtn.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View view) {
ViewPagerActivity.viewPager.setCurrentItem(ViewPagerActivity.index++);
}
});
return view;
}
}

Answer 1

This is probably happening because you are post-incrementing or post-decrementing the index instead of pre-incrementing or pre-decrementing it. I think that you should know how post and pre incrementation works, but here is something to read.
Basically the index that you are providing for the setCurrentItem is not evaluated on the spot as index + 1 or index - 1, but it gets its new value only after the execution of the line of code in which there is an expression index++ or index--. This is why you got that "lag" in switching pages that you have described.
So, in your listener for the rightBtn the code should probably be like this:
ViewPagerActivity.viewPager.setCurrentItem(++ViewPagerActivity.index);
The listener for the leftBtn will be analogous, but with --.