前幾天我分享了TabLayout選單製作，
今天要分享結合Fragment頁面容器，
做到點選切換的功能~
請先觀看:
[Android Studio菜鳥的學習分享]UI分享(一) - TabLayout & TabItem
[Android Studio菜鳥的學習分享]Fragment介紹
兩篇分享~

結果預覽:

新創建的三個Fragment頁面

BMI -> FragmentTest01Bmi
Order -> FragmentTest02Order
Setting -> FragmentTest03Setting

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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"
    android:orientation="vertical"
    tools:context=".MainActivity">

    <androidx.constraintlayout.widget.ConstraintLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent">

        <com.google.android.material.tabs.TabLayout
            android:id="@+id/tabLayoutMain"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent">

            <com.google.android.material.tabs.TabItem
                android:id="@+id/tabItemBMI"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:icon="@drawable/calculator"
                android:text="BMI計算機" />

            <com.google.android.material.tabs.TabItem
                android:id="@+id/tabItemOrder"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:icon="@drawable/orderfood"
                android:text="訂餐" />

            <com.google.android.material.tabs.TabItem
                android:id="@+id/tabItemSetting"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:icon="@drawable/gear"
                android:text="設定" />

        </com.google.android.material.tabs.TabLayout>

        <androidx.fragment.app.FragmentContainerView
            android:id="@+id/fragment_test"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            app:layout_constraintTop_toBottomOf="@id/tabLayoutMain"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent">

        </androidx.fragment.app.FragmentContainerView>

    </androidx.constraintlayout.widget.ConstraintLayout>

</LinearLayout>

MainActivity.java

package com.example.itthirty;

import androidx.appcompat.app.AppCompatActivity;
import androidx.fragment.app.FragmentManager;
import androidx.fragment.app.FragmentTransaction;
import android.os.Bundle;
import com.google.android.material.tabs.TabLayout;

public class MainActivity extends AppCompatActivity {


    private FragmentTest01Bmi fragmentTest01Bmi = new FragmentTest01Bmi();
    private FragmentTest02Order fragmentTest02Order = new FragmentTest02Order();
    private FragmentTest03Setting fragmentTest03Setting = new FragmentTest03Setting();
    private TabLayout tab_layout;
    int now = 0;

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

        // Step01-FragmentTransaction加入三個Fragment，暫時先隱藏另外兩張Fragment，只顯示BMI頁面:
        FragmentManager fragmentManager = getSupportFragmentManager();
        FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
        fragmentTransaction.add(R.id.fragment_test,fragmentTest01Bmi,"BMI");
        fragmentTransaction.add(R.id.fragment_test,fragmentTest02Order,"Order");
        fragmentTransaction.add(R.id.fragment_test,fragmentTest03Setting,"Setting");
        fragmentTransaction.hide(fragmentTest02Order);
        fragmentTransaction.hide(fragmentTest03Setting);
        fragmentTransaction.commit();

        // Step02-新增TabLayout的按下按鈕的監聽器:
        tab_layout = findViewById(R.id.tabLayoutMain);
        tab_layout.addOnTabSelectedListener(new TabLayout.OnTabSelectedListener() {
            //按下要做的事
            @Override
            public void onTabSelected(TabLayout.Tab tab) {
                // Step03-寫一個方法，tab.getPosition是按下哪個按鈕，將之傳入fragmentChange方法內:
                fragmentChange(tab.getPosition());
            }

            @Override
            public void onTabUnselected(TabLayout.Tab tab) {

            }

            @Override
            public void onTabReselected(TabLayout.Tab tab) {

            }
        });

    }

    // Step04-切換顯示方法撰寫:
    private void fragmentChange(int position){
        FragmentManager fragmentManager = getSupportFragmentManager();
        FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
        // Step05-先判斷目前的Fragment，並將之隱藏(now我預設為0，因為tab.Position的0為第一頁BMI):
        switch (now){
            case 0:
                fragmentTransaction.hide(fragmentTest01Bmi);
                break;
            case 1:
                fragmentTransaction.hide(fragmentTest02Order);
                break;
            case 2:
                fragmentTransaction.hide(fragmentTest03Setting);
                break;
        }
        // Step06-判斷使用者點選的選單按鈕，顯示相對應的Fragment:
        switch (position){
            case 0:
                fragmentTransaction.show(fragmentTest01Bmi);
                break;
            case 1:
                fragmentTransaction.show(fragmentTest02Order);
                break;
            case 2:
                fragmentTransaction.show(fragmentTest03Setting);
                break;
        }
        fragmentTransaction.commit();
        // Step06-更新目前所在的Fragment:
        now = position;
    }

}