Android 目标片段未出现在导航中

Question

I'm trying to use a navigation graph to navigate between two fragments. For some reason, when I call the navigate method with the graph's generated action, the destination fragment is being "created" but not appearing, while the start fragment just sits as if it's been inactivated but doesn't go away. I think I've followed all the steps listed on Google's documentation page for this , but I am probably missing something simple!

Here are my dependencies:

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation 'androidx.appcompat:appcompat:1.1.0'
    implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
    // ViewModel and LiveData
    def lifecycle_version = "2.1.0"
    implementation "androidx.lifecycle:lifecycle-extensions:$lifecycle_version"

    def nav_version = "2.1.0"
    implementation "androidx.navigation:navigation-fragment:$nav_version"
    implementation "androidx.navigation:navigation-ui:$nav_version"

    implementation 'androidx.legacy:legacy-support-v4:1.0.0'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'androidx.test:runner:1.2.0'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.2.0'
}

MainActivity

In my main activity class, I'm just defining a ModelView in OnCreate() :

public class MainActivity extends AppCompatActivity {
    MealDataModel mealDataModel;
   
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // Create the model on create of the main activity
        mealDataModel = ViewModelProviders.of(this).get(MealDataModel.class);
    }
}

And in my main activity layout, I'm just using the NavHostFragment:

...
<fragment
        android:id="@+id/fragment2"
        android:name="androidx.navigation.fragment.NavHostFragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:defaultNavHost="true"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:navGraph="@navigation/nav_graph" />
...

Navigation Graph

And here is my navigation graph:

<?xml version="1.0" encoding="utf-8"?>
<navigation 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:id="@+id/nav_graph"
    app:startDestination="@id/mealCostFrag">

    <fragment
        android:id="@+id/mealCostFrag"
        android:name="com.finley.program2.frag.MealCostFrag"
        android:label="fragment_meal_cost"
        tools:layout="@layout/fragment_meal_cost" >
        <action
            android:id="@+id/action_mealCostFrag_to_tipPercentFrag"
            app:destination="@id/tipPercentFrag" />
    </fragment>
    <fragment
        android:id="@+id/tipPercentFrag"
        android:name="com.finley.program2.frag.TipPercentFrag"
        android:label="fragment_tip_percent"
        tools:layout="@layout/fragment_tip_percent" />
</navigation>

Start Fragment

My start destination of the navigation graph is this fragment...:

public class MealCostFrag extends Fragment {
    private MealDataModel mealDataModel;
    private EditText etCost;
    private Button btnNext;
    private double cost;
    public MealCostFrag() {
        // Required empty public constructor
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        View myView = inflater.inflate(R.layout.fragment_meal_cost, container, false);
        mealDataModel = ViewModelProviders.of(getActivity()).get(MealDataModel.class);

        // Set listeners and such...
        etCost = myView.findViewById(R.id.editText01);
        btnNext= myView.findViewById(R.id.button01);
        btnNext.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                try {
                    cost = Double.parseDouble(etCost.getText().toString());
                    if (cost <= 0) throw new Exception();
                    mealDataModel.setMealCost(cost);
                    Log.i("COST FRAG","Navigating to tip pct with cost = " + mealDataModel.getMealCost());
                    Navigation.findNavController(v).navigate(R.id.action_mealCostFrag_to_tipPercentFrag);
                } catch (Exception e) {
                    Log.e("COST FRAG", "Invalid cost entered, cost = " + cost);
                    Toast.makeText(getContext(),
                            "Please enter a valid meal cost.", Toast.LENGTH_SHORT).show();
                }
            }
        });
        return myView;

    }

}

With this layout:

<?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"
    tools:context=".frag.MealCostFrag">

    <EditText
        android:id="@+id/editText01"
        android:layout_width="0dp"
        android:layout_height="55dp"
        android:layout_marginStart="64dp"
        android:layout_marginLeft="64dp"
        android:layout_marginTop="48dp"
        android:layout_marginEnd="16dp"
        android:layout_marginRight="16dp"
        android:ems="10"
        android:gravity="center_horizontal|center_vertical"
        android:hint="Cost"
        android:inputType="number|numberDecimal"
        android:textSize="24sp"
        app:layout_constraintEnd_toStartOf="@+id/button01"
        app:layout_constraintHorizontal_bias="1.0"/>

    <Button
        android:id="@+id/button01"
        android:layout_width="87dp"
        android:layout_height="46dp"
        android:layout_marginEnd="64dp"
        android:layout_marginRight="64dp"
        android:text="Next"
        app:layout_constraintBaseline_toBaselineOf="@+id/editText01"
        app:layout_constraintEnd_toEndOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

Destination fragment:

Here's the fragment I'm trying to navigate to is this, with an empty layout:

public class TipPercentFrag extends Fragment {
    private MealDataModel mealDataModel;
    public TipPercentFrag() {
        // Required empty public constructor
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        View myView = inflater.inflate(R.layout.fragment_meal_cost, container, false);
        mealDataModel = ViewModelProviders.of(getActivity()).get(MealDataModel.class);

        // Set listeners and such...
        Log.i("TIP PCT FRAG", "CreateView of tip pct frag called");

        return myView;

    }
}

What I find peculiar about this is the logs from inside the destination fragment's 'onCreateView()' method are being output, but it is just not displaying.

I know I've provided a massive wall of code for all of you to sift through, so I will be extra appreciative to anybody willing to spend some time helping me out. I will provide any extra necessary info upon request. Thank you all in advance, and hopefully it's a comically trivial error I'm making!

Answer 1

Just as I suspected, I was making a very simple mistake. The view I was inflating on my destination fragment was the view of my start fragment. Just a copy and paste error.

This:

// Inflate the layout for this fragment
        View myView = inflater.inflate(R.layout.fragment_meal_cost, container, false);
        mealDataModel = ViewModelProviders.of(getActivity()).get(MealDataModel.class);

Just needed to be this:

// Inflate the layout for this fragment
        View myView = inflater.inflate(R.layout.fragment_tip_percent, container, false);
        mealDataModel = ViewModelProviders.of(getActivity()).get(MealDataModel.class);

The lesson learned here is to closely examine every step of the process when debugging! Errors do typically hide in plain sight.

Answer 2

If you are using DataBinding please change inflater lines these lines

from

mDataBinding=DataBindingUtil.inflate(getLayoutInflater(),R.layout.fragment_date_sheet, container, false);

to

mDataBinding= DataBindingUtil.inflate(inflater,R.layout.fragment_date_sheet, container, false);