fix(app/library): add instance state handling

Signed-off-by: YuutaW <17158086+Trumeet@users.noreply.github.com>

4 files changed, 84 insertions, 31 deletions

diff --git a/app/src/main/java/moe/yuuta/flow/MainActivity.java b/app/src/main/java/moe/yuuta/flow/MainActivity.java
index cba4bbd..d827aa6 100644
--- a/app/src/main/java/moe/yuuta/flow/MainActivity.java
+++ b/app/src/main/java/moe/yuuta/flow/MainActivity.java
@@ -6,6 +6,7 @@ import android.util.Log;
 import android.view.LayoutInflater;
 import android.view.View;
 import android.view.ViewGroup;
+import android.widget.Button;
 import android.widget.TextView;
 
 import androidx.annotation.NonNull;
@@ -26,9 +27,9 @@ public class MainActivity extends AppCompatActivity {
         super.onCreate(savedInstanceState);
         AppCompatDelegate.setDefaultNightMode(AppCompatDelegate.MODE_NIGHT_YES);
         Log.i(TAG, "onCreate()");
-        if (mFragment == null) {
+        if (mFragment == null && savedInstanceState == null) {
             mFragment = new FlowFragment();
-            mFragment.setPages(Arrays.asList(new Page1(), new Page2(), new Page3()));
+            mFragment.setPages(Arrays.asList(new Page1(), new Page2(), new Page3(), new Page4()));
             getSupportFragmentManager()
                     .beginTransaction()
                     .replace(android.R.id.content, mFragment)
@@ -95,6 +96,34 @@ public class MainActivity extends AppCompatActivity {
         }
     }
 
+    public static class Page4 extends PageFragment {
+        public Page4() {
+            mInfo = new FlowInfo(new HeaderConfig("Page 4", "just a counter", false), null);
+        }
+
+        // TODO: The FlowFragment keeps the instance, maybe we don't want that happened to child fragments.
+        private int mCount;
+
+        @Nullable
+        @Override
+        public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
+            Button button = new Button(requireContext());
+            button.setMinHeight(20);
+            button.setText("+1s");
+            button.setOnClickListener(new View.OnClickListener() {
+                @Override
+                public void onClick(View v) {
+                    mCount++;
+                    HeaderConfig config = mInfo.getHeaderConfig();
+                    config.setSubtitleText(Integer.toString(mCount));
+                    mInfo.setHeaderConfig(config);
+                    getHostFragment().notifyCurrentFlowInfoUpdated();
+                }
+            });
+            return button;
+        }
+    }
+
     @Override
     public void onBackPressed() {
         if (mFragment != null && mFragment.onBackPressed()) return;
diff --git a/library/src/main/java/moe/yuuta/flow/FlowFragment.java b/library/src/main/java/moe/yuuta/flow/FlowFragment.java
index d7d4776..87deaf2 100644
--- a/library/src/main/java/moe/yuuta/flow/FlowFragment.java
+++ b/library/src/main/java/moe/yuuta/flow/FlowFragment.java
@@ -1,6 +1,7 @@
 package moe.yuuta.flow;
 
 import android.os.Bundle;
+import android.util.Log;
 import android.view.LayoutInflater;
 import android.view.View;
 import android.view.ViewGroup;
@@ -9,21 +10,25 @@ import androidx.annotation.NonNull;
 import androidx.annotation.Nullable;
 import androidx.constraintlayout.widget.ConstraintLayout;
 import androidx.fragment.app.Fragment;
-import androidx.fragment.app.FragmentManager;
-import androidx.fragment.app.FragmentPagerAdapter;
+import androidx.fragment.app.FragmentStatePagerAdapter;
 import androidx.viewpager.widget.ViewPager;
 
 import java.util.ArrayList;
 import java.util.List;
 
 /**
- * The Host fragment
+ * The Host fragment. It is marked as final because it was set to remain instance.
  */
-public class FlowFragment extends Fragment implements IFlowFragment, View.OnClickListener {
+public final class FlowFragment extends Fragment implements IFlowFragment, View.OnClickListener {
+    private static final String TAG = "FlowFragment";
+
     private List<PageFragment> mPages = new ArrayList<>(0);
     // true: after the view settles, update the UI immediately.
     private volatile boolean mUIUpdateScheduled;
 
+    private static final String ARG_CURRENT = FlowFragment.class.getName() + ".ARG_CURRENT";
+    private static final String ARG_UI_UPDATE_SCHEDULED = FlowFragment.class.getName() + ".ARG_UI_UPDATE_SCHEDULED";
+
     private Header mHeader;
     private ViewPager mPager;
     private NavigationBar mNav;
@@ -73,18 +78,17 @@ public class FlowFragment extends Fragment implements IFlowFragment, View.OnClic
         }
     }
 
-    @Nullable
     @Override
     public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
+        if (getView() != null) return getView();
         final View view = inflater.inflate(R.layout.fragment_flow, container, false);
         mHeader = new Header(view.<ConstraintLayout>findViewById(R.id.flow_host_header));
         mNav = new NavigationBar(view.<ConstraintLayout>findViewById(R.id.flow_host_nav));
         mPager = view.findViewById(R.id.flow_host_pager);
-        mPager.setAdapter(new FragmentPagerAdapter(getChildFragmentManager()) {
+        mPager.setAdapter(new FragmentStatePagerAdapter(getChildFragmentManager()) {
             @Override
             public Fragment getItem(int position) {
-                PageFragment fragment = mPages.get(position);
-                return fragment;
+                return mPages.get(position);
             }
 
             @Override
@@ -93,13 +97,11 @@ public class FlowFragment extends Fragment implements IFlowFragment, View.OnClic
             }
         });
         mPager.addOnPageChangeListener(mPageListener);
-        getChildFragmentManager().registerFragmentLifecycleCallbacks(mCallback, false);
         return view;
     }
 
     @Override
     public void onDestroyView() {
-        getChildFragmentManager().unregisterFragmentLifecycleCallbacks(mCallback);
         mPager.removeOnPageChangeListener(mPageListener);
         super.onDestroyView();
     }
@@ -107,6 +109,11 @@ public class FlowFragment extends Fragment implements IFlowFragment, View.OnClic
     @Override
     public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
         super.onViewCreated(view, savedInstanceState);
+        Log.d(TAG, "onViewCreated");
+        if (savedInstanceState != null) {
+            Log.d(TAG, "onViewCreated - restore " + savedInstanceState.getInt(ARG_CURRENT, -1));
+            mPager.setCurrentItem(savedInstanceState.getInt(ARG_CURRENT, 0));
+        }
         if (mUIUpdateScheduled) {
             updateUI();
             mUIUpdateScheduled = false;
@@ -143,16 +150,6 @@ public class FlowFragment extends Fragment implements IFlowFragment, View.OnClic
         mPager.setCurrentItem(index, true);
     }
 
-    private FragmentManager.FragmentLifecycleCallbacks mCallback = new FragmentManager.FragmentLifecycleCallbacks() {
-        @Override
-        public void onFragmentPreCreated(@NonNull FragmentManager fm, @NonNull Fragment f, @Nullable Bundle savedInstanceState) {
-            if (f instanceof PageFragment) {
-                final PageFragment pf = (PageFragment) f;
-                pf.setHostFragment(FlowFragment.this);
-            }
-        }
-    };
-
     private ViewPager.OnPageChangeListener mPageListener = new ViewPager.OnPageChangeListener() {
         @Override
         public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
@@ -191,4 +188,30 @@ public class FlowFragment extends Fragment implements IFlowFragment, View.OnClic
     public View.OnClickListener getGeneralFlowNavListener() {
         return this;
     }
+
+    @Override
+    public void onCreate(@Nullable Bundle savedInstanceState) {
+        super.onCreate(savedInstanceState);
+        // To save the child fragment instances
+        setRetainInstance(true);
+    }
+
+    @Override
+    public void onSaveInstanceState(@NonNull Bundle outState) {
+        super.onSaveInstanceState(outState);
+        Log.d(TAG, "save");
+        if (mPager != null) outState.putInt(ARG_CURRENT, mPager.getCurrentItem());
+        if (mPager != null) Log.d(TAG, "current " + mPager.getCurrentItem());
+        outState.putBoolean(ARG_UI_UPDATE_SCHEDULED, mUIUpdateScheduled);
+    }
+
+    @Override
+    public void onActivityCreated(@Nullable Bundle savedInstanceState) {
+        super.onActivityCreated(savedInstanceState);
+        Log.d(TAG, "onActivityCreated");
+        if (savedInstanceState != null) {
+            Log.d(TAG, "restore");
+            mUIUpdateScheduled = savedInstanceState.getBoolean(ARG_UI_UPDATE_SCHEDULED, false);
+        }
+    }
 }
diff --git a/library/src/main/java/moe/yuuta/flow/PageFragment.java b/library/src/main/java/moe/yuuta/flow/PageFragment.java
index dcc9f21..4e7996b 100644
--- a/library/src/main/java/moe/yuuta/flow/PageFragment.java
+++ b/library/src/main/java/moe/yuuta/flow/PageFragment.java
@@ -1,24 +1,25 @@
 package moe.yuuta.flow;
 
+import android.util.Log;
+
 import androidx.annotation.NonNull;
 import androidx.fragment.app.Fragment;
 
 public abstract class PageFragment extends Fragment {
-    private IFlowFragment mHostFragment;
-
     /**
      * Once mInfo is changed, you should call {@link IFlowFragment#notifyCurrentFlowInfoUpdated()} to publish it.
      * Note: it will be permanently change the recorded info.
      */
     protected FlowInfo mInfo;
 
-    final void setHostFragment(@NonNull IFlowFragment hostFragment) {
-        this.mHostFragment = hostFragment;
-    }
-
     @NonNull
     protected final IFlowFragment getHostFragment() {
-        return mHostFragment;
+        final Fragment parent = getParentFragment();
+        if (!(parent instanceof IFlowFragment)) {
+            throw new IllegalStateException("This fragment is not attached to a valid flow host");
+        }
+        Log.d("Page", "get");
+        return (IFlowFragment) parent;
     }
 
     /**
diff --git a/library/src/main/java/moe/yuuta/flow/widgets/FlowPager.java b/library/src/main/java/moe/yuuta/flow/widgets/FlowPager.java
index c821eef..e76f319 100644
--- a/library/src/main/java/moe/yuuta/flow/widgets/FlowPager.java
+++ b/library/src/main/java/moe/yuuta/flow/widgets/FlowPager.java
@@ -8,7 +8,7 @@ import android.view.View;
 
 import androidx.annotation.NonNull;
 import androidx.annotation.Nullable;
-import androidx.fragment.app.FragmentPagerAdapter;
+import androidx.fragment.app.FragmentStatePagerAdapter;
 import androidx.viewpager.widget.ViewPager;
 
 public class FlowPager extends ViewPager {
@@ -27,7 +27,7 @@ public class FlowPager extends ViewPager {
 
         if(null != getAdapter()) {
             int height = 0;
-            View child = ((FragmentPagerAdapter) getAdapter()).getItem(getCurrentItem()).getView();
+            View child = ((FragmentStatePagerAdapter) getAdapter()).getItem(getCurrentItem()).getView();
             if (child != null) {
                 child.measure(widthMeasureSpec, MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED));
                 height = child.getMeasuredHeight();