Initial support for predictive back gesture

https://developer.android.com/guide/navigation/predictive-back-gesture

app/src/main/java/com/hippo/ehviewer/ui/SettingsActivity.kt

@@ -57,7 +57,7 @@ class SettingsActivity : EhActivity() {
 
     override fun onOptionsItemSelected(item: MenuItem): Boolean {
         if (item.itemId == android.R.id.home) {
-            onBackPressed()
+            onBackPressedDispatcher.onBackPressed()
             return true
         }
         return super.onOptionsItemSelected(item)

app/src/main/java/com/hippo/ehviewer/ui/scene/GalleryListScene.java

@@ -91,6 +91,7 @@ import com.hippo.ehviewer.widget.SearchBar;
 import com.hippo.ehviewer.widget.SearchLayout;
 import com.hippo.scene.Announcer;
 import com.hippo.scene.SceneFragment;
+import com.hippo.scene.StageActivity;
 import com.hippo.util.ExceptionUtils;
 import com.hippo.view.BringOutTransition;
 import com.hippo.view.ViewTransition;
@@ -778,6 +779,21 @@ public final class GalleryListScene extends BaseScene
         return view;
     }
 
+    public boolean isBackpressCanPreviewLauncherStatus() {
+        if (null != mFabLayout && mFabLayout.isExpanded()) {
+            return false;
+        }
+        switch (mState) {
+            case STATE_NORMAL:
+                return getStackIndex() == 0;
+            case STATE_SIMPLE_SEARCH:
+            case STATE_SEARCH:
+            case STATE_SEARCH_SHOW_LIST:
+                return false;
+        }
+        return false;
+    }
+
     @Override
     public void onBackPressed() {
         if (null != mFabLayout && mFabLayout.isExpanded()) {
@@ -1184,6 +1200,7 @@ public final class GalleryListScene extends BaseScene
                     break;
             }
         }
+        ((StageActivity)requireActivity()).updateBackPressCallBackStatus();
     }
 
     @Override

app/src/main/java/com/hippo/scene/SceneFragment.java

@@ -61,6 +61,7 @@ public class SceneFragment extends Fragment {
         FragmentActivity activity = getActivity();
         if (activity instanceof StageActivity) {
             ((StageActivity) activity).startScene(announcer, horizontal);
+            ((StageActivity) activity).updateBackPressCallBackStatus();
         }
     }
 

app/src/main/java/com/hippo/scene/StageActivity.java

@@ -23,6 +23,7 @@ import android.os.Bundle;
 import android.util.Log;
 import android.view.View;
 
+import androidx.activity.OnBackPressedCallback;
 import androidx.annotation.NonNull;
 import androidx.annotation.Nullable;
 import androidx.fragment.app.Fragment;
@@ -31,6 +32,7 @@ import androidx.fragment.app.FragmentTransaction;
 
 import com.hippo.ehviewer.R;
 import com.hippo.ehviewer.ui.EhActivity;
+import com.hippo.ehviewer.ui.scene.GalleryListScene;
 import com.hippo.yorozuya.AssertUtils;
 import com.hippo.yorozuya.IntIdGenerator;
 
@@ -58,6 +60,8 @@ public abstract class StageActivity extends EhActivity {
     private final SceneViewComparator mSceneViewComparator = new SceneViewComparator();
     private int mStageId = IntIdGenerator.INVALID_ID;
 
+    private OnBackPressedCallback callback;
+
     public static void registerLaunchMode(Class<?> clazz, @SceneFragment.LaunchMode int launchMode) {
         if (launchMode != SceneFragment.LAUNCH_MODE_STANDARD &&
                 launchMode != SceneFragment.LAUNCH_MODE_SINGLE_TOP &&
@@ -140,6 +144,8 @@ public abstract class StageActivity extends EhActivity {
     protected final void onCreate(@Nullable Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
 
+        callback = new BackPressCallBack(false);
+        getOnBackPressedDispatcher().addCallback(callback);
         if (savedInstanceState != null) {
             mStageId = savedInstanceState.getInt(KEY_STAGE_ID, IntIdGenerator.INVALID_ID);
             ArrayList<String> list = savedInstanceState.getStringArrayList(KEY_SCENE_TAG_LIST);
@@ -247,6 +253,7 @@ public abstract class StageActivity extends EhActivity {
 
     public void startScene(Announcer announcer) {
         startScene(announcer, false);
+        updateBackPressCallBackStatus();
     }
 
     public void startScene(Announcer announcer, boolean horizontal) {
@@ -461,6 +468,7 @@ public abstract class StageActivity extends EhActivity {
             // TODO Call onNewArguments when view created ?
             scene.onNewArguments(args);
         }
+        updateBackPressCallBackStatus();
     }
 
     int getSceneIndex(SceneFragment scene) {
@@ -477,6 +485,7 @@ public abstract class StageActivity extends EhActivity {
 
     public void finishScene(SceneFragment scene) {
         finishScene(scene.getTag());
+        updateBackPressCallBackStatus();
     }
 
     private void finishScene(String tag) {
@@ -558,25 +567,51 @@ public abstract class StageActivity extends EhActivity {
         } else {
             fragmentManager.beginTransaction().detach(fragment).attach(fragment).commitAllowingStateLoss();
         }
+        updateBackPressCallBackStatus();
     }
 
-    @Override
-    public void onBackPressed() {
-        int size = mSceneTagList.size();
-        String tag = mSceneTagList.get(size - 1);
-        SceneFragment scene;
-        Fragment fragment = getSupportFragmentManager().findFragmentByTag(tag);
-        if (fragment == null) {
-            Log.e(TAG, "onBackPressed: Can't find scene by tag: " + tag);
-            return;
-        }
-        if (!(fragment instanceof SceneFragment)) {
-            Log.e(TAG, "onBackPressed: The fragment is not SceneFragment");
-            return;
+    private boolean shouldEnableBackPressCallBack() {
+        if (mSceneTagList.size() > 1)
+            return true;
+        if (mSceneTagList.size() == 0)
+            return false;
+        String tag = mSceneTagList.get(0);
+        GalleryListScene fragment = (GalleryListScene) getSupportFragmentManager().findFragmentByTag(tag);
+        if (fragment == null)
+            return false;
+        return !fragment.isBackpressCanPreviewLauncherStatus();
+    }
+
+    public void updateBackPressCallBackStatus() {
+        var isEnabled = callback.isEnabled();
+        var shouldEnable = shouldEnableBackPressCallBack();
+        if (isEnabled != shouldEnable)
+            callback.setEnabled(shouldEnable);
+    }
+
+    class BackPressCallBack extends OnBackPressedCallback {
+        public BackPressCallBack(boolean enabled) {
+            super(enabled);
         }
 
-        scene = (SceneFragment) fragment;
-        scene.onBackPressed();
+        @Override
+        public void handleOnBackPressed() {
+            int size = mSceneTagList.size();
+            String tag = mSceneTagList.get(size - 1);
+            SceneFragment scene;
+            Fragment fragment = getSupportFragmentManager().findFragmentByTag(tag);
+            if (fragment == null) {
+                Log.e(TAG, "onBackPressed: Can't find scene by tag: " + tag);
+                return;
+            }
+            if (!(fragment instanceof SceneFragment)) {
+                Log.e(TAG, "onBackPressed: The fragment is not SceneFragment");
+                return;
+            }
+
+            scene = (SceneFragment) fragment;
+            scene.onBackPressed();
+        }
     }
 
     @Override

app/src/main/java/com/hippo/widget/FabLayout.java

@@ -29,6 +29,7 @@ import androidx.annotation.NonNull;
 
 import com.google.android.material.floatingactionbutton.FloatingActionButton;
 import com.hippo.ehviewer.R;
+import com.hippo.scene.StageActivity;
 import com.hippo.yorozuya.AnimationUtils;
 import com.hippo.yorozuya.SimpleAnimatorListener;
 
@@ -298,6 +299,7 @@ public class FabLayout extends ViewGroup implements View.OnClickListener {
                 mOnExpandListener.onExpand(expanded);
             }
         }
+        ((StageActivity)getContext()).updateBackPressCallBackStatus();
     }