Restore Activity FAB's translationX on rotation

app/src/main/java/com/philliphsu/clock2/MainActivity.java

@@ -11,6 +11,7 @@ import android.support.v4.app.FragmentManager;
 import android.support.v4.app.FragmentPagerAdapter;
 import android.support.v4.content.ContextCompat;
 import android.support.v4.view.ViewPager;
+import android.util.Log;
 import android.util.SparseArray;
 import android.view.LayoutInflater;
 import android.view.MenuItem;
@@ -59,9 +60,25 @@ public class MainActivity extends BaseActivity {
 //    }
 
     @Override
-    protected void onCreate(Bundle savedInstanceState) {
+    protected void onCreate(final Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
-        // TODO: On device rotation, if we were last on stopwatch page, restore the fab's translationX.
+        final View rootView = ((ViewGroup) findViewById(android.R.id.content)).getChildAt(0);
+        // http://stackoverflow.com/a/24035591/5055032
+        // http://stackoverflow.com/a/3948036/5055032
+        // The views in our layout have begun drawing.
+        // There is no lifecycle callback that tells us when our layout finishes drawing;
+        // in my own test, drawing still isn't finished by onResume().
+        // Post a message in the UI events queue to be executed after drawing is complete,
+        // so that we may get their dimensions.
+        rootView.post(new Runnable() {
+            @Override
+            public void run() {
+                if (mViewPager.getCurrentItem() == mSectionsPagerAdapter.getCount() - 1) {
+                    // Restore the FAB's translationX from a previous configuration.
+                    mFab.setTranslationX(mViewPager.getWidth() / -2f + getFabPixelOffsetForXTranslation());
+                }
+            }
+        });
 
         // Create the adapter that will return a fragment for each of the three
         // primary sections of the activity.
@@ -76,8 +93,8 @@ public class MainActivity extends BaseActivity {
              */
             @Override
             public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
-//                Log.d(TAG, String.format("pos = %d, posOffset = %f, posOffsetPixels = %d",
+                Log.d(TAG, String.format("pos = %d, posOffset = %f, posOffsetPixels = %d",
-//                        position, positionOffset, positionOffsetPixels));
+                        position, positionOffset, positionOffsetPixels));
                 int pageBeforeLast = mSectionsPagerAdapter.getCount() - 2;
                 if (position <= pageBeforeLast) {
                     if (position < pageBeforeLast) {
@@ -101,22 +118,9 @@ public class MainActivity extends BaseActivity {
                         // is translated right, back to its original position.
                         float translationX = positionOffsetPixels / -2f;
                         // NOTE: You MUST scale your own additional pixel offsets by positionOffset,
-                        // or else the FAB will immediately translate by that many pixels, causing
+                        // or else the FAB will immediately translate by that many pixels, appearing
-                        // jitter as you scroll.
+                        // to skip/jump.
-                        final int margin;
+                        translationX += positionOffset * getFabPixelOffsetForXTranslation();
-                        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
-                            // Since each side's margin is the same, any side's would do.
-                            margin = ((ViewGroup.MarginLayoutParams) mFab.getLayoutParams()).rightMargin;
-                        } else {
-                            // Pre-Lollipop has measurement issues with FAB margins. This is
-                            // probably as good as we can get to centering the FAB, without
-                            // hardcoding some small margin value.
-                            margin = 0;
-                        }
-                        // Translation is done relative to a view's left position; by adding
-                        // an offset of half the FAB's width, we effectively rebase the translation
-                        // relative to the view's center position.
-                        translationX += positionOffset * (mFab.getWidth() / 2f + margin);
                         mFab.setTranslationX(translationX);
                     }
                 }
@@ -124,10 +128,15 @@ public class MainActivity extends BaseActivity {
 
             @Override
             public void onPageSelected(int position) {
+                Log.d(TAG, "onPageSelected");
                 if (position < mSectionsPagerAdapter.getCount() - 1) {
                     mFab.setImageDrawable(mAddItemDrawable);
                 }
                 Fragment f = mSectionsPagerAdapter.getFragment(mViewPager.getCurrentItem());
+                // NOTE: This callback is fired after a rotation, right after onStart().
+                // Unfortunately, the FragmentManager handling the rotation has yet to
+                // tell our adapter to re-instantiate the Fragments, so our collection
+                // of fragments is empty. You MUST keep this check so we don't cause a NPE.
                 if (f instanceof BaseFragment) {
                     ((BaseFragment) f).onPageSelected();
                 }
@@ -303,6 +312,28 @@ public class MainActivity extends BaseActivity {
         return super.onOptionsItemSelected(item);
     }
 
+    /**
+     * @return the positive offset in pixels required to rebase an X-translation of the FAB
+     * relative to its center position. An X-translation normally is done relative to a view's
+     * left position.
+     */
+    private float getFabPixelOffsetForXTranslation() {
+        final int margin;
+        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
+            // Since each side's margin is the same, any side's would do.
+            margin = ((ViewGroup.MarginLayoutParams) mFab.getLayoutParams()).rightMargin;
+        } else {
+            // Pre-Lollipop has measurement issues with FAB margins. This is
+            // probably as good as we can get to centering the FAB, without
+            // hardcoding some small margin value.
+            margin = 0;
+        }
+        // X-translation is done relative to a view's left position; by adding
+        // an offset of half the FAB's width, we effectively rebase the translation
+        // relative to the view's center position.
+        return mFab.getWidth() / 2f + margin;
+    }
+
     /**
      * A placeholder fragment containing a simple view.
      */

app/src/main/java/com/philliphsu/clock2/stopwatch/StopwatchFragment.java

@@ -109,15 +109,30 @@ public class StopwatchFragment extends RecyclerViewFragment<
     @Override
     public void onActivityCreated(@Nullable Bundle savedInstanceState) {
         super.onActivityCreated(savedInstanceState);
-        // TODO: Any better alternatives?
         // TOneverDO: Move to onCreate(). When the device rotates, onCreate() _is_ called,
         // but trying to find the FAB in the Activity's layout will fail, and we would get back
         // a null reference. This is probably because this Fragment's onCreate() is called
         // BEFORE the Activity's onCreate.
+        // TODO: Any better alternatives to control the Activity's FAB from here?
         mActivityFab = new WeakReference<>((FloatingActionButton) getActivity().findViewById(R.id.fab));
-        if (savedInstanceState != null) {
+        // There is no documentation for isMenuVisible(), so what exactly does it do?
+        // My guess is it checks for the Fragment's options menu. But we never initialize this
+        // Fragment with setHasOptionsMenu(), let alone we don't actually inflate a menu in here.
+        // My guess is when this Fragment becomes actually visible, it "hooks" onto the menu
+        // options "internal API" and inflates its menu in there if it has one.
+        //
+        // To us, this just makes for a very good visibility check.
+        if (savedInstanceState != null && isMenuVisible()) {
             // This is a pretty good indication that we just rotated.
-//            updateMiniFabs(); // TODO: Do we need this?
+            // isMenuVisible() filters out the case when you rotate on page 1 and scroll
+            // to page 2, the icon will prematurely change; that happens because at page 2,
+            // this Fragment will be instantiated for the first time for the current configuration,
+            // and so the lifecycle from onCreate() to onActivityCreated() occurs. As such,
+            // we will have a non-null savedInstanceState and this would call through.
+            //
+            // The reason when you open up the app for the first time and scrolling to page 2
+            // doesn't prematurely change the icon is the savedInstanceState is null, and so
+            // this call would be filtered out sufficiently just from the first check.
             updateFab();
         }
     }
@@ -232,8 +247,10 @@ public class StopwatchFragment extends RecyclerViewFragment<
 //                mProgressAnimator.resume();
 //            }
         }
-        updateAllFabs();
         savePrefs();
+        // TOneverDO: Precede savePrefs(), or else we don't save false to KEY_CHRONOMETER_RUNNING
+        /// and updateFab will update the wrong icon.
+        updateAllFabs();
     }
 
     @Override
@@ -306,7 +323,15 @@ public class StopwatchFragment extends RecyclerViewFragment<
 
     private void updateAllFabs() {
         updateMiniFabs();
-        updateFab();
+        // TODO: If we're calling this method, then chances are we are visible.
+        // You can verify this yourself by finding all usages.
+        // isVisible() is good for filtering out calls to this method when this Fragment
+        // isn't actually visible to the user; however, a side effect is it also filters
+        // out calls to this method when this Fragment is rotated. Fortunately, we don't
+        // make any calls to this method after a rotation.
+        if (isVisible()) {
+            updateFab();
+        }
     }
 
     private void updateMiniFabs() {
@@ -317,17 +342,7 @@ public class StopwatchFragment extends RecyclerViewFragment<
     }
 
     private void updateFab() {
-        // Avoid changing the icon in premature cases.
+        mActivityFab.get().setImageDrawable(isStopwatchRunning() ? mPauseDrawable : mStartDrawable);
-        // isVisible() is good for filtering out calls to this method when this Fragment
-        // isn't actually visible to the user; however, a side effect is it also filters
-        // out calls to this method when this Fragment is rotated.
-        // isMenuVisible() is good for the rotation case; however, a side effect is when you
-        // rotate on page 1 and scroll to page 2, the icon prematurely changes. Fortunately,
-        // for every page change after that, the icon no longer prematurely changes.
-        // TODO: If you can live with that, then move on.
-        if ((isVisible() || isMenuVisible()) && mActivityFab != null) {
-            mActivityFab.get().setImageDrawable(isStopwatchRunning() ? mPauseDrawable : mStartDrawable);
-        }
     }
 
     private void startNewProgressBarAnimator() {

app/src/main/res/layout-land/fragment_stopwatch.xml

@@ -0,0 +1,71 @@
+<?xml version="1.0" encoding="utf-8"?>
+<android.support.design.widget.CoordinatorLayout
+    xmlns:android="http://schemas.android.com/apk/res/android"
+    android:layout_width="match_parent"
+    android:layout_height="match_parent"
+    xmlns:app="http://schemas.android.com/apk/res-auto">
+
+    <LinearLayout
+        android:layout_width="match_parent"
+        android:layout_height="match_parent"
+        android:orientation="vertical">
+
+        <com.philliphsu.clock2.stopwatch.ChronometerWithMillis
+            android:id="@+id/chronometer"
+            android:layout_width="match_parent"
+            android:layout_height="wrap_content"
+            android:padding="16dp"
+            android:background="@color/colorPrimary"
+            android:gravity="center_horizontal"
+            android:textSize="@dimen/text_size_display_3"
+            style="@style/TextAppearance.AppCompat.Inverse"
+            android:layout_marginBottom="8dp"/>
+
+        <!-- RecyclerView -->
+        <include layout="@layout/fragment_recycler_view"/>
+
+    </LinearLayout>
+
+    <com.philliphsu.clock2.UntouchableSeekBar
+        android:id="@+id/seek_bar"
+        android:layout_width="match_parent"
+        android:layout_height="wrap_content"
+        app:layout_anchor="@id/chronometer"
+        app:layout_anchorGravity="bottom"
+        android:paddingStart="0dp"
+        android:paddingEnd="0dp"/>
+
+    <!-- TODO: dimen resource for height -->
+    <android.support.v7.widget.GridLayout
+        android:layout_width="match_parent"
+        android:layout_height="88dp"
+        android:layout_gravity="bottom"
+        app:columnCount="2">
+
+        <android.support.design.widget.FloatingActionButton
+            android:id="@+id/new_lap"
+            android:layout_width="wrap_content"
+            android:layout_height="wrap_content"
+            android:src="@drawable/ic_add_lap_24dp"
+            android:tint="@android:color/white"
+            android:visibility="invisible"
+            app:layout_columnWeight="1"
+            app:layout_gravity="center"
+            app:fabSize="mini"
+            app:backgroundTint="@color/colorPrimary"/>
+
+        <android.support.design.widget.FloatingActionButton
+            android:id="@+id/stop"
+            android:layout_width="wrap_content"
+            android:layout_height="wrap_content"
+            android:src="@drawable/ic_stop_24dp"
+            android:tint="@android:color/white"
+            android:visibility="invisible"
+            app:layout_columnWeight="1"
+            app:layout_gravity="center"
+            app:fabSize="mini"
+            app:backgroundTint="@color/colorPrimary"/>
+
+    </android.support.v7.widget.GridLayout>
+
+</android.support.design.widget.CoordinatorLayout>