Don't re-use mHistoryFragment

Make a new HistoryFragment every time we enter and exit history to work
around a bug on older platforms where a Fragment's internal state may
not be reset correctly when detached.

Also the HistoryFragment open/close transition is now properly animated
using a custom Animator returned by DragLayout.

Fixes: 33587141
Fixes: 33789131
Fixes: 33789337
Fixes: 34132294
Fixes: 34132998
Fixes: 34134349
Fixes: 34135442

Test: manually verified no crash occurs on API 22, 23, & 24 emulator
Change-Id: Iaefbe2bbf4ca186b6396397579887774ed5906d3

7 files changed, 230 insertions, 181 deletions

diff --git a/res/layout/activity_calculator_main.xml b/res/layout/activity_calculator_main.xml
index 4173d33..2e9d62d 100644
--- a/res/layout/activity_calculator_main.xml
+++ b/res/layout/activity_calculator_main.xml
@@ -28,6 +28,7 @@
     <FrameLayout
         android:id="@+id/history_frame"
         android:layout_width="match_parent"
-        android:layout_height="match_parent" />
+        android:layout_height="match_parent"
+        android:visibility="invisible" />
 
-</com.android.calculator2.DragLayout>
\ No newline at end of file
+</com.android.calculator2.DragLayout>
diff --git a/res/layout/fragment_history.xml b/res/layout/fragment_history.xml
index 78ba4cc..e5fe50e 100644
--- a/res/layout/fragment_history.xml
+++ b/res/layout/fragment_history.xml
@@ -19,7 +19,6 @@
     xmlns:app="http://schemas.android.com/apk/res-auto"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
-    android:background="@color/display_background_color"
     android:clipChildren="false"
     android:orientation="vertical">
 
@@ -40,9 +39,12 @@
         android:id="@+id/history_recycler_view"
         android:layout_width="match_parent"
         android:layout_height="match_parent"
+        android:background="@color/display_background_color"
         android:clipChildren="false"
         android:clipToPadding="false"
         android:paddingBottom="@dimen/history_divider_padding"
+        android:visibility="invisible"
         app:layoutManager="LinearLayoutManager"
         app:reverseLayout="true" />
+
 </LinearLayout>
diff --git a/res/layout/history_item.xml b/res/layout/history_item.xml
index eaa95ad..cf8b6c0 100644
--- a/res/layout/history_item.xml
+++ b/res/layout/history_item.xml
@@ -22,6 +22,7 @@
     android:paddingTop="@dimen/history_divider_padding"
     android:clipChildren="false"
     android:clipToPadding="false"
+    android:importantForAccessibility="no"
     android:orientation="vertical">
 
     <View
diff --git a/src/com/android/calculator2/Calculator.java b/src/com/android/calculator2/Calculator.java
index 970683b..5019fdf 100644
--- a/src/com/android/calculator2/Calculator.java
+++ b/src/com/android/calculator2/Calculator.java
@@ -34,6 +34,7 @@ import android.animation.PropertyValuesHolder;
 import android.app.ActionBar;
 import android.app.Activity;
 import android.app.FragmentManager;
+import android.app.FragmentTransaction;
 import android.content.ClipData;
 import android.content.DialogInterface;
 import android.content.Intent;
@@ -66,7 +67,6 @@ import android.view.ViewAnimationUtils;
 import android.view.ViewGroupOverlay;
 import android.view.ViewTreeObserver;
 import android.view.animation.AccelerateDecelerateInterpolator;
-import android.widget.FrameLayout;
 import android.widget.HorizontalScrollView;
 import android.widget.TextView;
 import android.widget.Toolbar;
@@ -252,7 +252,6 @@ public class Calculator extends Activity
     private CalculatorResult mResultText;
     private HorizontalScrollView mFormulaContainer;
     private DragLayout mDragLayout;
-    private FrameLayout mHistoryFrame;
 
     private ViewPager mPadViewPager;
     private View mDeleteButton;
@@ -280,8 +279,6 @@ public class Calculator extends Activity
     // Whether the display is one line.
     private boolean mIsOneLine;
 
-    private HistoryFragment mHistoryFragment = new HistoryFragment();
-
     /**
      * Map the old saved state to a new state reflecting requested result reevaluation.
      */
@@ -421,8 +418,6 @@ public class Calculator extends Activity
         mDragLayout.addDragCallback(this);
         mDragLayout.setCloseCallback(this);
 
-        mHistoryFrame = (FrameLayout) findViewById(R.id.history_frame);
-
         mFormulaText.setOnContextMenuClickListener(mOnFormulaContextMenuClickListener);
         mFormulaText.setOnDisplayMemoryOperationsListener(mOnDisplayMemoryOperationsListener);
 
@@ -596,8 +591,10 @@ public class Calculator extends Activity
     public boolean dispatchTouchEvent(MotionEvent e) {
         if (e.getActionMasked() == MotionEvent.ACTION_DOWN) {
             stopActionModeOrContextMenu();
-            if (mDragLayout.isOpen()) {
-                mHistoryFragment.stopActionModeOrContextMenu();
+
+            final HistoryFragment historyFragment = getHistoryFragment();
+            if (mDragLayout.isOpen() && historyFragment != null) {
+                historyFragment.stopActionModeOrContextMenu();
             }
         }
         return super.dispatchTouchEvent(e);
@@ -606,9 +603,9 @@ public class Calculator extends Activity
     @Override
     public void onBackPressed() {
         if (!stopActionModeOrContextMenu()) {
-            if (mDragLayout.isOpen()) {
-                if (!mHistoryFragment.stopActionModeOrContextMenu()) {
-                    mDragLayout.setClosed();
+            final HistoryFragment historyFragment = getHistoryFragment();
+            if (mDragLayout.isOpen() && historyFragment != null) {
+                if (!historyFragment.stopActionModeOrContextMenu()) {
                     removeHistoryFragment();
                 }
                 return;
@@ -1278,8 +1275,7 @@ public class Calculator extends Activity
     public boolean onOptionsItemSelected(MenuItem item) {
         switch (item.getItemId()) {
             case R.id.menu_history:
-                showHistoryFragment();
-                mDragLayout.setOpen();
+                showHistoryFragment(FragmentTransaction.TRANSIT_FRAGMENT_OPEN);
                 return true;
             case R.id.menu_leading:
                 displayFull();
@@ -1306,21 +1302,23 @@ public class Calculator extends Activity
 
     /* Begin override DragCallback methods */
 
-    @Override
     public void onStartDraggingOpen() {
-        showHistoryFragment();
+        mDisplayView.hideToolbar();
+        showHistoryFragment(FragmentTransaction.TRANSIT_NONE);
+    }
+
+    @Override
+    public void onInstanceStateRestored(boolean isOpen) {
     }
 
     @Override
     public void whileDragging(float yFraction) {
-        // no-op
     }
 
     @Override
     public boolean shouldCaptureView(View view, int x, int y) {
-        return mDragLayout.isMoving()
-                || mDragLayout.isOpen()
-                || mDragLayout.isViewUnder(mDisplayView, x, y);
+        return view.getId() == R.id.history_frame
+            && (mDragLayout.isMoving() || mDragLayout.isViewUnder(view, x, y));
     }
 
     @Override
@@ -1328,11 +1326,6 @@ public class Calculator extends Activity
         return mDisplayView.getMeasuredHeight();
     }
 
-    @Override
-    public void onLayout(int translation) {
-        mHistoryFrame.setTranslationY(translation + mDisplayView.getBottom());
-    }
-
     /* End override DragCallback methods */
 
     /**
@@ -1358,27 +1351,34 @@ public class Calculator extends Activity
         return true;
     }
 
-    private void showHistoryFragment() {
+    private HistoryFragment getHistoryFragment() {
+        final FragmentManager manager = getFragmentManager();
+        if (manager == null || manager.isDestroyed()) {
+            return null;
+        }
+        return (HistoryFragment) manager.findFragmentByTag(HistoryFragment.TAG);
+    }
+
+    private void showHistoryFragment(int transit) {
         final FragmentManager manager = getFragmentManager();
         if (manager == null || manager.isDestroyed()) {
             return;
         }
-        if (!prepareForHistory()) {
+
+        if (getHistoryFragment() != null || !prepareForHistory()) {
             return;
         }
-        if (!mDragLayout.isOpen()) {
-            stopActionModeOrContextMenu();
 
-            manager.beginTransaction()
-                    .replace(R.id.history_frame, mHistoryFragment, HistoryFragment.TAG)
-                    .addToBackStack(HistoryFragment.TAG)
-                    .commit();
-            manager.executePendingTransactions();
+        stopActionModeOrContextMenu();
+        manager.beginTransaction()
+                .replace(R.id.history_frame, new HistoryFragment(), HistoryFragment.TAG)
+                .setTransition(transit)
+                .addToBackStack(HistoryFragment.TAG)
+                .commit();
 
-            // When HistoryFragment is visible, hide all descendants of the main Calculator view.
-            mMainCalculator.setImportantForAccessibility(
-                    View.IMPORTANT_FOR_ACCESSIBILITY_NO_HIDE_DESCENDANTS);
-        }
+        // When HistoryFragment is visible, hide all descendants of the main Calculator view.
+        mMainCalculator.setImportantForAccessibility(
+                View.IMPORTANT_FOR_ACCESSIBILITY_NO_HIDE_DESCENDANTS);
         // TODO: pass current scroll position of result
     }
 
diff --git a/src/com/android/calculator2/DragController.java b/src/com/android/calculator2/DragController.java
index e90d1b4..8810e7d 100644
--- a/src/com/android/calculator2/DragController.java
+++ b/src/com/android/calculator2/DragController.java
@@ -94,9 +94,17 @@ public final class DragController {
     }
 
     public void animateViews(float yFraction, RecyclerView recyclerView) {
-        final HistoryAdapter.ViewHolder vh = (HistoryAdapter.ViewHolder)
-                recyclerView.findViewHolderForAdapterPosition(0);
-        if (yFraction > 0) {
+        if (mDisplayFormula == null
+                || mDisplayResult == null
+                || mToolbar == null
+                || mEvaluator == null) {
+            // Bail if we aren't yet initialized.
+            return;
+        }
+
+        final HistoryAdapter.ViewHolder vh =
+                (HistoryAdapter.ViewHolder) recyclerView.findViewHolderForAdapterPosition(0);
+        if (yFraction > 0 && vh != null) {
             recyclerView.setVisibility(View.VISIBLE);
         }
         if (vh != null && !EvaluatorStateUtils.isDisplayEmpty(mEvaluator)) {
@@ -123,40 +131,37 @@ public final class DragController {
                 mAnimationInitialized = true;
             }
 
-            if (mAnimationInitialized) {
-                result.setScaleX(mAnimationController.getResultScale(yFraction));
-                result.setScaleY(mAnimationController.getResultScale(yFraction));
+            result.setScaleX(mAnimationController.getResultScale(yFraction));
+            result.setScaleY(mAnimationController.getResultScale(yFraction));
 
-                formula.setScaleX(mAnimationController.getFormulaScale(yFraction));
-                formula.setScaleY(mAnimationController.getFormulaScale(yFraction));
+            formula.setScaleX(mAnimationController.getFormulaScale(yFraction));
+            formula.setScaleY(mAnimationController.getFormulaScale(yFraction));
 
-                formula.setPivotX(formula.getWidth() - formula.getPaddingEnd());
-                formula.setPivotY(formula.getHeight() - formula.getPaddingBottom());
+            formula.setPivotX(formula.getWidth() - formula.getPaddingEnd());
+            formula.setPivotY(formula.getHeight() - formula.getPaddingBottom());
 
-                result.setPivotX(result.getWidth() - result.getPaddingEnd());
-                result.setPivotY(result.getHeight() - result.getPaddingBottom());
+            result.setPivotX(result.getWidth() - result.getPaddingEnd());
+            result.setPivotY(result.getHeight() - result.getPaddingBottom());
 
-                formula.setTranslationX(mAnimationController.getFormulaTranslationX(yFraction));
-                formula.setTranslationY(mAnimationController.getFormulaTranslationY(yFraction));
+            formula.setTranslationX(mAnimationController.getFormulaTranslationX(yFraction));
+            formula.setTranslationY(mAnimationController.getFormulaTranslationY(yFraction));
 
-                result.setTranslationX(mAnimationController.getResultTranslationX(yFraction));
-                result.setTranslationY(mAnimationController.getResultTranslationY(yFraction));
+            result.setTranslationX(mAnimationController.getResultTranslationX(yFraction));
+            result.setTranslationY(mAnimationController.getResultTranslationY(yFraction));
 
-                formula.setTextColor((int) mColorEvaluator.evaluate(yFraction, mFormulaStartColor,
-                        mFormulaEndColor));
+            formula.setTextColor((int) mColorEvaluator.evaluate(yFraction, mFormulaStartColor,
+                    mFormulaEndColor));
 
-                result.setTextColor((int) mColorEvaluator.evaluate(yFraction, mResultStartColor,
-                        mResultEndColor));
+            result.setTextColor((int) mColorEvaluator.evaluate(yFraction, mResultStartColor,
+                    mResultEndColor));
 
-                date.setTranslationY(mAnimationController.getDateTranslationY(yFraction));
-                divider.setTranslationY(mAnimationController.getDateTranslationY(yFraction));
-            }
+            date.setTranslationY(mAnimationController.getDateTranslationY(yFraction));
+            divider.setTranslationY(mAnimationController.getDateTranslationY(yFraction));
         } else if (EvaluatorStateUtils.isDisplayEmpty(mEvaluator)) {
             // There is no current expression but we still need to collect information
             // to translate the other viewholders.
             if (!mAnimationInitialized) {
                 mAnimationController.initializeDisplayHeight();
-
                 mAnimationInitialized = true;
             }
         }
@@ -180,11 +185,8 @@ public final class DragController {
 
     /**
      * Reset all initialized values.
-     * If the DragLayout is closed, set recyclerview to INVISIBLE to avoid flickering.
      */
-    public void initializeAnimation(RecyclerView recyclerView, boolean isResult,
-            boolean oneLine, boolean isOpen) {
-        recyclerView.setVisibility(isOpen ? View.VISIBLE : View.INVISIBLE);
+    public void initializeAnimation(boolean isResult, boolean oneLine) {
         mAnimationInitialized = false;
         initializeController(isResult, oneLine);
     }
@@ -245,7 +247,7 @@ public final class DragController {
 
         public void initializeScales(AlignedTextView formula, CalculatorResult result) {
             // Calculate the scale for the text
-            mFormulaScale = (mDisplayFormula.getTextSize() * 1.0f) / formula.getTextSize();
+            mFormulaScale = mDisplayFormula.getTextSize() / formula.getTextSize();
         }
 
         public void initializeFormulaTranslationY(AlignedTextView formula,
@@ -281,38 +283,37 @@ public final class DragController {
         }
 
         public float getResultTranslationX(float yFraction) {
-            return (mResultTranslationX * yFraction) - mResultTranslationX;
+            return mResultTranslationX * (yFraction - 1f);
         }
 
         public float getResultTranslationY(float yFraction) {
-            return (mResultTranslationY * yFraction) - mResultTranslationY;
+            return mResultTranslationY * (yFraction - 1f);
         }
 
         public float getResultScale(float yFraction) {
-            return 1;
+            return 1f;
         }
 
         public float getFormulaScale(float yFraction) {
-            return mFormulaScale - (mFormulaScale * yFraction) + yFraction;
+            return mFormulaScale + (1f - mFormulaScale) * yFraction;
         }
 
         public float getFormulaTranslationX(float yFraction) {
-            return (mFormulaTranslationX * yFraction) -
-                    mFormulaTranslationX;
+            return mFormulaTranslationX * (yFraction - 1f);
         }
 
         public float getFormulaTranslationY(float yFraction) {
             // Scale linearly between -FormulaTranslationY and 0.
-            return (mFormulaTranslationY * yFraction) - mFormulaTranslationY;
+            return mFormulaTranslationY * (yFraction - 1f);
         }
 
         public float getDateTranslationY(float yFraction) {
             // We also want the date to start out above the visible screen with
             // this distance decreasing as it's pulled down.
             // Account for the scaled formula height.
-            return -mToolbar.getHeight() * (1 - yFraction)
+            return -mToolbar.getHeight() * (1f - yFraction)
                     + getFormulaTranslationY(yFraction)
-                    - mDisplayFormula.getHeight() /getFormulaScale(yFraction) * (1 - yFraction);
+                    - mDisplayFormula.getHeight() /getFormulaScale(yFraction) * (1f - yFraction);
         }
 
         public float getHistoryElementTranslationY(float yFraction) {
@@ -331,8 +332,7 @@ public final class DragController {
         public void initializeScales(AlignedTextView formula, CalculatorResult result) {
             final float textSize = mDisplayResult.getTextSize() * mDisplayResult.getScaleX();
             mResultScale = textSize / result.getTextSize();
-
-            mFormulaScale = 1;
+            mFormulaScale = 1f;
         }
 
         @Override
@@ -392,14 +392,14 @@ public final class DragController {
 
         @Override
         public float getFormulaScale(float yFraction) {
-            return 1;
+            return 1f;
         }
 
         @Override
         public float getDateTranslationY(float yFraction) {
             // We also want the date to start out above the visible screen with
             // this distance decreasing as it's pulled down.
-            return -mToolbar.getHeight() * (1 - yFraction)
+            return -mToolbar.getHeight() * (1f - yFraction)
                     + (mResultTranslationY * yFraction) - mResultTranslationY
                     - mDisplayFormula.getPaddingTop() +
                     (mDisplayFormula.getPaddingTop() * yFraction);
@@ -448,27 +448,27 @@ public final class DragController {
 
         @Override
         public float getResultTranslationX(float yFraction) {
-            return 0;
+            return 0f;
         }
 
         @Override
         public float getResultTranslationY(float yFraction) {
-            return 0;
+            return 0f;
         }
 
         @Override
         public float getFormulaScale(float yFraction) {
-            return 1;
+            return 1f;
         }
 
         @Override
         public float getDateTranslationY(float yFraction) {
-            return 0;
+            return 0f;
         }
 
         @Override
         public float getHistoryElementTranslationY(float yFraction) {
-            return -mDisplayHeight * (1 - yFraction) - mBottomPaddingHeight;
+            return -mDisplayHeight * (1f - yFraction) - mBottomPaddingHeight;
         }
 
         @Override
diff --git a/src/com/android/calculator2/DragLayout.java b/src/com/android/calculator2/DragLayout.java
index 9e83aa1..1848e38 100644
--- a/src/com/android/calculator2/DragLayout.java
+++ b/src/com/android/calculator2/DragLayout.java
@@ -16,6 +16,9 @@
 
 package com.android.calculator2;
 
+import android.animation.Animator;
+import android.animation.AnimatorListenerAdapter;
+import android.animation.ValueAnimator;
 import android.content.Context;
 import android.graphics.PointF;
 import android.graphics.Rect;
@@ -26,18 +29,17 @@ import android.support.v4.widget.ViewDragHelper;
 import android.util.AttributeSet;
 import android.view.MotionEvent;
 import android.view.View;
+import android.view.ViewGroup;
 import android.widget.FrameLayout;
-import android.widget.RelativeLayout;
 
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.concurrent.CopyOnWriteArrayList;
 
-public class DragLayout extends RelativeLayout {
+public class DragLayout extends ViewGroup {
 
-    private static final String TAG = "DragLayout";
-    private static final double AUTO_OPEN_SPEED_LIMIT = 800.0;
+    private static final double AUTO_OPEN_SPEED_LIMIT = 600.0;
     private static final String KEY_IS_OPEN = "IS_OPEN";
     private static final String KEY_SUPER_STATE = "SUPER_STATE";
 
@@ -51,8 +53,6 @@ public class DragLayout extends RelativeLayout {
     private final Map<Integer, PointF> mLastMotionPoints = new HashMap<>();
     private final Rect mHitRect = new Rect();
 
-    private int mDraggingState = ViewDragHelper.STATE_IDLE;
-    private int mDraggingBorder;
     private int mVerticalRange;
     private boolean mIsOpen;
 
@@ -68,28 +68,30 @@ public class DragLayout extends RelativeLayout {
     }
 
     @Override
-    protected void onLayout(boolean changed, int l, int t, int r, int b) {
-        super.onLayout(changed, l, t, r, b);
-        if (changed) {
-            for (DragCallback c : mDragCallbacks) {
-                c.onLayout(t - b);
-            }
-            if (mIsOpen) {
-                setOpen();
-            } else {
-                setClosed();
-            }
-        }
+    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
+        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
+        measureChildren(widthMeasureSpec, heightMeasureSpec);
     }
 
     @Override
-    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
-        int height = 0;
+    protected void onLayout(boolean changed, int l, int t, int r, int b) {
+        int displayHeight = 0;
         for (DragCallback c : mDragCallbacks) {
-            height += c.getDisplayHeight();
+            displayHeight = Math.max(displayHeight, c.getDisplayHeight());
+        }
+        mVerticalRange = getHeight() - displayHeight;
+
+        final int childCount = getChildCount();
+        for (int i = 0; i < childCount; ++i) {
+            final View child = getChildAt(i);
+
+            int top = 0;
+            if (child == mHistoryFrame) {
+                top = mDragHelper.getViewDragState() != ViewDragHelper.STATE_IDLE
+                        ? child.getTop() : (mIsOpen ? 0 : -mVerticalRange);
+            }
+            child.layout(0, top, child.getMeasuredWidth(), top + child.getMeasuredHeight());
         }
-        mVerticalRange = h - height;
-        super.onSizeChanged(w, h, oldw, oldh);
     }
 
     @Override
@@ -105,6 +107,11 @@ public class DragLayout extends RelativeLayout {
         if (state instanceof Bundle) {
             final Bundle bundle = (Bundle) state;
             mIsOpen = bundle.getBoolean(KEY_IS_OPEN);
+            mHistoryFrame.setVisibility(mIsOpen ? View.VISIBLE : View.INVISIBLE);
+            for (DragCallback c : mDragCallbacks) {
+                c.onInstanceStateRestored(mIsOpen);
+            }
+
             state = bundle.getParcelable(KEY_SUPER_STATE);
         }
         super.onRestoreInstanceState(state);
@@ -179,8 +186,9 @@ public class DragLayout extends RelativeLayout {
     }
 
     public boolean isMoving() {
-        return mDraggingState == ViewDragHelper.STATE_DRAGGING
-                || mDraggingState == ViewDragHelper.STATE_SETTLING;
+        final int draggingState = mDragHelper.getViewDragState();
+        return draggingState == ViewDragHelper.STATE_DRAGGING
+                || draggingState == ViewDragHelper.STATE_SETTLING;
     }
 
     public boolean isOpen() {
@@ -188,14 +196,54 @@ public class DragLayout extends RelativeLayout {
     }
 
     public void setOpen() {
-        mDragHelper.smoothSlideViewTo(mHistoryFrame, 0, mVerticalRange);
-        mHistoryFrame.setVisibility(VISIBLE);
-        mIsOpen = true;
+        if (!mIsOpen) {
+            mIsOpen = true;
+            mDragHelper.smoothSlideViewTo(mHistoryFrame, 0, 0);
+            mHistoryFrame.setVisibility(VISIBLE);
+        }
     }
 
     public void setClosed() {
-        mDragHelper.smoothSlideViewTo(mHistoryFrame, 0, 0);
-        mIsOpen = false;
+        if (mIsOpen) {
+            mIsOpen = false;
+            mDragHelper.smoothSlideViewTo(mHistoryFrame, 0, -mVerticalRange);
+            mHistoryFrame.setVisibility(View.INVISIBLE);
+
+            if (mCloseCallback != null) {
+                mCloseCallback.onClose();
+            }
+        }
+    }
+
+    public Animator createAnimator(final boolean toOpen) {
+        mHistoryFrame.setVisibility(VISIBLE);
+
+        final ValueAnimator animator = ValueAnimator.ofInt(mHistoryFrame.getTop(),
+                toOpen ? 0 : -mVerticalRange);
+        animator.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {
+            @Override
+            public void onAnimationUpdate(ValueAnimator animator) {
+                final int top = (int) animator.getAnimatedValue();
+                mHistoryFrame.offsetTopAndBottom(top - mHistoryFrame.getTop());
+
+                for (DragCallback c : mDragCallbacks) {
+                    // Top is between [-mVerticalRange, 0].
+                    c.whileDragging(1f + (float) top / mVerticalRange);
+                }
+            }
+        });
+        animator.addListener(new AnimatorListenerAdapter() {
+            @Override
+            public void onAnimationEnd(Animator animator) {
+                if (toOpen) {
+                    setOpen();
+                } else {
+                    setClosed();
+                }
+            }
+        });
+
+        return animator;
     }
 
     public void setCloseCallback(CloseCallback callback) {
@@ -228,6 +276,9 @@ public class DragLayout extends RelativeLayout {
         // Callback when a drag to open begins.
         void onStartDraggingOpen();
 
+        // Callback in onRestoreInstanceState.
+        void onInstanceStateRestored(boolean isOpen);
+
         // Animate the RecyclerView text.
         void whileDragging(float yFraction);
 
@@ -235,43 +286,26 @@ public class DragLayout extends RelativeLayout {
         boolean shouldCaptureView(View view, int x, int y);
 
         int getDisplayHeight();
-
-        void onLayout(int translation);
     }
 
     public class DragHelperCallback extends ViewDragHelper.Callback {
         @Override
         public void onViewDragStateChanged(int state) {
-            if (state == mDraggingState) {
-                // No change.
-                return;
-            }
-            if ((mDraggingState == ViewDragHelper.STATE_DRAGGING
-                    || mDraggingState == ViewDragHelper.STATE_SETTLING)
-                    && state == ViewDragHelper.STATE_IDLE) {
-                // The view stopped moving.
-                if (mDraggingBorder == 0) {
+            // The view stopped moving.
+            if (state == ViewDragHelper.STATE_IDLE) {
+                if (mDragHelper.getCapturedView().getTop() < -(mVerticalRange / 2)) {
                     setClosed();
-                    mHistoryFrame.setVisibility(GONE);
-                    if (mCloseCallback != null) {
-                        mCloseCallback.onClose();
-                    }
-                } else if (mDraggingBorder == mVerticalRange) {
+                } else {
                     setOpen();
                 }
-            } else if (state == ViewDragHelper.STATE_DRAGGING && !mIsOpen) {
-                onStartDragging();
             }
-            mDraggingState = state;
         }
 
         @Override
         public void onViewPositionChanged(View changedView, int left, int top, int dx, int dy) {
-            mDraggingBorder = top;
-
-            // Animate RecyclerView text.
             for (DragCallback c : mDragCallbacks) {
-                c.whileDragging(top / (mVerticalRange * 1.0f));
+                // Top is between [-mVerticalRange, 0].
+                c.whileDragging(1f + (float) top / mVerticalRange);
             }
         }
 
@@ -300,27 +334,32 @@ public class DragLayout extends RelativeLayout {
 
         @Override
         public int clampViewPositionVertical(View child, int top, int dy) {
-            final int topBound = getPaddingTop();
-            final int bottomBound = mVerticalRange;
-            return Math.min(Math.max(top, topBound), bottomBound);
+            return Math.max(Math.min(top, 0), -mVerticalRange);
+        }
+
+        @Override
+        public void onViewCaptured(View capturedChild, int activePointerId) {
+            super.onViewCaptured(capturedChild, activePointerId);
+
+            if (!mIsOpen) {
+                mIsOpen = true;
+                onStartDragging();
+            }
         }
 
         @Override
         public void onViewReleased(View releasedChild, float xvel, float yvel) {
-            boolean settleToOpen = false;
-            final float threshold = mVerticalRange / 2;
+            final boolean settleToOpen;
             if (yvel > AUTO_OPEN_SPEED_LIMIT) {
                 // Speed has priority over position.
                 settleToOpen = true;
             } else if (yvel < -AUTO_OPEN_SPEED_LIMIT) {
                 settleToOpen = false;
-            } else if (mDraggingBorder > threshold) {
-                settleToOpen = true;
-            } else if (mDraggingBorder < threshold) {
-                settleToOpen = false;
+            } else {
+                settleToOpen = releasedChild.getTop() > -(mVerticalRange / 2);
             }
 
-            if (mDragHelper.settleCapturedViewAt(0, settleToOpen ? mVerticalRange : 0)) {
+            if (mDragHelper.settleCapturedViewAt(0, settleToOpen ? 0 : -mVerticalRange)) {
                 ViewCompat.postInvalidateOnAnimation(DragLayout.this);
             }
         }
diff --git a/src/com/android/calculator2/HistoryFragment.java b/src/com/android/calculator2/HistoryFragment.java
index 883f229..64a94f9 100644
--- a/src/com/android/calculator2/HistoryFragment.java
+++ b/src/com/android/calculator2/HistoryFragment.java
@@ -16,8 +16,9 @@
 
 package com.android.calculator2;
 
+import android.animation.Animator;
 import android.app.Fragment;
-import android.graphics.Color;
+import android.app.FragmentTransaction;
 import android.os.Bundle;
 import android.support.v4.content.ContextCompat;
 import android.support.v7.widget.RecyclerView;
@@ -49,7 +50,6 @@ public class HistoryFragment extends Fragment implements DragLayout.DragCallback
     @Override
     public void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
-
         mAdapter = new HistoryAdapter(mDataSet);
     }
 
@@ -59,6 +59,9 @@ public class HistoryFragment extends Fragment implements DragLayout.DragCallback
         final View view = inflater.inflate(
                 R.layout.fragment_history, container, false /* attachToRoot */);
 
+        mDragLayout = (DragLayout) container.getRootView().findViewById(R.id.drag_layout);
+        mDragLayout.addDragCallback(this);
+
         mRecyclerView = (RecyclerView) view.findViewById(R.id.history_recycler_view);
         mRecyclerView.addOnScrollListener(new RecyclerView.OnScrollListener() {
             @Override
@@ -110,10 +113,6 @@ public class HistoryFragment extends Fragment implements DragLayout.DragCallback
         final boolean isResultLayout = activity.isResultLayout();
         final boolean isOneLine = activity.isOneLine();
 
-        mDragLayout = (DragLayout) activity.findViewById(R.id.drag_layout);
-        mDragLayout.removeDragCallback(this);
-        mDragLayout.addDragCallback(this);
-
         if (mEvaluator != null) {
             initializeController(isResultLayout, isOneLine);
 
@@ -135,9 +134,8 @@ public class HistoryFragment extends Fragment implements DragLayout.DragCallback
                 newDataSet.add(null);
             }
             final boolean isEmpty = newDataSet.isEmpty();
-            mRecyclerView.setBackgroundColor(isEmpty
-                    ? ContextCompat.getColor(activity, R.color.empty_history_color)
-                    : Color.TRANSPARENT);
+            mRecyclerView.setBackgroundColor(ContextCompat.getColor(activity,
+                    isEmpty ? R.color.empty_history_color : R.color.display_background_color));
             if (isEmpty) {
                 newDataSet.add(new HistoryItem());
             }
@@ -153,37 +151,43 @@ public class HistoryFragment extends Fragment implements DragLayout.DragCallback
     @Override
     public void onStart() {
         super.onStart();
+
         final Calculator activity = (Calculator) getActivity();
-        // The orientation may have changed.
-        mDragController.initializeAnimation(mRecyclerView,
-                activity.isResultLayout(), activity.isOneLine(), mDragLayout.isOpen());
+        mDragController.initializeAnimation(activity.isResultLayout(), activity.isOneLine());
     }
 
     @Override
-    public void onDestroyView() {
-        final DragLayout dragLayout = (DragLayout) getActivity().findViewById(R.id.drag_layout);
-        if (dragLayout != null) {
-            dragLayout.removeDragCallback(this);
+    public Animator onCreateAnimator(int transit, boolean enter, int nextAnim) {
+        if (enter) {
+            if (transit == FragmentTransaction.TRANSIT_FRAGMENT_OPEN) {
+                return mDragLayout.createAnimator(true /* toOpen */);
+            } else {
+                return null;
+            }
+        }
+        return mDragLayout.createAnimator(false /* toOpen */);
+    }
+
+    @Override
+    public void onDestroy() {
+        super.onDestroy();
+
+        if (mDragLayout != null) {
+            mDragLayout.removeDragCallback(this);
         }
 
         // Note that the view is destroyed when the fragment backstack is popped, so
         // these are essentially called when the DragLayout is closed.
         mEvaluator.cancelNonMain();
-
-        super.onDestroyView();
     }
 
     private void initializeController(boolean isResult, boolean isOneLine) {
         mDragController.setDisplayFormula(
                 (CalculatorFormula) getActivity().findViewById(R.id.formula));
-
         mDragController.setDisplayResult(
                 (CalculatorResult) getActivity().findViewById(R.id.result));
-
         mDragController.setToolbar(getActivity().findViewById(R.id.toolbar));
-
         mDragController.setEvaluator(mEvaluator);
-
         mDragController.initializeController(isResult, isOneLine);
     }
 
@@ -211,15 +215,22 @@ public class HistoryFragment extends Fragment implements DragLayout.DragCallback
     }
 
     @Override
+    public void onInstanceStateRestored(boolean isOpen) {
+        if (isOpen) {
+            mRecyclerView.setVisibility(View.VISIBLE);
+        }
+    }
+
+    @Override
     public void whileDragging(float yFraction) {
-        mDragController.animateViews(yFraction, mRecyclerView);
+        if (isVisible() || isRemoving()) {
+            mDragController.animateViews(yFraction, mRecyclerView);
+        }
     }
 
     @Override
     public boolean shouldCaptureView(View view, int x, int y) {
-        return view.getId() == R.id.history_frame
-                && mDragLayout.isViewUnder(view, x, y)
-                && !mRecyclerView.canScrollVertically(1 /* scrolling down */);
+        return !mRecyclerView.canScrollVertically(1 /* scrolling down */);
     }
 
     @Override
@@ -227,10 +238,5 @@ public class HistoryFragment extends Fragment implements DragLayout.DragCallback
         return 0;
     }
 
-    @Override
-    public void onLayout(int translation) {
-        // no-op
-    }
-
     /* End override DragCallback methods. */
 }