fix an issue that a data in cookies may be lost

AnkiDroid/src/main/java/com/ichi2/anki/AbstractFlashcardViewer.java

@@ -913,6 +913,9 @@ public abstract class AbstractFlashcardViewer extends NavigationDrawerActivity {
 
         pauseTimer();
         mSoundPlayer.stopSounds();
+
+        // Prevent loss of data in Cookies
+        CompatHelper.getCompat().flushWebViewCookies();
     }
 
 
@@ -939,6 +942,11 @@ public abstract class AbstractFlashcardViewer extends NavigationDrawerActivity {
         if (mUnmountReceiver != null) {
             unregisterReceiver(mUnmountReceiver);
         }
+        // WebView.destroy() should be called after the end of use
+        // http://developer.android.com/reference/android/webkit/WebView.html#destroy()
+        mCardFrame.removeAllViews();
+        destroyWebView(mCard);
+        destroyWebView(mNextCard);
     }
 
 
@@ -1491,6 +1499,16 @@ public abstract class AbstractFlashcardViewer extends NavigationDrawerActivity {
     }
 
 
+    private void destroyWebView(WebView webView) {
+        if (webView != null) {
+            webView.stopLoading();
+            webView.setWebChromeClient(null);
+            webView.setWebViewClient(null);
+            webView.destroy();
+        }
+    }
+
+
     protected void showEaseButtons() {
         Resources res = getResources();
 
@@ -2192,7 +2210,7 @@ public abstract class AbstractFlashcardViewer extends NavigationDrawerActivity {
             mNextCard.loadDataWithBaseURL(mBaseUrl + "__viewer__.html", mCardContent.toString(), "text/html", "utf-8", null);
             mNextCard.setVisibility(View.VISIBLE);
             mCardFrame.removeView(mCard);
-            mCard.destroy();
+            destroyWebView(mCard);
             mCard = mNextCard;
             mNextCard = createWebView();
             mNextCard.setVisibility(View.GONE);

AnkiDroid/src/main/java/com/ichi2/anki/AnkiDroidApp.java

@@ -176,6 +176,9 @@ public class AnkiDroidApp extends Application {
         // Configure WebView to allow file scheme pages to access cookies.
         CompatHelper.getCompat().enableCookiesForFileSchemePages();
 
+        // Prepare Cookies to be synchronized between RAM and permanent storage.
+        CompatHelper.getCompat().prepareWebViewCookies(this.getApplicationContext());
+
         // Set good default values for swipe detection
         final ViewConfiguration vc = ViewConfiguration.get(this);
         DEFAULT_SWIPE_MIN_DISTANCE = vc.getScaledPagingTouchSlop();

AnkiDroid/src/main/java/com/ichi2/compat/Compat.java

@@ -60,5 +60,7 @@ public interface Compat {
     void openUrl(AnkiActivity activity, Uri uri);
     void supportAddContentMenu(final DeckPicker a);
     Intent getPreferenceSubscreenIntent(Context context, String subscreen);
+    void prepareWebViewCookies(Context context);
+    void flushWebViewCookies();
 }
 

AnkiDroid/src/main/java/com/ichi2/compat/CompatV10.java

@@ -14,6 +14,7 @@ import android.support.annotation.NonNull;
 import android.support.v4.content.ContextCompat;
 import android.view.View;
 import android.view.WindowManager;
+import android.webkit.CookieSyncManager;
 import android.widget.EditText;
 import android.widget.LinearLayout;
 import android.widget.RemoteViews;
@@ -73,6 +74,18 @@ public class CompatV10 implements Compat {
         Timber.w("Cookies not supported in API version %d", CompatHelper.getSdkVersion());
     }
 
+    // CookieSyncManager is need to be initialized before use.
+    // Note: CookieSyncManager is deprecated since API level 21, but still need to be used here.
+    public void prepareWebViewCookies(Context context) {
+        CookieSyncManager.createInstance(context);
+    }
+
+    // A data of cookies may be lost when an application exists just after it was written.
+    // Below API level 21, this problem can be solved by using CookieSyncManager.sync().
+    // Note: CookieSyncManager is deprecated since API level 21, but still need to be used here.
+    public void flushWebViewCookies() {
+        CookieSyncManager.getInstance().sync();
+    }
 
     // Below API level 16, widget dimensions cannot be adjusted
     public void updateWidgetDimensions(Context context, RemoteViews updateViews, Class<?> cls) {

AnkiDroid/src/main/java/com/ichi2/compat/CompatV21.java

@@ -2,11 +2,13 @@
 package com.ichi2.compat;
 
 import android.annotation.TargetApi;
+import android.content.Context;
 import android.content.res.TypedArray;
 import android.view.View;
+import android.webkit.CookieManager;
 
-/** Implementation of {@link Compat} for SDK level 19 */
-@TargetApi(19)
+/** Implementation of {@link Compat} for SDK level 21 */
+@TargetApi(21)
 public class CompatV21 extends CompatV19 implements Compat {
     @Override
     public void setSelectableBackground(View view) {
@@ -16,4 +18,17 @@ public class CompatV21 extends CompatV19 implements Compat {
         view.setBackgroundResource(ta.getResourceId(0, 0));
         ta.recycle();
     }
+
+    // On API level 21 and higher, CookieManager will be set automatically, so there is nothing to do here.
+    @Override
+    public void prepareWebViewCookies(Context context) {
+
+    }
+
+    // A data of cookies may be lost when an application exists just after it was written.
+    // On API level 21 and higher, this problem can be solved by using CookieManager.flush().
+    @Override
+    public void flushWebViewCookies() {
+        CookieManager.getInstance().flush();
+    }
 }