Add ContributionsActivityPagerAdapter inner class to manage view pager

app/src/main/java/fr/free/nrw/commons/contributions/ContributionsActivity.java

@@ -3,7 +3,11 @@ package fr.free.nrw.commons.contributions;
 import android.content.Intent;
 import android.os.Bundle;
 import android.support.design.widget.TabLayout;
+import android.support.v4.app.Fragment;
+import android.support.v4.app.FragmentManager;
+import android.support.v4.app.FragmentPagerAdapter;
 import android.support.v4.view.ViewPager;
+import android.util.Log;
 
 import javax.inject.Inject;
 
@@ -13,6 +17,7 @@ import fr.free.nrw.commons.BuildConfig;
 import fr.free.nrw.commons.R;
 import fr.free.nrw.commons.auth.AuthenticatedActivity;
 import fr.free.nrw.commons.auth.SessionManager;
+import fr.free.nrw.commons.nearby.NearbyFragment;
 import fr.free.nrw.commons.upload.UploadService;
 
 import static android.content.ContentResolver.requestSync;
@@ -31,7 +36,7 @@ public  class       ContributionsActivity
     public Intent uploadServiceIntent;
     public boolean isAuthCookieAcquired = false;
 
-    //public ContributionsActivityPagerAdapter contributionsActivityPagerAdapter;
+    public ContributionsActivityPagerAdapter contributionsActivityPagerAdapter;
     private final int CONTRIBUTIONS_TAB_POSITION = 0;
     private final int NEARBY_TAB_POSITION = 1;
 
@@ -90,4 +95,105 @@ public  class       ContributionsActivity
 
     }
 
+    public class ContributionsActivityPagerAdapter extends FragmentPagerAdapter {
+        FragmentManager fragmentManager;
+        private boolean isContributionsListFragment = true; // to know what to put in first tab, Contributions of Media Details
+        public ContributionsFragment contributionsFragment;
+        public NearbyFragment nearbyFragment;
+
+        public ContributionsActivityPagerAdapter(FragmentManager fragmentManager) {
+            super(fragmentManager);
+            this.fragmentManager = fragmentManager;
+        }
+
+        @Override
+        public int getCount() {
+            return 2;
+        }
+
+        /*
+         * Do not use getItem method to access fragments on pager adapter. User reference vairables
+         * instead.
+         * */
+        @Override
+        public Fragment getItem(int position) {
+            switch (position){
+                case 0:
+                    ContributionsFragment retainedContributionsFragment = getContributionsFragment(0);
+                    if (retainedContributionsFragment != null) {
+
+                        /**
+                         * ContributionsFragment is parent of ContributionsListFragment and
+                         * MediaDetailsFragment. If below decides which child will be visible.
+                         */
+                        if (isContributionsListFragment) {
+                            // TODO: Neslihan retainedContributionsFragment.setContributionsListFragment();
+                        } else {
+                            // TODO: Neslihan retainedContributionsFragment.setMediaDetailPagerFragment();
+                        }
+                        contributionsFragment = retainedContributionsFragment;
+                        return retainedContributionsFragment;
+                    }
+
+                    // If we reach here, retainedContributionsFragment is null
+                    contributionsFragment = new ContributionsFragment();
+                    String contributionsFragmentTag = makeFragmentName(R.id.pager, 0);
+                    fragmentManager.beginTransaction()
+                            .replace(R.id.pager, contributionsFragment, contributionsFragmentTag)
+                            .addToBackStack(contributionsFragmentTag)
+                            .commit();
+
+                    return contributionsFragment;
+                case 1:
+                    if (getNearbyFragment(1) != null) {
+                        return getNearbyFragment(1);
+                    }
+                    return new NearbyFragment();// nearby places needs photo
+                default:
+                    return null;
+            }
+        }
+
+        /**
+         * Generates fragment tag with makeFragmentName method to get retained contributions fragment
+         * @param position index of tabs, in our case 0 or 1
+         * @return
+         */
+        private ContributionsFragment getContributionsFragment(int position) {
+            String tag = makeFragmentName(R.id.pager, position);
+            return (ContributionsFragment)fragmentManager.findFragmentByTag(tag);
+        }
+
+        /**
+         * Generates fragment tag with makeFragmentName method to get retained nearby fragment
+         * @param position index of tabs, in our case 0 or 1
+         * @return
+         */
+        private NearbyFragment getNearbyFragment(int position) {
+            String tag = makeFragmentName(R.id.pager, position);
+            return (NearbyFragment)fragmentManager.findFragmentByTag(tag);
+        }
+
+        /**
+         * A simple hack to use retained fragment when getID is called explicitly, if we don't use
+         * this method, a new fragment will be initialized on each explicit calls of getID
+         * @param viewId id of view pager
+         * @param index index of tabs, in our case 0 or 1
+         * @return
+         */
+        public String makeFragmentName(int viewId, int index) {
+            return "android:switcher:" + viewId + ":" + index;
+        }
+
+        /**
+         * In first tab we can have ContributionsFragment or Media details fragment. This method
+         * is responsible to update related boolean
+         * @param isContributionsListFragment true when contribution fragment should be visible, false
+         *                                means user clicked to MediaDetails
+         */
+        private void updateContributionFragmentTabContent(boolean isContributionsListFragment) {
+            this.isContributionsListFragment = isContributionsListFragment;
+        }
+    }
+
 }