Use callback pattern in Welcome activity/adapter.

Author: dbrant

app/src/main/java/fr/free/nrw/commons/WelcomeActivity.java

@@ -13,6 +13,7 @@ public class WelcomeActivity extends BaseActivity {
 
     @BindView(R.id.welcomePager) ViewPager pager;
     @BindView(R.id.welcomePagerIndicator) CirclePageIndicator indicator;
+    private WelcomePagerAdapter adapter = new WelcomePagerAdapter();
 
     @Override
     public void onCreate(Bundle savedInstanceState) {
@@ -24,11 +25,19 @@ public void onCreate(Bundle savedInstanceState) {
         }
         ButterKnife.bind(this);
 
-        setUpAdapter();
+        pager.setAdapter(adapter);
+        indicator.setViewPager(pager);
+        adapter.setCallback(new WelcomePagerAdapter.Callback() {
+            @Override
+            public void onYesClicked() {
+                finish();
+            }
+        });
     }
 
-    private void setUpAdapter() {
-        pager.setAdapter(new WelcomePagerAdapter());
-        indicator.setViewPager(pager);
+    @Override
+    public void onDestroy() {
+        adapter.setCallback(null);
+        super.onDestroy();
     }
 }

app/src/main/java/fr/free/nrw/commons/WelcomePagerAdapter.java

@@ -1,6 +1,6 @@
 package fr.free.nrw.commons;
 
-import android.app.Activity;
+import android.support.annotation.Nullable;
 import android.support.v4.view.PagerAdapter;
 import android.view.LayoutInflater;
 import android.view.View;
@@ -11,6 +11,11 @@
 
 public class WelcomePagerAdapter extends PagerAdapter {
     private static final int PAGE_FINAL = 4;
+    private Callback callback;
+
+    public interface Callback {
+        void onYesClicked();
+    }
 
     static final int[] PAGE_LAYOUTS = new int[]{
             R.layout.welcome_wikipedia,
@@ -20,6 +25,10 @@ public class WelcomePagerAdapter extends PagerAdapter {
             R.layout.welcome_final
     };
 
+    public void setCallback(@Nullable Callback callback) {
+        this.callback = callback;
+    }
+
     @Override
     public int getCount() {
         return PAGE_LAYOUTS.length;
@@ -48,14 +57,16 @@ public void destroyItem(ViewGroup container, int position, Object obj) {
         container.removeView((View) obj);
     }
 
-    public static class ViewHolder {
-        public ViewHolder(View view) {
+    class ViewHolder {
+        ViewHolder(View view) {
             ButterKnife.bind(this, view);
         }
 
         @OnClick(R.id.welcomeYesButton)
-        void onClicked(View view) {
-            ((Activity) view.getContext()).finish();
+        void onClicked() {
+            if (callback != null) {
+                callback.onYesClicked();
+            }
         }
     }
 }