Refactored welcome activity to decouple the view pager (#481)

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

@@ -0,0 +1,72 @@
+package fr.free.nrw.commons;
+
+import android.app.Activity;
+import android.content.Context;
+import android.support.v4.view.PagerAdapter;
+import android.view.LayoutInflater;
+import android.view.View;
+import android.view.ViewGroup;
+
+import butterknife.ButterKnife;
+import butterknife.OnClick;
+
+public class WelcomePagerAdapter extends PagerAdapter {
+
+    private Context context;
+
+    private static final int PAGE_FINAL = 4;
+
+    static final int[] PAGE_LAYOUTS = new int[]{
+            R.layout.welcome_wikipedia,
+            R.layout.welcome_do_upload,
+            R.layout.welcome_dont_upload,
+            R.layout.welcome_image_details,
+            R.layout.welcome_final
+    };
+
+    public WelcomePagerAdapter(Context context) {
+        this.context = context;
+    }
+
+    @Override
+    public int getCount() {
+        return PAGE_LAYOUTS.length;
+    }
+
+    @Override
+    public boolean isViewFromObject(View view, Object object) {
+        return (view == object);
+    }
+
+    @Override
+    public Object instantiateItem(ViewGroup container, int position) {
+        LayoutInflater inflater = LayoutInflater.from(context);
+        ViewGroup layout = (ViewGroup) inflater.inflate(PAGE_LAYOUTS[position], container, false);
+
+        if (position == PAGE_FINAL) {
+            ViewHolder holder = new ViewHolder(layout, context);
+            layout.setTag(holder);
+        }
+        container.addView(layout);
+        return layout;
+    }
+
+    @Override
+    public void destroyItem(ViewGroup container, int position, Object obj) {
+        container.removeView((View) obj);
+    }
+
+    public static class ViewHolder {
+        private Context context;
+
+        public ViewHolder(View view, Context context) {
+            ButterKnife.bind(this, view);
+            this.context = context;
+        }
+
+        @OnClick(R.id.welcomeYesButton)
+        void onClicked() {
+            ((Activity) context).finish();
+        }
+    }
+}