Android Image Slideshow using ViewPager with PagerAdapter

We’ve already discussed ViewPager in depth in one of my earlier posts. We saw how to use it with specific PagerAdapter implementations like FragmentPagerAdapter and FragmentStatePagerAdapter that works with fragments but we can also use it to inflate any other View or ViewGroup (with standard View hierarchy) by hooking it up with PagerAdapter itself. In this tutorial we’ll just discuss how to how to hook a PagerAdapter to a ViewPager to create an Image slideshow.

Setting Up

Firstly, we’ll need to have a ViewPager element in our layout file at res/layout/activity_main.xml:

[xml]
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
android:paddingBottom="@dimen/activity_vertical_margin"
android:id="@+id/relativeLayout">

<android.support.v4.view.ViewPager xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/pager"
android:layout_width="match_parent"
android:layout_height="match_parent">
</android.support.v4.view.ViewPager>

</RelativeLayout>
[/xml]

Next, in our onCreate() Activity method, we’ll instantiate our custom PagerAdapter implementation and bind it to our ViewPager object.

[java]
mCustomPagerAdapter = new CustomPagerAdapter(this);

mViewPager = (ViewPager) findViewById(R.id.pager);
mViewPager.setAdapter(mCustomPagerAdapter);
[/java]

In our Activity class we’ll have a mResources instance variable that’ll contain an array of drawable resource IDs:

[java]
int[] mResources = {
R.drawable.first,
R.drawable.second,
R.drawable.third,
R.drawable.fourth,
R.drawable.fifth,
R.drawable.sixth
};
[/java]

I downloaded six images from LoremPixel of size 400×600 and put them into the res/drawable directory. We’ll use these images in our carousel.

Finally it’s time to write our custom pager adapter implementation that’ll do the real job of populating content pages within our ViewPager:

[java]
class CustomPagerAdapter extends PagerAdapter {

Context mContext;
LayoutInflater mLayoutInflater;

public CustomPagerAdapter(Context context) {
mContext = context;
mLayoutInflater = (LayoutInflater) mContext.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
}

@Override
public int getCount() {
return mResources.length;
}

@Override
public boolean isViewFromObject(View view, Object object) {
return view == ((LinearLayout) object);
}

@Override
public Object instantiateItem(ViewGroup container, int position) {
View itemView = mLayoutInflater.inflate(R.layout.pager_item, container, false);

ImageView imageView = (ImageView) itemView.findViewById(R.id.imageView);
imageView.setImageResource(mResources[position]);

container.addView(itemView);

return itemView;
}

@Override
public void destroyItem(ViewGroup container, int position, Object object) {
container.removeView((LinearLayout) object);
}
}
[/java]

Let’s discuss the four methods that we’ve overridden (mandatory):

• getCount() – This method should return the number of views available, i.e., number of pages to be displayed/created in the ViewPager.
• instantiateItem() – This method should create the page for the given position passed to it as an argument. In our case, we inflate() our layout resource to create the hierarchy of view objects and then set resource for the ImageView in it. Finally, the inflated view is added to the container (which should be the ViewPager) and return it as well.
• destroyItem() – Removes the page from the container for the given position. We simply removed object using removeView() but could’ve also used removeViewAt() by passing it the position.
• isViewFromObject() – The object returned by instantiateItem() is a key/identifier. This method checks whether the View passed to it (representing the page) is associated with that key or not. It is required by a PagerAdapter to function properly. For our example, the implementation of this method is really simple, we just compare the two instances and return the evaluated boolean.

Here’s how res/layout/pager_item.xml will look like which is inflated in the instantiateItem() method:

[xml]
<?xml version="1.0" encoding="utf-8"?>

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical" android:layout_width="match_parent"
android:layout_height="match_parent">

<ImageView
android:layout_width="match_parent"
android:layout_height="match_parent"
android:id="@+id/imageView" />
</LinearLayout>
[/xml]

Now if you go and test your app in the emulator or a device, you’ll be able to play with a nice slideshow based on ViewPager with 6 pages each showing up an image from the drawable resources directory.

It is important to understand that as the user navigates to a particular page, the one next to it is generated by calling instantiateItem() while the one before the previous one gets destroyed by calling destroyItem(). This caching limit (destruction and rebuilding limit) can be specified by the setOffscreenPageLimit() method on the ViewPager object which is set to 1 by default. Increasing this value to a higher number leads to a smoother navigation as far as the animations and interactions are concerned as everything is retained in memory but then it can also cause a memory overhead affecting the app’s performance. So you’ve to find the perfect balance in your case.

Hope that helps!