Activity Archive

During my last project I’ve encountered two problems which caused my app to crash which I did not expect to happen because the exact same code was working within other activities. These crashes were triggered by simple things like a press on the search button of the mobile phone or the creation of a dialog. When analyzing this behavior I’ve identified a nested TabHost View (a TabHost containing another TabHost which contains the Activity) as the cause for the crashes.

One TabHost contains another TabHost. The last TabHost contains the actual content.The problematic setup is shown in the picture to the left. As you can see the outer TabHost contains a Tab which Activity itself is the inner TabHost. The content of this TabHost will be placed between both tabs. So this is a design which doesn’t seem to be so out of the world.

In the following I’m going to point out the two problems and the solutions which I’ve found so that you might be able to solve similar problems. To demonstrate the problems I’ve created an example project which causes the errors and one which contains the proposed solutions where the functions are working.

In this post I will introduce a library which you can use to display elements from any java.util.Collection (e.g. LinkedList, ArrayList, HashSet, Queue, Stack, TreeSet,…) within a ListView or a Spinner. It’s even possible to use this library to implement any layout you want for your entries (e.g. a multi-line entry, an entry with including an image, etc…).

So this library is a very generic approach to display various data sets with custom layouts in a Spinner or a ListView.

To demonstrate the basic usage of this adapter I’ve created a simple example application which is using the library for a spinner adapter for a HashSet of UUIDs and a list adapter to display the single parts of the UUID together with an image.

Feel free to download the library and to reuse it within your own projects. You can also examine the source code at our Google code repository (http://code.google.com/p/android-collections-list-and-spinner-adapter/) by browsing the source or by checking it out using svn.

Basic Usage

Initialization

To ensure an easy usage of the library the CollectionsAdapter provides constructors which are closely related to the default android ArrayAdapter. The basic usage includes the use of the adapter with a simple TextView resource id or with a layout resource id and a TextView resource id which is located within this layout. When you are using this methods the objects toString method will be used to populate the give TextView element with text.

The corresponding constructors of the com.appsolut.adapter.collections. CollectionsAdapter<E>  class have the following signature:

public CollectionsAdapter(Context context, int textViewResourceId, Collection<E> content)
public CollectionsAdapter(Context context, int layoutResourceId, int textViewResourceId, Collection<E> content)

Where the parameters should be:

    • context – The context for the adapter. This could be your Activity.
    • layoutResourceId – If you are using the constructor which also uses a give layout resource id this parameter should point to this layout.
    • textViewResourceId – The resource id which is referring to the text view element used to fill in the text.
    • content – The collection containing the data. The collection can be modified afterwards.

 

In the example project this is demonstrated by the adapter for the spinner:

final Set<UUID> spinnerContent = new HashSet<UUID>();
for(int i = 0; i < 10; i++) {
	spinnerContent.add(UUID.randomUUID());
}
final CollectionsAdapter<UUID> spinnerAdapter = new CollectionsAdapter<UUID>(this, android.R.layout.simple_spinner_item, spinnerContent);
final Spinner spinner = (Spinner)findViewById(R.id.exampleSpinner);
spinner.setAdapter(spinnerAdapter);

Modifying the Data

If you later on want to modify the data you can either do this directly within the corresponding collection and then calling the notifyDataSetChanged() of the adapter to signal that you have modified the data or you can use the provided methods within the adapter which will take care of this. The signature of these methods is just like within the Collection interface:

      • public boolean add(E object)
      • public boolean addAll(Collection<? extends E> c)
      • public boolean remove(E object)
      • public boolean removeAll(Collection<?> c)
      • public void clear()

The boolean return value of the add and remove methods indicate if the data of your collection has been actually changed by the method call.

So as you can see the basic usage of the adapter is very simple.

 

Defining custom Layouts

If you want to use custom layouts within the adapter you can use the third available constructor of the com.appsolut.adapter.collections. CollectionsAdapter<E> class:

public CollectionsAdapter(Context context, Collection<E> content, ICollectionsAdapterViewFactory<E> viewFactory)

This constructor requires a context, the collection which will be used as data basis for the adapter and an com.appsolut.adapter.collections.view.ICollectionsAdapterViewFactory<E> viewFactory. The ICollectionsAdapterViewFactory<E> interface defines a method which will be called to display an element. During this method call you can create your own layout:

public View getView(int position, View convertView, ViewGroup parent, E item, LayoutInflater inflater, Context context)

If you have already created custom views for an adapter the method should look familiar to you. There are just some new parameters which ensure an easy creation of the view. If custom views within an adapter are completely new you should check out my previous posting on this topic first (http://xinfo.de/2011/03/using-custom-layouts-for-spinner-or-listview-entries-in-android/) and the return here.

So let’s take a look at the parameters of the method:

      • position – indicates the position of the element within the data set which should be displayed
      • convertView – “The old view to reuse, if possible. Note: You should check that this view is non-null and of an appropriate type before using. If it is not possible to convert this view to display the correct data, this method can create a new view.” – taken from the original Android API
      • parent – The parent view to which you should attach your view
      • item – The item which should be displayed by this view
      • inflater – The layout inflater service used to inflate xml layouts
      • context – The current context

In the example project I’ve implemented such an ICollectionsAdapterViewFactory which will populate a layout consisting of an ImageView and two TextViews:

public class NumberedViewFactory<T> implements
		ICollectionsAdapterViewFactory<T> {

	@Override
	public View getView(int position, View convertView,
			ViewGroup parent, T item, LayoutInflater inflater,
			Context context) {
		View layoutView;
		if (convertView == null) {
			layoutView = inflater.inflate(R.layout.listview_entry, parent, false);
		} else {
			layoutView = convertView;
		}
		final TextView segmentText = (TextView)layoutView.findViewById(R.id.textSegment);
		segmentText.setText(item.toString());

		final TextView segmentNumber = (TextView)layoutView.findViewById(R.id.segmentNumber);
		segmentNumber.setText(Integer.toString(position) + ". ");

		final ImageView imageView = (ImageView)layoutView.findViewById(R.id.image);
		if(position%2 == 0) {
			imageView.setBackgroundResource(android.R.drawable.presence_online);
		} else {
			imageView.setBackgroundResource(android.R.drawable.presence_invisible);
		}

		return layoutView;
	}

}

Summary

I hope you can use this flexible library to populate your ListViews and Spinners with various data sets. If you have any problems or suggestions on how to improve this library feel free to post comments.

by Kevin Kratzer

When developing Apps for the Android OS you might end up in the situation where you have common Activities or Services which could be reused in multiple Apps with just a little modification. As you don’t want to copy these classes into every single project – which would lead to a hardly maintainable code – you would seek a way to reference the source of these classes from multiple projects. For non-android applications an approach for this problem would be to break up the code into multiple libraries so that the required functionality can be referenced from multiple projects. As long as you are not interacting with external resources this attempt is also possible in Android projects (build a jar and reference it from the projects). But if you are using external resources these classes can’t be used within a library. This is caused by the fact that the Android SDK won’t generate matching ids within the “R” class for external resources which are included in a referenced jar file. In this post I want to show you different solutions for this problem which can avoid the necessity to copy the source code into the projects.