ArrayAdapter (basic android adapter)

Introduction

Adapter is a bridge between the data source (Array object, List object…) and the user interface. The role of the adapter object (implements the Adapter interface) is to read data from different data sources, and based on them fill View objects members of a ViewGroup.
ViewGroup: ListView, RecyclerView, GridView, Spinner, Gallery whose contents are populated using adapters are collectively called “AdapterView”.

Using the adapter as a mediator between the “model” (data) and the View is a variation of the MVC pattern called MVA (ModelViewAdapter). This means that the Model and the View never communicate with each other, but through the Adapter (which is really just an “event-driven Controller”). The advantage of this pattern is that the View does not have to know anything about the model, and in this way a better separation of responsibilities is achieved.

There are several different adapters, and their choice depends on the AdapterView it is used for. The simplest adapter is “ArrayAdapter”, which takes data in the form of “ArrayList” and fills with it the content of View elements that are part of the container (“ListView”, GridView, Spinner). The “RecyclerView” uses its specialized “RecyclerView.adapter”, while the “ViewPager” uses the “PagerAdapter”.

ArrayAdapter creation procedure

The basic default Android Adapter is an “ArrayAdapter” that uses some of the default layouts built into Android.

a) Defining the data to be displayed

First we need data to be stored in a View, ArrayAdapter has different constructors, and therefore can receive different types of data:

• Array
• ArrayList
• Resource XML

b) Creating an AdapterView that should accept data

It has already been mentioned that AdapterView is actually a ViewGroup (ListView, RecyclerView…) whose content is filled with data with the help of an adapter, and it is necessary to define an AdapterView within XML.

Example

NOTE:
If we extend the activity with ListActivity (or for fragments with ListFragment), then we don’t have to define any layout, because in that case the activity (fragment) already contains by default a ListView that can receive data as a root view. In addition, ListActivity and ListFragment also allow us to “override” (override) the onListItemClick() method and thus define the action that will be performed by clicking on one of the list members. See the unified code from the example here.

c) Creating an adapter instance

There are multiple “ArrayAdapter” constructors that can accept different data types, although a generalized constructor might look like this:

Legend:

×

• Context
• Some predefined layout that comes with android (eg “R.layout.simple_list_item_1” see picture) or some custom layout. You can view the list of predefined layouts here.
• Data to be inserted yes (list, array…)

Example

ArrayAdapter requires that the array element type be declared as View (in this example as String).

NOTE:
ArrayAdapter by default in the background for each array item calls the “toString()” method and thus generates a TextView into which it puts the content of the array member. If we want a more complicated layout that has more elements (eg ImageView), then we need to create a custom ArrayAdapter which will be explained in another article.

In addition to this way of creating adapters where the method constructor is used, we can also use its method to create adapters
method createFromResource(). See all ArrayAdapter and RecyclerView.adapter methods here.

d) Connecting adapter instance to AdapterView

Connecting the adapter towith the corresponding View, it is done with the method setAdapter() (setListAdapter() in “ListActivity”).

With this, we have enabled the adapter to pick up data from a resource and insert it as a String into each AdapterView element that is styled with the selected predefined android layout (in the example: “android.R.layout.simple_list_item_1”)

f) Defining the click listener

In order to activate the action after clicking on one of the list elements, it is necessary to implement the AdapterView.OnItemClickListener interface and override the onItemClick() callback method. The method accepts four parameters, the first of which represents the parent view (in this example ListView).

See the unified code from the example here.