Android Activity launch modes

We often confused with Android Activity Launch Mode when we see android:launchMode attribute associated with <activity> element in manifest file, it defines the way of associated task. A task is a collection of activities that users interact with when performing any job. Task uses STACK to maintain the history of activites.

Android Activity has four Launch Mode as described below.

“standard” (Default Launch Mode)

<activity android:launchMode="standard" />

A new instance of activity is created every time in the task from which it was started. Multiple instances of activity can be create and each instance may belongs to different task.

“singleTop”

<activity android:launchMode="singleTop" />

If instance of activity is present on top of Task stack, a new instance will not be created and system will route activity intent information through onNewIntent(). If it is not present on top, a new instance of activity is created. Multiple instance of same activity can be created and each instance may belong to different task and pushed at the top of the stack.

“singleTask”

<activity android:launchMode="singleTask" />

A new task will be created and a new instance of the activity will be pushed at root of new task. If activity instance exists on the separate task then system routes the call to existing instance through onNewIntent() method. Only one instance will exist at a time. So clear the other existing instance at the top of singleTask instance and pushed its instance at top into the stack.

“singleInstance”

<activity android:launchMode="singleInstance" />

Only one instance of activity will exist at a time. System will not launch any other activity into task holding this type. It is always a single member of its task and activities started from here will open into separate task. means only one instance of activity will be in single task at a time.

Even multiple time using singleInstance instance of the activity. Always single instance is available for the same into separate stack. 

RecyclerView and CardView on Android

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If you are interested in building an Android app that makes use of lists to display data, Android Lollipop features two new widgets to make your life easier,RecyclerView and CardView. Using these widgets, it is very easy to give your app a look and feel that conforms to the guidelines mentioned in Google's material design specification.

Prerequisites

To follow along, you should be using the latest version of Android Studio. You can get it from the Android Developer website.

1. Support Older Versions

At the time of writing, less than 2% of Android devices run Android Lollipop. However, thanks to the v7 Support Library, you can use the RecyclerView andCardView widgets on devices that run older versions of Android by adding the following lines to the dependencies section in your project's build.grade file:

1 2	compile 'com.android.support:cardview-v7:21.0.+' compile 'com.android.support:recyclerview-v7:21.0.+'

2. Creating a CardView

A CardView is a ViewGroup. Like any other ViewGroup, it can be added to yourActivity or Fragment using a layout XML file.

To create an empty CardView, you would have to add the following code to your layout XML as shown in the following snippet:

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<android.support.v7.widget.CardView         xmlns:card_view="http://schemas.android.com/apk/res-auto"         android:layout_width="match_parent"         android:layout_height="wrap_content"> </android.support.v7.widget.CardView>

As a more realistic example, let us now create a LinearLayout and place aCardView inside it. The CardView could represent, for example, a person and contain the following:

• a TextView to display the name of the person
• a TextView to display the person's age
• an ImageView to display the person's photo

This is what the XML would look like:

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<?xml version="1.0" encoding="utf-8"?> <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"     android:layout_width="match_parent" android:layout_height="match_parent"     android:padding="16dp"     >     <android.support.v7.widget.CardView         android:layout_width="match_parent"         android:layout_height="wrap_content"         android:id="@+id/cv"         >         <RelativeLayout             android:layout_width="match_parent"             android:layout_height="wrap_content"             android:padding="16dp"             >             <ImageView                 android:layout_width="wrap_content"                 android:layout_height="wrap_content"                 android:id="@+id/person_photo"                 android:layout_alignParentLeft="true"                 android:layout_alignParentTop="true"                 android:layout_marginRight="16dp"                 />             <TextView                 android:layout_width="wrap_content"                 android:layout_height="wrap_content"                 android:id="@+id/person_name"                 android:layout_toRightOf="@+id/person_photo"                 android:layout_alignParentTop="true"                 android:textSize="30sp"                 />             <TextView                 android:layout_width="wrap_content"                 android:layout_height="wrap_content"                 android:id="@+id/person_age"                 android:layout_toRightOf="@+id/person_photo"                 android:layout_below="@+id/person_name"                 />         </RelativeLayout>     </android.support.v7.widget.CardView> </LinearLayout>

If this XML is used as the layout of an Activity, with the TextView and ImageViewfields are set to meaningful values, then this is how it would render on an Android device:

3. Creating a RecyclerView

Step 1: Defining It in a Layout

Using a RecyclerView instance is slightly more complicated. However, defining it in a layout XML file is quite simple. You can define it in a layout as follows:

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<android.support.v7.widget.RecyclerView     android:layout_width="match_parent"     android:layout_height="match_parent"     android:id="@+id/rv"     />

To obtain a handle to it in your Activity, use the following snippet:

1	RecyclerView rv = (RecyclerView)findViewById(R.id.rv);

If you are sure that the size of the RecyclerView won't be changing, you can add the following to improve performance:

1	rv.setHasFixedSize(true);

Step 2: Using a LayoutManager

Unlike a ListView, a RecyclerView needs a LayoutManager to manage the positioning of its items. You could define your own LayoutManager by extending theRecyclerView.LayoutManager class. However, in most cases, you could simply use one of the predefined LayoutManager subclasses:

• LinearLayoutManager
• GridLayoutManager
• StaggeredGridLayoutManager

In this tutorial, I am going to use a LinearLayoutManager. ThisLayoutManager subclass will, by default, make your RecyclerView look like aListView.

1 2	LinearLayoutManager llm = new LinearLayoutManager(context); rv.setLayoutManager(llm);

Step 3: Defining the Data

Just like a ListView, a RecyclerView needs an adapter to access its data. But before we create an adapter, let us create data that we can work with. Create a simple class to represent a person and then write a method to initialize a List ofPerson objects:

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class Person {     String name;     String age;     int photoId;     Person(String name, String age, int photoId) {         this.name = name;         this.age = age;         this.photoId = photoId;     } } private List<Person> persons; // This method creates an ArrayList that has three Person objects // Checkout the project associated with this tutorial on Github if // you want to use the same images. private void initializeData(){     persons = new ArrayList<>();     persons.add(new Person("Emma Wilson", "23 years old", R.drawable.emma));     persons.add(new Person("Lavery Maiss", "25 years old", R.drawable.lavery));     persons.add(new Person("Lillie Watts", "35 years old", R.drawable.lillie)); }

Step 4: Creating an Adapter

To create an adapter that a RecyclerView can use, you must extendRecyclerView.Adapter. This adapter follows the view holder design pattern, which means that it you to define a custom class that extends RecyclerView.ViewHolder. This pattern minimizes the number of calls to the costly findViewById method.

Earlier in this tutorial, we already defined the XML layout for a CardView that represents a person. We are going to reuse that layout now. Inside the constructor of our custom ViewHolder, initialize the views that belong to the items of ourRecyclerView.

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public class RVAdapter extends RecyclerView.Adapter<RVAdapter.PersonViewHolder>{     public static class PersonViewHolder extends RecyclerView.ViewHolder {              CardView cv;         TextView personName;         TextView personAge;         ImageView personPhoto;         PersonViewHolder(View itemView) {             super(itemView);             cv = (CardView)itemView.findViewById(R.id.cv);             personName = (TextView)itemView.findViewById(R.id.person_name);             personAge = (TextView)itemView.findViewById(R.id.person_age);             personPhoto = (ImageView)itemView.findViewById(R.id.person_photo);         }     } }

Next, add a constructor to the custom adapter so that it has a handle to the data that the RecyclerView displays. As our data is in the form of a List of Person objects, use the following code:

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List<Person> persons; RVAdapter(List<Person> persons){     this.persons = persons; }

RecyclerView.Adapter has three abstract methods that we must override. Let us start with the getItemCount method. This should return the number of items present in the data. As our data is in the form of a List, we only need to call the sizemethod on the List object:

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@Override public int getItemCount() {     return persons.size(); }

Next, override the onCreateViewHolder method. As its name suggests, this method is called when the custom ViewHolder needs to be initialized. We specify the layout that each item of the RecyclerView should use. This is done by inflating the layout using LayoutInflater, passing the output to the constructor of the customViewHolder.

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@Override public PersonViewHolder onCreateViewHolder(ViewGroup viewGroup, int i) {     View v = LayoutInflater.from(viewGroup.getContext()).inflate(R.layout.item, viewGroup, false);     PersonViewHolder pvh = new PersonViewHolder(v);     return pvh; }

Override the onBindViewHolder to specify the contents of each item of theRecyclerView. This method is very similar to the getView method of a ListView's adapter. In our example, here's where you have to set the values of the name, age, and photo fields of the CardView.

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@Override public void onBindViewHolder(PersonViewHolder personViewHolder, int i) {     personViewHolder.personName.setText(persons.get(i).name);     personViewHolder.personAge.setText(persons.get(i).age);     personViewHolder.personPhoto.setImageResource(persons.get(i).photoId); }

Finally, you need to override the onAttachedToRecyclerView method. For now, we can simply use the superclass's implementation of this method as shown below.

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@Override public void onAttachedToRecyclerView(RecyclerView recyclerView) {     super.onAttachedToRecyclerView(recyclerView); }

Step 5: Using the Adapter

Now that the adapter is ready, add the following code to your Activity to initialize and use the adapter by calling the adapter's constructor and the RecyclerView'ssetAdapter method:

1 2	RVAdapter adapter = new RVAdapter(persons); rv.setAdapter(adapter);

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Step 6: Compile and Run

When you run the RecyclerView example on an Android device, you should see something similar to the following result.

Conclusion

In this tutorial, you have learned how to use the CardView and RecyclerViewwidgets that were introduced in Android Lollipop. You have also seen examples of how to make use of these widgets in Material Design apps. Note that even though aRecyclerView can do almost everything a ListView can, for small datasets, using aListView is still preferable as it requires fewer lines of code.

You can refer to the Android Developers Reference for more information about theCardView and RecyclerView classes.