In Android app development, the layout of the screen is a crucial element that significantly affects the user experience. In this course, we will deeply explore how to construct screens using views in Android app development with Java. We will examine the various types of views, their usage, the development of custom views, and the screen composition using XML layout files step by step.
1. What is a View in Android?
A View is the fundamental element that makes up the user interface of an Android application. All UI elements displayed on the screen, such as buttons, text, and images, are views. Android provides the View class and various subclasses that inherit from it to effectively handle these views.
1.1 Types of Views
There are various types of views in Android. Here are the most commonly used view classes:
TextView: A view that displays text.EditText: A text box that can receive user input.Button: A clickable button.ImageView: A view that displays an image.LinearLayout: A layout that arranges child views either vertically or horizontally.
2. XML Layout Files
The UI of Android apps is mainly defined in XML files. These XML files represent the view hierarchy and allow you to set properties for each view. To create an XML layout file, follow these steps.
2.1 Creating a Layout File
1. Locate the res/layout folder in the project.
2. Right-click and select New > Layout Resource File.
3. Enter the file name and click OK.
2.2 Structure of XML Layout
The basic structure of an XML layout file is as follows:
<?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:orientation="vertical">
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Hello, World!" />
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Click Me!" />
</LinearLayout>
The example above defines vertically arranged text and a button using LinearLayout. The layout_width and layout_height properties of each view set the size of the view.
3. Dynamically Creating Views in Java
While defining layouts through XML is good, sometimes it’s necessary to dynamically create views through Java code. Let’s look at how to dynamically create views in Java.
3.1 Creating Basic Views
LinearLayout layout = new LinearLayout(this);
layout.setOrientation(LinearLayout.VERTICAL);
TextView textView = new TextView(this);
textView.setText("Hello, World!");
Button button = new Button(this);
button.setText("Click Me!");
layout.addView(textView);
layout.addView(button);
setContentView(layout);
In the code above, we created a LinearLayout, then created a TextView and a Button and added them to the layout. Finally, we display this layout on the screen using the setContentView method.
3.2 Event Handling
To handle events such as button clicks, you can set listeners. Here’s how to handle button click events:
button.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Toast.makeText(getApplicationContext(), "Button Clicked!", Toast.LENGTH_SHORT).show();
}
});
4. Structuring Content with Layouts
Android applications increasingly demand more complex layouts beyond simple screen configurations. Let’s explore how to apply various combinations of layouts.
4.1 ConstraintLayout
ConstraintLayout allows you to define the relationships between components to easily create complex UIs. Here’s a basic usage example.
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:layout_width="match_parent"
android:layout_height="match_parent">
<TextView
android:id="@+id/textView"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:text="Hello, World!"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintEnd_toEndOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>
4.2 RecyclerView
When you need to display a lot of data, it’s advisable to use RecyclerView. It helps efficiently layout many items. Let’s also look at the basic usage of RecyclerView.
Creating a RecyclerView Adapter
public class MyAdapter extends RecyclerView.Adapter {
private String[] mData;
public static class ViewHolder extends RecyclerView.ViewHolder {
public TextView textView;
public ViewHolder(View v) {
super(v);
textView = v.findViewById(R.id.textView);
}
}
public MyAdapter(String[] data) {
mData = data;
}
@Override
public MyAdapter.ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
View v = LayoutInflater.from(parent.getContext()).inflate(R.layout.text_view_item, parent, false);
return new ViewHolder(v);
}
@Override
public void onBindViewHolder(ViewHolder holder, int position) {
holder.textView.setText(mData[position]);
}
@Override
public int getItemCount() {
return mData.length;
}
}
The code above demonstrates how to create an adapter for RecyclerView. This adapter receives a data array and sets the data for each view.
5. Creating Custom Views
There are times when you need to create custom views to meet specific UI/UX requirements. The process for creating custom views is as follows.
5.1 Create a Custom View Class
public class MyCustomView extends View {
public MyCustomView(Context context, AttributeSet attrs) {
super(context, attrs);
// Initialization work
}
@Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
// Code to draw the view
}
}
The code above shows the basic configuration of a custom view that inherits from the View class. The onDraw method can be used to perform direct drawing operations.
Conclusion
In this course, we explored how to compose screens using views in Android app development with Java. We covered various methods to effectively build the user interface of Android applications, from static composition using XML layout files to dynamic creation through Java code, as well as developing complex layouts and custom views.
Based on what you learned in this course, try applying various views and layouts that your app needs. Android app development is continuously evolving, and it is important to learn new technologies and methods in a timely manner. We will return with more in-depth topics in the future.