[JAVA] Android app: Try to summarize events and listeners

Last time A newcomer tries to summarize Android views (beginner Android application development)

Continued. .. ..

Introduction

It's been three months since I joined the company. I am studying android application development and I am summarizing it.

This time, about events and listeners. Since it is a demon super basic, I would appreciate it if you think that it is for super beginners. ..

Words coming this time

--Event If you tap the youtube app icon, youtube will open. The user operation of tapping this app is called an event.

• Event handler If you tap the youtube app, youtube will open. The event is to tap this app. The event handler is the process of opening youtube.

--Listener android keeps an eye on youtube so that it can be touched at any time. Keeping an eye on this is called a listener.

Last words

--View Screen parts

--Activity The screen itself

Anyway sample

environment

androidstudio 3.6.2 openjdk version "11.0.6"

Screen to make

Create an app that simply displays the entered characters with a button tap.

Button tap = event Processing to display the entered characters below = event handler Watch if tapped android function = listener

sample

The import statement is omitted because it will be long.

activity_main.xml

<LinearLayout ～～ Omitted ～～>

    <EditText
        android:id="@+id/etName"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:inputType="text"/>
    <Button
        android:id="@+id/btClick"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="GoGo!"/>
    <TextView
        android:id="@+id/tvOutput"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="25dp"
        android:text=""
        android:textSize="25dp"/>
</LinearLayout>

MainActivity.java

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        //Get Button object which is a display button
        Button btClick = findViewById(R.id.btClick);
        //Create an instance of the listener class
        HelloListener listener = new HelloListener();
        //Set listener on display button
        btClick.setOnClickListener(listener);
    }

    private class HelloListener implements View.OnClickListener{
        @Override
        public void onClick(View view){
            //Get EditText object which is a name input field
            EditText input = findViewById(R.id.etName);
            //Get a TextView object to display a message
            TextView output = findViewById(R.id.tvOutput);
            //Store the input name string in inputStr
            String inputStr = input.getText().toString();
            //Show message
            output.setText(inputStr + "← Entered text");
            }
        }
    }

Important points for beginners

Unlike the last time, it is divided into java class (MainActivity) and XML class (activity_main). Think of the Java class as creating the listener settings and processing, and the xml as creating the screen configuration (activity).

The important thing this time is ・ Where are the above two files linked? -How to set listeners and write event handlers

is.

Linking java file and screen parts

MainActivity.java

//Get Button object which is a display button
Button btClick = findViewById(R.id.btClick);

activity_main.xml

<Button
        android:id="@+id/btClick"

The usual pattern to get screen parts in java class. R doesn't care now. It can be obtained by findViewById (R.id. Screen part ID name). Store it in a variable of Button object.

Listener settings and event handlers

Create a listener class → Set the listener in the prepared btnClick → Describe the event handler in the listener class

MainActivity.java

//Create an instance of the listener class
HelloListener listener = new HelloListener();
//Set listener on display button
btClick.setOnClickListener(listener);

With this description, we have gone as far as setting the listener for the button. Enter the event handler in the listener class below.

MainActivity.java

private class HelloListener implements View.OnClickListener{
        @Override
        public void onClick(View view){
            //Get EditText object which is a name input field
            EditText input = findViewById(R.id.etName);
            //Get a TextView object to display a message
            TextView output = findViewById(R.id.tvOutput);
            //Store the input name string in inputStr
            String inputStr = input.getText().toString();
            //Show message
            output.setText(inputStr + "← Entered text");
            }
        }

The screen parts are acquired by findViewId and displayed in TextView.

Please think that the code below, which is not explained, is magical for now. I will write it when I explain the life cycle of the activity later.

 @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

At the end

Writing a commentary will deepen your understanding. .. Whether or not it can be written in an easy-to-understand manner. .. ..

I will devote myself. Please forgive me that Japanese is messy.

Next time I will write a screen transition!