Android fundamentals 02.1:Activities and intents

Android fundamentals 02.1:Activities and intents

이 포스팅은 아래 구글 코드랩을 개인 학습용으로 정리한 글입니다.

• 2.1: 활동 및 인텐트

1. Welcome

Introduction

• Activity

  • represents a single screen in your app
  • your user can perform a single, focused task such as taking a photo, sending an email, or viewing a map
  • usually presented to the user as a full-screen window

• An app usually consists of multiple screens(activities) that are loosely bound to each other

• Main activity

  • presented to the user when the app is launched
  • The main activity can then start other activities to perform different actions

➖

• Each time a new activity starts, the previous activity is stopped

  • the system preserves the previous activity in the back stack

• When a new activity starts, that new activity is pushed onto the back stack and takes user focus (top of the back stack)

• When the user is done with the current activity and presses the Back button,

  • the current activity is popped from the stack and destroyed
  • the previous activity resumes (top of the back stack)

➖

• An activity is started or activated with an intent

• Intent:

  • an asynchronous message
  • you can use in your activity to request an action from another activity, or from some other app component
  • you can use an intent to start one activity from another activity, and to pass data between activities

➖

• An Intent can be explicit or implicit
• An explicit intent:
  • you know the target of that intent.
  • you already know the fully qualified class name of that specific activity.
• An implicit intent:
  • you do not have the name of the target component
  • but, you have a general action to perform

2. App overview

• You build the app in three stages

• In the first stage:

  • create an app whose main activity contains one button, Send
  • When the user clicks this button, your main activity uses an intent to start the second activity

• In the second stage:

  • add an EditText view to the main activity
  • The user enters a message and clicks Send
  • The main activity uses an intent to start the second activity and send the user's message to the second activity
  • The second activity displays the message it received

• In the final stage:

  • add an EditText and a Reply button to the second activity
  • The user can now type a reply message and tap Reply, and the reply is displayed on the main activity
  • you use an intent to pass the reply back from the second activity to the main activity

3. Task 1: Create the TwoActivities project

1.1 Create the TwoActivities project

1.2 Define the layout for the main Activity

1.3 Define the Button action

3. Inside launchSecondActivity(), add a Log statement that says "Button Clicked!"

Log.d(LOG_TAG, "Button clicked!");

4. At the top of the MainActivity class, add a constant for the LOG_TAG variable
   -> This constant uses the name of the class itself as the tag.

private static final String LOG_TAG = MainActivity.class.getSimpleName();

➕In kotlin:

private val TAG : String = MainActivity::class.java.simpleName.toString()

MainActivity.kt

package googleCodelabs.TwoActivities

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.util.Log
import android.view.View
import androidx.core.content.PackageManagerCompat.LOG_TAG

private val TAG : String = MainActivity::class.java.simpleName.toString()

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
    }

    fun launchSecondActivity(view: View) {
        Log.d(TAG, "Button clicked!")
    }
}

4. Task 2: Create and launch the second Activity

• Each new activity has its own layout and Java files, separate from those of the main activity.

• They also have their own $<$activity$>$ elements in the AndroidManifest.xml file.

  • As with the main activity, new activity implementations extend from the AppCompatActivity class.

➖

• you can define an activity as a parent of another activity in the AndroidManifest.xml file
  -> Parent-child relationship
  -> enables Android to add navigation hints(such as left-facing arrows) in the title bar for each activity

➖

• In this task you add a second activity to our app, with its own layout.

• You modify the AndroidManifest.xml file
  -> to define the main activity as the parent of the second activity.

• Then you modify the launchSecondActivity() method in MainActivity
  -> to include an intent that launches the second activity when you click the button.

2.1 Create the second Activity

2.2 Modify the AndroidManifest.xml file

2. Find the $<$activity$>$ element that Android Studio created for the second Activity.

  <activity
            android:name=".SecondActivity"
            android:exported="false">
            <meta-data
                android:name="android.app.lib_name"
                android:value="" />
  </activity>

3. Replace the entire $<$activity$>$ element with the following:

  <activity android:name=".SecondActivity"
            android:label = "Second Activity"
            android:parentActivityName=".MainActivity">
            <meta-data
                android:name="android.support.PARENT_ACTIVITY"
                android:value=
                    "googleCodelabs.TwoActivities.MainActivity" />
  </activity>

• label attribute:
  adds the title of the Activity to the app bar.

• parentActivityName attribute:
  you indicate that the main activity is the parent of the second activity.

• This relationship is used for Up navigation in your app
  -> the app bar for the second activity will have a left-facing arrow
  -> the user can navigate "upward" to the main activity

• $<$meta-data$>$ element
  you provide additional arbitrary information about the activity in the form of key-value pairs.

• In this case, the metadata attributes do the same thing as the android:parentActivityName attribute
  -> they define a relationship between two activities for upward navigation.

• These metadata attributes are required for older versions of Android
  -> the android:parentActivityName attribute is only available for API levels 16 and higher.

2.3 Define the layout for the second Activity

4. Add the android:layout_marginLeft="8dp" attribute to the TextView to complement the layout_marginStart attribute for older versions of Android.

2.4 Add an Intent to the main Activity

2. Create a new Intent in the launchSecondActivity() method.

• The Intent constructor takes two arguments for an explicit Intent
  -> an application Context
  -> the specific component that will receive that Intent

Intent intent = new Intent(this, SecondActivity.class);

➕ In kotlin:

val intent = Intent(this, SecondActivity::class.java)

3. Call the startActivity() method with the new Intent as the argument

startActivity(intent);

MainActivity.kt

private val TAG : String = MainActivity::class.java.simpleName.toString()

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
    }

    fun launchSecondActivity(view: View) {
        Log.d(TAG, "Button clicked!")
        val intent = Intent(this, SecondActivity::class.java)
        startActivity(intent)
    }
}

5. Task 3: Send data from the main Activity to the second Activity

• You can also use an intent to send data from one activity to another while launching it.

➖

• Your intent object can pass data to the target activity in two ways:
  -> in the data field or in the intent extras.

• intent data: a URI indicating the specific data to be acted on.

• intent extras: you can put additional information into extras
  -> if the information you want to pass is not a URI
  -> if you want to send more than one piece of information

➖

• intent extra: key/value pairs in a Bundle.
  (Bundle: a collection of data, stored as key/value pairs.)

• To pass information from one activity to another:
  -> you put keys and values into the intent extra Bundle from the sending activity
  -> then get them back out again in the receiving activity

➖

• In this task, you modify the explicit intent in MainActivity
  -> to include additional data(in this case, a user-entered string) in the intent extra Bundle.

• You then modify SecondActivity
  -> to get that data back out of the intent extra Bundle
  -> and display it on the screen.

3.1 Add an EditText to the MainActivity layout

3.2 Add a string to the Intent extras

MainActivity.kt

private val TAG : String = MainActivity::class.java.simpleName.toString()

//public
val EXTRA_MESSAGE : String = "googleCodelabs.TwoActivities.extra.MESSAGE"

class MainActivity : AppCompatActivity() {

    lateinit var mMessageEditText : EditText

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        mMessageEditText = findViewById(R.id.editText_main)
    }

    fun launchSecondActivity(view: View) {
        Log.d(TAG, "Button clicked!")
        val intent = Intent(this, SecondActivity::class.java)

        val message = mMessageEditText.text.toString()
        intent.putExtra(EXTRA_MESSAGE, message)
        startActivity(intent)
    }
}

3.3 Add a TextView to SecondActivity for the message

3.4 Modify SecondActivity to get the extras and display the message

2. Get the Intent that activated this Activity:

Intent intent = getIntent();

➕In Kotlin:

val intent: Intent = intent

3. Get the string containing the message from the Intent extras using the MainActivity.EXTRA_MESSAGE static variable as the key:

String message = intent.getStringExtra(MainActivity.EXTRA_MESSAGE);

➕In Kotlin:

 val message : String? = intent.getStringExtra(googleCodelabs.TwoActivities.EXTRA_MESSAGE)

SecondActivity.kt

class SecondActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_second)

        //Get the Intent that activated this Activity
        val intent: Intent = intent
        val message : String? = intent.getStringExtra(EXTRA_MESSAGE)

        val textView = findViewById<TextView>(R.id.text_message)
        textView.text = message
    }

6. Task 4: Return data back to the main Activity

4.1 Add an EditText and a Button to the SecondActivity layout

4.2 Create a response Intent in the second Activity

8. Set the result to RESULT_OK
   -> to indicate that the response was successful.
   -> The Activity class defines the result codes (including RESULT_OK and RESULT_CANCELLED.)

setResult(RESULT_OK,replyIntent);

9. Call finish() to close the Activity and return to MainActivity.

finish();

SecondActivity.kt

//public
val EXTRA_REPLY : String = "googleCodelabs.TwoActivities.extra.REPLY"

class SecondActivity : AppCompatActivity() {

    lateinit var mReply : EditText

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_second)

        //Get the Intent that activated this Activity
        val intent: Intent = intent
        val message : String? = intent.getStringExtra(EXTRA_MESSAGE)

        val textView = findViewById<TextView>(R.id.text_message)
        textView.text = message
    }

    fun returnReply(view: View) {
        mReply = findViewById(R.id.editText_second)
        val reply : String? = mReply.text.toString()

        val replyIntent = Intent()
        replyIntent.putExtra(EXTRA_REPLY, reply)

        setResult(RESULT_OK, replyIntent)
        finish()
    }

4.3 Add TextView elements to display the reply

4.4 Get the reply from the Intent extra and display it

• In this task you modify the app to start SecondActivity expecting a result
  -> to extract that return data from the Intent
  -> and to display that data in the TextView elements

MainActivity.kt

//public
val EXTRA_MESSAGE : String = "googleCodelabs.TwoActivities.extra.MESSAGE"

//public
val TEXT_REQUEST : Int = 1

class MainActivity : AppCompatActivity() {
...

	fun launchSecondActivity(view: View) {
        Log.d(TAG, "Button clicked!")
        val intent = Intent(this, SecondActivity::class.java)

        val message = mMessageEditText.text.toString()
        intent.putExtra(EXTRA_MESSAGE, message)

        startActivityForResult(intent, TEXT_REQUEST)
 	}
    
	override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) {
        super.onActivityResult(requestCode, resultCode, data)

        if(requestCode == TEXT_REQUEST){
            if(resultCode == RESULT_OK){
                val reply = data?.getStringExtra(EXTRA_REPLY)
                mReplyHeadTextView.visibility = View.VISIBLE
                mReplyTextView.visibility = View.VISIBLE
                mReplyTextView.text = reply
            }
        }
    }
 ...
 }

8. Coding challenge

📌참고자료

• What is the best way to define log TAG constant in Kotlin?

How to define log TAG constant:

• In Java:

private static final String TAG = MyClass.class.getSimpleName();

• In Kotlin:

private val TAG = MyClass::class.java.simpleName