This course provides an in-depth explanation and practical examples on how to control activities when developing Android apps using Kotlin.<\/p>\n<\/header>\n An activity is a basic component that constitutes the UI of an Android app. Each activity represents a single screen that interacts with the user, and multiple activities are used to implement the app’s functionality.<\/p>\n For example, a login screen, main screen, settings screen, etc., can each be composed of independent activities. Each activity has a lifecycle, which plays an important role in managing state changes in the app.<\/p>\n<\/section>\n The lifecycle of an activity consists of the following main methods:<\/p>\n By appropriately utilizing these lifecycle methods, you can effectively manage the state of the app.<\/p>\n<\/section>\n Now, let’s create a simple Android app. We will look at how to create a basic activity using Kotlin.<\/p>\n Open Android Studio and create a new project. Choose Kotlin<\/strong> as the programming language and select the basic template of Empty Activity<\/strong>. We’ll set the project name as HelloWorld<\/strong>.<\/p>\n In the above code, we created a MainActivity<\/strong> class and overridden various lifecycle methods to define actions to be performed at each state.<\/p>\n Now let’s modify the activity_main.xml<\/strong> file to define the UI.<\/p>\n In the above XML code, we used a TextView<\/strong> to display the text “Hello World!” at the center of the screen.<\/p>\n<\/section>\n In Android apps, it is common to use multiple activities to construct the user interface. In this section, we will look at how to navigate between activities.<\/p>\n We will add a new activity. Let’s create an activity named SecondActivity<\/strong> and set its contents as follows:<\/p>\n We will also modify the corresponding XML layout file activity_second.xml<\/strong>.<\/p>\n Now let’s add code to MainActivity<\/strong> that allows navigation to SecondActivity<\/strong>. We will create a button and set it to switch to the new activity when clicked.<\/p>\n When the button is clicked, it will transition to SecondActivity<\/strong>.<\/p>\n<\/section>\n Passing data between activities is also one of the important functions. Here, we will explain how to return results.<\/p>\n First, we will modify SecondActivity<\/strong> to return a result.<\/p>\n Now, let’s modify MainActivity<\/strong> to receive results.<\/p>\n Now MainActivity<\/strong> can receive results from SecondActivity<\/strong> and reflect it in the UI.<\/p>\n<\/section>\n In Android, you can set flags in the intent when starting an activity to control its behavior.<\/p>\n Some of the most commonly used flags are:<\/p>\n In the above code, using FLAG_ACTIVITY_CLEAR_TOP<\/strong> will remove existing activities in the stack when starting SecondActivity<\/strong>.<\/p>\n<\/section>\n You can apply various themes and styles to activities to adjust the UI. Here, we will explain how to create and apply a custom theme.<\/p>\n Open the res\/values\/styles.xml file and add a new theme.<\/p>\n Apply the new theme to MainActivity<\/strong> in the AndroidManifest.xml file.<\/p>\n In the above code, we set CustomTheme<\/strong> to be applied to the activity.<\/p>\n<\/section>\n There are several ways to end an activity. By default, you can call the finish()<\/strong> method to terminate the current activity.<\/p>\n It is also important to note that it is automatically handled when the user presses the back button to close an activity.<\/p>\n In the above code, clicking the button will terminate the current activity.<\/p>\n<\/section>\n1. What is an Activity?<\/h2>\n
2. Activity Lifecycle<\/h2>\n
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3. Creating a Basic Activity<\/h2>\n
3.1 Creating a Project<\/h3>\n
3.2 Writing Activity Code<\/h3>\n
\n package com.example.helloworld\n\n import android.os.Bundle\n import androidx.appcompat.app.AppCompatActivity\n\n class MainActivity : AppCompatActivity() {\n override fun onCreate(savedInstanceState: Bundle?) {\n super.onCreate(savedInstanceState)\n setContentView(R.layout.activity_main)\n }\n\n override fun onStart() {\n super.onStart()\n \/\/ Actions to perform when the activity starts\n }\n\n override fun onResume() {\n super.onResume()\n \/\/ Actions to perform when the activity is in the foreground\n }\n\n override fun onPause() {\n super.onPause()\n \/\/ Actions to perform when the activity is paused\n }\n\n override fun onStop() {\n super.onStop()\n \/\/ Actions to perform when the activity is no longer visible\n }\n\n override fun onDestroy() {\n super.onDestroy()\n \/\/ Actions to perform when the activity is destroyed\n }\n }\n <\/code><\/pre>\n3.3 Setting XML Layout<\/h3>\n
\n <?xml version=\"1.0\" encoding=\"utf-8\"?>\n <RelativeLayout xmlns:android=\"http:\/\/schemas.android.com\/apk\/res\/android\"\n android:layout_width=\"match_parent\"\n android:layout_height=\"match_parent\">\n\n <TextView\n android:id=\"@+id\/textView\"\n android:layout_width=\"wrap_content\"\n android:layout_height=\"wrap_content\"\n android:text=\"Hello World!\"\n android:layout_centerInParent=\"true\" \/>\n\n <\/RelativeLayout>\n <\/code><\/pre>\n4. Navigating Between Activities<\/h2>\n
4.1 Creating a New Activity<\/h3>\n
\n package com.example.helloworld\n\n import android.os.Bundle\n import androidx.appcompat.app.AppCompatActivity\n\n class SecondActivity : AppCompatActivity() {\n override fun onCreate(savedInstanceState: Bundle?) {\n super.onCreate(savedInstanceState)\n setContentView(R.layout.activity_second)\n }\n }\n <\/code><\/pre>\n4.2 Setting XML Layout<\/h3>\n
\n <?xml version=\"1.0\" encoding=\"utf-8\"?>\n <RelativeLayout xmlns:android=\"http:\/\/schemas.android.com\/apk\/res\/android\"\n android:layout_width=\"match_parent\"\n android:layout_height=\"match_parent\">\n\n <TextView\n android:layout_width=\"wrap_content\"\n android:layout_height=\"wrap_content\"\n android:text=\"This is the Second Activity\"\n android:layout_centerInParent=\"true\" \/>\n\n <\/RelativeLayout>\n <\/code><\/pre>\n4.3 Adding Code for Activity Transition<\/h3>\n
\n <Button\n android:id=\"@+id\/button\"\n android:layout_width=\"wrap_content\"\n android:layout_height=\"wrap_content\"\n android:text=\"Go to Second Activity\"\n android:layout_below=\"@id\/textView\"\n android:layout_centerHorizontal=\"true\"\/> \n <\/code><\/pre>\n\n \/\/ Add to the onCreate method of MainActivity\n val button: Button = findViewById(R.id.button)\n button.setOnClickListener {\n val intent = Intent(this, SecondActivity::class.java)\n startActivity(intent)\n }\n <\/code><\/pre>\n5. Passing Results Between Activities<\/h2>\n
5.1 Setting Up the Activity to Return Results<\/h3>\n
\n \/\/ SecondActivity code\n button.setOnClickListener {\n val resultIntent = Intent()\n resultIntent.putExtra(\"result\", \"Result from Second Activity\")\n setResult(RESULT_OK, resultIntent)\n finish()\n }\n <\/code><\/pre>\n5.2 Setting Up the Activity to Receive Results<\/h3>\n
\n \/\/ Add to MainActivity code\n val startForResult = registerForActivityResult(ActivityResultContracts.StartActivityForResult()) { result ->\n if (result.resultCode == Activity.RESULT_OK) {\n val data: Intent? = result.data\n val resultText = data?.getStringExtra(\"result\")\n \/\/ Set result to TextView\n }\n }\n\n button.setOnClickListener {\n val intent = Intent(this, SecondActivity::class.java)\n startForResult.launch(intent)\n }\n <\/code><\/pre>\n6. Setting Activity Flags<\/h2>\n
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6.1 Example of Flags<\/h3>\n
\n val intent = Intent(this, SecondActivity::class.java)\n intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP)\n startActivity(intent)\n <\/code><\/pre>\n7. Applying Activity Themes and Styles<\/h2>\n
7.1 Setting the Theme<\/h3>\n
\n <style name=\"CustomTheme\" parent=\"Theme.AppCompat.Light.NoActionBar\">\n <item name=\"colorPrimary\">#FF5722<\/item>\n <item name=\"colorPrimaryDark\">#E64A19<\/item>\n <item name=\"colorAccent\">#FFC107<\/item>\n <\/style>\n <\/code><\/pre>\n7.2 Applying the Theme<\/h3>\n
\n <activity\n android:name=\".MainActivity\"\n android:theme=\"@style\/CustomTheme\">\n <\/activity>\n <\/code><\/pre>\n8. Ending an Activity<\/h2>\n
\n button.setOnClickListener {\n finish()\n }\n <\/code><\/pre>\n