Basics

There are two tools: NDK and SDK. Each tool has a version.

Android has a concept of Android native API level.

There are three environment variables to set:

• ANDROID_NDK_ROOT

• ANDROID_SDK_ROOT

• ANDROID_HOME

The name of the app is determined by <application android:label in AndroidManifest.xml. We can use @string/app_name.

The title of the nav bar is determined by <activity android:label in AndroidManifest.xml.

Button click:

val button: Button = findViewById(R.id.button1)
button1.setOnclickListener {
  Toast.makeText(this, "Clicked me!", Toast.LengthShort).show()
}

The menu is added in res/menu The directory res/menu is created by us manually. After creating the directory, we can right click it, and select menu resource file. If we name it main, it will create a file main.xml inside res/menu. Inside main.xml, each menu item contains something like below:

<item android:id="@+id/add_item" android:title="Add" />

After creating main.xml, click the menu Code -> Override Method, select onCreateOptionsMenu:

menuInflater.inflace(R.menu.main, menu)
return true

To define a function when a menu item is clicked, click the menu Code->Override Method, select onOptionsItemSelected.

when (item.itemId) {
  R.id.add_item -> { xxx }
  R.id.remove_item -> { xxx }
}
return true

To destroy an activity, call finish().

TextView

It is a label in other GUI.

<TextView
  android:id="@+id/textView"
  android:text="hello world"
  android:layout_width="match_parent"
  android:layout_height="wrap_content"
/>

To change the alignment, use android:gravity.

android:gravity="center"
// top, bottom, center, start, end
// center_vertical|center_horizontal

To change the color, use android::textColor

android::textColor="#00ff00"

To change the font size, use android::textSize="24sp"

Button

<Button
  android:id="@+id/button"
  android:text="Button"
  android:textAllCaps="false"
  android:layout_width="match_parent"
  android:layout_height="wrap_content"
/>

EditText

<EditText
  android:id="@+id/editText"
  android:layout_width="match_parent"
  android:layout_height="wrap_content"
  android:hint="this is some hint"
  android:maxLines="10"
/>

To get the text of EditText in the kotlin code, use:

val inputText = editText.text.toString()

LinearLayout

<LinearLayout
  android:orientation="vertical"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
/>

orientation can also be horizontal.

view binding

Edit app/build.gradle, add the following:

android { ...
  kotlinOptions {
    jvmTarget = '1.8'
  }
  buildFeatures {
    viewBinding true
  }
}

Then in the main activity, use:

// activity_main.xml -> AcitvityMainBinding
import com.bignerdranch.android.geoquiz.databinding.ActivityMainBinding

private lateinit var binding: ActivityMainBinding
binding = ActivityMainBinding.inflate(layoutInflater)
setContentView(binding.root)

binding.trueButton.setOnClickListener { view: View -> }

binding.falseButton.setOnClickListener { view: View -> }