9-2 Android 2

Rotaion and Layouts

• Rotation causes a configuration change to the device
• land-scape version of layout is just a different XML layout
• it can be created as follows from intelliJ
  

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User Interface Style Language

CSS is most well-known example

Android Themes and Styles

create <style> definitions in “res/values/styles.xml”

<resources>
  <style name="BigCentred" parent="TextAppearance.AppCompat">
    <item name="android:textSize">28sp</item>
    <item name="android:background">#cccccc</item>
    <item name="android:text">default</item>
  	...
  </style>
  <style name="BigCentred.Blue">
    <item name="android:background">#0000ff</item>
    <item name="android:text">blue</item>
  </style>
  ...
</resources>

apply styles to views in layout

<TextView style="@style/BigCentred" />
<TextView style="@style/BigCentred.Blue" />

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Multi-Screen Applications with Activities

Android travels between activities through Intent

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

You can pass key-value pairs to the activities

intent.putExtra("key", value)
////////
// use whatever is applicable getIntExtra, etc.
intent.getStringExtra("key") 

You can get result from the activity

val startForResult = registerForActivityResult(StartActivityForResult()){ result ->
	// result.data is of type intent
    ...
}

startForResult.launch(intent)

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Return to previous activity

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you can open other applications using intents

val intent = Intent(Intent.ACTION_VIEW, Uri.parse("geo:0,0?q=honolulu"))
startActivity(intent)

** geo: is a deeplink of google map

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Save state

on configuration change, or when application goes to background, the app can be killed and recreated when it gets pull back to the foreground.

so there needs a way to save application state independent from the application's life cycle.

methods of saving states:

1. viewModel - saved in memory
2. save instance state - save data to 'disk'
3. persistent storage - save data to 'disk'

How to save data using the second method - save instance state

var counter = 0
...
override fun onCreate(inState: Bundle?) {
  ...
  if (inState != null) {
    with (inState) {
      counter = getInt("COUNTER")
  }
}

override fun onSaveInstanceState(outState: Bundle) {
  with (outState) {
  	putInt("COUNTER", counter)
  }
  super.onSaveInstanceState(outState)
}

override fun onRestoreInstanceState(inState: Bundle) {
  super.onRestoreInstanceState(inState)
  with (inState) {
  	counter = getInt("COUNTER")
  }
}