Android Java reference

A reference Android project in Java for
learning purposes for the course PMP(Programing for Mobile Platforms)

Generated with Empty activity for API 21: Android 5.0 Lollipop.

Notes

Project structure

app
    src
        main (the source code is placed here)
        res (xml and other resources that the application uses)
            drawable
            drawable-hdpi (situational-specific folder for screens with hdpi)
            layout
            layout-land (situational-specific folder for landscape layouts)
            values
                strings.xml (GUI components can read text data to display from this file)
            values-mk (situational-specific folder for language specific values
            styles (for containign xml files with styles)
    build.gradle

Activity represents one particular screen on which particular UI elements(called views) can be drawn
Views - are the building blocks for building UI
- ViewGroup subclass for layouts
- other subclasses for widgets(buttons, text fields, images, etc)
Events are used a lot in android programming
- when view is clicked, touch gesture, data from request is received, timers
For basic debugging, use Toast.makeText(this, "text", time).show()
- Toast is a short message displayed on the screen - used for non-essential information
Usage findViewById
- TextView tw = findViewById(R.id.idOfView)
Box model of widget:
- content size
  - attributes layout_width and layout_height, possible values in dp, match_parent or wrap_content
- padding
  - attributes padding, paddingLeft, paddingRight, paddingBottom, paddingTom
- border
- margin
  - attributes layout_margin, layout_marginLeft, layout_marginRight, layout_marginBottom, layout_marginTom
XML files for describing layouts are placed in res/layout
Linear layout
- basic viewGroup for horizontal or vertical placement of views
- set gravity attribute to specify alignment direction for the whole layout
- set layout_gravity attribute to specify alignment direction for the specific view
- set weight attribute with value k to specify the fraction k/total of the total size of the layout
Grid layout
- places views in grid cells
- attributes android_rowCount and android_colCount
- attributes for vies layout_row and layout_col
Relative layout
- places views relative to parent ViewGroup or other views
- attributes set on views: layout_below, layout_above, layout_toLeftOf, layout_toRightOf
Button view
- key attributes: clickable, id, onClick, text
ImageButton view
- bonus attribute: src
ImageView
- key attributes: id, src
RadioButton view
- need to be nested in RadioGroup ViewGrup
- key attributes: checked, clickable, id, onClick, text
Android apps can read data from internal and external storage
Internal storage refers to the data that is private to each application
One application cannot write to internal data storage of another application
Some classes that are useful for reading and writing storage data
- java.io.File
- java.io.InputStream
- java.io.OutputStream
- java.util.Scanner
- java.io.BufferedReader
- java.io.PrintStream
Some methods that Android provides to get file handles in internal storage
- getFilesDir() - internal storage directory
- getCacheDir() - temp directory
- openFileInput("name", mode) - opens file for reading
- openFileOutput("name", mode) - opens file for writing
Some methods that Android provides to get file handles in external storage
- getExternalFileDir()

To use external storage, web data or camera the application needs to specify that it has to get permission from the system

that is done in AndroidManifest.xml

<uses-permission
        android:name="android.permission.READ_EXTERNAL_STORAGE"/>
 <uses-permission
        android:name="android.permission.Write_EXTERNAL_STORAGE"/>
 <uses-permission
        android:name="android.permission.INTERNET"/>
 <uses-permission
        android:name="android.permission.CAMERA"/>

ListView View
- can contain list of selectable entries
- key xml attributes:
  - android:clickable
  - android:entries
  - android:id
- adapt list structure to ListView:
  - ArrayAdapter<String> lstAdapter = new ArrayAdapter<>(activity, layout, array)
    - activity is usually this
    - default layout for list item: android.R.layout.simple_list_item_1
    - array: String[], ArrayList etc
  - when data is changed in array, the AttayAdapter instance needs to be notified
- events
  - setOnItemClickListener
  - setOnItemLongClickListener
  - setOnItemSelectedListener
- custom layout
  - write short layout file
  - tell ArrayAdapter to use it and tell how to supply data to the layout
RecyclerView
- displays large set of data
- recycles unused ViewHolders, so the method onBindViewHolder might be more used than OnCreateViewHolder
- key classes
  - RecyclerView
  - RecyclerView.ViewHolder
  - RecyclerView.Adapter
  - LayoutManager
Activity lifecycle
- onCreate()
- onStart()
- onRestoreSavedInstance() - when restoring from some hibernating state
- onResume()
- onSaveInstanceState()
- onPause()
- onStop()
- when users goes back to activity
  - onRestart()
  - onStart()
  - and then the rest
- onDestroy()
- when we move across activities from one application
  the lifecycle methods onPause() and onResume()
- when we move to other applications the lifecycle methods onStop() and onRestart() are used
- when the activity is destroyed and created the
  lifecycle methods onDestroy() and onCreate() are used
Intents are used to specify the desired action or activity to launch,
can be thought of as bridge between activities.
- they can store extra data to pass as parameters to activity
- started activity can return data back to the caller
- explicit intents start a particular activity
- implicit intents specify a particular action that other application can handle
Some concepts tied to activities
- activity stack: activities are pushed and removed from stack
  when one activity is started from another activity and that activity is closed
  - can set the parent activity explicitly
- toolbar - a top level UI for menu of actions in activity
  - contain some actions, and other actions in a dropdown

Check rotation state

getResources().getConfiguration().orientation == Configuration.ORIENTATION_PORTRAIT

Fragments are reusable segment of Android UI that is created always inside
activity or other fragment.
- can solve the problem with redundant layouts
Fragments have similar lifecycle methods as activities with some additional
- onAttach() - when fragment is glued to its surrounding activity
- OnCreateView() - method that returns the view of the fragment
- onActivityCreated() - method that indicates surrounding activity is ready
- onDetach() - when fragment is being detached
Fragments extends the classFragment and must override the method onCreate
Fragments can access activity's data with getActivity()
- can also read the intent of that activity
Fragments can communicate with other fragments that the activity contains using
getActivity().getFragmentManager().findFragmentById()
Fragment's layout file can contain root element FrameLayout or any other layout
On activity layout file, fragmens can be placed with the element <fragment>
- key attributes:
  - id
  - name is name of the fragment class
Styles are placed inside styles project directory
- define particular style with <style name="SomeName">...</style
- inside <style> define properties using
  <item name="some view property">someValue</item>
Some types of dialogs:
- AlertDialog, CharacterPickerDialog, DatePickerDialog,
  ProgressDialog, TimePickerDialog
Parts of a dialog
- Icon, title, message, negative button, positive button, neutral button
Dialogs can be used
- by building with Builder(example ```AlertDialog.Builder``)
- by creating custom dialogs
Some libraries for android development
- picasso - for loading images from internet and caching
- butterknife - simplify usage of widgets
- Espresso - powerful DSL for Android integration testing
On 160 dpi screen, 1 dp == 1 pixel
- on 320 dpi, 1 dp == 2 pixel
- on 480 dpi, 1 dp == 3 pixel
To check android version, use Build
- example Build.VERSION_SDK_INT

To check existence of device hardware

PackageManager pm = getPackageManager();
pm.hasSystemFeature(PackageManager.FEATURE_MICROPHONE);

WebView - a view that displays web page
Firebase
- Backend-As-A-Service
- built by SF-based Google subsidiary
- cloud database
- API to access data from Android, IOS, Java, Javascript
- REST API
- receive notifications for data changes
- cloud scaling
- login/auth
- database is represented as tree structure and map of key-value pairs
Service in context of Android is a task that is running in background
when started by an application.
- example playing music or listening for new data changes
Services are useful for long running tasks or providing functionality to other applications.
Service lifecycle
- startService()
- onCreate()
- onStartCommand()
- onDestroy()
Service classes extend the class Service
Parts of Notifications
- large icon, small icon, content title, content text, content subtext,
  actions, time, number of content info

matevskial/android-java-pmp-reference

Android Java reference

Notes