Last updated: Saturday 22nd September 2018, 7:30 PT, AD


Android Notes

Picture of launcher screen on an Android phone

Contents

Android Developer Fundamentals - Videos

Installing Android Studio 
Running Android Studio
Android Developer Guide
My Android Blog
Android Studio 3.1.x — How to fix it
What is an Android app?
Check out an existing Android app from Version Control (GitHub)
How to copy an existing Android project to a new project (ALWAYS: File -> Sync Project with Gradle Files before running app!)
AndroidManifest.xml
Intents and Intent Filters
What happens when you click on an app's launch icon?
Android Activity Lifecycle
What is a View?
How to show the bounds of layouts in an app
Using different branches of app code stored on GitHub (Git)
Images for different resolutions and devices
Convert text to mp3 to make audio resource file
Save data using SQLite
MySQL for reference
sqlite3
sqlite3 commands
Run a command script in sqlite3
Create an SQLite database and table and populate with data from text file (in csv format)
Create an SQLite database from a text file
Create an SQLite database using DB Browser for SQLite
Writing the result of an SQLite query to a text file
SQLite Indexing
SQLite Primary key and other Constraints
Android Content Provider

More learning resources...











































What is an Android app?

Android Developer Fundamentals - Videos

Picture of launcher screen on an Android phone


An Android app is a software application 
running on the Android platform. 
In other words, an Android app
is a small program that runs
on your Android phone or tablet. 
You may have an app for online banking,
an app for emailing.
You can download many free apps 
from the Google Play Store.

Watch the video below to see a running app...



Because the Android platform is built for 
mobile devices, 
a typical Android app is designed for 
a smartphone or a tablet PC running 
on the Android OS.

Android apps can be written using Kotlin, 
Java, and C++ languages. 
The Android SDK tools compile your code 
along with any data and resource files 
into an APK, an Android package.
The Android package is an archive file 
with an .apk suffix. 

One APK file contains all the contents 
of an Android app and is the file 
that Android-powered devices 
use to install the app.

Each Android app lives in its own 
security sandbox, protected by the following 
Android security features:

The Android operating system 
is a multi-user Linux system 
in which each app is a different user.
By default, the system assigns each app 
a unique Linux user ID 
(the ID is used only by the system 
and is unknown to the app). 
The system sets permissions for all 
the files in an app so that only 
the user ID assigned to that app 
can access them. Each process has 
its own virtual machine (VM), 
so an app's code runs in isolation 
from other apps. By default, 
every app runs in its own Linux process.
 
The Android system starts the process 
when any of the app's components need 
to be executed, and then shuts down 
the process when it's no longer needed 
or when the system must 
recover memory for other apps.

The Android system implements 
the principle of least privilege. 
That is, each app, by default, 
has access only to the components 
that it requires to do its work and no more. 
This creates a very secure environment 
in which an app cannot access 
parts of the system 
for which it is not given permission. 
However, there are ways for an app 
to share data with other apps 
and for an app to access system services...

Reference 
Another Developer Guide 
Android Tutorial 1
Android Tutorial 2






Check out an existing Android app
from Version Control (GitHub)
Go to the GitHub project you want to clone, click on the "Clone or Download" button, copy the URL shown in the GitHub textbox... then open Android Studio and from the first screen click on "Check out an existing Android app from Version Control" and select "GitHub" from the list of version controls... Paste in the URL copied from GitHub. The Parent Directory and Directory Name boxes should be automatically filled in for you. Then click on the Clone button at the bottom of the screen... Check_out_project_from_Version_Control_1.png Click on "Yes" on the following screen... Check_out_project_from_Version_Control_2.png Click on "OK" on the following screen... Check_out_project_from_Version_Control_3.png IMPORTANT: If you are presented with the "Edit configuration" window shown below, click on the "Update Project" link, then "Run". Check_out_project_from_Version_Control_4.png Using the method above, clone and open the GitHub project: ud839_CustomAdapter_Example and study the files while reading the following...

AndroidManifest.xml


Every app project must have an 
AndroidManifest.xml file 
(with precisely that name) 
at the root of the project source set. 

If you're using Android Studio 
to build your app, 
the manifest file is created for you, 
and most of the essential 
manifest elements 
are added as you build your app 
(especially when using code templates).

The manifest file describes 
essential information about your app 
to the Android build tools (Gradle), 
the Android operating system, 
and Google Play. 
Examples of types of information stored 
are values for app: icon, label, 
theme and main activity.


For each app component 
that you create in your app, 
you must declare 
a corresponding XML element 
in AndroidManifest.xml:
e.g. Activity, Service, 
BroadcastReceiver or ContentProvider...

Contents of AndroidManifest.xml



Merge multiple manifest files

Your APK file can contain 
just one AndroidManifest.xml file, 
but your Android Studio project may contain 
several—provided by the main source set, 
build variants, and imported libraries. 
So when building your app, 
the Gradle build merges all manifest files
into a single manifest file 
that's packaged into your APK.

The manifest merger tool combines 
all XML elements from each file 
by following some merge heuristics 
and by obeying merge preferences 
that you have defined with 
special XML attributes. 

Reference    


Intents and Intent Filters



An intent starts an activity, a service or a broadcast.



1.  Starting an activity

An activity represents 
a single screen in an app. 
You can start a new activity 
by passing an intent to startActivity(),
e.g. startActivity(numbersIntent); 
The intent describes the activity to start 
and carries any necessary data.

If you want to receive a result 
from the activity when it finishes, 
call startActivityForResult(). 
Your activity receives the result 
as a separate Intent object 
in your activity's onActivityResult() callback. 
For more information, see the Activities guide.


2.  Starting a service

A Service is a component that 
performs operations in the background 
without a user interface. 


3.  Starting a broadcast

A broadcast is a message 
that any app can receive. 
The system delivers various broadcasts 
for system events, 
such as when the system boots up 
or the device starts charging. 
You can deliver a broadcast to other apps 
by passing an Intent to sendBroadcast() 
or sendOrderedBroadcast().




How Intent Filters work diagram

Figure 1. How an implicit intent is delivered 
through the system to start another activity: 
[1] Activity A creates an Intent 
with an action description 
and passes it to startActivity(). 
[2] The Android System searches all apps 
for an intent filter that matches the intent. 
When a match is found, 
[3] the system starts 
the matching activity (Activity B) 
by invoking its onCreate() method 
and passing it the Intent.

Picture of implicit intents code


Sometimes the intention 
is to start an activity such as
displaying a web page at a certain URL.
Such an activity will require a URI, a  URL or a URN.

How an explicit intent is delivered 
through the system to start another activity: 


// Executed in an Activity, so 'this' is the Context (i.e. scope of the activity)
// The fileUrl is a string URL, such as "http://www.example.com/image.png"
Intent downloadIntent = new Intent(this, DownloadService.class);
downloadIntent.setData(Uri.parse(fileUrl));
startService(downloadIntent);


Reference    


What happens 
when you click 
on an app's launch icon?



In Android, the launcher is your phone's
home screen - the screen where you 
launch your apps...

Picture of launcher screen on an Android phone

the icon launcher (ic_launcher.png)
is the icon you click on
to launch the app...


Launcher app calls 
startActivity() with an intent 
(as defined in the AndroidManifest.xml file).
[action = Intent.ACTION_MAIN, 
category = Intent.CATEGORY_LAUNCHER 
flag = Intent.FLAG_ACTIVITY_NEW_TASK].

Regarding 
Intent.FLAG_ACTIVITY_NEW_TASK 
from docs:

When using this flag, 
if a task is already running 
for the activity you are now starting, 
then a new activity will not be started; 
instead, the current task 
will simply be brought 
to the front of the screen 
with the state it was last in.

Reference

What happens at launch time?

What happens when you start an app? 










Android Activity Lifecycle


Activity Lifecycle 

Google created the following figure showing 
the Android activity lifecycle 
the methods related to every transition. 

In most cases you would need to 
take care only about things that happen 
at the beginning in onCreate(). 
The Android system 
takes care about the rest.


Diagram of Android Activity Lifecycle



An application might consist of 
just one activity -
or it may contain several. 
Typically, one of the activities 
is marked as the first one 
that should be presented to the user 
when the application is launched. 
This information is contained in
the manifest file. 


Moving from one activity to another 
is accomplished by 
having the current activity start the next one.






What is a View?

Views, Layouts and Resources


Diagram of Android Views, ViewGroups and Layouts


Common Android Views - Cheat Sheet pdf


View objects are the basic building blocks 
of User Interface(UI) elements in Android.
A view is a screen on your phone.
View is a simple rectangle box 
which responds to the user's actions.
Examples are TextView, EditText, ImageView, 
Button, CheckBox etc..
View refers to the android.view.View class, 
which is the base class of all UI classes.
View class represents 
the basic building block 
for user interface components. 
A View occupies a rectangular area 
on the screen and is responsible 
for drawing and event handling. 
View is the base class for widgets, 
which are used to create interactive 
UI components such as buttons, 
text fields, etc.
    
View (TextView)    A user interface element 
that displays text to the user. 
To provide user-editable text, see EditText. 

View (ImageView)    Displays image 
resources, for example Bitmap 
or Drawable resources. 
ImageView is also commonly used to 
apply tints to an image 
and handle image scaling.

View (EditText)    A user interface 
element that allows user-editable text.

View (Button)     

View (CheckBox)   A checkbox 
is a specific type of two-states button 
that can be either checked or unchecked. 

View (ViewGroup) is an invisible container. 
It holds Views and ViewGroups.
For example, LinearLayout is the ViewGroup 
that contains other views 
such as Button views, 
TextViews and other Layouts.
LinearLayout is a layout that 
arranges other views either 
horizontally in a single column 
or vertically in a single row.
ViewGroup is the base class 
for Layouts.
A ViewGroup is a special view 
that can contain other views 
(called children.) The view group 
is the base class 
for layouts and views containers. 
This class also defines the 
ViewGroup.LayoutParams class 
which serves as 
the base class for layouts parameters.
 
View (ListView) displays a
vertically-scrollable collection of views, 
where each view is positioned immediately 
below the previous view in the list. 
For a more modern, flexible, 
and performant approach 
to displaying lists, use RecyclerView.
A ListView object is an adapter view 
that does not know the details, 
such as type and contents, 
of the views it contains. 
Instead a ListView object requests 
views on demand 
from a ListAdapter as needed, 
such as to display new views 
as the user scrolls up or down.


Android Layouts are made from Views

Android's res/layout/activity_main.xml
is the layout for the main activity 
of the app.

You can view the text of the layout files
and see the Preview at the same time by 
clicking the Preview tab on the right hand side 
of the Android Studio screen when
a layout file is currently in view...

The LinearLayout class

The ConstraintLayout class  
The ConstraintLayout view container 
is the preferred ViewGroup for performance reasons.


Mastering the ConstraintLayout


How to show the bounds of layouts in an app

On your phone or tablet, 
go to Settings -> Developer Options
-> Show Layout Bounds

Screenshot_Settings_Developer_Options_Show_Layout_Bounds







Using different branches
of app code 
stored on GitHub (Git)




GitHub screen for git branches

This is assuming that Git is installed 
and an account on GitHub is current.

To import a project branch from GitHub into Android Studio,
this is one method that works for me:
at the very bottom line of any Android Studio screen,
on the right hand side 
shows a link to all git branches available 
at the GitHub repository for the current app...


Android Studio screen for git branches



On the very bottom line on screen above
you will see the word "Git:"...


Click on the word Git: 
at the bottom right of the screen


Android Studio screen for git branches



Simply select the branch 
you want to import 
into Android Studio.





IMPORTANT NOTE WHEN IMPORTING 
A GITHUB BRANCH

The code will run on your device, 
however you may notice that the 
AppCompatActivity class 
is highlighted in red
in MainActivity.java with the error: 
'Cannot resolve symbol AppCompatActivity'.
There may be other symbols 
that are highlighted in red.
This means that the current project 
is out of sync with the Gradle files.
To correct this, from Android Studio,
simply click on the main File menu 
then select:
"Sync Project with Gradle Files".







Images for different resolutions
and devices











SQLite

SQL is query language. 
Sqlite is embeddable relational database management system. 
Unlike other databases (like SQL Server and MySQL) 
SQLite does not support stored procedures. 
SQLite is file-based, unlike other databases, 
like SQL Server and MySQL which are server-based.

Save data using SQLite 
SQLite comes with the Android SDK and is located in /tools folder of your install. You can add this folder to your PATH variable and thus open a database file from every command prompt. Remember that there are some older Android ROMs which don't come with an SQLite Editor installed. You would have to install SQLite manually in that case. Working with databases from within your app should work seemlessly as it's a basic feature of the Android OS. The following is a very useful tutorial on how to make an app to create a database and store to the database data entered on screen: All code is there so I suggest try it: Android Studio example app using SQLite. Use sqlite3 to inspect Android Studio databases from the command line. anne@Inspiron-15R:~$ sqlite3 SQLite tutorial

Run a command script in sqlite3

Execute SQL statements from a file

Suppose we have a file named commands.txt 
in the c:/sqlite/ folder 
with the following content:


SELECT albumid, title
FROM albums
ORDER BY title
LIMIT 10;

To execute the SQL statements in the commands.txt file, 
you use the .read FILENAME command as follows:


sqlite> .mode column
sqlite> .read c:/sqlite/commands.txt
156         ...And Justice For All
257         20th Century Masters -
296         A Copland Celebration,
94          A Matter of Life and D
95          A Real Dead One
96          A Real Live One
285         A Soprano Inspired


NOTE: The above is taken from: 
sqlitetutorial.net/sqlite-commands









Create an SQLite database and table and populate with data from text file (in csv format)


If you want to delete 
any previous questions.db database, 
simply navigate to the file on your file system,
and delete the file.

To open the database questions.db 
(or create a new one if it doesn't aleady exist)...


anne@Inspiron-15R:~$ sqlite3 questions.db



If you put the following code in a text file 
(e.g. commands.txt), then you can
execute the textfile commands through sqlite3
by following the instructions 
shown below the code.




CREATE TABLE questions_mc (
 question_id integer,
 question_text text ,
 answer_1 text NOT NULL,
 answer_2 text NOT NULL,
 answer_3 text NOT NULL,
 answer_4 text NOT NULL,
 correct_answer text NOT NULL,
 category text NOT NULL,
 difficulty text NOT NULL
);


Note that questions.csv is a simple text file
saved with the extension .csv
and has comma separated values.

This is the contents of questions.csv


1,What is the data type of the following value: 3.142,string,boolean,float,int,float,data types,easy
2,What is the data type of the following value: 3,string,boolean,float,int,float,data types,easy



sqlite> .read commands.txt
sqlite> .mode csv
sqlite> .import questions.csv questions_mc
sqlite> select * from questions_mc;

Read this about .mode:

.mode MODE ?TABLE?     Set output mode where MODE is one of:
                         ascii    Columns/rows delimited by 0x1F and 0x1E
                         csv      Comma-separated values
                         column   Left-aligned columns.  (See .width)
                         html     HTML <table> code
                         insert   SQL insert statements for TABLE
                         line     One value per line
                         list     Values delimited by "|"
                         quote    Escape answers as for SQL
                         tabs     Tab-separated values
                         tcl      TCL list elements

the above from: https://www.sqlite.org/cli.html 

Tutorial on creating a table in SQLite

Create an SQLite database from a text file

Say you have a file text.txt of CSV format:

name1,content1
name2,content2

Try the commands below 
to import the data in test.txt 
into a new database called test.db

D:\test>sqlite3 test.db
SQLite version 3.6.23
Enter ".help" for instructions
Enter SQL statements terminated with a ";"
sqlite> create table myTable(nameOfText TEXT, contents TEXT);
sqlite> .separator ","
sqlite> .import test.txt myTable
sqlite> select * from myTable;
name1,content1
name2,content2
sqlite>






Create an SQLite database using DB Browser for SQLite

Ubuntu -> Software -> DB Browser for SQLite

Usage:

https://www.youtube.com/watch?v=2pbL7gTHMRw











Writing the result of an SQLite query to a text file

6. Writing results to a file

By default, sqlite3 sends query results to standard output. 
You can change this using the ".output" and ".once" commands. 
Just put the name of an output file as an argument to .output 
and all subsequent query results will be written to that file. 
Or use the .once command instead of .output 
and output will only be redirected for the single next command 
before reverting to the console. 
Use .output with no arguments 
to begin writing to standard output again. 

For example:

sqlite> .mode list
sqlite> .separator |
sqlite> .output test_file_1.txt
sqlite> select * from tbl1;
sqlite> .exit
$ cat test_file_1.txt
hello|10
goodbye|20
$
If the first character of the ".output" or ".once" filename 
is a pipe symbol ("|") 
then the remaining characters
are treated as a command 
and the output is sent to that command. 

This makes it easy 
to pipe the results of a query 
into some other process. 

For example, the "open -f" command on a Mac 
opens a text editor 
to display the content 
that it reads from standard input. 

So to see the results of a query in a text editor, 
one could type:

sqlite3> .once '|open -f'
sqlite3> SELECT * FROM bigTable;

If the ".output" or ".once" commands 
have an argument of "-e" 
then output is collected into a temporary file 
and the system text editor is invoked 
on that text file. 

Thus, the command ".once -e" achieves 
the same result as ".once '|open -f'" 
but with the benefit of being portable across all systems.

If the ".output" or ".once" commands 
have a "-x" argument, 
that causes them to accumulate output 
as Comma-Separated-Values (CSV) 
in a temporary file, 
then invoke the default system utility 
for viewing CSV files 
(usually a spreadsheet program) on the result. 

This is a quick way 
of sending the result of a query 
to a spreadsheet for easy viewing:

sqlite3> .once -x
sqlite3> SELECT * FROM bigTable;

The ".excel" command is an alias for ".once -x". 
It does exactly the same thing.


NOTE: The above is taken from: 
sqlite.org







Indexing

Indexing is a way of sorting a number of records
on multiple fields. 
Creating an index on a field in a table 
creates another data structure 
which holds the field value, 
and a pointer to the record it relates to. 
This index structure is then sorted, 
allowing Binary Searches to be performed on it.


NOTE: The above is taken from: 
sqlite.org







SQLite Primary key, ROWID and Constraints

Whenever you create a table without the WITHOUT ROWID constraint,
you get an implicit auto increment column called rowid.
Such a table is known as a rowid table.

The true primary key for a rowid table is the rowid.
rowid is a pseudocolumn that 
uniquely defines a single row in a database table. 
The term pseudocolumn is used because 
you can refer to ROWID in the WHERE clauses 
of a query 
as you would refer to a column stored in your database; 
the difference is 
you cannot insert, update, or delete ROWID values.



You won't see it if you do this:
SELECT * FROM tablename;

You have to specifically ask for it:
SELECT rowid,otherfield FROM tablename;

rowid

https://www.sqlite.org/rowidtable.html

https://www.sqlite.org/lang_createtable.html#rowid








Android Content Providers

A content provider is only required 
if you need to share data 
between multiple applications.

If you don't need to share data 
amongst multiple applications 
you can use a database directly 
via SQLiteDatabase. 

If you want to harness the power
of a content provider (e.g. query
and send results to file), you would
create a contact provider and link it
to your pre-made database
originally imported from a csv file.

A content provider component supplies data 
from one application to other applications on request. 
Such requests are handled by the methods 
of the ContentResolver class. 



Picture of Content Provider Diagram

An android app 
that displays Android Operating System flavors
from data accessed
via content provider and SQLite.
https://www.tutorialspoint.com/android/android_content_providers.htm How to create a content provider... More learning resources... Udacity Android Studio Android Developer Guides Android App Development on Ubuntu Linux Android Developer Fundamentals Course – Practicals Android Developer Fundamentals Course – Concepts Android Developer Fundamentals - Videos






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