The entire content is inspired by ,
Google's udacity course Android Basics: User Interface. Its first part of the five parts Android Basics Nanodegree Program series , This course is for basics of Android and Java programming as , and take the first step on your journey to becoming an Android developer, you can learn more about this here.
So I would be writing a Blog series where I will cover everything that I have learned ,and explain it in easy way.

If you are thinking of starting an Android Development you can check this blog,

In this article,

We cover about the basic knowledge of how to place the layouts on the page to create image, text , button on the phone screen.

So without much late, let's get into topic..

• We use XML (Xtensible Markup Language) to build Layout or define an UI in the Android app ,like TextView how to place and where to place something
• This resource are always maintained separately in various sub-directories under res/ directory of the project.

What is Layout🤔

Every individual view make up what we call layout of the app of what the user sees on the screen.

The basic building block for user interface is a View object which is created from the View class and 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.

The ViewGroup is a subclass of View and provides invisible container that hold other Views or other ViewGroups and define their layout properties.

View

A rectangle on the screen that shows up in a piece of information or some content
It can be image text buttonor anything a app can display

⭕Some View attributes

• android:id

View object may have a unique ID assigned to it which will identify the View uniquely. The syntax for an ID, inside an XML tag is
For assigning the View ID name

android:id="@+id/text_id"

For using its id name to as a reference

android:id="@id/text_id"

• layout_width & layout_height

These are ViewGroup Layout Params
Its specifies the basic width & height of the view respectively.This is a required attribute for any view inside of a containing layout manager.

This is actually an attributes of ViewGroup and used by the view to tell there parent view how they want to be set

The attributes value may be a dimension (such as "12dp") which is hardcoded value or one of the special constants..

android:layout_width="match_parent"
     android:layout_height="wrap_content"

Density Independent Pixel (dp)

First lets know about what is Pixel❓

The one small dot or illuminated area on screen is called 1 pixel
Screens are made up of rows and columns of 1000' of these pixel.
Now,
Device can range in screen density which is numbers of pixel per inch are on the screen , Hences devices with are varies in low,medium, high and extra high resolution.
If we use pixel(px) as a unit it would appear very smaller on high resolution device as there screen density is more

So its better to use density independent pixel which is also a unit and are independent of pixel ,to achieve same physical size of Views , across of different screen densites .

Hardcoding❓

• Its set if instructions given into the app we use to give the fixed value to the View attributes.
• Its disadvantages is, its fixed we have to keep on updating according to the content .
• Hence we should use some special constant like match_parent wrap_content which is flexible and set accordingly .

⭕Android UI Controls

There are number of UI controls provided by Android that allow you to build the graphical user interface for your app.

🔘 Now we will learn abouts some important Views and there attributes

1.TextView Attributes

• android:text
  Text to display.

• android:textAllCaps
  Present the text in ALL CAPS. Possible value either "true" or "false".

• android:textSize

Size of the text. Recommended dimension type for text is "sp" for scaled-pixels (example: 15sp).

We use attributes values unit as sp here because its changes to with the preference of user for the font size, which is setting app of the Android devices

• android:textStyle

Style (bold, italic, bolditalic) for the text. You can use or more of the following values separated by '|'.

normal - 0
bold - 1
italic - 2

• android:password
  Whether the characters of the field are displayed as password dots instead of themselves. Possible value either "true" or "false".

• android:textColor
  For setting the color of color of the text .

  HexColor Code

Android has set of color defined byt not all ,hence from material design we can use color code called hex color code is given a specific hexadecimal digits start with "#" sign.
Eg:- #FF0000-> Red
#000000-> Black
#FFFFFF-> White ,etc

• Similarly many more attributes you can check on documentation

<TextView
      android:id="@+id/text_id"
      android:layout_width="300dp"
      android:layout_height="wrap_content"
      android:text="Hello_world" 
      android:textAllCaps="true"
      android:textColor="@android:color/holo_blue_dark
      android:textSize="50sp"/>

2.ImageView Attributes

• android:src Sets a drawable as the content of this ImageView. How to insert and image file ❓
  • First of all download an image , save it somewhere in local
  • Now, in the directories app/res/drawable ,we put here every images used in the app
  • For placing here we have to right click on drawable folder, click on reveal in explorer
  • Now, just drag and drop the image file here in the drawable folder
  • Inside the ImageView use this attributes android:src to refer our image in the drawable folder
  • @ is used for the referencing a resource in an android app, here drawable is the resource type, and then we write the filename of the specific image

android:src="@ drawable/image_name

There are many various other resources like static content that your code uses, such as bitmaps, colors, layout definitions, user interface strings, animation instructions, and more. These resources are always maintained separately in various sub-directories under res/ directory of the project.

MyProject/
   app/
      manifest/
         AndroidManifest.xml
   java/
      MyActivity.java  
      res/
         drawable/  
            image_name.png  
         layout/  
            activity_main.xml
            info.xml
         values/  
            strings.xml

• android:scaleType Controls how the image should be resized or moved to match the size of this ImageView. It has some constant attributes values Lets check some of its ,
  • center Center the image in the view, but perform no scaling.
  • centerCrop Its scales the image to fit, the bounds of the view And it also maintain the aspect ratio of the image so that it didn't get distorted and then it crop of the edges and centres the image .
• Similarly Many more attributes you can check onDocumentation

<ImageView
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:scaleType="centerCrop"
         android:src="@drawable/image_name />

3.Button Attributes

• Button, is a component which can be pressed or clicked by the user to perform an action. It has the same properties as a TextView, with a few Button specific properties. The main usage of the Button view is that whenever we click a button, we can set a method that will handle that specific button request and will carry out the necessary action.

Below there is XML, how to define a button view in your android application:

<Button
    android:id="@+id/button_id"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:text="Click me"
    android:textColor="@android:color/holo_blue_dark" />

public class MyActivity extends Activity {
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);

         setContentView(R.layout.content_layout_id);

         final Button button = findViewById(R.id.button_id);
         button.setOnClickListener(new View.OnClickListener() {
             public void onClick(View v) {
                 // Code Perform action on click
             }
         });
     }
 }

The above code is in activity class its creates an instance of View.OnClickListener and wires the listener to the button using setOnClickListener .

Therefore, whenever we press the button with id button_id, the above method is called which executes the code written in onClick(View) method

• Using android:onClick attributes

We can also assign a method directly in the layout XML while defining the button using, android:onClick attribute

<Button
    android:id="@+id/button_id"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:text="Click Me"
    android:textColor="@android:color/holo_blue_dark"
    android:onClick="study" />

When user will click on the Button defined in the above layout xml file, then Android system will call study(View) method, defined in MainActivity.java file. In order for this to work, the method must be public and accept a View type as its only parameter.

public void study(View view) {

    //Perform action on click   
}

• Similarly many more attributes you can check on documentation

ViewGroup

• It is a root view ,but that big as it contain the other view inside itself .
• It's called the parent view and the view inside its are called its child view of the view group ,can be TextView , Button , ImageView,etc.
• The ViewGroup has also attributes ,such as layout_height, layout_width, background color ,etc.
• We call it as a containers for the other views .

Types of ViewGroup

There are number of Layouts provided by Android which you will use in almost all the Android applications to provide different view, look and feel.
Some of them are ,

1. Linear Layout
   LinearLayout is a view group that aligns all children in a single direction, vertically or horizontally.

2. Relative Layout
   RelativeLayout is a view group that displays child views in relative to the parent or the other child view.

3. Table Layout
   TableLayout is a view that groups views into rows and columns.

4. Frame Layout
   The FrameLayout is a placeholder on screen that you can use to display a single view.

5. Constraint Layout
   Constraint Layout allows us to create large and complex layouts with a flat view hierarchy, and also allows you to position and size widgets in a very flexible way.

That's all for today!

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