aadilayub / android-basic-concepts

Notes from the Android Basics Nanodegree. For personal reference

Concepts to Revise:

• Activity Lifecycle
• ViewGroups
• RelativeLayout
• LinearLayout
• Styles
• Fragments
• MediaPlayers and AudioFocus
• Activities
• Contexts
• ArrayAdapter
• Intents

Android UI Overview

The user interface (UI) that appears on the screen of an Android device consists of a hierarchy of objects called views - every element of the screen is a View . The View class represents the basic building block for all UI components, and the base class for classes that provide interactive UI components such as buttons, checkboxes, and text entry fields. Commonly used View subclasses include:

• TextView for displaying text.
• EditText to enable the user to enter and edit text.
• Button and other clickable elements (such as RadioButton, Checkbox, and Spinner) to provide interactive behavior.
• ScrollView and RecyclerView to display scrollable items.
• ImageView for displaying images.
• ConstraintLayout and LinearLayout for containing other View elements and positioning them.

The Java code that displays and drives the UI is contained in a class that extends Activity. An Activity is usually associated with a layout of UI views defined as an XML (eXtended Markup Language) file. This XML file is usually named after its Activity and defines the layout of View elements on the screen.

For example, the MainActivity code in the Hello World app displays a layout defined in the activity_main.xml layout file, which includes a TextView with the text "Hello World".

In more complex apps, an activity might implement actions to respond to user taps, draw graphical content, or request data from a database or the Internet.

Views and Layouts

A layout defines a single UI in your app. In Android, all UIs are built using a heirarchy of View and ViewGroup objects.

A View usually draws something the user can see and interact with like a TextView, ImageView , or Button, whereas a ViewGroup is an invisible container that defines a layout structure for other View and ViewGroup objects.

Examples of ViewGroups are LinearLayout, RelativeLayout, and ConstraintLayout.

LinearLayout

In a LinearLayout, like the name suggests, all the elements are displayed in a single direction either vertically or horizontally, and this behavior is specified in the android:orientation attribute.

Positioning

LinearLayout is required to have these attributes set:

• android:layout_width
• android:layout_height
• android:orientation

Other layout-related attributes include:

• android:layout_gravity: This attribute is used with a UI element to control where the element is arranged within its parent. For example, the following attribute centers the UI element horizontally within the parent ViewGroup:

android:layout_gravity="center_horizontal"

• android:layout_weight: This is useful for equally (or proportionally) spacing out views in a layout. To space out views equally set their height or width to 0dp, and assign each view a layout weight of 1. A related value is android:weightsum. Let's say you want to proportionally space out two views. You could set weightsum to 4, and set the layout weight of the views to 3 and 1 respectively. This will make on view take up 75% of the space, and the other one 25%.

RelativeLayout

Another useful ViewGroup for layout is RelativeLayout, which you can use to position child View elements relative to each other or to the parent. The attributes you can use with RelativeLayout include the following:

• android:layout_toLeftOf: Positions the right edge of this View to the left of another View (identified by its ID).
• android:layout_toRightOf: Positions the left edge of this View to the right of another View (identified by its ID).
• android:layout_centerHorizontal: Centers this View horizontally within its parent.
• android:layout_centerVertical: Centers this View vertically within its parent.
• android:layout_alignParentTop: Positions the top edge of this View to match the top edge of the parent.
• android:layout_alignParentBottom: Positions the bottom edge of this View to match the bottom edge of the parent.

ConstraintLayout

Style-related attributes

You specify style attributes to customize the appearance of a View. A View that doesn't have style attributes, such as android:textColor, android:textSize, and android:background, takes on the styles defined in the app's theme.

The following are common style-related attributes:

• android:background: Specifies a color or drawable resource to use as the background.
• android:text: Specifies the text to display in the view.
• android:textColor: Specifies the text color.
• android:textSize: Specifies the text size.
• android:textStyle: Specifies the text style, such as bold.