# Activities

Subclasses of **android.app.Activity**

- Present a visual UI
- Each activity has its own 'window'
    - Only one 'window' on screen at once
    - **Granular management** of the resources of an application
    - UI layout - a 'view'
        - Specified in a separate XML file
        - Constructed programmatically
    - Apps usually have several activities

#### Android UI

An activity has a window associated with it - usually full screen.

![img](img/k.png)

#### View Hierarchy

There are 3 types of views:

1. Those that display something (`Views`)
2. Those that do something (`Widgets`)
3. Those that lay out sub-views (`ViewGroups`)

##### View Principles

Mobile devices are rarely the same, i.e. they have different screen sizes & resolutions

* Layout should **adapt** to the screen it is inflated in
    * No hard-coding, layouts are defined in hierarchies and relationships.
* Different layouts for orientation

###### ViewGroups

* Frame Layout
    * Simplest - contains a single object
* Linear Layout
    * Aligns all children in a single direction
* Table Layout
    * Positions children into columns and rows
* Constraint Layout
    * Allows children to specify their position relative to their parent view
* Scroll View
    * A vertical scrolling view; only contains a single element

###### Views Widgets

* Views that the user can optionally interact with
    * Buttons, text view, calendar viewer, image viewer, etc.
* Generate UI events

###### Views Parameters

* width or height
    * Use percentages - relative to parent or screen width, etc.
* ID
    * Used to generate a Java member variable we can refer to within the program
* Basic binding of methods
    * Used to automatically bind UI events to methods

###### Views Resources

* Layouts are an example of a *resource*
    * Static content used by components at run-time
    * Can be referenced by ID

Data Binding - involves a lot of boilerplate code

`@{}` notation - receives data changes

`@={}` - receives data changes to the property and listens to user updates

##### Model-View-ViewModel

![img](img/l.png)

## The Manifest

A *list* of the application's components