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Android User Interface

The Android device screen serves as the primary visual interface for users to interact with an application. A well-designed user interface (UI) should be visually appealing, intuitive, consistent, and responsive. It must incorporate essential accessibility features and support device orientation changes and cross-device compatibility.

The UI comprises various graphical user interface elements created using View and ViewGroup objects in Java or Kotlin. These elements include controls like buttons and text boxes, along with images, fonts, colors, themes, and other visual components. They are organized within invisible containers called layouts, which are ViewGroup objects responsible for positioning their child View objects (UI elements) on the screen.

The static aspects of the user interface, excluding dynamic functionalities, can be conveniently designed using XML files or programmatically with Java or Kotlin. XML files define the structure and appearance of the UI elements and are converted into Java or Kotlin objects at runtime for rendering on the screen.

Basic UI Layouts

  • Linear Layout - Aligns child views in a single direction, either horizontally or vertically. It is ideal for creating simple linear arrangements of views.
  • Relative Layout - Positions child views relative to sibling views or the parent container. It provides flexibility for arranging views based on their relationships to each other.
  • Table Layout - Organizes child views in rows and columns, similar to a table. It is useful for creating grid-like structures with consistent cell sizes.
  • Frame Layout - Stacks child views at specific locations on the screen, allowing views to overlap. It is commonly used for displaying a single view or layering multiple views.
  • Absolute Layout - Positions child views at exact coordinates on the screen. While offering precise control, it is generally not recommended due to its lack of adaptability to different screen sizes.
  • Constraint Layout - Arranges child views using constraints relative to other views and the parent layout. It provides advanced flexibility for designing responsive layouts across various screen sizes and orientations. Constraint Layout is an enhanced version of Relative Layout.
  • Coordinator Layout - Manages complex interactions and arrangements between child views, enabling dynamic behavior such as animations and scrolling effects. It is often used for advanced UI designs with interactive components.

ViewGroup is the base class for all layouts that define how views are arranged within its invisible container. For example, a LinearLayout can contain various views such as Buttons, TextViews, etc. A ViewGroup can also nest another ViewGroup within it (e.g., a LinearLayout inside a ConstraintLayout). To display the UI of any layout, it must be inflated in the corresponding Java/Kotlin class, converting it into a hierarchy of View objects that form the UI. For inflating layouts in an Activity, override the onCreate method and use the setContentView method. For a Fragment, override onCreateView and use a layout inflater to retrieve the defined view hierarchy from the XML file.

Using the Data Binding library simplifies the process of inflating layouts by automatically generating binding classes that provide direct access to views and data within the layout. To enable Data Binding, update the app's build.gradle file. Instead of a standard layout XML file, create a Data Binding layout by enclosing the existing layout in a <layout> tag. The library automatically generates a binding class based on the layout file's name. In an Activity or Fragment, use the generated binding class to inflate the layout without requiring the findViewById method. Specific views can be accessed directly using the binding object (an instance of the binding class).

Input Controls

  • Android Button
  • Image Button
  • Radio Button
  • Switch Button
  • Toggle Button
  • Edit Text
  • Text View
  • Check Box
  • Rating Bar
  • Spinner
  • Date Picker
  • Time Picker
  • Toast Message
  • Alert Dialog
  • Snackbar
  • Sliding Menu with WebView
  • Dropdown Menu
  • Action Bar/App Bar
  • Toolbar
  • Action Bar Tabs
  • Design Support Library
  • Navigation Drawer View
  • Floating Action Button (FAB)
  • Floating Label for EditText

View is the base class for most of these controls. After an XML layout is inflated in the corresponding Activity or Fragment using Java or Kotlin, instances of view objects can be created to access specific views by their View ID. This is typically done using methods such as findViewById(), View Binding, or Data Binding. An event listener, such as a click listener, can then be attached to a view to perform specific actions when the user interacts with it.

App Widgets

App widgets are another important UI component. They are self-contained, providing specific functionality that can be embedded within an application's user interface or placed on the device's home screen. Widgets enable quick access to certain features or information of an app directly, offering "micro-interactions" with the app's functionality. They come in various forms and sizes, ranging from simple controls like buttons and text views to more complex elements like calendars, weather forecasts, music players, and news feeds. Widgets can be moved around on the home screen and are often resizable, allowing users to adjust their dimensions based on preferences.

To add a widget to your home screen:

  • Touch and hold on an empty space on your device's home screen.
  • Tap Widgets from the menu that appears.
  • Select the app and the specific widget of your choice from its sub-menu.
  • Tap and hold the widget, then drag it to the desired location on your home screen.

For an example of a home screen widget that makes a network call, check out: Widget to fetch JSON data.

Custom User Interface

Custom Layout

Creating custom layouts requires a thorough understanding of Android's view hierarchy, layout calculations, and UI design requirements. To create custom layouts in Android, you can extend the ViewGroup class, allowing you to modify the appearance and behavior of standard layouts. Here are the main steps involved:

  • Extend the ViewGroup class: Create a new class that extends ViewGroup. This class will serve as the container for your custom layout.
  • Override the onLayout() method: Define how child views should be positioned within the layout. Specify the coordinates and sizes of each child view based on your desired arrangement.
  • Override the onMeasure() method: This method determines the size of the custom layout by calculating the dimensions of its child views. Use getMeasuredWidth() and getMeasuredHeight() to obtain these measurements.
  • Override additional methods as needed: Depending on your layout's requirements, you may override methods related to touch events or custom drawing on the canvas.

Custom Views

Custom views are used to enhance the appearance and behavior of default views or to create entirely new UI components. There are several approaches to building custom views:

  • Extend an existing Android widget: Subclass an existing widget, such as TextView, Button, or ImageView. This approach is useful for adding specific functionality or modifications to an existing view. Place necessary setup code in the constructor to ensure reusability.
  • Extend the base View class: For more control, extend the base View class or one of its subclasses, such as ViewGroup. Override methods like onDraw() to handle custom drawing on the canvas and onTouchEvent() to handle touch interactions. This approach is ideal for creating highly customized views that differ significantly from standard widgets.
  • Create Compound Views: Combine multiple views into a single reusable component. A compound view serves as a container for related child views, encapsulating their layout and behavior in a custom class.

Each approach has its advantages, and the choice depends on your application's design and functionality requirements. Custom views enable the creation of unique and tailored UI components to enhance the user experience.

UI Styles and Themes

Material Design

Material Design provides a consistent and intuitive user interface design across different platforms and devices. Some key aspects include:

  • Real-world Inspiration: Material Design draws inspiration from the physical world, utilizing visual cues that mimic the behavior of real objects and materials. It simplifies real-world interactions and animations to create a familiar and intuitive user experience.
  • Responsive Animations and Transitions: Material Design incorporates responsive animations and transitions to provide smooth and meaningful visual feedback to user interactions. These animations help convey the relationships between different UI elements and enhance the overall user experience.
  • Grid-based Layout: Material Design emphasizes a grid-based layout system. This approach creates a sense of order and consistency in the placement of elements within the interface, ensuring better alignment and organization across various screen sizes and orientations.
  • Colors, Typography, and Depth Effects: Material Design introduces a comprehensive color palette, typography guidelines, and depth effects such as lighting and shadows. These elements work together to create a visually appealing and cohesive design. Depth effects add a sense of hierarchy and focus to UI elements.
  • New Widgets: Material Design introduces new UI widgets that enhance the user interface. For example, the CardView widget allows for the display of important information within cards, maintaining a consistent look and feel. The Toolbar widget replaces the traditional ActionBar, providing more flexibility in customization and placement within activities. The RecyclerView widget is an advanced version of ListView, offering improved performance, item animations, and more customizable layout options.
  • Z-axis Dimension: In Material Design, views in Android have a 3D environment. Along with the X and Y dimensions, the Z dimension is also considered. This allows for the creation of layered and depth-aware interfaces, enabling visual elements to appear on different planes.

For more details about Material Design in Android and some examples, visit: Android Material Design.

Jetpack Compose

The recent arrival of Google's Android framework, Jetpack Compose, marks a significant update to UI development. It allows developers to build responsive, modular, declarative, and consistent UI designs programmatically using Kotlin, eliminating the need for XML layouts.

Main Features of Jetpack Compose

  • Composable Kotlin Functions: UI components are created using composable Kotlin functions marked with the @Composable annotation. These functions define and render UI components, automatically updating when the underlying data or state changes. UI hierarchies are constructed by nesting composable functions.
  • Instant Code Changes: The @Preview annotation lets developers preview composable functions within Android Studio using stub data. This feature enables viewing code changes without running an emulator or device. Additionally, Live Edit allows real-time code changes to be viewed directly on a device or emulator using real data.
  • Interoperability with XML: Jetpack Compose supports interoperability with existing Android Views and ViewGroups. Compose components can be embedded within traditional View-based layouts, and vice versa. This flexibility is particularly beneficial for large, complex projects where incremental migration to Compose is preferred.
  • Advanced Animations: Jetpack Compose provides robust APIs for creating animations, transitions, and motion effects. These APIs simplify defining complex animations for interactive and visually engaging UI experiences.
  • Accessibility Support: Compose emphasizes accessibility with built-in support for screen readers, keyboard navigation, and more. Developers can annotate composable functions with accessibility properties to ensure inclusivity and usability for all users.
  • Styling and Themes: Compose uses modifiers to style and configure composables, enabling changes to size, layout, appearance, and interactions. It includes a ready-to-use implementation of Material Design, built around Color, Typography, and Shape. Lists, animations, and other UI components are supported out of the box.
  • State Management: Compose updates UI reactively based on state changes. The remember function retains state across recompositions, while rememberSaveable preserves state across configuration changes. For managing state, mutableStateOf creates a mutable state holder that triggers recomposition on value changes. For complex scenarios, ViewModel can be used. Lifecycle-aware features such as OnLifecycleEvent and LaunchedEffect handle side effects and activity lifecycle integration.
  • Automation Testing: Traditional testing tools like Espresso, JUnit, Mockito, and Robolectric are compatible with Compose. Additionally, Compose provides its own testing tools, such as ComposeTestRule and Compose testing APIs, for isolating and testing UI components effectively.

Jetpack Compose Sample Codes