Android Basics. Android UI Architecture. Android UI 1

Transcription

1 Android Basics Android UI Architecture Android UI 1

2 Android Design Constraints Limited resources like memory, processing, battery à Android stops your app when not in use Primarily touch interaction à Android provides more kinds of events Application Model à Android uses a full-screen application model Android UI 2

3 App Components - Essential building blocks of an Android app - Each component is an entry point through which the system or a user can enter your app Components Activity Service Content provider Broadcast receiver Description Single-screen of an application Long-running background process Provides shared set of application data Responds to system broadcast events Android UI 3

4 Activities A standard application component is an Activity - Typically represents a single screen - Main entry point (equivalent to main() method) - For most purposes, this is your application class Android UI 4

5 Managing the activity lifecycle An activity goes through a number of states over the course of its lifetime A series of callbacks is used to handle transitions between states - oncreate() - onstart() - onresume() - onpause() - onstop() - onrestart() - ondestroy() Android UI 5

6 Activity Lifecycle Activities have an explicit lifecycle - One activity runs at a time, others are paused in the background. - As users navigate through your application, they switch activities. Every activity has a state: run, paused, or stopped. - Changing state fires a corresponding activity method. Running Paused / Stopped Android UI 6

7 Interrupted Workflow Applications can stop at any time (i.e. user quits, OS kills it). - Each activity needs to manage its own state - Activities have methods for saving and restoring state Android UI 7

8 Intents An Intent is a messaging object you can use to request an action from another app component - Starting an activity - Starting a service - Delivering a broadcast We use intents to pass data between activities. - a data structure holding an abstract description of an action Android UI 8

9 Intents Use Activity startactivity() method to launch with intent.explicit (named activity) vs. implicit (capabilities, e.g. camera) Android UI 9

10 Fragments Fragments can be thought of as portions of a UI An activity can contain (and manage) multiple fragments. - Fragments have their own lifecycle, and their own layout. - Alternative to multiple activities Android UI 10

11 Views (what android calls a widget) android.view.view Base widget class (drawing and event handling) Subclasses: android.widget.button android.widget.imageview android.widget.progressbar Android.widget.TextView... android.view.viewgroup Abstract container class Includes layout functionality directly Subclasses: LinearLayout, RelativeLayout, GridLayout, Android UI 11

12 Layouts Defines the structure for a user interface Built using a hierarchy of View and ViewGroup A View usually draws something the user can see and interact with A ViewGroup is an invisible container that defines the layout structure for View and other ViewGroup objects Android UI 12

13 Common Layouts Each subclass of the ViewGroup class provides a unique way to display the views you nest within it Linear Layout A layout that organizes its children into a single horizontal or vertical row Relative Layout Enables us to specify the location of child objects relative to each other or to the parent. Grid View Displays items in a two-dimensional, scrollable grid Android UI 13

14 UI Definition and Layout Layout can be handled in one of two ways: - Programmatically. You write code to instantiate ViewGroups, Views and bind them together (like in Java Swing). - Use XML to describe your layout. In XML describe the screen elements (view groups and views) along with properties, and then tell your application to dynamically load it. Using XML is the preferred way - Android Studio includes a GUI builder to make this easier! Android UI 14

15 xml layout <?xml version="1.0" encoding="utf-8"?> <RelativeLayout xmlns:android=" xmlns:tools=" android:layout_width="match_parent" android:layout_height="match_parent" android:paddingbottom="16dp" tools:context=".mainactivity" > <Spinner android:layout_width="96dp" android:layout_height="wrap_content" android:layout_alignparentright="true" /> <Button android:layout_width="96dp" android:layout_height="wrap_content" android:layout_alignparentright="true" /> </RelativeLayout> Android UI 15

16 Pixel density The number of pixels that fit into an inch is referred to as pixel density. ex. 160dpi = 160pixel per inch High-density screens have more pixels per inch than low-density ones. Screen density = screen width (or height) in pixels / screen width (or height) in inches Android UI 16

17 dp Layout Units Uses Density-independent pixels (dp), pronounced dips A dp is equal to one physical pixel on a screen with 160 dpi To calculate dp: dp = (width in pixels * 160) / screen density Android UI 17

18 Events Android uses the Java event model with additional mobile events - Event listener: interface for specific type of event - Event handler: registered callback method to handle the event Event Listener Event Handler Type of event OnClickListener() onclick() Touch, click OnLongClickListener() onlongclick() Press and hold ontouchlistener() ontouch() Generic touch events; can be used for touch_up, second_touch Android UI 18