Day 3-Being Active Through Activities

Today We’re Covering…

• Application Structure
• Android Application Anatomy
• Activity
• Layout
• Using Layouts from Activity
• Activity Lifecycle

Project Structure…

Created Project has the following structure

Project Structure

-Source (src)

-Generated Class (gen)

-Android 1.6 library

-Assets (assets)

-Resource(res)

    -drawable-hdpi

    -drawable-ldpi

    -drawable-mdpi

    -layout

    -values

-AndroidMenifest.xml

-default.properties

-Source (src)

We have used only one class here which is an Activity named HalloActivity. We’ll describe about Activity in detail with lifecycle shortly. For now we can consider Activity as Android analogue for the window or dialog in a desktop application. It can load view from xml layout (here main.xml under res/layout folder)

In the HelloActivity class, the screen layout is set by main.xml given below:

-res/layout/main.xml

1.UI Layout can be defined from source code using View  or by layout xmls.

2.The layout xml can be generated by visual tool given by ADT

-res/drawable

1.From Android 1.6 to support different screen sizes and screen densities graphic files are kept in 3 different folders drawable-hdpi, drawable-ldpi and drawable-mdpi

2.In our current project, they contain only default icon file with different dimensions to support devices with different screen resolution.

-assets

Holds other static files you wish packaged with the application for deployment onto the device. In this project, we have none

-gen/R.java

-values/strings.xml

-AndroidMenifest.xml

XML file describing the application being built and what components – activities, services, etc. – are being supplied by that application

Let’s Build Something Useful…

Objective:  To get hands-on experience of building something useful more than just “Hallo World”.

Plan:  We’ll create a project to show how the basic building block Activity and some UI elements work.

Output:  User will push a button and see current time.

We’ll learn how to:

1.Design UI from layout XML

2.Set the layout in an Activity

3.And make UI elements in action

Steps:

1.Creating project

2.Design UI

3.Add functionality to UI

4.Run the application

Creating the Project:

Start Eclipse and go to New>Project>Android Project

Designing Layout

Edit layout/main.xml using the visual tool given by ADT by adding a TextView and a Button in a LinearLayout (more on layouts will be covered later)

The output xml is like below. We can directly edit layout xml to design the UI.

Properties were set from the visual tool.

Let’s infuse life to UI

Activity with UI elements declared

Initializing UI elements

Auto-generated R.java

Now:

1.Adding Button action listener

2.A method for getting time from Date class

Complete Activity

Creating Run Configuration and Run

So, you can now create your own application

Android Activities

Activity

Activity provides a user generally with an interactive screen to do something like: Dialing the phone, View a map List of something for user to select or Anything you want your user to do

•An application usually consists of multiple activities.

•Typically, one activity in an application is specified as the "main" activity, which is presented to the user when launching the application for the first time. (which is specified in AndroidMenifest.xml) 

The  element specifies that this is the "main" entry point to the application.

The  element specifies that this activity should be listed in the system's application

launcher (to allow users to launch this activity).

Creating Activity

We have already created Activities. But how did we create it?

Let’s revisit…

1.We created subclass of Activity base class

2.We implemented one callback method onCreate

Open Questions:

1.What is creating subclass???

2.What is callback methods???

Next Step: Implementing User Interface

1.Design res/layout/yourlayout.xml

2.Use Views from Activity class

Next Step: Manifest

Next Step: Starting Activity

We can start another activity by calling startActivity(), passing it an Intent that describes the

activity you want to start. 

Intent intent = new Intent(this, ToActivity.class);

startActivity(intent);

Android Application Anatomy

Activity Lifecycle

We'll discuss Activity Lifecycle in detail in some future session