Lecture 2 - The wonderful work of Android
The Android experience revolves around the design of a beautiful UI that users find intuitive, easy to use and navigate through and simple. The design of a great looking UI is an art and requires exceptionally good UI design skills. We will not teach that in this class but we will gain skills of putting together a nice, clean and simple UI.What this lecture will teach you
- Quick tour of the phone and user interface
- Development cycle
- Android frameworking
- Class information
- Dartmouth biorhythm project
- Getting started
Standard UI Screens
There are number parts of the UI that you are already using; that is the home, all apps and recent app screens.Home screen
When you unlock the phone or touch the home button on the navigation bar you see the home screen, as shown below. Above the system bar you can see the the favorites tray with various icons of your favorite apps (e.g., phone, mail, all apps button, messaging and web).
BeWell app
This is a side story: note, I'm running the BeWell app that we developed. It is available from Google play. It comprises of a nice aquatic animation that runs as active wallpaper on the home screen. The basic idea of BeWell is it continuously measures your activity (sitting, walking, running, etc.), the amount of sleep you get each night and how social you are. It infers all these human behaviors by using embedded sensors in the phones -- this is called smartphone sensing. The user does nothing, just carries and use the phone as normal. Here is the cool thing: the clown fish represents the level of user activity (swims faster or slower), the size of the school blue fish represents how much you are surrounded by conversation, and the ambient light reflects how much sleep you had last night - bright means you had a good 8 hours and the darker it gets the less sleep you had). This is a complex continuous sensing app that includes multiple activities and services. Importantly a digital arts student (undergrad) did the UI design -- nice hey.All app screen
If you touch on the all apps button (circled above) in the favorite tray on the home screen you wil come to the all apps screen. Users can swipe through multiple app and widget screens and drag and drop items to customize the all app screens.
Recent apps screen
The apps that have been used and are currently stopped can be viewed by touching the recent app button on the navigation bar, as circled in the diagram above. You can switch back to an app that you have previously used by simply touching it. Note, that applications are stacked in reverse chronological order with the most recently used app at the bottom.
Status bar and notification screen
The status bar highlight on the home screen but visible in all the screens above displays notifications on the left and status on the right. Status include includes time, battery, cell signal strength, bluetooth enabled. The notifications on the right (in the example screens shown above) show wifi, x, mail, bewell, voicemail, usb active, music. The phone is informing me that there is stuff for me to look at -- mail to read, voicemail, etc. These are pending notification of events that the user should take a peek at.If you drag or swipe down the status bar it provides a whole lot more information on these notifications -- they are expanded from the simple icons in the status bar.
Facebook UI
Android allows you to design UIs using graphical tools and xml. It also provides a number of widgets and standard layouts for the design of simple clean UIs. Here is an example that shows some standard UI components that you will use.Consider the Facebook app. It uses the action bar -- in fact it uses a split action bar with a main action bar and the a split action bar -- as shown in the figure below. You are use to using this navigation scheme as a Facebook user.
Action bar
A quick note on the action bar. The action bar is configurable from app to app and provides a semi-standard way to build out a simple and clean app UI to handle user actiona dn app navigation.The action bar provides a common and consistent interface for Android users. It can be used and adapted by many apps. We will show the email app below. We will use action bars and the ActionBar APIs to set up some clean UIs.
Gmail UI
The second app is the gmail UI --shown below. This time the split action bar is located at the bottom of the screen. In addition there is a view control which I have circled.
App development cycle
As coders you are aware of the development cycle of any software but the unique part of Android is the publishing part -- see the development cycle below.
Some factoids on Androids
- Java: Android applications are written in the Java
- filename.apk: The Android SDK tools compile the code—along with any data and resource files (layouts, strings)—into an Android PacKage (filename.apk). When you download an app from Google Play or install it yourself (from email, using ADT) you are installing the apk file.
- UI design: Android separates the UI design from the program so they can be developed independently. In this manner you could theoretically completely change the UI without changing a line of Java code (not true in reality). The UI is designed using a graphical tool or Extensible Markup Language - XML:``XML is is a markup language that defines a set of rules for encoding documents in a format that is both human-readable and machine-readable''
What's in an app: application components
When you develop apps you will be using five key building blocks -- some apps may have all these components and others not -- really depends on what type of app you are planning on building:- activities: An activity is the key execution component. You need at least one activity in an app that deals with the UI but you probably will have multiple activity. This is the entry point to the application -- think of main() in C. An activity might handle one or more screens in terms of UI control. For example, the camera app might have one activity to allow you to focus and take a photo and the start another activity to manage the storage locally and presentation of the photo to view. Each activity is independent of each other but could be coupled by control or data.
- fragments: Fragments are new to Android but very important in programming the UI. I think of them as mini-activities. In implementation there are considerable differences; for example, an activity has to be specified in the manifest, while fragments do not because they can only exist in the context of an activity and are coupled to the activity state. While fragments are strongly integrated into the UI you can use fragments without a view or UI components. Fragments can implement a number of application behaviors, such as, code to handle listeners, files, database, etc. These types of fragments can be thought of as invisible worker for the activity -- more later on this. For now we will just consider fragments when applied to the UI.
- services: MyRuns need to read the location (e.g., GPS, WiFi, cellular) periodically and pass the location to an activity. The code that periodically interacts with the location manager is a service: a component that runs in the background detached from the main application activities -- sort of does its own thing that wants to sync to exchange information. Typically services are long running programs that don't need to interact with the UI. Other examples of services could be music which the user is listening to while reading CS65 notes, the Dartmouth Biorhythm Project code runs as a service in the background. Typically an activity will control a service -- that is, start it, pause it, bind and get data from it.
- content providers: Apps share data. The nice thing about Android is you can not only call internal apps such as the camera but you can get data from apps that you might need for your application. For example, I want to develop an app that reads your contact lists and add them to your social networks (not always a good idea). To do this you need to interact with the content provider of the contacts information. And assuming the user give you permission (at the time of install) you can interact with the content provider to get the data. Not only can apps get data (send query get response) but they can update the data -- add a contact for example.
- broadcast receivers: If an service for example (the MyRuns location service) has data it can initiate broadcasts (something like ``I got the location for anyone interested''). The other end of that are components (e.g., an activity) that are broadcast receivers. Typically, broadcast receivers don't interact with the UI but commonly create a status bar notification to alert the user when a broadcast event occurs. So the broadcast announcement/receiver infrastructure is a way to delegate a task and the make sure components interested in an event (e.g., download completed, email to read, missed call) get that event to process.
Android software framework
Android has a complex and deep set of components in its framework. The best way to understand this abstract stack is to use it.Each Android app runs in its own security sandbox:
The Android operating system is a multi-user Linux system in which each application is a different user.
By default, the system assigns each application a unique Linux user ID (the ID is used only by the system and is unknown to the application). The system sets permissions for all the files in an application so that only the user ID assigned to that application can access them.
Each process has its own virtual machine (VM), so an application's code runs in isolation from other applications.
By default, every application runs in its own Linux process. Android starts the process when any of the application's components need to be executed, then shuts down the process when it's no longer needed or when the system must recover memory for other applications.
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