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Android Development

Android Development 27
Università Degli Studi di Parma Distributed Systems Group Android Development Lecture 2 Android Platform Marco Picone 2012Università Degli Studi di Parma Distributed Systems Group Lecture Summary The Android Platform Dalvik Virtual Machine Application Sandbox Security and Permissions Traditional Programming compared to Android Activities Services Content Providers Intents Broadcast Receivers Android Application Structure Android Application Manifest Model View Controller User Interface 2 Marco Picone 2012Università Degli Studi di Parma Distributed Systems Group The Android Platform Android is a software stack for mobile devices that includes an operating system, middleware and key applications. The Android SDK provides the tools and APIs necessary to begin developing applications on the Android platform using the Java programming language. It is a Linuxbased operating system for mobile devices such as smartphones and tablet computers. It is developed by the Open Handset Alliance led by Google. The Linux 2.6 kernel handles core system services and acts as hardware abstraction layer (HAL) between the physical hardware and the Android Software Stack. Kernel handles: Application permissions and security ‣ Lowlevel energy management ‣ Process management and Threading ‣ Networking ‣ Display, keypad input, camera, Flash memory, audio and binder (IPC) driver access ‣ Source: Wikipedia, AWA, http://developer.android.com/guide/developing/tools/aidl.html 3 Marco Picone 2012Università Degli Studi di Parma Distributed Systems Group Android Application A Android Application A Android Application A Written Using Android Written Using Android Written Using Android Java Framework Java Framework Java Framework DALVIK Virtual Machine DALVIK Virtual Machine DALVIK Virtual Machine Linux User Linux User Linux User A B C Linux 2.6 Operating System (Hardware Abstraction Layer) Security Process I/O Other Devices Memory Management Network Power WiFi, Bluetooth, Camera, Audio, Telephony, Flash, Management Stack Display, Keyboard, Management Device Sensors Binder IPC Touchscreen Physical Hardware Source: AWAChapter1 4 Marco Picone 2012Università Degli Studi di Parma Distributed Systems Group Dalvik Virtual Machine It is the Android Virtual Machine, based on the Java VM but optimized for mobile devices. Has a small memory footprint, and multiple instances of Dalvik VM can run concurrently on the device. Programs are mainly written in a dialect of Java and compiled to bytecode. They are converted from Java Virtual Machinecompatible .class files to Dalvikcompatible .dex (Dalvik Executable) files before installation on a device. The compact Dalvik Executable format is designed to be suitable for systems that are constrained in terms of memory and processor speed. Each Android application runs in a separate process, with its own instance of the Dalvik virtual machine. Android application life cycle (next lecture) enables the OS to enhance the performance of the garbage collection and how it manages memory recovery across multiple heaps. Source: PAChapter 3 AWA Chapter 1 5 Marco Picone 2012Università Degli Studi di Parma Distributed Systems Group Application Sandbox Android security is strictly related to restriction and security levels of the Linux operating system and in particular on process and userlevel boundaries. App A Android creates a new user for each application vendor. Each application it is executed with different user privileges (expect File DB SMS Logs for those signed by the same vendor). Sensors Network .... Files owned by one application are, by default, inaccessible by other applications. The SDK provides Content Provider and Intent to allows Android applications to communicate and exchange data. OS App B This solution has been adopted to provide an high security in a world of numerous third part small applications from File DB SMS Logs different vendors. Sensors Network .... The sandbox approach is a common solution for mobile operating systems such as iOS. Source: PAChapter 3 http://www.ibm.com/developerworks/xml/library/xandroidsecurity/ http://source.android.com/tech/ 6 Marco Picone 2012 security/index.htmltheapplicationsandboxUniversità Degli Studi di Parma Distributed Systems Group Application Sandbox Same User ID App Sandbox: Linux user ID 54321 App Sandbox: Linux user ID 12345 Application Application Linux User ID: 54321 Linux User ID: 12345 Resources Resources Linux user ID: 54321 Linux user ID: 12345 File DB SMS Logs File DB SMS Logs Network Sensors .... Network Sensors .... Source: PAChapter 3 http://www.ibm.com/developerworks/xml/library/xandroidsecurity/ http://source.android.com/tech/ 7 Marco Picone 2012 security/index.htmltheapplicationsandboxUniversità Degli Studi di Parma Distributed Systems Group Application Sandbox Same User ID App Sandbox: Linux user ID 12345 Application Application Linux User ID: 12345 Linux User ID: 12345 Resources Linux user ID: 12345 File DB SMS Logs File DB SMS Logs Network Sensors .... Network Sensors .... Source: PAChapter 3 http://www.ibm.com/developerworks/xml/library/xandroidsecurity/ http://source.android.com/tech/ 8 Marco Picone 2012 security/index.htmltheapplicationsandboxUniversità Degli Studi di Parma Distributed Systems Group Security and Permissions Android platform adopts several security and integrity measures to ensure that the user’s data is secure and that the device is note subject to malware. Application as Operating System Users (Dalvik instance and sandboxes) Application Permissions Each application registers for specific privileges to access shared resources. Some of these privileges are ‣ related for example to phone functionalities, make calls, access network file and system, control the camera or the other hardware sensors. Additional permission are dedicated to access private and personal information, such as preferences, ‣ user’s contacts or location. Limited AdHoc Permissions Applications that use a Content Provider to openly share specific information to other applications can ‣ define some onthefly permission to grant o revoke access to a resource. Application Signing All application are signed with a certificate to allow the users to know that the application is authentic. ‣ Source: http://developer.android.com/guide/topics/security/security.html 9 Marco Picone 2012Università Degli Studi di Parma Distributed Systems Group Traditional Programming compared to Android OS Process Traditional OS applications: use a single entry point (traditionally called main). ‣ Application The OS loads the program into a process and main()... then start executing it (Fig. A). OS Process Java VM Java based applications: Application main()... are managed in a different way. A Java VM is ‣ instantiated in a dedicated process to load all Thread Thread classes used by the application and execute it. Source: http://developer.android.com/guide/topics/fundamentals.html 10 Marco Picone 2012Università Degli Studi di Parma Distributed Systems Group Traditional Programming compared to Android Android introduces a more complex system supporting multiple application entry points. Each component is a OS Process different point through which the system can enter your application. Not all components are actual entry points Dalvik VM for the user and some depend on each other, but each one exists as its own entity and plays a specific role each Application with multiple entry points one is a unique building block that helps define your application's overall behavior. There are four different types of application components. Activity 1 Activity 2 Activity 3 Each type serves a distinct purpose and has a distinct Thread Thread Thread lifecycle that defines how the component is created and destroyed. Main application entry points are: Service 1 Service 2 Activities ‣ (No UI) (No UI) Thread Thread Services ‣ Content Providers ‣ Broadcast Receivers ‣ Source: http://developer.android.com/guide/topics/fundamentals.html 11 Marco Picone 2012Università Degli Studi di Parma Distributed Systems Group Activities Is both a unit of user interaction (typically associated Activity 1 back button to a View) and a unit of execution. To create an activity, you must create a subclass of Activity 2 Activity (or a subclass of it such as MapActivity). 1 Multiple Activities (related also to different Activity 3 applications) are organized in a stack structure where new elements can be pushed or removed after View 2 specific user actions (UI interaction or back button). Although activities work together to form a cohesive View 3 user experience in application, each one is independent of the others. As such, a different application can start any one of these activities. For example, a camera application can start the activity in the email application that composes new mail, in startActivity() order for the user to share a picture. Source: http://developer.android.com/guide/topics/fundamentals.html Marco Picone 2012 12Università Degli Studi di Parma Distributed Systems Group Activity Life Cycle Activity 4 Top Activity. View 4 User can interact with it Activity 3 Second Activity in the stack. If the user clicks back or the Top Activity is destroyed View 2 the user can see and interact with it again Activity 2 Activity in the middle of the stack. User cannot interact with it until every activity above it View 2 is destroyed. Activity 1 Activity at the bottom of the stack. If Activities above it use too many resources it will be View 1 destroyed. Source: AWA Chapter 4 13 Marco Picone 2012Università Degli Studi di Parma Distributed Systems Group Killable Method Description Next after Activity Life Cycle Called when the activity is first created. This is where onCreate() you should do all of your normal static set up — create No onStart() views, bind data to lists, and so on. Called after the activity has been stopped, just prior onRestart() No onStart() to it being started again. onResume() Called just before the activity becomes visible to the onStart() or user. onStop() Called just before the activity starts interacting with onResume() No onPause() the user. At this point the activity is at the top of the activity stack, with user input going to it. Called when the system is about to start resuming another activity. This method is typically used to onResume() commit unsaved changes to persistent data, stop or onPause() animations and other things that may be consuming CPU, Yes onStop() and so on. It should do whatever it does very quickly, because the next activity will not be resumed until it returns. Called when the activity is no longer visible to the onRestart() user. This may happen because it is being destroyed, or onStop() Yes or because another activity (either an existing one or a onDestroy() new one) has been resumed and is covering it. Called before the activity is destroyed. This is the final call that the activity will receive. It could be onDestroy() called either because the activity is finishing, or Yes nothing because the system is temporarily destroying this instance of the activity to save space. Source: http://developer.android.com/guide/topics/fundamentals/activities.html 14 Marco Picone 2012Università Degli Studi di Parma Distributed Systems Group Saving Activity State When the system destroys an activity in order to recover memory, the Activity object is destroyed, so the system cannot simply resume it with its state intact. If the user navigates back to the destroyed Activity the system needs to recreate the object. The user is unaware that the system destroyed the activity and recreated it and, thus, probably expects the activity to be exactly as it was. In this situation Android OS provides a method called onSaveInstanceState() ensure that important information about the activity state is preserved. Source: http://developer.android.com/guide/topics/fundamentals/activities.html 15 Marco Picone 2012Università Degli Studi di Parma Distributed Systems Group Service The Android Service class is for background operations that are active but not visible to the user. It is used too perform a longerrunning operation while not interacting with the user or to supply functionality for other applications to use (chat service, HTTP fetcher, GPS Location tracking). Additionally, a component can bind to a service to interact with it and even perform interprocess communication (IPC). For example, a service might handle network transactions, play music, perform file I/ O, or interact with a content provider, all from the background. Has a life cycle similar to the one presented for the Activity and has two main method to control its life such as start, stop and restart. Source: http://developer.android.com/guide/topics/fundamentals/services.html 16 Marco Picone 2012Università Degli Studi di Parma Distributed Systems Group Content Providers Content Providers (CPs) manage access to a structured set of Android Application 2 Android Application 3 data (Files or Databases). Activity Activity They encapsulate the data, and provide mechanisms for defining data security. Content providers are the standard interface that connects data in one process with code running in another process. Other applications can access information of a content provider through a specific URI ( content:// ). Android Application 1 Typical provided operations are: Activity 1 Create Insert ‣ Content Provider Read Query ‣ Activity 2 Update ‣ Delete ‣ Android OS provides some builtin CPs for Browser, Calendar, File SQLite Remote Contacts, Call Log, Media and Setting allowing the access to those data from other active applications. Source: PAChapter3 http://developer.android.com/guide/topics/providers/contentproviders.html 17 Marco Picone 2012Università Degli Studi di Parma Distributed Systems Group Intents Intents are asynchronous messages allowing OS components to request functionality from other system elements. For example an Activity can send an Intent to Message List Activity the Android system to starts another Activity. View WebView Message Intents can be used to signal to the Android system that a Open Website Activity Intent Intent certain event has occurred. Other components in Android View can register to this event and will get notified. Message Activity Intents are send to the Android system. Depending on how the Intent was constructed the Android system will run an receiver determination and determine what to do. View Video Intent An Intent can also contain data. This data can be used by the receiving component. For example your application Video View can calls via an Intent a browser component. As data is it Activity may send the URL to the browser component. Android supports explicit and implicit Intents. Source: http://developer.android.com/guide/topics/intents/intentsfilters.html / http://www.vogella.de/articles/AndroidIntent/ 18 Marco Picone 2012 article.htmlUniversità Degli Studi di Parma Distributed Systems Group Broadcast Receiver Represent a variant of communication between different process based on Intent objects. An application or directly the system Many broadcasts originate from the system—for example, a broadcast announcing that the screen has turned off, the battery is low, or a picture was captured. Applications can also initiate broadcasts for example, to let other applications know that some data has been downloaded to the device and is available for them to use. They do not display a user interface, but may create a status bar notification to alert the user when a broadcast event occurs. To simplify a broadcast receiver is just a "gateway" to other components and is intended to do a very minimal amount of work. For instance, it might initiate a service to perform some work based on the event. Source: PAChapter3 / http://developer.android.com/guide/topics/fundamentals.html 19 Marco Picone 2012Università Degli Studi di Parma Distributed Systems Group Android Application Structure Android Manifest.xml res/ layout/ application layout files ‣ drawable/ images, patches, drawable, xml ‣ raw/ data files that can be loaded as streams ‣ values/ xml files with strings, number values used in the code for ‣ example to localize the application in difference languages src/ java/package/directories ‣ gen/ directory generated by Eclipse and Android SDK 20 Marco Picone 2012Università Degli Studi di Parma Distributed Systems Group Android Application Context The Application Context is central entity accessible by all top applications containing global information about an application and the environment. It is an abstract class provided by Android OS allowing to access applicationspecific resources and classes, as well as upcalls for applicationlevel operations such as launching activities, broadcasting and receiving intents. Methods in the Context can be directly called by Activity and Service classes because they directly extend Context class. Some examples of Context features are: retrieving application resources ‣ accessing application preferences ‣ lunch activities ‣ request a system service (for example Location Service) ‣ manage private application files, directories and databases ‣ inspect and enforce application permissions ‣ Source: Wikipedia, AWA, http://developer.android.com/guide/developing/tools/aidl.html 21 Marco Picone 2012Università Degli Studi di Parma Distributed Systems Group Android Application Manifest xml version="1.0" encoding="utf8" manifest xmlns:android="http://schemas.android.com/apk/res/android" package="it.unipr.ce.dgt.android" android:versionCode="1" android:versionName="1.0" Android Application describes their content in an usessdk android:minSdkVersion="7" / XML file called AndroidManifest.xml. usespermission android:name="android.permission.ACCESSCOARSELOCATION" / usespermission android:name="android.permission.ACCESSFINELOCATION" / usespermission android:name="android.permission.ACCESSLOCATION" / usespermission android:name="android.permission.ACCESSNETWORKSTATE" / Using this file the developer can declare the usespermission android:name="android.permission.GPS" / usespermission android:name="android.permission.INTERNET" / presence of Activities, Services, Content Provides, usespermission android:name="android.permission.WRITEEXTERNALSTORAGE" / Broadcast Receiver, Permissions and other application android:icon="drawable/icon" android:label="string/appname" useslibrary android:name="com.google.android.maps" / elements such as code version, app name, SDK activity android:name=".WarningMessageActivity"/activity activity android:name=".IncomingWarningMessageActivity"/activity requirements and . activity android:name=".PeerInfoActivity"/activity activity android:name=".DGTDroidActivity" android:label="string/appname" Manifest files all applications are shared with the intentfilter action android:name="android.intent.action.MAIN" / operating system allowing to load and execute category android:name="android.intent.category.LAUNCHER" / /intentfilter them in a managed environment. /activity /application /manifest 22 Marco Picone 2012Università Degli Studi di Parma Distributed Systems Group R Object / AUTOGENERATED FILE. DO NOT MODIFY. This class was automatically generated by the Eclipse and Android SDK work together to create aapt tool from the resource data it found. It should not be modified by hand. the directory gen. It contains a specific class / called R, which is located inside the Java package it.unipr.ce.dgt.android; application package named in the Android public final class R Manifest. public static final class attr public static final class drawable This class contains fields that uniquely identify all public static final int accidentmarker=0x7f020000; ... resources in the application package structure. public static final class id public static final int accidentButton=0x7f050018; Using the Context object ( Context.getResources ) ... it is possible to obtain an instance of public static final class layout android.content.res.Resources that directly public static final int alertmessagedialog=0x7f030000; ... contains application resources. public static final class string public static final int appname=0x7f040001; R object is also used to retrieve UI layout files and .... related UI elements. 23 Marco Picone 2012Università Degli Studi di Parma Distributed Systems Group Application Logs Android framework provides a structured way to allows developer to log event related to his application using the class called Log. Log class provides five static log method with the same structure: Log.v(TAG,”log”) VERBOSE ‣ Log.d(TAG,”log”) DEBUG ‣ Log.i(TAG,”log”) INFO ‣ Log.w(TAG,”log”) WARNING ‣ Log.e(TAG,”log”) ERROR ‣ The order in terms of verbosity, from least to most is ERROR, WARN, INFO, DEBUG, VERBOSE. Verbose should never be compiled into an application except during development. Debug logs are compiled in but stripped at runtime. Error, warning and info logs are always kept. A good approach is to declare a TAG constant in you class and use it in your log calls. private static final String TAG = "MyActivity"; 24 Marco Picone 2012Università Degli Studi di Parma Distributed Systems Group Model View Controller ModelViewController (MVC) is a software architectural pattern used to isolates the user Redraw OS Events interface (input and presentation) from the "logic" of an application (the application logic for the user). The use of MVC permitting independent development, testing and maintenance of different Controller View component. Android UI Framework is, like other Java framework, organized around a common MVC pattern. Invalidate Update It provides structure and tools for building a Model Controller that handle use input (key pressed and screen taps) and a View that renders graphical information to the screen. Source: PA Chapter 7 Marco Picone 2012 25Università Degli Studi di Parma Distributed Systems Group Model The model manages the behavior and data of the application. It responds to requests for information about its state (usually from the view), and responds to instructions to change state (usually from the controller). In eventdriven systems, the model notifies observers (usually views) when the information changes so that they can react. The model is all about application data. It provides all methods needed to access application data related contained for example in a database, a file or a generic data structure. While particular application’s View and Controller will necessary reflect the Model they manipulate, a single Model might be used by several different application. Simple examples could be the list of contact addresses or the database of music on your device. Applications such as a MP3 Player and MP3 Convert have different functionalities but are both based on the same model related to MP3 file format. Marco Picone 2012Università Degli Studi di Parma Distributed Systems Group View View 3 The View is the visualization of the model. Generally View 2 speaking a view is an application’s component View 1 responsible for rendering the display, sending audio to the speaker, generate tactile feedbacks, and so on. Row View 1 In Android framework, like other existing platforms, the framework paints the screen by walking the view tree, Row View 2 asking each component to draw itself in a preorder Table View traversal. Each View draw itself and then asks each of its children to do the same. A View object is a data structure whose properties store the layout parameters and content for a specific Button View rectangular area of the screen. A View object handles its own measurement, layout, drawing, focus change, scrolling, and key/gesture interactions for the rectangular area of the screen in which it resides. Table Row Row Button View 1 View 1 2 View As an object in the user interface, a View is also a point of interaction for the user and the receiver of the interaction events. draw() draw() draw() draw() draw() Source: http://developer.android.com/guide/topics/ui/howandroiddraws.html Marco Picone 2012Università Degli Studi di Parma Distributed Systems Group Controller Row View 1 Row View 2 Table View The Controller is the application’s portion that responds to external events: screen tap, key pressed, incoming Button Click Event call, etc. It is implemented as an event queue. Each external Button View action is represented as a unique event in the queue. The framework removes each event from the queue and dispatch it calling the right handler method of the Click view related to the event. Event Queue Event For example in an MP3 player application, for instance, when the user taps a Play/Pause button, the related event is dispatched to that button object and its handler can update the Model to resume playing some Controller previously selected song. onClick(View v) Marco Picone 2012Università Degli Studi di Parma Distributed Systems Group Android User Interface The user interface of Android application is built using View and ViewGroup objects. The framework provides several types of views and view groups, each of which is a descendant of the View class. The View class is the base for subclasses called "widgets," which offer fully implemented UI objects, like text fields and buttons. The ViewGroup class serves as the base for subclasses called "layouts," which offer different kinds of layout architecture, like linear, tabular and relative. A View object is a data structure whose properties store the layout parameters and content for a specific rectangular area of the screen. A View object handles its own measurement, layout, drawing, focus change, scrolling, and key/gesture interactions for the rectangular area of the screen in which it resides. As an object in the user interface, a View is also a point of interaction for the user and the receiver of the interaction events. Source: http://developer.android.com/guide/topics/ui/index.html 29 Marco Picone 2012Università Degli Studi di Parma Distributed Systems Group View Hierarchy On the Android platform, you define an Activity's UI using a hierarchy of View Activity’s UI and ViewGroup nodes. This hierarchy tree can be as simple or complex as you setContentView(main.xml) need it to be, and you can build it up using Android's set of predefined widgets and layouts, or with custom Views that you create yourself. View objects are leaves in the tree, ViewGroup objects are branches in the tree (see the View Hierarchy figure above). In order to attach the view hierarchy tree to the screen for rendering, your Activity must call the setContentView() method and pass a reference to the root main.xml node object (main layout xml). As previously described The Android system receives this reference and uses it to invalidate, measure, and draw the tree. The root node of the hierarchy requests that its child nodes draw themselves — in turn, each view group node is responsible for calling upon each of its own child views to draw themselves. Because these are drawn inorder, if there are elements that overlap positions, the last one to be drawn will lie on top of others previously drawn to that space. Source: http://developer.android.com/guide/topics/ui/index.html 30 Marco Picone 2012Università Degli Studi di Parma Distributed Systems Group Layout xml version="1.0" encoding="utf8" LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"               android:layoutwidth="fillparent"               android:layoutheight="fillparent"               android:orientation="vertical"     TextView android:id="+id/text"               android:layoutwidth="wrapcontent"               android:layoutheight="wrapcontent"               android:text="Hello, I am a TextView" /     Button android:id="+id/button"             android:layoutwidth="wrapcontent"             android:layoutheight="wrapcontent"             android:text="Hello, I am a Button" / /LinearLayout The most common way to define your layout and express the view hierarchy is with an XML layout file. XML offers a humanreadable structure for the layout, much like HTML. Each element in XML is either a View or ViewGroup object (or descendant thereof). There are a variety of ways in which you can layout your views. Using more and different kinds of view groups, you can structure child views and view groups in an infinite number of ways. Some predefined view groups offered by Android (called layouts) include LinearLayout, RelativeLayout, TableLayout, GridLayout and others. Each offers a unique set of layout parameters that are used to define the positions of child views and layout structure. 31 Marco Picone 2012Università Degli Studi di Parma Distributed Systems Group Layout Attributes Every View and ViewGroup object supports their own variety of XML attributes. Some attributes are specific to a View object (for example, TextView supports the textSize attribute), but these attributes are also inherited by any View objects that may extend this class. Some are common to all View objects, because they are inherited from the root View class (like the id attribute). Other attributes are considered "layout parameters," which are attributes that describe certain layout orientations of the View object, as defined by that object's parent ViewGroup object. Layout attributes can be changed directly in the XML file or using the dedicated framework API when the View object reference has been retrieved in the code. 32 Marco Picone 2012Università Degli Studi di Parma Distributed Systems Group Layout ID Attribute Any View object may have an integer ID associated with it, to uniquely identify the View within the tree. When the application is compiled, this ID is referenced as an integer, but the ID is typically assigned in the layout XML file as a string, in the id attribute. The atsymbol () at the beginning of the string indicates that the XML parser should parse and expand the rest of the ID string and identify it as an ID resource. The plussymbol (+) means that this is a new resource name that must be created and added to our resources (in the R.java file). When the resource is added to the R object it can be used to retrieve the View element in an Activity using the method context.findViewById(R.resourceid); Button android:id="+id/mybutton"         android:layoutwidth="wrapcontent" Button myButton = (Button) findViewById(R.id.mybutton);         android:layoutheight="wrapcontent"         android:text="string/mybuttontext"/ 33 Marco Picone 2012Università Degli Studi di Parma Distributed Systems Group Widgets A widget is a View object that serves as an interface for interaction with the user. Android provides a set of fully implemented widgets, like buttons, checkboxes, and textentry fields. Some widgets provided by the platform are more complex, like a date picker, a clock, and zoom controls. There is also the opportunity to do something more customized creating a user defined actionable elements. The developer can define he/she own View object or can extend and combine existing widgets. An example of custom components is user defined Custom Buttons, Progress Bar, Chart View, Map Marker etc .... Source: Custom Components (http://developer.android.com/guide/topics/ui/customcomponents.html) / ChartDroid(http:// 34 Marco Picone 2012 code.google.com/p/chartdroid/wiki/Screenshots), / achartEngine (http://code.google.com/p/achartengine/)Università Degli Studi di Parma Distributed Systems Group View Write incoming data into Is associated to a View app storage space Row View 1 according to model Row View 2 structure Retrieve Object instances related to Table View UI Elements Model Activity Update UI elements Read Model structure and retrieve stored data Button View Create the link between UI and Controller Controller onClick(View v) 35 Marco Picone 2012Università Degli Studi di Parma Distributed Systems Group Activity xml version="1.0" encoding="utf8" LinearLayout xmlns:android="http://schemas.android.com/ An Activity is both a unit of user interaction (typically apk/res/android" android:layoutwidth="fillparent" associated to a View) and a unit of execution. android:layoutheight="fillparent" android:orientation="vertical" Inside the override method onCreate(...) the developer TextView android:id="+id/mainTextView" can set the layout View associated to the Activity android:layoutwidth="fillparent" calling setContentView(R.layout.layoutfile) using android:layoutheight="wrapcontent" android:text="string/hello" / the shared R object to specify the resource file /LinearLayout containing the layout. public class HelloWorldActivity extends Activity When the root view has been set it is possible to / Called when the activity is first created. / retrieve objects associated to the UI elements through Override public void onCreate(Bundle savedInstanceState) the method findViewById(R.id.resourceid). super.onCreate(savedInstanceState); setContentView(R.layout.main); A View object like TextView can be used to updated the user interface, to get values from input fields or to set Event listeners using provided get and set methods. TextView mainTextView = (TextView)findViewById(R.id.mainTextView); mainTextView.setText("Hello World :D"); 36 Marco Picone 2012Università Degli Studi di Parma Distributed Systems Group Input Events Views/widgets and generally UI are used not only to show contents and information but have the important role to allow the user to interact with the application performing actions. To be informed of user input events, the developer needs to do one of two things: Define an event listener and register it with the View. The View class contains a collection of nested interfaces named ‣ OnsomethingListener, each with a callback method called Onsomething(). For example, View.OnClickListener (for handling "clicks" on a View), View.OnTouchListener (for handling touch screen events in a View), and View.OnKeyListener (for handling device key presses within a View). In order to be notified when the view is "clicked" (such as when a button is selected), implement OnClickListener and define its onClick() callback method (where you perform the action upon click), and register it to the View with setOnClickListener(). Override an existing callback method for the View. This is what you should do when you've implemented your own View ‣ class and want to listen for specific events that occur within it. // Create an anonymous implementation of OnClickListener private OnClickListener mCorkyListener = new OnClickListener()     public void onClick(View v)       // do something when the button is clicked     ; protected void onCreate(Bundle savedValues)     // Capture our button from layout     Button button = (Button)findViewById(R.id.corky);     // Register the onClick listener with the implementation above     button.setOnClickListener(mCorkyListener); 37 Marco Picone 2012Università Degli Studi di Parma Distributed Systems Group Button Events public class MyActivity extends Activity      protected void onCreate(Bundle icicle)          super.onCreate(icicle);          setContentView(R.layout.contentlayoutid);          final Button button = (Button) findViewById(R.id.buttonid);          button.setOnClickListener(new View.OnClickListener()              public void onClick(View v)                  // Perform action on click                        );         38 Marco Picone 2012Università Degli Studi di Parma Distributed Systems Group Launch / Finish Activity The startActivity(Intent) method is used to start a new activity, which will be placed at the top of the activity stack. It takes a single argument, an Intent, which describes the activity to be executed  startActivity(new Intent(getApplicationContext(), MyActivity.class)); Sometimes you want to get a result back from an activity when it ends. For example, you may start an activity that lets the user pick a person in a list of contacts; when it ends, it returns the person that was selected. To do this, you call the startActivityForResult(Intent, int) version with a second integer parameter identifying the call. The result will come back through your onActivityResult(int, int, Intent) method. When an activity exits, it can call setResult(int) to return data back to its parent. It must always supply a result code, which can be the standard results RESULTCANCELED, RESULTOK, or any custom values starting at RESULTFIRSTUSER. In addition, it can optionally return back an Intent containing any additional data it wants. All of this information appears back on the parent's Activity.onActivityResult(), along with the integer identifier it originally supplied. When the activity is done the developer can call the method finish() to close it. The ActivityResult is propagated back to whoever launched you via onActivityResult(). 39 Marco Picone 2012Università Degli Studi di Parma Distributed Systems Group Coming Up Next Lecture Android Graphical User Interface 1 ‣ Homework Review Hello Android Application ‣ .... ‣ 40 Marco Picone 2012Università Degli Studi di Parma Distributed Systems Group Android Development Lecture 2 Android Platform Marco Picone 2012
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