Objects First with Java Class Business © David J. Barnes and Michael Kölling

User Interface The UI for an Android app is built using a hierarchy of View  and ViewGroup objects.  View objects are usually UI widgets such as buttons or textfields  ViewGroup objects are invisible view containers that define how the child views are laid out, such as in a grid or a vertical list.

Assignment?

Activity An Activity is the main object in an Android app. Lifecycle:

Activity Created Redefine onCreate() Good time to Define the interface with setContentView() Initialize any parts of the interface, including callback assignments Start any necessary threads/services (more on this later)

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Activity Started Created activities are also started Redefine onStart() Good time to Read preferences Probe resources

Resume an Activity This means the activity has been pulled to the foreground and the user can interact with it. New activities are also resumed. Define onResume() to do something if necessary Good time to Check any indicators (e.g. email) and refresh user interface items Obtain system resources (e.g. camera)

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Activity Paused Redefine onPause() Activity is obscured Good time to Reset user interface indicators Pause any realtime actions Release system resources (e.g. camera)

Activity Stopped Redefine onStop() Activity is completely hidden and considered to be in the background onPause() is called first Good time to act as if process terminated Write user data Write preferences Act like the app is shut down

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Activity Restarted Redefine onRestart() Activity is resurrected Good time to act as if process were created Refresh user data and preferences Refresh the interface

Activity Destroyed Redefine onDestroy() Activity is completely removed from the system Good time to Reset system components

Example: Rotation Rotation causes a number of events to take place: App is stopped: pause, then stop (save state) App is restarted: restart, then resumed (reinstate state)

Dialogs To display dialog boxes, do two things: Define onCreateDialog() and pay attention to the parameter. Create a dialog box and return it. Call showDialog() with an integer to indicate which dialog you want.

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Menus Override the onCreateOptions() call Example: public boolean onCreateOptionsMenu(Menu menu) { MenuInflater inflater = getMenuInflater(); inflater.inflate(R.menu.opening_menu, menu); return true; }

Menus Define your menu in XML spec file <menu> <item android:id="@+id/opening_menu_new_item" android:icon="@drawable/ic_menu_sun" android:title="New" /> <item android:id="@+id/opening_menu_import_item" android:icon="@drawable/ic_menu_filter" android:title="Import"> </item>

Response to Menu Events Redefine onOptionsItemSelected() public boolean onOptionsItemSelected(MenuItem item){ switch (item.getItemId()) { case R.id.item1: return true; case R.id.item2: startActivity(new Intent(getApplicationContext(), ActivityTwo.class)); default: return super.onContextItemSelected(item); 12: } 13: }

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Other Programmable Events Examples onPrepareDialog() before onCreateDialog() onCreateContextMenu() for popup menus onContextItemSelected() for popup menu items selected onCreateOptionsMenu() to create menus onOptionsItemSelected() from other menus Etc…etc…etc…

Machine Problem #4

Starting New Activities A common action for an activity is to start another activity. When another activity is started, the current activity is stopped and the new activity is created/started/resumed.

Starting New Activities: Intents Intents are communications about classes or executables or data Example: communicate which activity to start by creating an intent that indicates the class to use

Starting New Activities Two ways to start an activity To start an independent activity that will run, use startActivity() To start an activity that will return information to the current activity, use startActivityForResult() Both calls use intents to indicate which activity should start up

startActivity() Uri smsUri = Uri.parse("tel:"+grades.getStudent(itemPosition). getMobilePhoneNumber()); Intent sendIntent = new Intent(Intent.ACTION_VIEW, smsUri); sendIntent.putExtra("address", grades.getStudent(itemPosition).getMobilePhoneNumber()); sendIntent.putExtra("sms_body", "type message here"); sendIntent.setType("vnd.android-dir/mms-sms"); startActivity(sendIntent);

startActivityForResult() Using this call, one gives the intent AND a returning value that identifies the activity. When the Activity is completed, the method onActivityResult() is called.

startActivityForResult() Intent gameIntent = new Intent(this, GameDisplay.class); startActivityForResult( gameIntent, R.id.class_display_games);

startActivityForResult() protected void onActivityResult( int requestCode, int resultCode, Intent intent) { super.onActivityResult( requestCode, resultCode, intent); if (resultCode != RESULT_OK) return; if (intent != null) { if (requestCode == R.id.class_display_games) { … }

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Returning From an Activity To halt an Activity, call finish() To return to the Activity that created the current Activity, call setResult(): setResult(RESULT_OK, result); This means automatic return with the ID that it was created with.

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Information in Intents You can insert information into an intent to communicate with the activity that is being started. The typical way is to Create a Bundle Put things into the Bundle Add the Bundle to the Intent through putExtras()

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Retrieving Information Use the getIntent() call to retrieve the Intent Use the get…() call to get the extra you are looking for

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Better Ways to Organize the Code There are several times when the code is organized around an integer return value. onCreateDialog() onOptionsItemSelected() onActivityResult() This results in a string of “if…then…else” code that just stinks.

Better Ways to Organize the Code Use a table of classes. Do a table lookup of the code and Java reflection to call the right class.

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Files Files are done "normally" from the Java perspective The location of files is not normal Because of security, "file space" is buried deep in the Android file system.

Open local file FileInputStream classFile = openFileInput("ClassLocations"); … classFile.close(); FileOutputStream file = openFileOutput("ClassLocations"); file.close();

Open Local File Do not open an absolute path name unless you know where your app is stored. Can get the path as String f = getFilesDir().getAbsolutePath();

Open online file DefaultHttpClient httpclient = new DefaultHttpClient(); HttpGet httppost = new HttpGet("http://www.urlOfThePageYouWantToRead.nl/text.txt"); HttpResponse response = httpclient.execute(httppost); HttpEntity ht = response.getEntity(); BufferedHttpEntity buf = new BufferedHttpEntity(ht); InputStream is = buf.getContent(); BufferedReader r = new BufferedReader(new InputStreamReader(is));