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Developing Mobile Applications
Week 2
Activity Lifecycle
Steve Jones
Department of Computer Science
University of Waikato
[email protected]
Activity Lifecycle
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Today
Android application components
Understanding the activity lifecycle
Coping with orientation changes
Adding an Action Bar to PropertyFinder
Activity Lifecycle
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Application/project components
Creating an Android Application project produced
all Java source code goes here
you organise it into sensible packages
the Java file was created for us and contains a trivial Activity class
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Application/project components
Creating an Android Application project produced
the Java library for the version of Android that we are compiling against
automatically added to project and Java build path
Activity Lifecycle
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Application/project components
Creating an Android Application project produced
defines characteristics of the application
Activity Lifecycle
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Application/project components
Creating an Android Application project produced
Java classes automatically generated by the build process
treat as READ ONLY
files are automatically added when an executable is built
the installer (.apk) file will appear here
treat as READ ONLY
proguard deals with things relevant to a public release (shrinking,
obfuscating, optimising)
project properties are things like target API level
treat both as READ ONLY
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Application/project components
Creating an Android Application project produced
things you need to bundle with the app but aren’t standard Android
resources (eg you might want to bundle a pre-constructed database)
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Application/project components
Creating an Android Application project produced
additional Java libraries required by the application
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Application/project components
Creating an Android Application project produced
definitions for layouts, constant values (eg UI strings), icons, images,
menu content etc
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Application/project components
Creating an Android Application project produced
application icons for different screen
densities
definition of the UI layout
definition of application’s menu content
definition of string constants used in the
app and UI styles/themes used by the app
some styles/themes are Android version
dependent : v11/v14 used when device
supports them
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Application/project components
Building an executable creates (in the ‘bin’ folder)
standard class files produced by the Java compiler
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Application/project components
Building an executable creates (in the ‘bin’ folder)
‘dexed’ versions of the Java class files
Android uses its own virtual machine called Dalvik
Java byte code is converted to Dalvik byte code (a dalvik executable)
Android Java class libraries are actually a subset of Apache Harmony
(open source Java runtime)
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Application/project components
Building an executable creates (in the ‘bin’ folder)
‘dexed’ versions of additional Java libraries (none here)
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Application/project components
Building an executable creates (in the ‘bin’ folder)
the application package file that is loaded on to a device for installation
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Application/project components
Building an executable creates (in the ‘bin’ folder)
a zip archive of the application’s resources
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Application/project components
The .apk file is just a zip archive containing the files
needed for installation on device
•  APK installer on device unzips and places files into
appropriate locations
the real executable
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Application/project components
During development we are mainly concerned with
•  making sure AndroidManifest.xml correctly describes
the application
•  adding Java code to the ‘src’ folder
•  adding resources (icons, images, layout defns, menus,
constants etc) to the ‘res’ folder
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AndroidManifest.xml
An app needs a special file AndroidManifest.xml that describes its
requirements, capabilities and components
<uses-xxx>
uses-feature entries describe the features that a device should have for
the app to work (eg a camcorder app needs a device to have a
camera)
uses-permission entries describe the privileges that the app wants to
be granted (eg network access, read/write SD card, initiate phone
calls, use location sensors). Users need to agree during installation
uses-sdk entry states the minimum Android version that is required for
the app to work (eg it uses a widget that was introduced in Android 4
so won't work on 1.x-3.x devices)
The Play Store can filter available apps by comparing these
characteristics to the user's device
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AndroidManifest.xml
An app needs a special file AndroidManifest.xml that
describes its requirements, capabilities and
components
<application>
•  configures attributes of the application eg
•  icon, label, UI theme
•  defines the components that the application consists of
eg
•  activities, services, data providers
•  defines entry points into the application (one or more
activities)
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PropertyFinder1 AndroidManifest.xml
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
the min/max version that you know
package="org.stevej.android.propertyfinder1"
you app works correctly on
android:versionCode="1"
android:versionName="1.0" >
whether the application’s data can be
<uses-sdk
backed up using the Android backup
android:minSdkVersion="8"
infrastructure
android:targetSdkVersion="16" />
refer to definitions in resource files
<application
android:allowBackup="true"
android:icon="@drawable/ic_launcher"
the application consists of this one
android:label="@string/app_name"
activity
android:theme="@style/AppTheme" >
<activity
android:name="org.stevej.android.propertyfinder1.PropertyFinderActivity"
android:label="@string/app_name" >
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
this is the main activity of the
</activity>
application and can be accessed by
</application>
launching the app from the home
screen/app list
</manifest> Activity Lifecycle
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Activities
An activity is the equivalent of a screen/window in a conventional GUI, and an
application includes one or more of them
An SMS app could have activities for viewing a message list, showing a
conversation, creating a message etc
Important differences
• 
only one activity visible at a time
• 
you need to be aware of the stack of activities
• 
the system may kill activities that aren't visible
• 
you can allow other applications to launch your application's activities
• 
your application can launch activities belonging to other applications
So the SMS app could launch the 'Add contact' activity of the Contacts app, and
the Contacts app could launch the 'Create message' activity of the SMS app
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Intents
An intent is a message that is published to activate application
components (activities), and contains a bundle of information needed for
the activation
So an SMS app could publish an intent containing
•  the identifier of the 'Create contact' activity of the Contacts app
•  the phone number of the contact
and the system handles the activation of the target activity
It can be cleverer than that… publish an intent containing
•  a generic identifier for a 'Create contact' action (no specific app/activity)
•  the phone number of the contact
and the system handles the activation of an activity that can carry out the
action (providing the user with a choice if there's more than one – there
may be multiple contact manager apps on the device)
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Activity lifecycle
Only 1 activity from 1
application is visible to the user
Your application’s current
activity can be interrupted,
killed, recreated, restarted,
resumed by the system
Activities need to cope with
these changes in state
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PropertyFinderActivity.java
A basic activity
all of your activities will extend
android.app.Activity
package org.stevej.android.propertyfinder1;
import android.app.Activity;
import android.os.Bundle;
import android.view.Menu;
called automatically when the activity
is created (in this case, the application
public class PropertyFinderActivity extends Activity {
is launched)
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
set the activity’s UI content to be
setContentView(R.layout.activity_property_finder);
that defined in res/layout/
}
activity_property_finder.xml
@Override
public boolean onCreateOptionsMenu(Menu menu) {
getMenuInflater().inflate(R.menu.activity_property_finder, menu);
called automatically if a menu has been defined
return true;
menu layout is created from
}
/res/menu/activity_property_finder.xml defn
and added to the application menu (handle to it
}
provided by menu)
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Activity Lifecycle
Activity lifecycle
You can override all lifecycle methods in your Java
code (eg add this to PropertyFinderActivity.java)
@Override
protected void onPause() {
super.onPause();
Log.d("PropertyFinderActivity", "onPause");
} to add behaviour for that point in the lifecycle
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Restoring state
Our activity current has a text label
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".PropertyFinderActivity" >
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_centerHorizontal="true"
android:layout_centerVertical="true"
android:text="@string/hello_world" />
</RelativeLayout> What if we add an editable text box?
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Restoring state
Our activity currently has a text label. What if we add
an editable text box?
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".PropertyFinderActivity" >
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_centerHorizontal="true"
android:layout_centerVertical="true"
android:text="@string/hello_world" />
<EditText
android:layout_width=”match_parent"
android:layout_height="wrap_content” />
</RelativeLayout> Type in some text, then rotate the device…
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Restoring state
Type in some text, then rotate the device…
Ooops! We’ve lost the text! Activity Lifecycle
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Restoring state
Why has the text disappeared?
Add logging to onCreate, override onDestroy and add
logging
Activity Lifecycle
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
Log.d("PropertyFinderActivity", "onCreate");
setContentView(R.layout.activity_property_finder);
}
@Override
protected void onDestroy() {
super.onDestroy();
Log.d("PropertyFinderActivity", "onDestroy");
} 29
Restoring state
Why has the text disappeared?
Orientation changes (amongst many other things)
cause the current activity to be destroyed and
recreated!
We need to store the state of the text box before the
activity is destroyed and restore it when the activity
is recreated
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Restoring state
Actually most widgets (such as EditText) do this
automatically
The problem is that the system can only save the
state of a widget if it has a unique id
Add one
<EditText
android:id="@+id/text_field"
android:layout_width="match_parent"
android:layout_height="wrap_content" /> It is good practice to always give UI elements unique
ids
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Restoring state
Add this
Activity Lifecycle
<EditText
android:id="@+id/text_field"
android:layout_width="match_parent"
android:layout_height="wrap_content" /> 32
Restoring state
BUT
• 
• 
• 
• 
• 
what if the user enters text
then the phone rings
then the user starts up several more apps
the system kills your activity to free memory
the user returns to your app
Text is lost
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Restoring state
savedInstanceState… recall that onCreate has an
argument
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
Log.d("PropertyFinderActivity", "onCreate");
setContentView(R.layout.activity_property_finder);
}
This provides a mechanism by which we can
explicitly store/restore activity state
We override the activity’s onSaveInstanceState
method
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Restoring state
We override the activity’s onSaveInstanceState
method
finds the UI component with the given ID
a key-value map of the data to be saved
in the activity’s view hiearchy
@Override
public void onSaveInstanceState(Bundle outState) {
EditText text_field = (EditText) findViewById(R.id.text_field);
outState.putString("user_text", text_field.getText().toString());
super.onSaveInstanceState(outState);
}
add our own data to the map
This is automatically called by the system before the
activity is destroyed
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Restoring state
And then check if we have saved state in onCreate
If so, use it to initialise the text field
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
Log.d("PropertyFinderActivity", "onCreate");
setContentView(R.layout.activity_property_finder);
if (savedInstanceState != null) {
String user_text = savedInstanceState.getString("user_text");
EditText text_field = (EditText) findViewById(R.id.text_field);
text_field.setText(user_text);
}
}
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Restoring state
You should ensure that the app saves and restores
state that is critical to the user seamlessly resuming
use of the app
This will probably include data that is not
automatically saved by a UI widget
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The Action Bar
The application’s menu bar
It can support
• 
• 
• 
• 
• 
navigation within the app
displaying of navigation context cues
drop down navigation list
buttons to access common application functionality
inclusion of widgets (such as search box)
(Prior to Android 3.0 there was no consistent place to
do this, and user needed to explicitly tap the device’s
menu button to get the application menu)
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The Action Bar
4 commonly used segments
the app icon
view selector
action buttons
overflow menu
if we have multiple
screens we can
show a < and allow
a click to move the
user back up the
screen hierarchy
drop down menu to
support navigation to
different views within
the application
the key actions that the
user can carry out in
the app
other, less frequently
used actions (eg
Settings, About)
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The Action Bar
When we created the PropertyFinder project we got
•  /res/menu/activity_property_finder.xml
<menu xmlns:android="http://schemas.android.com/apk/res/android" >
<item
android:id="@+id/menu_settings"
android:orderInCategory="100"
android:showAsAction="never"
android:title="@string/menu_settings"/>
</menu>
•  if we had several items we could group them by assigning an
android:menuCategory value
•  and order them within the category
•  we can make items always appear in the overflow menu ie
never as an action button
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The Action Bar
When we created the PropertyFinder project we got
•  /res/menu/activity_property_finder.xml
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The Action Bar
Add some useful items
<menu xmlns:android="http://schemas.android.com/apk/res/android" >
<item
android:id="@+id/action_search"
android:actionViewClass="android.widget.SearchView"
android:icon="@android:drawable/ic_menu_search"
android:showAsAction="collapseActionView|ifRoom"
android:title="@string/action_bar_search"/>
<item
android:id="@+id/action_map"
android:icon="@android:drawable/ic_menu_mapmode"
android:showAsAction="ifRoom"
android:title="@string/action_bar_map"/>
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The Action Bar
Item attributes
<menu xmlns:android="http://schemas.android.com/apk/res/android" >
<item
android:id="@+id/action_search"
android:actionViewClass="android.widget.SearchView"
android:icon="@android:drawable/ic_menu_search"
android:showAsAction="collapseActionView|ifRoom"
android:title="@string/action_bar_search"/>
<item
this action displays a search widget
the system handles its construction
and display
the icon is built into android
search widget collapsed by default,
display the search icon if there is room
in the action bar (if not it will go in the
overflow menu0
android:id="@+id/action_map"
android:icon="@android:drawable/ic_menu_mapmode"
android:showAsAction="ifRoom"
android:title="@string/action_bar_map"/>
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The Action Bar
Add some useful items
•  the system automatically creates the share menu
entries when action for menu item defined as provided
by built-in ShareActionProvider
<item
android:id="@+id/action_share"
android:actionProviderClass="android.widget.ShareActionProvider"
android:icon="@android:drawable/ic_menu_share"
android:showAsAction="ifRoom"
android:title="@string/action_bar_share"/>
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The Action Bar
Add some useful items
•  we can have sub menus
<item
android:id="@+id/action_sort"
android:icon="@android:drawable/ic_menu_sort_by_size"
android:showAsAction="ifRoom"
android:title="@string/action_bar_sort">
<menu>
<item
android:id="@+id/action_sort_price_asc"
android:icon="@drawable/ic_action_price_asc"
android:title="@string/action_bar_sort_price_asc"/>
<item
there aren’t built in icons for these
android:id="@+id/action_sort_price_desc"
create them ourselves and add to /res/
android:icon="@drawable/ic_action_price_desc"
drawable folders
android:title="@string/action_bar_sort_price_desc"/>
<item
android:id="@+id/action_sort_alpha"
android:icon="@android:drawable/ic_menu_sort_alphabetically"
android:title="@string/action_bar_sort_alpha"/>
</menu>
</item>
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The Action Bar
Add some useful items
<item
android:id="@+id/settings"
android:icon="@android:drawable/ic_menu_preferences"
android:showAsAction="never"
android:title="@string/action_bar_settings">
</item>
<item
android:id="@+id/action_legal"
android:orderInCategory="100"
android:showAsAction="never"
android:title="@string/action_bar_legal"/>
</menu>
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The Action Bar
Some screenshots are with device in portrait
orientation, others landscape
•  in portrait the action bar has been automatically split
between the top and bottom of the screen
•  achieved by setting an application attribute in the
manifest
<application
android:allowBackup="true"
android:icon="@drawable/ic_launcher"
android:label="@string/app_name”
android:uiOptions="splitActionBarWhenNarrow" >
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The Action Bar
What about the app icon and view navigation menu?
•  app icon automatically added – we don’t need to declare
it in the menu XML
•  same for the view menu, but in this case we need to
configure and populate it from Java code – items added
at run time (later lecture)
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The Action Bar
Can configure aspects in Java code
private void setUpActionBar() {
final ActionBar actionBar = getActionBar();
actionBar.setDisplayShowTitleEnabled(false);
actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_LIST);
}
•  don’t show the app title (just the icon)
•  make the view navigation a list (menu)
•  the alternative is tabs, which isn’t really appropriate here
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The Action Bar
Ooops
•  remember the Action Bar was introduced in Android 3.0
(API level 11)
•  change AndroidManifest.xml
•  android:minSdkVersion="11"
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Homework
Read
•  http://developer.android.com/tools/help/logcat.html
•  http://developer.android.com/guide/components/
activities.html
•  http://developer.android.com/guide/topics/ui/
actionbar.html
Override all activity lifecycle methods in the
PropertyFinderActivity, output a log message for
each, investigate when each method is called
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Next
List views and adapters
Fragments
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