The Big Nerd Ranch Guide to the Android Activity Lifecycle
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This chapter is from the book
This chapter is from the book
This chapter is from the book
The Activity Lifecycle
Every instance of Activity has a lifecycle.During this lifecycle, an activity transitions between three possible states: running, paused, and stopped.For each transition, there is an Activity method that notifies the activity of the change in its state.Figure 3.1 shows the activity lifecycle, states, and methods.
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Figure 3.1 Activity state diagram
Subclasses of Activity can take advantage of the methods named in Figure 3.1 to get work done at critical transitions in the activity's lifecycle.
You are already acquainted with one of these methods -onCreate(Bundle).The OS calls this method after the activity instance is created but before it is put on screen.
Typically, an activity overrides onCreate(...) to prepare the specifics of its user interface:
- inflating widgets and putting them on screen (in the call to (setContentView(int))
- getting references to inflated widgets
- setting listeners on widgets to handle user interaction
- connecting to external model data
It is important to understand that you never call onCreate(...) or any of the other Activity lifecycle methods yourself.You override them in your activity subclasses, and Android calls them at the appropriate time.
Logging the Activity Lifecycle
In this section, you are going to override lifecycle methods to eavesdrop on QuizActivity's lifecycle.Each implementation will simply log a message informing you that the method has been called.
Making log messages
In Android, the android.util.Log class sends log messages to a shared system-level log.Log has several methods for logging messages . Here is the one that you will use most often in this book:
public static int d(String tag, String msg)
The d stands for “debug” and refers to the level of the log message.(There is more about the Log levels in the final section of this chapter.)The first parameter identifies the source of the message, and the second is the contents of the message.
The first string is typically a TAG constant with the class name as its value.This makes it easy to determine the source of a particular message.
In QuizActivity.java, add a TAG constant to QuizActivity:
Listing 3.1 Adding TAG constant (QuizActivity.java)
public class QuizActivity extends Activity {
private static final String TAG = "QuizActivity";
...
}
Next, in onCreate(...), call Log.d(...) to log a message.
Listing 3.2 Adding log statement to onCreate(...) (QuizActivity.java)
public class QuizActivity extends Activity {
...
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
Log.d(TAG, "onCreate(Bundle) called");
setContentView(R.layout.activity_quiz);
...
This code may cause an error regarding the Log class.If so, press Command+Shift+O (Ctrl+Shift+O) to organize your imports.Eclipse will then ask you to choose which class to import. Choose android.util.Log.
Now override five more methods in QuizActivity:
Listing 3.3 Overriding more lifecycle methods (QuizActivity.java)
} // End of onCreate(Bundle)
@Override
public void onStart() {
super.onStart();
Log.d(TAG, "onStart() called");
}
@Override
public void onPause() {
super.onPause();
Log.d(TAG, "onPause() called");
}
@Override
public void onResume() {
super.onResume();
Log.d(TAG, "onResume() called");
}
@Override
public void onStop() {
super.onStop();
Log.d(TAG, "onStop() called");
}
@Override
public void onDestroy() {
super.onDestroy();
Log.d(TAG, "onDestroy() called");
}
}
Notice that you call the superclass implementations before you log your messages. These superclass calls are required.Calling the superclass implementation before you do anything else is critical in onCreate(...);the order is less important in the other methods.
You may have been wondering about the @Override annotation. This asks the compiler to ensure that the class actually has the method that you are attempting to override.For example, the compiler would be able to alert you to the following misspelled method name:
public class QuizActivity extends Activity {
@Override
public void onCreat(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_quiz);
}
...
The Activity class does not have an onCreat(Bundle) method, so the compiler will complain.Then you can fix the typo rather than accidentally implementing QuizActivity.onCreat(Bundle).
Using LogCat
To access to the log while the application is running, you can use LogCat, a log viewer included in the Android SDK tools.
To get to LogCat, select Window → Show View → Other... In the Android folder, find and select LogCat and click OK (Figure 3.2).
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Figure 3.2 Finding LogCat
LogCat will open in the right half of your screen and annoyingly shrink your editor.It would be better if LogCat were at the bottom of the workbench window.
To get it there, drag from the tab of the LogCat view to the toolbar at the bottom-right corner of the workbench.
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Figure 3.3 Drag from LogCat tab to bottom-right toolbar
The LogCat view will close, and its icon (a horizontal Andy with Nyan rainbow feet) will appear in the toolbar.Click the icon, and LogCat will reopen at the bottom of the window.
Your Eclipse workbench should now look something like Figure 3.4.You can resize the panes in LogCat by dragging their boundaries.This is true for any of the panes in the Eclipse workbench.
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Figure 3.4 Eclipse workbench - now with LogCat
Run GeoQuiz, and messages will start appearing fast and furiously in LogCat.Most of the messages will be system output.Scroll to the bottom of the log to find your messages.In LogCat's Tag column, you will see the TAG constant you created for QuizActivity.
(If you do not see any messages in LogCat, LogCat may be monitoring the wrong device.Select Window → Show View → Other... and open the Devices view.Select the device you are currently running on and then return to LogCat.)
To make your messages easier to find, you can filter the output using the TAG constant.In LogCat, click the green + button at the top of the lefthand pane to create a message filter. Name the filter QuizActivity and enter QuizActivity in the by Log Tag: field (Figure 3.5).
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Figure 3.5 Creating a filter in LogCat
Click OK, and a new tab will open showing only messages tagged QuizActivity (Figure 3.6).Three lifecycle methods were called after GeoQuiz was launched and the initial instance of QuizActivity was created.
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Figure 3.6 Launching GeoQuiz creates, starts, and resumes an activity
(If you are not seeing the filtered list, select the QuizActivity filter from LogCat's lefthand pane.)
Now let's have some fun. Press the Back button on the device and then check LogCat.Your activity received calls to onPause(), onStop(), and onDestroy().
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Figure 3.7 Pressing the Back button destroys the activity
When you pressed the Back button, you told Android, “I'm done with this activity, and I won't need it anymore.”Android then destroyed your activity.This is Android's way of being frugal with your device's limited resources.
Relaunch GeoQuiz. Press the Home button and then check LogCat.Your activity received calls to onPause() and onStop(), but not onDestroy().
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Figure 3.8 Pressing the Home button stops the activity
On the device, pull up the task manager. On newer devices, press the Recents button next to the Home button (Figure 3.9).On devices without a Recents button, long-press the Home button.
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Figure 3.9 Home, Back, and Recents buttons
In the task manager, press GeoQuiz and then check LogCat. The activity was started and resumed, but it did not need to be created.
Pressing the Home button tells Android, “I'm going to go look at something else, but I might come back.”Android pauses and stops your activity but tries not to destroy it in case you come back.
However, a stopped activity's survival is not guaranteed. When the system needs to reclaim memory, it will destroy stopped activities.
Finally, imagine a small pop-up window that only partially covers the activity.When one of these appears, the activity behind it is paused and cannot be interacted with.The activity will be resumed when the pop-up window is dismissed.
As you continue through the book, you will override the different activity lifecycle methods to do real things for your application.When you do, you will learn more about the uses of each method.