Android学习路线（十二）Activity生命周期——启动一个Activity

先占个位置，过会儿来翻译，:p

Unlike other programming paradigms in which apps are launched with a main()method, the Android system initiates code in an Activity instance
by invoking specific callback methods that correspond to specific stages of its lifecycle. There is a sequence of callback methods that start up an activity and a sequence of callback methods that tear down an activity.

This lesson provides an overview of the most important lifecycle methods and shows you how to handle the first lifecycle callback that creates a new instance of your activity.

Understand the Lifecycle Callbacks

As the user begins to leave the activity, the system calls other methods that move the activity state back down the pyramid in order to dismantle the activity. In some cases, the activity will move only part way down the pyramid and wait (such as when the user
switches to another app), from which point the activity can move back to the top (if the user returns to the activity) and resume where the user left off.

Figure 1. A simplified illustration of the Activity lifecycle, expressed as a step pyramid. This shows how, for every callback used to take the activity a step toward the Resumed state at the top, there‘s a callback
method that takes the activity a step down. The activity can also return to the resumed state from the Paused and Stopped state.

Depending on the complexity of your activity, you probably don‘t need to implement all the lifecycle methods. However, it‘s important that you understand each one and implement those that ensure your app behaves the way users expect. Implementing your activity
lifecycle methods properly ensures your app behaves well in several ways, including that it:

• Does not crash if the user receives a phone call or switches to another app while using your app.
• Does not consume valuable system resources when the user is not actively using it.
• Does not lose the user‘s progress if they leave your app and return to it at a later time.
• Does not crash or lose the user‘s progress when the screen rotates between landscape and portrait orientation.

As you‘ll learn in the following lessons, there are several situations in which an activity transitions between different states that are illustrated in figure 1. However, only three of these states can be static. That is, the activity can exist in one of only
three states for an extended period of time:

Resumed

In this state, the activity is in the foreground and the user can interact with it. (Also sometimes referred to as the "running" state.)

Paused

In this state, the activity is partially obscured by another activity—the other activity that‘s in the foreground is semi-transparent or doesn‘t cover the entire screen. The paused activity does not receive user input and
cannot execute any code.

Stopped

In this state, the activity is completely hidden and not visible to the user; it is considered to be in the background. While stopped, the activity instance and all its state information such as member variables is retained,
but it cannot execute any code.

The other states (Created and Started) are transient and the system quickly moves from them to the next state by calling the next lifecycle callback method. That is, after the system calls onCreate(),
it quickly callsonStart(),
which is quickly followed by onResume().

That‘s it for the basic activity lifecycle. Now you‘ll start learning about some of the specific lifecycle behaviors.

Specify Your App‘s Launcher Activity

You can define which activity to use as the main activity in the Android manifest file, AndroidManifest.xml,
which is at the root of your project directory.

The main activity for your app must be declared in the manifest with an <intent-filter> that
includes the MAINaction
and LAUNCHER category.
For example:

<activity android:name=".MainActivity" android:label="@string/app_name">
    <intent-filter>
        <action android:name="android.intent.action.MAIN" />
        <category android:name="android.intent.category.LAUNCHER" />
    </intent-filter>
</activity>

Note: When you create a new Android project with the Android SDK tools, the default project files include anActivity class
that‘s declared in the manifest with this filter.

If either the MAIN action
or LAUNCHER category
are not declared for one of your activities, then your app icon will not appear in the Home screen‘s list of apps.

Create a New Instance

You must implement the onCreate() method
to perform basic application startup logic that should happen only once for the entire life of the activity. For example, your implementation of onCreate() should
define the user interface and possibly instantiate some class-scope variables.

For example, the following example of the onCreate() method
shows some code that performs some fundamental setup for the activity, such as declaring the user interface (defined in an XML layout file), defining member variables, and configuring some of the UI.

TextView mTextView; // Member variable for text view in the layout

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    // Set the user interface layout for this Activity
    // The layout file is defined in the project res/layout/main_activity.xml file
    setContentView(R.layout.main_activity);
   
    // Initialize member TextView so we can manipulate it later
    mTextView = (TextView) findViewById(R.id.text_message);
   
    // Make sure we‘re running on Honeycomb or higher to use ActionBar APIs
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {
        // For the main activity, make sure the app icon in the action bar
        // does not behave as a button
        ActionBar actionBar = getActionBar();
        actionBar.setHomeButtonEnabled(false);
    }
}

Caution: Using the SDK_INT to
prevent older systems from executing new APIs works in this way on Android 2.0 (API level 5) and higher only. Older versions will encounter a runtime exception.

Once the onCreate() finishes
execution, the system calls the onStart() and onResume() methods
in quick succession. Your activity never resides in the Created or Started states. Technically, the activity becomes visible to the user when onStart() is
called, but onResume() quickly
follows and the activity remains in the Resumed state until something occurs to change that, such as when a phone call is received, the user navigates to another activity, or the device screen turns off.

In the other lessons that follow, you‘ll see how the other start up methods, onStart() and onResume(),
are useful during your activity‘s lifecycle when used to resume the activity from the Paused or Stopped states.

Note: The onCreate() method
includes a parameter called savedInstanceState that‘s discussed in the latter lesson about Recreating
an Activity.

Figure 2. Another illustration of the activity lifecycle structure with an emphasis on the three main callbacks that the system calls in sequence when creating a new instance of the activity: onCreate(), onStart(),
and onResume(). Once this sequence of callbacks
complete, the activity reaches the Resumed state where users can interact with the activity until they switch to a different activity.

Destroy the Activity

Most apps don‘t need to implement this method because local class references are destroyed with the activity and your activity should perform most cleanup during onPause() and onStop().
However, if your activity includes background threads that you created during onCreate() or
other long-running resources that could potentially leak memory if not properly closed, you should kill them during onDestroy().

@Override
public void onDestroy() {
    super.onDestroy();  // Always call the superclass
   
    // Stop method tracing that the activity started during onCreate()
    android.os.Debug.stopMethodTracing();
}

Note: The system calls onDestroy() after
it has already called onPause() and onStop() in
all situations except one: when you call finish() from
within the onCreate() method.
In some cases, such as when your activity operates as a temporary decision maker to launch another activity, you might call finish() from
withinonCreate() to
destroy the activity. In this case, the system immediately calls onDestroy() without
calling any of the other lifecycle methods.

Android学习路线（十二）Activity生命周期——启动一个Activity