參考：《Professional Android 4 Application Development》

深入瞭解Android Activity

每一個Android Activity都對應於一個使用者介面（UI）。每個Android Application都有一個main Activity，而這個main Activity大多由多個Fragment組成，而這些Fragment後面往往都由一個或多個secondary Activity支援。當用戶在不同的介面（視窗）切換時，Android會生成對應的新Activity。

建立Activity

Android提供了Activity類，在編寫自己的

package cn.jubincn.activities;

import android.app.Activity;

import android.os.Bundle;

public class MyActivity extends Activity {

/** Called when the activity is first created. */

@Override

public void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

}

}

空白的Activity對應的是一個空白螢幕，因此我們需要新增Fragment，Layout和View等UI元素來定製自己的Activity。大部分Activity會佔據整個螢幕，除此之外，還有一些半透明的Activity和浮動的Activity。

Android應用程式中，使用者互動介面和資料的顯示是由View來提供的。Android中的layout類，也叫ViewGroup，可以將View打包進行管理。Fragment則用來組織介面元素，從而更方便地適配不同的介面。給Activity設定介面的方法有兩種，一種是建立View類，將其設定到Activity中；另一種是將layout

@Override

public void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

TextView textView = new TextView(this);

setContentView(textView);

}

@Override

public void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.main);

}

<activity android:label="@string/app_name" android:name=".MyActivity">

<intent-filter>

<action android:name="android.intent.action.MAIN" />

<category android:name="android.intent.category.LAUNCHER" />

</intent-filter>

</activity>

Activity生命週期

Activity的生命週期可以決定Process的優先順序，並且Activity需要對生命週期事件進行適當地響應才能提供更好的使用者體驗。

Activity Stacks

Activity的狀態決定於它在Activity Stack中的位置。Activity Stack遵循Stack的“先進後出”的規則：當一個Activity剛被建立時，它會移到棧頂；若使用者點選”go back”按鈕，或關閉當前Activity時，棧頂的Activity會被彈出，第二個Activity成為新的棧頂Activity。

Activity State

Activity的生命週期中，有這幾種狀態：

• Active：在棧頂的Activity，具有最高的優先順序，Android會竭力滿足它對資源的需求。當其他Activity變為Active狀態時，此Activity的狀態會變為Paused。
• Paused：處於這個狀態的Activity的部分或全部對使用者可見，但無法接受使用者的輸入。
• Stopped：當Activity對使用者不可見時，它將轉為Stopped狀態。此時儘管Activity仍會保留在記憶體中，但當系統資源緊張時，它會被回收掉。
• Inactive：Activity在啟動之前，或被kill之後，處於Inactive狀態。

監聽並響應Activity State的變化

Activity需要在State發生變化時作出相應的反應，為此Android為Activity的生命週期事件提供了很多相關的ActionHandler，下圖展示了Activity的幾個生命週期：

對應的程式碼：

package com.paad.activities;
import android.app.Activity;
import android.os.Bundle;
public class MyStateChangeActivity extends Activity {
// Called at the start of the full lifetime.
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
// Initialize Activity and inflate the UI.
}
// Called after onCreate has finished, use to restore UI state
@Override
public void onRestoreInstanceState(Bundle savedInstanceState) {
super.onRestoreInstanceState(savedInstanceState);
// Restore UI state from the savedInstanceState.
// This bundle has also been passed to onCreate. Will only be called if the Activity has been killed by the system since it was last visible.
}
// Called before subsequent visible lifetimes for an Activity process.
@Override
public void onRestart(){
super.onRestart();
// Load changes knowing that the Activity has already been visible within this process.
}
// Called at the start of the visible lifetime.
@Override
public void onStart(){
super.onStart();
// Apply any required UI change now that the Activity is visible.
}
// Called at the start of the active lifetime.
@Override
public void onResume(){
super.onResume();
// Resume any paused UI updates, threads, or processes required
// by the Activity but suspended when it was inactive.
}
// Called to save UI state changes at the end of the active lifecycle.
@Override
public void onSaveInstanceState(Bundle savedInstanceState) {
// Save UI state changes to the savedInstanceState.
// This bundle will be passed to onCreate and onRestoreInstanceState if the process is killed and restarted by the run time.
super.onSaveInstanceState(savedInstanceState);
}
// Called at the end of the active lifetime.
@Override
public void onPause(){
// Suspend UI updates, threads, or CPU intensive processes that don’t need to be updated when the Activity isn't the active foreground Activity.
super.onPause();
}
// Called at the end of the visible lifetime.
@Override
public void onStop(){
// Suspend remaining UI updates, threads, or processing that aren’t required when the Activity isn’t visible.
// Persist all edits or state changes
// as after this call the process is likely to be killed.
super.onStop();
}
// Sometimes called at the end of the full lifetime.
@Override
public void onDestroy(){
// Clean up any resources including ending threads,
// closing database connections etc.
super.onDestroy();
}
}