Activity是一个应用程序组件，提供用户与程序交互的界面。 Android四大组件 . Activity . Service . BroadcastReceiver . Content Provider 创建使用Activity 1.继承Android的Activity类 public class MainActivity extends AppCompatActivity {
2.重写方法 @Override protected void onCreate(Bundle savedInstanceState) {
3.设置显示布局 setContentView(R.layout.activity_main);
4.在AndroidManifest文件中，注册Activity AndroidManifest.xml文件 有个Activity标签
android:name Activity的标识 com.example.hello1包名 MainActivity文件
<activity android:name="com.example.hello1.MainActivity" android:label="@string/app_name" > <intent-filter> 若这个Activity是主入口，还要设置action和category <action android:name="android.intent.action.MAIN" /> <category android:name="android.intent.category.LAUNCHER" /> </intent-filter> </activity>activity生命周期： 从 创建 到 销毁。 onCreate() 创建 onStart() 运行 onResume() 获取焦点 onPause() 失去焦点 onStop() 暂停 onDestroy() 销毁 onRestart()
Activity的状态： 1.活动状态（Active/Running）Activity处于界面最顶端，获取加点 2.暂停状态（Paused）Activity失去加点，但对用户可见 3.停止状态（Stopped）Activity被完全遮挡，但保留所有状态和成员信息 4.非活动状态（killeed）Activity被停止
package com.example.hello1; import android.support.v7.app.ActionBarActivity; import android.support.v7.app.ActionBar; import android.support.v4.app.Fragment; import android.os.Bundle; import android.util.Log; import android.view.LayoutInflater; import android.view.Menu; import android.view.MenuItem; import android.view.View; import android.view.ViewGroup; import android.os.Build; public class MainActivity extends ActionBarActivity { final String TAG = "tag";//定义常量 @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); Log.i(TAG, "MainActivity --> onCreate"); if (savedInstanceState == null) { getSupportFragmentManager().beginTransaction() .add(R.id.container, new PlaceholderFragment()) .commit(); } } @Override protected void onStart() { // TODO Auto-generated method stub super.onStart(); Log.i(TAG, "MainActivity --> onStart"); } @Override protected void onResume() { // TODO Auto-generated method stub super.onResume(); Log.i(TAG, "MainActivity --> onResume"); } @Override protected void onPause() { // TODO Auto-generated method stub super.onPause(); Log.i(TAG, "MainActivity --> onPause"); } @Override protected void onStop() { // TODO Auto-generated method stub super.onStop(); Log.i(TAG, "MainActivity --> onStop"); } @Override protected void onDestroy() { // TODO Auto-generated method stub super.onDestroy(); Log.i(TAG, "MainActivity --> onDestroy"); } @Override protected void onRestart() { // TODO Auto-generated method stub super.onRestart(); Log.i(TAG, "MainActivity --> onRestart"); } @Override public boolean onCreateOptionsMenu(Menu menu) { // Inflate the menu; this adds items to the action bar if it is present. getMenuInflater().inflate(R.menu.main, menu); return true; } @Override public boolean onOptionsItemSelected(MenuItem item) { // Handle action bar item clicks here. The action bar will // automatically handle clicks on the Home/Up button, so long // as you specify a parent activity in AndroidManifest.xml. int id = item.getItemId(); if (id == R.id.action_settings) { return true; } return super.onOptionsItemSelected(item); } /** * A placeholder fragment containing a simple view. */ public static class PlaceholderFragment extends Fragment { public PlaceholderFragment() { } @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { View rootView = inflater.inflate(R.layout.fragment_main, container, false); return rootView; } } }