activity的生命周期

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activity的生命周期
（1）onCreate：当activity被创建的时候调用
（2）onStart：当activity变成用户可见的时候调用
（3）onResume：界面获取焦点的时候调用的方法
（4）onPause：界面失去焦点（控件不能得到点击事件）
（5）onStop：界面不可见

package org.gentry.testnetwork;import android.app.Activity;import android.app.AlertDialog;import android.app.AlertDialog.Builder;import android.content.Context;import android.content.DialogInterface;import android.content.DialogInterface.OnClickListener;import android.content.Intent;import android.net.ConnectivityManager;import android.net.NetworkInfo;import android.os.Bundle;import android.widget.Toast;public class MainActivity extends Activity {@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);// 检查用户的网络情况（不适用于4.0以上）ConnectivityManager cm = (ConnectivityManager) this.getSystemService(Context.CONNECTIVITY_SERVICE);NetworkInfo info = cm.getActiveNetworkInfo();if (info != null && info.isConnected()) {Toast.makeText(this, "网络可用", Toast.LENGTH_SHORT).show();} else {Toast.makeText(this, "网络不可用", Toast.LENGTH_SHORT).show();// 定向用户到系统网络设置的界面Intent intent = new Intent();intent.setClassName("com.android.settings","com.android.settings.WirelessSettings");startActivity(intent);}}@Overrideprotected void onStart() {super.onStart();// 检查用户的网络情况（不适用于4.0以上）ConnectivityManager cm = (ConnectivityManager) MainActivity.this.getSystemService(Context.CONNECTIVITY_SERVICE);NetworkInfo info = cm.getActiveNetworkInfo();if (info != null && info.isConnected()) {Toast.makeText(MainActivity.this, "网络可用", Toast.LENGTH_SHORT).show();} else {Toast.makeText(MainActivity.this, "网络不可用", Toast.LENGTH_SHORT).show();// 1. 创建对话框的构造器AlertDialog.Builder builder = new Builder(this);builder.setTitle("提醒");builder.setMessage("当前网络不可用，点击确定请设置网络");builder.setPositiveButton("确定", new OnClickListener() {@Overridepublic void onClick(DialogInterface dialog, int which) {// 定向用户到系统网络设置的界面Intent intent = new Intent();intent.setClassName("com.android.settings","com.android.settings.WirelessSettings");startActivity(intent);}});builder.create().show();}}}

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