在我们开发的过程中，经常遇到app进入后台，或者切换到前台的情况。
比如：

1. 从后台切换到前台显示全屏广告实践
2. 来电显示从后台切换到前台

如图显示：压后台10秒钟，自动弹出

实现主要分为两个步骤：

第一步通过ActivityLifecycleCallbacks判断是否在前后台显示

private void registerActivityLifecycleCallbacks() {registerActivityLifecycleCallbacks(new ActivityLifecycleCallbacks() {@Overridepublic void onActivityCreated(Activity activity, Bundle savedInstanceState) {}@Overridepublic void onActivityStarted(Activity activity) {mCount++;if (!mFront) {mFront = true;Log.e("", "AppContext------->处于前台");}}@Overridepublic void onActivityResumed(Activity activity) {}@Overridepublic void onActivityPaused(Activity activity) {}@Overridepublic void onActivityStopped(Activity activity) {mCount--;if (mCount == 0) {mFront = false;Log.e("", "AppContext------->处于后台");timerTask();}}@Overridepublic void onActivitySaveInstanceState(Activity activity, Bundle outState) {}@Overridepublic void onActivityDestroyed(Activity activity) {}});}

第二部通过

 List<ActivityManager.RunningTaskInfo> taskInfoList = activityManager.getRunningTasks(20)

的RunningTasks，

ActivityManager的moveTaskToFront(rti.id, ActivityManager.MOVE_TASK_WITH_HOME);

方法将应用唤起到前台

@SuppressLint("NewApi")public static void isRunningForegroundToApp1(Context context, final Class Class) {ActivityManager activityManager = (ActivityManager) context.getSystemService(Context.ACTIVITY_SERVICE);List<ActivityManager.RunningTaskInfo> taskInfoList = activityManager.getRunningTasks(20);/**枚举进程*/for (ActivityManager.RunningTaskInfo taskInfo : taskInfoList) {//*找到本应用的 task，并将它切换到前台if (taskInfo.baseActivity.getPackageName().equals(context.getPackageName())) {Log.e("timerTask", "timerTask  pid " + taskInfo.id);Log.e("timerTask", "timerTask  processName " + taskInfo.topActivity.getPackageName());Log.e("timerTask", "timerTask  getPackageName " + context.getPackageName());activityManager.moveTaskToFront(taskInfo.id, ActivityManager.MOVE_TASK_WITH_HOME);Intent intent = new Intent(context, Class);intent.addCategory(Intent.CATEGORY_LAUNCHER);intent.setAction(Intent.ACTION_MAIN);intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_RESET_TASK_IF_NEEDED);context.startActivity(intent);break;}}}

然后通过Intent 跳转到对应界面 Class的启动模式一定是singleTask

 Intent intent = new Intent(context, Class);intent.addCategory(Intent.CATEGORY_LAUNCHER);intent.setAction(Intent.ACTION_MAIN);intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_RESET_TASK_IF_NEEDED);context.startActivity(intent);

完成代码如下：

package com.click.backdesk;import android.annotation.SuppressLint;
import android.app.Activity;
import android.app.ActivityManager;
import android.app.Application;
import android.content.Context;
import android.content.Intent;
import android.os.Build;
import android.os.Bundle;
import android.util.Log;import androidx.annotation.RequiresApi;import java.util.List;
import java.util.Timer;
import java.util.TimerTask;public class MyAppliction extends Application {public static MyAppliction sInstance;public static int mCount;public static boolean mFront;//是否前台private static Timer mTimer;static int cntStart = 0;static int waitTime = 10;static TimerTask mTask = null;protected void attachBaseContext(Context base) {super.attachBaseContext(base);}@Overridepublic void onCreate() {super.onCreate();sInstance = this;registerActivityLifecycleCallbacks();}private void registerActivityLifecycleCallbacks() {registerActivityLifecycleCallbacks(new ActivityLifecycleCallbacks() {@Overridepublic void onActivityCreated(Activity activity, Bundle savedInstanceState) {}@Overridepublic void onActivityStarted(Activity activity) {mCount++;if (!mFront) {mFront = true;Log.e("", "AppContext------->处于前台");}}@Overridepublic void onActivityResumed(Activity activity) {}@Overridepublic void onActivityPaused(Activity activity) {}@Overridepublic void onActivityStopped(Activity activity) {mCount--;if (mCount == 0) {mFront = false;Log.e("", "AppContext------->处于后台");timerTask();}}@Overridepublic void onActivitySaveInstanceState(Activity activity, Bundle outState) {}@Overridepublic void onActivityDestroyed(Activity activity) {}});}private static void timerTask() {if (mTimer == null && mTask == null) {mTimer = new Timer();mTask = new TimerTask() {@RequiresApi(api = Build.VERSION_CODES.Q)@Overridepublic void run() {cntStart++;Log.e("timerTask", "timerTask   " + cntStart);if (waitTime == cntStart) {
//                        go();isRunningForegroundToApp1(sInstance.getBaseContext(), FristActivity.class);cancelTimer();Log.e("timerTask", "timerTask 30  " + cntStart);}}};mTimer.schedule(mTask, 0, 1000);}}@SuppressLint("NewApi")public static void isRunningForegroundToApp1(Context context, final Class Class) {ActivityManager activityManager = (ActivityManager) context.getSystemService(Context.ACTIVITY_SERVICE);List<ActivityManager.RunningTaskInfo> taskInfoList = activityManager.getRunningTasks(20);/**枚举进程*/for (ActivityManager.RunningTaskInfo taskInfo : taskInfoList) {//*找到本应用的 task，并将它切换到前台if (taskInfo.baseActivity.getPackageName().equals(context.getPackageName())) {Log.e("timerTask", "timerTask  pid " + taskInfo.id);Log.e("timerTask", "timerTask  processName " + taskInfo.topActivity.getPackageName());Log.e("timerTask", "timerTask  getPackageName " + context.getPackageName());activityManager.moveTaskToFront(taskInfo.id, ActivityManager.MOVE_TASK_WITH_HOME);Intent intent = new Intent(context, Class);intent.addCategory(Intent.CATEGORY_LAUNCHER);intent.setAction(Intent.ACTION_MAIN);intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_RESET_TASK_IF_NEEDED);context.startActivity(intent);break;}}}private static void cancelTimer() {cntStart = 0;if (mTimer != null) {mTimer.cancel();mTimer = null;}if (mTask != null) {mTask.cancel();mTask = null;}}
}

<uses-permission android:name="android.permission.PACKAGE_USAGE_STATS" /><uses-permission android:name="android.permission.REORDER_TASKS" /><applicationandroid:name=".MyAppliction"android:allowBackup="true"android:icon="@mipmap/ic_launcher"android:label="@string/app_name"android:roundIcon="@mipmap/ic_launcher_round"android:supportsRtl="true"android:theme="@style/AppTheme"><activity android:name=".MainActivity"><intent-filter><action android:name="android.intent.action.MAIN" /><category android:name="android.intent.category.LAUNCHER" /></intent-filter></activity><activityandroid:name=".FristActivity"android:launchMode="singleTask" /></application>