方法一.

看看Google工程师是怎么判断的（来源于Android源码中的Call应用，AsyncTask中的onPostExecute片段）：

@Override
protected void onPostExecute(Void result) {
    final Activity activity = progressDialog.getOwnerActivity();

    if (activity == null || activity.isDestroyed() || activity.isFinishing()) {
        return;
    }

    if (progressDialog != null && progressDialog.isShowing()) {
        progressDialog.dismiss();
    }
}

多了一个isDestroyed()的判断

方法二.定义ActivityCollector

import android.annotation.TargetApi;
import android.app.Activity;
import android.os.Build;

import java.util.HashMap;
import java.util.LinkedHashMap;
import java.util.Map.Entry;
import java.util.Set;

/**
 * Author: jack
 * Description:管理所有的栈中的Activity
 */
public class ActivityCollector {

    /**
     * 存放activity的列表
     */
    public static HashMap<Class<?>, Activity> activities = new LinkedHashMap<>();

    /**
     * 添加Activity
     *
     * @param activity
     */
    public static void addActivity(Activity activity, Class<?> clz) {
        activities.put(clz, activity);
    }

    /**
     * 判断一个Activity 是否存在
     *
     * @param clz
     * @return
     */
    @TargetApi(Build.VERSION_CODES.JELLY_BEAN_MR1)
    public static <T extends Activity> boolean isActivityExist(Class<T> clz) {
        boolean res;
        Activity activity = getActivity(clz);
        if (activity == null) {
            res = false;
        } else {
            if (activity.isFinishing() || activity.isDestroyed()) {
                res = false;
            } else {
                res = true;
            }
        }

        return res;
    }

    /**
     * 获得指定activity实例
     *
     * @param clazz Activity 的类对象
     * @return
     */
    public static <T extends Activity> T getActivity(Class<T> clazz) {
        return (T) activities.get(clazz);
    }

    /**
     * 移除activity,代替finish
     *
     * @param activity
     */
    public static void removeActivity(Activity activity) {
        if (activities.containsValue(activity)) {
            activities.remove(activity.getClass());
        }
    }

    /**
     * 移除所有的Activity
     */
    public static void removeAllActivity() {
        if (activities != null && activities.size() > 0) {
            Set<Entry<Class<?>, Activity>> sets = activities.entrySet();
            for (Entry<Class<?>, Activity> s : sets) {
                if (!s.getValue().isFinishing()) {
                    s.getValue().finish();
                }
            }
        }
        activities.clear();
    }
}

在 App中的BaseActivity中

public  class BaseActivity extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        ActivityCollector.addActivity(this, getClass());
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        ActivityCollector.removeActivity(this);
    }
}

然后所有的Activity都去继承 BaseActvity 就可以使 ActivityCollector起作用了。

好处

1. 在应用的任何地方都可以去获取到 Activity实例,从而操作指定的 View对象

MainActivity mainActivity = ActivityCollector.getActivity(MainActivity.class);
if (mainActivity!=null) mainActivity.main_radio.check(R.id.radio_button1);

2. 判断一个 Activity是否存在

ActivityCollector.isActivityExist(MainActivity.class); 

参考资料

• 如何判断Activity是否在运行？
• ActivityManager 来实现，请戳我的博客：ActivityManager.