Android开发中需要用到当前 activity实例，如何获取当前正在显示的Activity实例

1.保存当前Activity实例

import android.app.Activity;
import java.lang.ref.WeakReference;

public class MyActivityManager {
    private static MyActivityManager sInstance = new MyActivityManager();
    private WeakReference<Activity> sCurrentActivityWeakRef;
    private MyActivityManager() {
    }

    public static MyActivityManager getInstance() {
        return sInstance;
    }

    public Activity getCurrentActivity() {
        Activity currentActivity = null;
        if (sCurrentActivityWeakRef != null && sCurrentActivityWeakRef.get() != null) {
            currentActivity = sCurrentActivityWeakRef.get();
        }
        return currentActivity;
    }

    public void setCurrentActivity(Activity activity) {
        sCurrentActivityWeakRef = new WeakReference<Activity>(activity);
    }
}

2.监听Activity的生命周期

在Application的onCreate方法中监听Activity的生命周期

private void initLifeCycle() {
        registerActivityLifecycleCallbacks(new ActivityLifecycleCallbacks() {
            @Override
            public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
                MyActivityManager.getInstance().setCurrentActivity(activity);
            }

            @Override
            public void onActivityStarted(Activity activity) {
            }

            @Override
            public void onActivityResumed(Activity activity) {
                MyActivityManager.getInstance().setCurrentActivity(activity);
            }

            @Override
            public void onActivityPaused(Activity activity) {
            }

            @Override
            public void onActivityStopped(Activity activity) {
            }

            @Override
            public void onActivitySaveInstanceState(Activity activity, Bundle outState) {
            }

            @Override
            public void onActivityDestroyed(Activity activity) {
            }
        });
    }

3.使用

activity = MyActivityManager.getInstance().getCurrentActivity();

注意：很多人在onResume方法中对当前实例对象进行赋值，原理上是对的，其实有隐患，储存当前activity的单例类，可能在任意一个地方被调用，比如在某个activity的oncreate方法里执行，这时候会发生取错，进而发生很多不知名bug，正确做法，应该是oncreate和onResume方法里都对单例类进行赋值，保证随时取到的都是最新的正确值。