Develop Reference

After Activity.onDestroy() method, why I can still get this activity's instance?

I have created a class to cache Activities, like:
public class ActivityList {
public static List<WeakReference<Activity>> list = new LinkedList<>();
public static void put(Activity activity) {
list.add(new WeakReference<>(activity));
}
}
and, the onDestroy method:
#Override
protected void onDestroy() {
super.onDestroy();
Log..e("test", "call onDestroy");
}
Secondly, I enable the "Don't keep activities" in System's developer options, to make sure an activity will be killed after I left
Then, I start an Activity and put an instance into then ActivityList class
ActivityList.put(this);
When I left this activity to another activity, the LogCat shows the first activity called onDestroy(). Then in then second activity, I get the first activity's instance like:
WeakReference<Activity> weakReference = ActivityList.list.get(0);
Activity activity = weakReference.get();
activity.runOnUiThread(() -> Toast.makeText(this, "HHHH", Toast.LENGTH_SHORT).show());
Then code works well ?!
In my view, the first activity called onDestroy() then I cannot use it anymore, so the Toast should NOT show. AND after onDestroy(), there are no any strong references refer to the activity, it should be GC by system.
So, Why does it work?

Weak references should be cleared at Garbage Collection,but finish an activity may not trigger GC.

Cleanly binding/unbinding to a Service in an Application

I have an Android application that is binding to a persistent service (once started with startService()).
The service is an integral part of the application and thus is used in almost every Activity. Hence I want to bind to the service just once (instead of binding/unbinding in every Activity) and keep the binding during the lifetime of my application.
I've extended from Application and bind to the service in Application#onCreate(). However I now have the problem that I don't know when my application exists since Application#onTerminate() is never called, see JavaDoc:
This method is for use in emulated process environments. It will never
be called on a production Android device, where processes are removed
by simply killing them; no user code (including this callback) is
executed when doing so.
So how do I cleanly unbind from a service bound in Application?

I solved this problem by counting the references to the service binding in the Application. Every Activity has to call acquireBinding() in their onCreate() methods and call releaseBinding() in onDestroy(). If the reference counter reaches zero the binding is released.
Here's an example:
class MyApp extends Application {
private final AtomicInteger refCount = new AtomicInteger();
private Binding binding;
#Override
public void onCreate() {
// create service binding here
}
public Binding acquireBinding() {
refCount.incrementAndGet();
return binding;
}
public void releaseBinding() {
if (refCount.get() == 0 || refCount.decrementAndGet() == 0) {
// release binding
}
}
}
// Base Activity for all other Activities
abstract class MyBaseActivity extend Activity {
protected MyApp app;
protected Binding binding;
#Override
public void onCreate(Bundle savedBundleState) {
super.onCreate(savedBundleState);
this.app = (MyApp) getApplication();
this.binding = this.app.acquireBinding();
}
#Override
public void onDestroy() {
super.onDestroy();
this.app.releaseBinding();
}
}

From Sven's answer:
I solved this problem by counting the references to the service
binding in the Application. Every Activity has to call
acquireBinding() in their onCreate() methods and call releaseBinding()
in onDestroy(). If the reference counter reaches zero the binding is
released.
I agree, BUT you shouldn't do it in onDestroy - that will often not get called.
Instead I suggest the following (based on your code sample)...
// Base Activity for all other Activities
abstract class MyBaseActivity extend Activity {
protected MyApp app;
protected Binding binding;
#Override
public void onCreate(Bundle savedBundleState) {
super.onCreate(savedBundleState);
this.app = (MyApp) getApplication();
this.binding = this.app.acquireBinding();
}
#Override
protected void onPause() {
super.onPause();
// Pre-HC, activity is killable after this.
if ((11 > Build.VERSION.SDK_INT) && (isFinishing()))
onFinishing();
}
#Override
protected void onStop() {
super.onStop();
if ((10 < Build.VERSION.SDK_INT) && (isFinishing()))
onFinishing();
}
protected void onFinishing() {
// Do all activity clean-up here.
this.app.releaseBinding();
}
}
BUT, my use of isFinishing() is just a thought - I'm not certain that it is reliable. Perhaps onPause/onStop get called with isFinishing() false, but then the activity gets killed - and your releaseBinding() never gets called.
If you get rid of the isFinishing check I think you need to move the acquireBinding() call from onCreate to onStart/onResume (depending on sdk version), to ensure that your ref count doesn't get messed up.
Who knew that releasing your app's service would be so complicated!

Is unbinding necessary at all in this case? The application gets killed anyway. I tried implementing a sample application doing this without unbinding and it seems to work properly.

Does the handler use the bind thread If i use a handler from anoter activity?

That's the cuestion , i have:
Activity A
Activity B
From A i want to communicate with B , then i use a Handler created on B to do it this way:
ActivityB.handlerB.sendMessage(msg);
Then what i don't know if it would use the ActivityA thread , or the ActivityB thread .
In order to let ActivityB Thread do the work , Is it well done?

Unless ActvityB created handlerB using a non-default Looper, all messages sent to handlerB will only deliver their messages on the same thread that handlerB was created within.
So, for example, if ActivityB created handler in a very typical way, from an activity event callback, handlerB will deliver its messages to the default UI thead.
So, if ActivityB is made like this, ActivityA is safe to call handlerB.sendMessage():
public class ActivityB extends Activity {
Handler handlerB;
public void onCreate(Bundle bundle) {
this.handlerB = new Handler() {
public void handleMessage(Message message) {
;; // Do something
}
};
}
}
So, for most circumstances, your answer is: Yes.

Android, start service using thread and warn activity when it's done

I have a service which has a method that downloads an image from an URL and returns an Uri.
That service will get more complex when it has all the intended features. Therefore,
I'm invoking its methods within a thread.
My problem is how to warn the activity that the service has done it's work.
I could change a class isFinished variable but the activity had to be constantly checking
for its value.
I just want the service to tell the activity that it's work is done and the resources are
available for use.
I thought something in the lines of the service calling stopSelf() and the activity was
warned through "onServiceDisconnected" but that didn't seem very "political correct".
Thanks in advance

There are two ways to do it.
1. You can start your activity using by firing an intent.
2. You can Broadcast an intent and write receiver for it in your app when your receiver receives intent and onreceive method is called in this method you can start your activity using intent.
cheers...

public class MyActivity extends Activity{
public MyActivity() {
...
MyThread thread = new MyThread(this);
thread.start();
}
public void onFinishedThread(...) {
}
}
class MyThread extends Thread {
MyActivity activity;
public MyThread(MyActivity activity) {
this.activity = activity;
}
public void run() {
// do work
...
this.activity.onFinishedThread(...);
}
}

How to force main Acivity to wait for subactivity in Android?

I am calling a subactivity from main activity. This subactivity should take few numbers from user (i'm using Edit text control to achieve this), save them to static variable in another class and terminate. I want main activity to wait for subactivity but both are just running simultaneously. Even doing sth like that doesn't help:
Thread t = new Thread(new Runnable(){
public void run(){
Log.v("==================", "run "+new Date());
startActivityForResult(new Intent(ctx,myCustomSubactivity.class),1);
} });
Log.v("==================", "calling run "+new Date());
t.start();
try {
t.join();
} catch (InterruptedException e) {Log.v("==================", "can't join");}
Log.v("==================", "back from activity "+new Date());
do you know how to force main activity to wait? Thread.wait() method is not supported in Android(program throws error).

May be I'm missing something but why don't just use startActivityForResult and onActivityResult mechanism? You could get result from you subactivity from intent it was resulted with.
Edit: BTW as far as I understand, if you will run Object.wait() from Activity code if will hold UI tread whitch can result in Application not responding error.

I agree with Nikolay this is definitely the android way to do this.
Start the subactivity with startActivityForResult in the sub activity use setResult to add an result code and an intent with all the numbers you need in the data bundle.
In your first activity overwrite onActivityResult and retrieve the numbers from the Intent.
If you use the static variable this seems easier in the first moment but it is very insecure and there are some cases this may not work. If your program is send to the background your activities will be saved but if the phone runs low on memory the system will close your program and after the user resumes it everything looks like the moment the user left it but the static variables will be recreated to their initialization value.
Try to get used to the way the android activity lifecycle works. Using this approach will result in fewer used memory and a much better user experience.

Check out the Notepad example, it covers exactly this situation. And as others have said, the Android way is to have your first activity start up your second activity (not sub-activity!) and asynchronously listen for a response (not pause or wait, no need for joining, etc.).

Well... you can do it like this (btw, there's not straight forward way):
Have a singleton class, let's call it Monitor:
public class Singleton
{
private Singleton() { }
private static Singleton instance = new Singleton();
public static Singleton getInstance() {
return instance;
}
}
public class ParentActivity extends Activity
{
private void startAndWait()
{
Intent i = new Intent();
// initialize i
startActivityForResult(i);
Singleton si = Singleton.getInstance();
synchronized(si)
{
si.wait();
}
//do remaining work
}
}
public class ChildActivity extends Activity
{
protected void onCreate(Bundle savedInstance)
{
//do all the work
Singleton si = Singleton.getInstance();
synchronized(si)
{
si.notify();
}
}
}

I'm not here to judge if it's a good pattern or not but if you really need an activity to wait for a sub-activity, you can try this approach:
define an object (lock) over which the two activities get synchronized; this can (should) also work as the object to exchange data between those two activities and thus should be defined as static
in parent activity, start an async task (as the UI main thread cannot be in waiting state)
in the async task, start your sub-activity
the async task waits on the lock till it gets notified
the sub-activity does whatever it needs and notifies the waiting thread when it finishes
I did a similar thing in my app and IMHO had a good reason for this (not to bother a user with login screen upon app start or resume, the app tries to re-use credentials stored in a secured place and only in case it fails, it shows this login screen. So yes, basically any activity in my app can get "paused" and waits till the user provides correct credentials in the login activity upon which the login screen finishes and the app continues exactly where it got paused (in the parent activity).
In the code it would be something like this:
ParentActivity:
public class ParentActivity extends Activity {
private static final String TAG = ParentActivity.class.getSimpleName();
public static class Lock {
private boolean condition;
public boolean conditionMet() {
return condition;
}
public void setCondition(boolean condition) {
this.condition = condition;
}
}
public static final Lock LOCK = new Lock();
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.parent_layout);
// do whatever logic you need and anytime you need to stat sub-activity
new ParentAsyncTask().execute(false);
}
private class ParentAsyncTask extends AsyncTask<Boolean, Void, Boolean> {
#Override
protected Boolean doInBackground(Boolean... params) {
// do what you need and if you decide to stop this activity and wait for the sub-activity, do this
Intent i = new Intent(ParentActivity.this, ChildActivity.class);
startActivity(i);
synchronized (LOCK) {
while (!LOCK.conditionMet()) {
try {
LOCK.wait();
} catch (InterruptedException e) {
Log.e(TAG, "Exception when waiting for condition", e);
return false;
}
}
}
return true;
}
}
}
ChildActivity:
public class ChildActivity extends Activity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.child_layout);
// do whatever you need in child activity, but once you want to finish, do this and continue in parent activity
synchronized (ParentActivity.LOCK) {
ParentActivity.LOCK.setCondition(true);
ParentActivity.LOCK.notifyAll();
}
finish();
// if you need the stuff to run in background, use AsyncTask again, just please note that you need to
// start the async task using executeOnExecutor method as you need more executors (one is already occupied), like this:
// new ChildAsyncTask().executeOnExecutor(ChildAsyncTask.THREAD_POOL_EXECUTOR, false);
}
}