Develop Reference

Matter of Handlers execution in a sequence

I took this snipet from a site explaining handler in Android (a threading thing).
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Thread myThread = new Thread(new Runnable() {
#Override
public void run() {
for (int i = 0; i < 4; i++) {
try {
TimeUnit.SECONDS.sleep(2);
} catch (InterruptedException e) {
e.printStackTrace();
}
if (i == 2) {
mUiHandler.post(new Runnable() {
#Override
public void run() {
Toast.makeText(MyActivity.this, "I am at the middle of background task",
Toast.LENGTH_LONG)
.show();
}
});
}
}//ends for()
// THE SECOND HANDLER, RIGHT HERE!
mUiHandler.post(new Runnable() {
#Override
public void run() {
Toast.makeText(MyActivity.this,
"Background task is completed",
Toast.LENGTH_LONG)
.show();
}
});
} //ends run()
});
myThread.start();
Judging from the task outputted in the second executed Handler, which is
Toast.makeText(MyActivity.this,
"Background task is completed",
Toast.LENGTH_LONG)
.show();
Seems like the writer of the article is pretty much sure that the second Handler will be executed last.
My question is that whether it's true that the second Handler will be executed last just after the first Handler finishes its job. Though, when I ran it multiple times, yes, it's executed last. In my mind, since the Handler is done in the background Thread then we're not supposed to know (even predict) which one the those two tasks the Handler will execute first. I need an explanation, thank you in advance.

My question is that whether it's true that the second handler will be
executed last just after the first handler finishes its job.
A Handler instance is associated to a single Thread (also called a message queue).
Runnables are executed on this Thread sequentially.
Calling post() will put the Runnable at the end of that queue, so yes, the second Runnable will be executed after the first one.

The outermost anonymous Runnable, the one passed to the new Thread(...) constructor, is what runs in the background thread. Everything inside that runnable will execute sequentially - one instruction after the other.
Since that runnable has a for loop and only after that the final toast appears you're guaranteed it'll run after the loop body.

There are not two handlers in play, just a single handler on the UI thread (mUiHandler). Your secondary thread is creating Runnable objects and posting them to the Handler. They will be executed by the handler in the order they are posted. Since the loop of the thread executes and posts first then the thread finishes by posting the "second" Runnable, that second one will always execute last, relative to the other things posted within the loop.

How can I use if/while statements in Android SDK?

I am very new to Android SDK so that question may be very simple.
I try to make am app that shows the current time. I know how to get the current time but i somehow need to update the time. So i try putting a while/if statement that updates the time outside onCreate() and inside main Activity class but 4 errors pop up saying "unexpected end of Decleration.
I cant really find a way to solve this so any help would be much appreciated.

I get the idea that a longer example will be more useful for you.
Make a layout for the application, call the file activity_main.xml.
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="#dimen/activity_vertical_margin"
android:paddingLeft="#dimen/activity_horizontal_margin"
android:paddingRight="#dimen/activity_horizontal_margin"
android:paddingTop="#dimen/activity_vertical_margin"
tools:context="com.example.timedemo.MainActivity">
<TextView
android:id="#+id/textClock"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentLeft="true"
android:layout_alignParentStart="true"
android:layout_alignTop="#+id/textView" />
</LinearLayout>
Make a file called MainActivity.java, make it extends Activity (or something else that extends Activity)
Init your layout in the onCreate method.
Use a handler to run code at a delayed time, instead of halting the thread.
public class MainActivity extends AppCompatActivity {
//Handler can be used to send Runnables (code to run) to a specific Thread.
//In this case the UI-thread.
Handler handler = new Handler();
//TextView variable defined in Class-scope.
TextView myTextClock;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
//Load the layout from activity_main.xml into this Activity.
setContentView(R.layout.activity_main);
//Find the textclock that is mentioned in the activity_main.xml
//Use the ID to find the right View.
//As you can see in the xml-file, the id is 'textClock'.
//Looks like this in the XML --> <TextView android:id="#+id/textClock" />
myTextClock = (TextView) this.findViewById(R.id.textClock);
//Tell the Handler to execute this code at an interval.
handler.postDelayed(codeToRun, 1000);
}
//The runnable contains the code that will be run every second.
Runnable codeToRun = new Runnable() {
#Override
public void run() {
updateTime();
}
};
public void updateTime(){
//Code to update the Clock on the UI-thread
//see: http://developer.android.com/reference/java/text/SimpleDateFormat.html
DateFormat sdf = DateFormat.getDateTimeInstance();
myTextClock.setText(sdf.format(new Date()));
//Make sure it runs the next time too.
handler.postDelayed(codeToRun, 1000);
}
}
Hope this helps you get on the right path.

I'd suggest you use ScheduledThreadPoolExecutor class instead of a while loop. There is an example in this tutorial on how to create a clock app. It can be useful as a starting point.
class ClockTask implements Runnable {
#Override
public void run() {
updateClock();
}
}
ScheduledThreadPoolExecutor se = new ScheduledThreadPoolExecutor(1);
se.scheduleAtFixedRate(new ClockTask(), 0, 1000, TimeUnit.MICROSECONDS);

I'm not sure how you plan to make this, but keep in mind that you do not stall the Main Thread. So avoid a construction like this:
while(true){ Thread.sleep(1000); updateTime(); }
A better approach would to use Handler, for example:
Handler handler = new Handler();
handler.postDelayed(new Runnable() {
#Override
public void run() {
updateTime();
}
}, 1000);
http://developer.android.com/reference/android/os/Handler.html#postDelayed(java.lang.Runnable,%20long)
Hope this helps ;-)

Android - efficiently schedule a task periodically?

I've got this code to schedule a task every so often:
final Handler handler = new Handler();
Runnable update = new Runnable() {
#Override
public void run() {
try{
runOnUiThread(new Runnable(){
public void run(){
lbl.setText(info);
cpb.setProgress(Float.valueOf(textInfo);
}
});
handler.postDelayed(this, 1000);
}
catch (Exception e) {
// TODO: handle exception
}
}
};
I want to constantly update the UI (every second, etc). The code works, however, the interface starts lagging. After the code iterates the task a few times the interface stops responding.
Is there any way I can schedule a task to repeat periodically without overloading the memory and without the interface lagging.

Assuming lbl is a TextView and cpb is a ProgressBar, your code will not considerably lag any device as it is. The problem lies somewhere else. Also, you appear to have missed a closing bracket on (Float.valueOf(textInfo);.
As an aside, you are unnecessarily using runOnUiThread inside the Runnable from what I can see. When you create a new Handler() it is implicitly linked to the calling thread's Looper, which I am assuming is the UI thread. In which case, the update Runnable will already be running on the UI thread. EDIT: This should not be the cause of the lag for the record, since iirc runOnUiThread checks if it is being executed on the UI thread then just runs it immediately, without doing another post.

Getting rid of the black screen

This question has probably been asked a lot many times.. Yes! I am facing the so called "black screen problem"..
My first activity is which the main activity, in which login is done, and this activity is followed by the next one which is a simple summary report of the user's account.
Now, for this report, i fetch the dataset, parse it and use it to build the table dynamically.
The villan black screen appears here!! The report, as in the table is rendered properly, but upon that screen, this black tranluscent layer keeps appearing.
I tried everything, using handlers, asynctask, translucent theme but nothing helps!! The villan still appears when my summary report loads. It goes away if i press the "Back" button, and the screen appears normal, as it is expected to be when it loads the first time.. I cant figure out what exactly is going wrong, whether, its my coding approach(dynamically generating the table) or it is an emulator problem or what.
My emulator details are as follows:
CPU:ARM
Target: Android 2.3.3
skin: WVGA800
sd card:1024M
Someone please rescue me!!
EDIT:
In my Second activity i do the following:
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_summary_report);
ad = new AlertDialog.Builder(this).create();
boolean is = IsConnectedToNetwork();
if (is == true){
Handler execWS = new Handler();
execWS.post(callWS);//Fetch Data via web service and parse it
if(result != null){
Handler genUI = new Handler();
genUI.post(createUI);// generate the table based on "result". ie. in a for loop i create textviews and append it to the table layout
}
}
else{
Error = "Connection Failed. Try Again!";
}
if(Error.length() != 0){
ad.setMessage(Error);
ad.setTitle("Error..");
ad.show();
}
}
My Xml layout for the second activity..
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:baselineAligned="false"
android:gravity="center"
android:orientation="vertical">
<TableLayout
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:padding="5dip">
<ImageView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center_horizontal"
android:contentDescription="#string/Logo"
android:src="#drawable/complogo"
android:gravity="top"/>
<TextView
android:id="#+id/lblLoginInfo"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center"
android:gravity="center_horizontal"/>
</TableLayout>
<TableLayout
android:id="#+id/tblSummRep"
android:layout_width="wrap_content"
android:layout_height="0dp"
android:layout_weight="4"
android:padding="5dip">
</TableLayout>
<TableLayout
android:layout_width="match_parent"
android:layout_height="0dp"
android:layout_weight="1"
android:padding="5dip">
<TextView
android:id="#+id/AppName"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:gravity="center"
android:text="#string/AppName"
android:textColor="#ff6600" />
<TextView
android:id="#+id/AppVersion"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:gravity="center"
android:text="#string/AppVersion"
android:textColor="#ff6600" />
<TextView
android:id="#+id/Disclaimer"
android:gravity="center"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="#string/Disclaimer"
android:textColor="#ff6600" />
</TableLayout>
</LinearLayout>
I append the textview to "tblSummRep"..
UPDATE
my asynctask class..
private class ShareWork extends AsyncTask<Void, Void, Boolean>{
ProgressDialog pDlg;
//String[][] result = null;
protected void onPreExecute(){
pDlg.setMessage("Please wait while the Report Loads...");
pDlg.show();
}
#Override
protected Boolean doInBackground(Void... params) {
boolean RetVal = false;
//my fetch code
return RetVal;
}
protected void onPostExecute(Boolean value){
if(value == true){
pDlg.setMessage("Please try again later!!");
pDlg.show();
}
else{
createUI.run();
}
}
}

I do not know what do you refer to by "black screen problem", but you state that you are fetching data (I guess from a remote server) and that happens. That sounds to me about blocking the UI thread. In which method of the activity are you fetching the data (maybe in the onCreate)? You should not perform time-consuming actions on the graphical thread, such as querying a remote server (which could take some seconds). In these cases always an AsyncTask should be used. The user should receive some feedback about what is going on in the background, so using a ProgressBar is normally recommendable (or at least a spinner).
You can take a look about how to solve a similar problem with an AsyncTask here.
Update
You have just posted your code. The problem lies (as I presumed) in this line of code:
execWS.post(callWS);//Fetch Data via web service and parse it
I guess this is a WS call. You are blocking here the UI thread. Create an AsyncTask the same way as the one of the link I provided, and upon completion of the task execute the rest of the code (display error or the dynamic table with the data).
Your AsyncTask could look like this (I have not tried it):
private class LoadTableTask extends AsyncTask<URL, Integer, Object> {
protected Object doInBackground(URL... urls) {
Handler execWS = new Handler();
execWS.post(urls[0]);//Fetch Data via web service and parse it
return result;
}
protected void onProgressUpdate(Integer... progress) {
//Not used in your case. It would be a good idea to create an undefined progress dialog in your case
}
protected void onPostExecute(Object result) {
if(result != null){
Handler genUI = new Handler();
genUI.post(createUI);// generate the table based on "result". ie. in a for loop i create textviews and append it to the table layout
}
}
}
Your onCreate() method would be replaced by:
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_summary_report);
ad = new AlertDialog.Builder(this).create();
boolean is = IsConnectedToNetwork();
if (is == true){
new LoadTableTask().execute(callWS);
}
else{
Error = "Connection Failed. Try Again!";
}
if(Error.length() != 0){
ad.setMessage(Error);
ad.setTitle("Error..");
ad.show();
}
}

What exactly does the post method do?

I've encountered a very weird feature.
When I'm trying to run an animation on the main thread, it does not start.
When I run said animation using
getView().post(new Runnable() {
#Override
public void run() {
getView().startAnimation(a);
}
});
It does start.
I've printed the CurrentThread before starting the animation and both print main.
Obviously, I am missing something here, as both should start the animation on the main thread...
My guess is that as post adds the task to the queue, it starts at a more "correct time", but I would love to know what happens here at more depth.
EDIT:
Let me clear things up - my question is, why starting the animation on post causes it to start, when starting the animation on the main thread does not.

post :post causes the Runnable to be added to the message queue,
Runnable : Represents a command that can be executed. Often used to run code in a different Thread.
run () : Starts executing the active part of the class' code. This method is called when a thread is started that has been created with a class which implements Runnable.
getView().post(new Runnable() {
#Override
public void run() {
getView().startAnimation(a);
}
});
code : getView().startAnimation(a);
in your code,
post causes the Runnable (the code will be run a in different thread) to add the message queue.
So startAnimation will be fired in a new thread when it is fetched from the messageQueue
[EDIT 1]
Why do we use a new thread instead of UI thread (main thread)?
UI Thread :
When application is started, Ui Thread is created automatically
it is in charge of dispatching the events to the appropriate widgets
and this includes the drawing events.
It is also the thread you interact with Android widgets with
For instance, if you touch the a button on screen, the UI thread
dispatches the touch event to the widget which in turn sets its
pressed state and posts an invalidate request to the event queue. The
UI thread dequeues the request and notifies the widget to redraw
itself.
What happens if a user press a button which will do longOperation ?
((Button)findViewById(R.id.Button1)).setOnClickListener(
new OnClickListener() {
#Override
public void onClick(View v) {
final Bitmap b = loadImageFromNetwork();
mImageView.setImageBitmap(b);
}
});
The UI freezes. The program may even crash.
public void onClick(View v) {
new Thread(new Runnable() {
public void run() {
final Bitmap b = loadImageFromNetwork();
mImageView.setImageBitmap(b);
}
}).start();
}
It breaks the android rule that never update UI directly from worker thread
Android offers several ways to access the UI thread from other threads.
Activity.runOnUiThread(Runnable)
View.post(Runnable)
View.postDelayed(Runnable, long)
Handler
Like below,
View.post(Runnable)
public void onClick(View v) {
new Thread(new Runnable() {
public void run() {
final Bitmap b = loadImageFromNetwork();
mImageView.post(new Runnable() {
public void run() {
mImageView.setImageBitmap(b);
}
});
}
}).start();
}
Handler
final Handler myHandler = new Handler(Looper.getMainLooper());
(new Thread(new Runnable() {
#Override
public void run() {
final Bitmap b = loadImageFromNetwork();
myHandler.post(new Runnable() {
#Override
public void run() {
mImageView.setImageBitmap(b);
}
});
}
})).start();
}
For more info
http://android-developers.blogspot.com/2009/05/painless-threading.html
http://www.aviyehuda.com/blog/2010/12/20/android-multithreading-in-a-ui-environment/

Is this being done on onCreate or onCreateView? If so, the app might not be in a state where the View is attached to the window. A lot of algorithms based on View metrics may not work since things like the View's measurements and position may have not been calculated. Android animations typically require them to run through UI math
View.post actually queues the animation on the View's message loop, so once the view gets attached to the window, it executes the animation instead of having it execute manually.
You are actually running things on the UI thread, but at a different time

Have a look here for a good answer. view.post() is the same as handler.post() pretty much. It goes into the main thread queue and gets executed after the other pending tasks are finished. If you call activity.runOnUiThread() it will be called immediately on the UI thread.

The problem I think could be the life-cycle method where you are calling the post() method. Are you doing it in onCreate()? if so look at what I found in the activity's onResume() documentation:
onResume()
Added in API level 1 void onResume () Called after
onRestoreInstanceState(Bundle), onRestart(), or onPause(), for your
activity to start interacting with the user. This is a good place to
begin animations, open exclusive-access devices (such as the
camera), etc.
https://developer.android.com/reference/android/app/Activity.html#onResume()
So, as Joe Plante said, maybe the view is not ready to start animations at the moment you call post(), so try moving it to onResume().
PD: Actually if you do move the code to onResume() then I think you can remove the post() call since you are already in the ui-thread and the view should be ready to start animations.