A handler is a routine/function/method which is specialized in a certain type of data or focused on certain special tasks.

Examples:

 - Event handler - Receives and digests events and signals from the
   surrounding system (e.g. OS or GUI).

 - Memory handler - Performs certain special tasks on memory.

 - File input handler - A function receiving file input and performing
   special tasks on the data, all depending on context of course.

Code that&#39;s associated with and triggered by the occurrence of a specific event, like an incoming message, a thrown exception, a signal sent to a process, a network I/O request completing, or a mouse click on a user interface element.  It&#39;s a very generic term.

I think it&#39;s a very general term, without a &#39;hard&#39; definition.  The meaning is highly contextual, varies depending on the general code design.

For me, it usually means some code that is called from an inner core and is supposed to do some things and return.  That &#39;inner&#39; part can have several &#39;handlers&#39; available, and chooses which one to call.

In some cases, you define some API to make those handlers mostly interchangeable, so the caller can pick one from a table and use the same code to call any of them.  OOP helps a lot here.

The following code has a simple binding which binds the Text of the TextBlock named MyTextBlock to TextBox&#39;s Text and ToolTip property using the exact same Binding notation:

    &lt;StackPanel&gt;
        &lt;TextBlock x:Name=&quot;MyTextBlock&quot;&gt;Foo Bar&lt;/TextBlock&gt;
        &lt;TextBox    Text=&quot;{Binding ElementName=MyTextBlock, Path=Text, StringFormat=&#39;It is: \{0\}&#39;}&quot;
                 ToolTip=&quot;{Binding ElementName=MyTextBlock, Path=Text, StringFormat=&#39;It is: \{0\}&#39;}&quot; /&gt;
    &lt;/StackPanel&gt;

The binding also uses the [StringFormat property introduced with .NET 3.5 SP1][1] which is working fine for the above Text property but seems to be broken for the ToolTip. The expected result is &quot;It is: Foo Bar&quot; but when you hover over the TextBox, the ToolTip shows only the binding value, not the string formatted value. Any ideas?


  [1]: http://blogs.msdn.com/llobo/archive/2008/05/19/wpf-3-5-sp1-feature-stringformat.aspx

WPF binding with StringFormat doesn&#39;t work on ToolTips

How do I stop a Java process gracefully in Linux and Windows?

When does `Runtime.getRuntime().addShutdownHook` get called, and when does it not?

What about finalizers, do they help here?

Can I send some sort of signal to a Java process from a shell?

I am looking for preferably portable solutions.

How to stop java process gracefully?

I am trying to learn some programming related terms, and I often come over the word &quot;handler&quot;. Can anyone please explain what it means and when to use it?

What is a handler?

<p>I am trying to learn some programming related terms, and I often come over the word "handler". Can anyone please explain what it means and when to use it?</p>


I&#39;m converting my code from using `Handler` to `AsyncTask`. The latter is great at what it does - asynchronous updates and handling of results in the main UI thread.  What&#39;s unclear to me is how to handle exceptions if something goes haywire in `AsyncTask#doInBackground`. 

The way I do it is to have an error Handler and send messages to it. It works fine, but is it the &quot;right&quot; approach or is there better alternative?

Also I understand that if I define the error Handler as an Activity field, it should execute in the UI thread.  However, sometimes (very unpredictably) I will get an Exception saying that  code triggered from `Handler#handleMessage` is executing on the wrong thread. Should I initialize error Handler in `Activity#onCreate` instead? Placing `runOnUiThread` into `Handler#handleMessage` seems redundant but it executes very reliably.

AsyncTask and error handling on Android

I want to be able to call the following method after a specified delay.
 In objective c there was something like:

    [self performSelector:@selector(DoSomething) withObject:nil afterDelay:5];

Is there an equivalent of this method in android with java?
For example I need to be able to call a method after 5 seconds.

    public void DoSomething()
    {
         //do something here
    }

How to call a method after a delay in Android

I have an issue with the following error in Android:

&gt; CalledFromWrongThreadException;: Only
&gt; the original thread that created a
&gt; view hierarchy can touch its views

It appears to happen when I try to update a Textview in my Activity, the call to update the TextView is from within my Activity but I still get the above error.

I have it like this:

onCreate()
-sets up the buttons and the text view.

onStateChange() - a listener for notifications about state changes, when this gets notification if changes the TextView to say some different text.

When I get notification of a new text I try to change the TextView as so:

    ((TextView)findViewById(R.id.title)).setText(&quot;Some Text&quot;);

But I get the above Error.

From googling it, it appears I should use a handler to change the TextView or maybe use AsyncTask?

Could anyone explain which one would be better to use and why?

**EDIT: ADDED CODE SNIPPETS:**

---

		 public void onCreate(Bundle savedInstanceState) {
		        super.onCreate(savedInstanceState);
		        
				requestWindowFeature(Window.FEATURE_CUSTOM_TITLE);	

				setContentView(R.layout.my);
		        
				getWindow().setFeatureInt(Window.FEATURE_CUSTOM_TITLE, R.layout.my_title);	

				((TextView)findViewById(R.id.time)).setText(&quot;Hello Text&quot;);

				
				findViewById(R.id.keyboardimage).setOnClickListener(new OnClickListener() {
		            public void onClick(View v) {
		            	
		            	Intent dialIntent = new Intent(Intent.ACTION_DIAL, Uri.parse(&quot;tel:&quot;));
		        		startActivity(dialIntent);
		        		
		                    dispatchKeyEvent(new KeyEvent(KeyEvent.ACTION_DOWN,KeyEvent.FLAG_SOFT_KEYBOARD));
		                    dispatchKeyEvent(new KeyEvent(KeyEvent.ACTION_UP, KeyEvent.KEYCODE_BACK));       
		            }
		    });
				
		 }


---

    //CallBacks from running Service

    private final ICallDialogActivity.Stub iCallDialogActivity = new ICallDialogActivity.Stub(){

    @Override
    public void onStateChanged(int callState)
					throws RemoteException {	
				switch(callState){
				case GlobalData.CALL_STATUS_IDLE:
					 
					break;
					
				case GlobalData.CALL_STATUS_DISCONNECTING:
					byeSetup();
					break;
        } 

    };

---

    public void byeSetup(){
		    	
		    	((TextView)findViewById(R.id.time)).setText(&quot;Bye Text&quot;);
		    	
		    	findViewById(R.id.keyboardimage).setOnClickListener(new OnClickListener() {
		            public void onClick(View v) {
		            	//Void the Button
		            }});
    }


CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch views

I have investigated this problem for months now, came up with different solutions to it, which I am not happy with since they are all massive hacks. I still cannot believe that a class that flawed in design made it into the framework and no-one is talking about it, so I guess I just must be missing something.

The problem is with `AsyncTask`. According to the documentation it 

&gt; &quot;allows to perform background
&gt; operations and publish results on the
&gt; UI thread without having to manipulate
&gt; threads and/or handlers.&quot;

The example then continues to show how some exemplary `showDialog()` method is called in `onPostExecute()`. This, however, seems **entirely contrived** to me, because showing a dialog always needs a reference to a valid `Context`, and an AsyncTask *must never hold a strong reference to a context object*.

The reason is obvious: what if the activity gets destroyed which triggered the task? This can happen all the time, e.g. because you flipped the screen. If the task would hold a reference to the context that created it, you&#39;re not only holding on to a useless context object (the window will have been destroyed and *any* UI interaction will fail with an exception!), you even risk creating a memory leak.

Unless my logic is flawed here, this translates to: `onPostExecute()` is entirely useless, because what good is it for this method to run on the UI thread if you don&#39;t have access to any context? You can&#39;t do anything meaningful here.

One workaround would be to not pass context instances to an AsyncTask, but a `Handler` instance. That works: since a Handler loosely binds the context and the task, you can exchange messages between them without risking a leak (right?). But that would mean that the premise of AsyncTask, namely that you don&#39;t need to bother with handlers, is wrong. It also seems like abusing Handler, since you are sending and receiving messages on the same thread (you create it on the UI thread and send through it in onPostExecute() which is also executed on the UI thread).

To top it all off, even with that workaround, you still have the problem that when the context gets destroyed, you have *no record* of the tasks it fired. That means that you have to re-start any tasks when re-creating the context, e.g. after a screen orientation change. This is slow and wasteful.

My solution to this (as [implemented in the Droid-Fu library][1]) is to maintain a mapping of `WeakReference`s from component names to their current instances on the unique application object. Whenever an AsyncTask is started, it records the calling context in that map, and on every callback, it will fetch the current context instance from that mapping. This ensures that you will never reference a stale context instance *and* you always have access to a valid context in the callbacks so you can do meaningful UI work there. It also doesn&#39;t leak, because the references are weak and are cleared when no instance of a given component exists anymore.

Still, it is a complex workaround and requires to sub-class some of the Droid-Fu library classes, making this a pretty intrusive approach.

**Now I simply want to know:** Am I just massively missing something or is AsyncTask really entirely flawed? How are your experiences working with it? How did you solve these problem?

Thanks for your input.


  [1]: https://github.com/mttkay/droid-fu/

Is AsyncTask really conceptually flawed or am I just missing something?

I have an **&quot;open&quot;** animation and am using `Handler.postDelayed(Runnable, delay)` to trigger a **&quot;close&quot;** animation after a short delay. However, during the time between open and close, there is possibly another animation triggered by a click.

My question is, how would I cancel the **&quot;close&quot;** animation in the handler?

How to remove a runnable from a handler object added by postDelayed?

What is the difference in meaning between &#39;semantics&#39; and &#39;syntax&#39;?  What are they?

Also, what&#39;s the difference between things like &quot;semantic website vs. normal website&quot;, &quot;semantic social networking vs. normal social networking&quot; etc.


Are semantics and syntax the same?

Which types of objects fall into the domain of &quot;subscriptable&quot;?

What does it mean if a Python object is &quot;subscriptable&quot; or not?

I quite often see on the Internet various complaints that other peoples examples of currying are not currying, but are actually just partial application.

I&#39;ve not found a decent explanation of what partial application is, or how it differs from currying. There seems to be a general confusion, with equivalent examples being described as currying in some places, and partial application in others.

Could someone provide me with a definition of both terms, and details of how they differ?

What is the difference between currying and partial application?

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Content Type	Original Author	Original Content on Stackoverflow
Question	tji	View Question on Stackoverflow
Solution 1 - Handler	sharkin	View Answer on Stackoverflow
Solution 2 - Handler	An̲̳̳drew	View Answer on Stackoverflow
Solution 3 - Handler	Javier	View Answer on Stackoverflow

What is a handler?

Handler Problem Overview

Handler Solutions

Solution 1 - Handler

Solution 2 - Handler

Solution 3 - Handler

How to stop java process gracefully?

WPF binding with StringFormat doesn't work on ToolTips

Attributions