Wednesday, February 22, 2012

Anroid PDF Widget - As on Date 02/22/2012

As of 02/22/2012 there is no widget/Android View available in Android SDK which could simply render/display a .pdf file in an standalone app. The alternative way one can work are:

1. If its only about viewing - Then Install a 3rd party PDF Viewer App like that of Adobe's PDF Viewer or any other similar PDF Viewing application and then from within your app with the help of Intents launch the file by giving a URI in an OnClick Listener() which essentially will look up for any applications present on the device with a capability of displaying a PDF.

Make sure to catch the exception and display appropriate message to the user, in the event when there are no PDF Viewing applications present on the device.

2. Saving PDF's locally in a standalone app - Get the PDF data in the form of bytes, store it in a persistent database and then form the pdf and launch for display(the way it has been described in the first point).

3. Or write an application similar to PDF Viewing application as one by Adobe's.

Note - iOS although has a capability of displaying a .pdf file in a webview and renders just fine. However upon launching a PDF in an Android Webview or a WebBrowser, a user ends up downloading it to the mobile device's disk space instead of displaying it. This is just how the Android Browser/Webview behaves in the case of PDF's.

Activities Lifecycle Overlap/ Coordinating activities

When one activity starts another, they both experience lifecycle transitions. The first activity pauses and stops (though, it won't stop if it's still visible in the background), while the other activity is created. In case these activities share data saved to disc or elsewhere, it's important to understand that the first activity is not completely stopped before the second one is created. Rather, the process of starting the second one overlaps with the process of stopping the first one.

The order of lifecycle callbacks is well defined, particularly when the two activities are in the same process and one is starting the other. Here's the order of operations that occur when Activity A starts Acivity B:

1. Activity A's onPause() method executes.
2. Activity B's onCreate(), onStart(), and onResume() methods execute in sequence. (Activity B now has user focus.)
3. Then, if Activity A is no longer visible on screen, its onStop() method executes.

This predictable sequence of lifecycle callbacks allows you to manage the transition of information from one activity to another. For example, if you must write to a database when the first activity stops so that the following activity can read it, then you should write to the database during onPause() instead of during onStop().

Android Runtime Concepts/Dalvik VM/Linux Kernel/ Application Fundamentals

Android is a software stack for mobile devices that includes an operating system, middleware and key applications. The Android SDK provides the tools and APIs necessary to begin developing applications on the Android platform using the Java programming language


1. Application framework enabling reuse and replacement of components
2. Dalvik virtual machine optimized for mobile devices
3. Integrated browser based on the open source WebKit engine
4. Optimized graphics powered by a custom 2D graphics library; 3D graphics based on the OpenGL ES 1.0 specification (hardware acceleration optional)
5. SQLite for structured data storage
6. Media support for common audio, video, and still image formats (MPEG4, H.264, MP3, AAC, AMR, JPG, PNG, GIF)
7. GSM Telephony (hardware dependent)
8. Bluetooth, EDGE, 3G, and WiFi (hardware dependent)
9. Camera, GPS, compass, and accelerometer (hardware dependent)
10. Rich development environment including a device emulator, tools for debugging, memory and performance profiling, and a plugin for the Eclipse IDE


Android includes a set of C/C++ libraries used by various components of the Android system. These capabilities are exposed to developers through the Android application framework. Some of the core libraries are listed below:

System C library - a BSD-derived implementation of the standard C system library (libc), tuned for embedded Linux-based devices
Media Libraries - based on PacketVideo's OpenCORE; the libraries support playback and recording of many popular audio and video formats, as well as static image files, including MPEG4, H.264, MP3, AAC, AMR, JPG, and PNG
Surface Manager - manages access to the display subsystem and seamlessly composites 2D and 3D graphic layers from multiple applications
LibWebCore - a modern web browser engine which powers both the Android browser and an embeddable web view
SGL - the underlying 2D graphics engine
3D libraries - an implementation based on OpenGL ES 1.0 APIs; the libraries use either hardware 3D acceleration (where available) or the included, highly optimized 3D software rasterizer
FreeType - bitmap and vector font rendering
SQLite - a powerful and lightweight relational database engine available to all applications

Android Runtime/Dalvik VM

Android includes a set of core libraries that provides most of the functionality available in the core libraries of the Java programming language.

Every Android application runs in its own process, with its own instance of the Dalvik virtual machine. Dalvik has been written so that a device can run multiple VMs efficiently. The Dalvik VM executes files in the Dalvik Executable (.dex) format which is optimized for minimal memory footprint. The VM is register-based, and runs classes compiled by a Java language compiler that have been transformed into the .dex format by the included "dx" tool.

The Dalvik VM relies on the Linux kernel for underlying functionality such as threading and low-level memory management.

Linux Kernel

Android relies on Linux version 2.6 for core system services such as security, memory management, process management, network stack, and driver model. The kernel also acts as an abstraction layer between the hardware and the rest of the software stack.

Application Fundamentals

Android applications are written in the Java programming language. The Android SDK tools compile the code—along with any data and resource files—into an Android package, an archive file with an .apk suffix. All the code in a single .apk file is considered to be one application and is the file that Android-powered devices use to install the application.

Once installed on a device, each Android application lives in its own security sandbox:

- The Android operating system is a multi-user Linux system in which each application is a different user.
- By default, the system assigns each application a unique Linux user ID (the ID is used only by the system and is unknown to the application). The system sets permissions for all the files in an application so that only the user ID assigned to that application can access them.
- Each process has its own virtual machine (VM), so an application's code runs in isolation from other applications.
- By default, every application runs in its own Linux process. Android starts the process when any of the application's components need to be executed, then shuts down the process when it's no longer needed or when the system must recover memory for other applications.

In this way, the Android system implements the principle of least privilege. That is, each application, by default, has access only to the components that it requires to do its work and no more. This creates a very secure environment in which an application cannot access parts of the system for which it is not given permission.

However, there are ways for an application to share data with other applications and for an application to access system services:

- It's possible to arrange for two applications to share the same Linux user ID, in which case they are able to access each other's files. To conserve system resources, applications with the same user ID can also arrange to run in the same Linux process and share the same VM (the applications must also be signed with the same certificate).
- An application can request permission to access device data such as the user's contacts, SMS messages, the mountable storage (SD card), camera, Bluetooth, and more. All application permissions must be granted by the user at install time.

That covers the basics regarding how an Android application exists within the system. The rest is:

- The core framework components that define your application.
- The manifest file in which you declare components and required device features for your application.
- Resources that are separate from the application code and allow your application to gracefully optimize its behavior for a variety of device configurations.

- Android applications are composed of one or more application components (activities, services, content providers, and broadcast receivers)
- Each component performs a different role in the overall application behavior, and each one can be activated individually (even by other applications)
- The manifest file must declare all components in the application and should also declare all application requirements, such as the minimum version of Android required and any hardware configurations required
- Non-code application resources (images, strings, layout files, etc.) should include alternatives for different device configurations (such as different strings for different languages and different layouts for different screen sizes)

Service killed by Android System, Notifications/Toasts via a Service, Stopping a Service manually

Android Service killed by system:

In a scenario where one is implementing a Service, and where that Service is required to run forever, the question comes what if the System itself destroys/kills the Service started by the app in extreme conditions - Answer to such a case is below -

The Android system will force-stop a service only when memory is low and it must recover system resources for the activity that has user focus. If the service is bound to an activity that has user focus, then it's less likely to be killed, and if the service is declared to run in the foreground (discussed later), then it will almost never be killed. Otherwise, if the service was started and is long-running, then the system will lower its position in the list of background tasks over time and the service will become highly susceptible to killing—if your service is started, then you must design it to gracefully handle restarts by the system. If the system kills your service, it restarts it as soon as resources become available again (though this also depends on the value you return from onStartCommand(), as discussed later).

The onStartCommand() method must return an integer. The integer is a value that describes how the system should continue the service in the event that the system kills it (as discussed above, the default implementation for IntentService handles this for you, though you are able to modify it). The return value from onStartCommand() must be one of the following constants:

If the system kills the service after onStartCommand() returns, do not recreate the service, unless there are pending intents to deliver. This is the safest option to avoid running your service when not necessary and when your application can simply restart any unfinished jobs.

If the system kills the service after onStartCommand() returns, recreate the service and call onStartCommand(), but do not redeliver the last intent. Instead, the system calls onStartCommand() with a null intent, unless there were pending intents to start the service, in which case, those intents are delivered. This is suitable for media players (or similar services) that are not executing commands, but running indefinitely and waiting for a job.

If the system kills the service after onStartCommand() returns, recreate the service and call onStartCommand() with the last intent that was delivered to the service. Any pending intents are delivered in turn. This is suitable for services that are actively performing a job that should be immediately resumed, such as downloading a file.

ForeGround Services:

A foreground service is a service that's considered to be something the user is actively aware of and thus not a candidate for the system to kill when low on memory. A foreground service must provide a notification for the status bar, which is placed under the "Ongoing" heading, which means that the notification cannot be dismissed unless the service is either stopped or removed from the foreground.

For example, a music player that plays music from a service should be set to run in the foreground, because the user is explicitly aware of its operation. The notification in the status bar might indicate the current song and allow the user to launch an activity to interact with the music player.

To request that your service run in the foreground, call startForeground(). This method takes two parameters: an integer that uniquely identifies the notification and the Notification for the status bar.

To remove the service from the foreground, call stopForeground(). This method takes a boolean, indicating whether to remove the status bar notification as well. This method does not stop the service. However, if you stop the service while it's still running in the foreground, then the notification is also removed.

Sending Notifications to the User

Once running, a service can notify the user of events using Toast Notifications or Status Bar Notifications.

A toast notification is a message that appears on the surface of the current window for a moment then disappears, while a status bar notification provides an icon in the status bar with a message, which the user can select in order to take an action (such as start an activity).

Usually, a status bar notification is the best technique when some background work has completed (such as a file completed downloading) and the user can now act on it. When the user selects the notification from the expanded view, the notification can start an activity (such as to view the downloaded file).

Stopping a Service manually:

A started service must manage its own lifecycle. That is, the system does not stop or destroy the service unless it must recover system memory and the service continues to run after onStartCommand() returns. So, the service must stop itself by calling stopSelf() or another component can stop it by calling stopService().

Once requested to stop with stopSelf() or stopService(), the system destroys the service as soon as possible.

However, if your service handles multiple requests to onStartCommand() concurrently, then you shouldn't stop the service when you're done processing a start request, because you might have since received a new start request (stopping at the end of the first request would terminate the second one). To avoid this problem, you can use stopSelf(int) to ensure that your request to stop the service is always based on the most recent start request. That is, when you call stopSelf(int), you pass the ID of the start request (the startId delivered to onStartCommand()) to which your stop request corresponds. Then if the service received a new start request before you were able to call stopSelf(int), then the ID will not match and the service will not stop.

Caution: It's important that your application stops its services when it's done working, to avoid wasting system resources and consuming battery power. If necessary, other components can stop the service by calling stopService(). Even if you enable binding for the service, you must always stop the service yourself if it ever received a call to onStartCommand().

Note: Although a started service is stopped by a call to either stopSelf() or stopService(), there is not a respective callback for the service (there's no onStop() callback). So, unless the service is bound to a client, the system destroys it when the service is stopped—onDestroy() is the only callback received.

Also, keep in mind that any service, no matter how it's started, can potentially allow clients to bind to it. So, a service that was initially started with onStartCommand() (by a client calling startService()) can still receive a call to onBind() (when a client calls bindService()).

Tuesday, February 21, 2012

Android Service v/s Thread

When to use a Service or a Worker Thread in Android:

Basics about a Service: A Service is an application component that can perform long-running operations in the background and does not provide a user interface. Another application component can start a service and it will continue to run in the background even if the user switches to another application. Additionally, a component can bind to a service to interact with it and even perform interprocess communication (IPC). For example, a service might handle network transactions, play music, perform file I/O, or interact with a content provider, all from the background.

A service can essentially take two forms:


A service is "started" when an application component (such as an activity) starts it by calling startService(). Once started, a service can run in the background indefinitely, even if the component that started it is destroyed. Usually, a started service performs a single operation and does not return a result to the caller. For example, it might download or upload a file over the network. When the operation is done, the service should stop itself.


A service is "bound" when an application component binds to it by calling bindService(). A bound service offers a client-server interface that allows components to interact with the service, send requests, get results, and even do so across processes with interprocess communication (IPC). A bound service runs only as long as another application component is bound to it. Multiple components can bind to the service at once, but when all of them unbind, the service is destroyed.
Although this documentation generally discusses these two types of services separately, your service can work both ways—it can be started (to run indefinitely) and also allow binding. It's simply a matter of whether you implement a couple callback methods: onStartCommand() to allow components to start it and onBind() to allow binding.

However an IntentService works in a different way and spawns a worker thread:(details below):

Traditionally, there are two classes you can extend to create a started service:


This is the base class for all services. When you extend this class, it's important that you create a new thread in which to do all the service's work, because the service uses your application's main thread, by default, which could slow the performance of any activity your application is running.


This is a subclass of Service that uses a worker thread to handle all start requests, one at a time. This is the best option if you don't require that your service handle multiple requests simultaneously. All you need to do is implement onHandleIntent(), which receives the intent for each start request so you can do the background work.

The following sections describe how you can implement your service using either one for these classes.

Extending the IntentService class

Because most started services don't need to handle multiple requests simultaneously (which can actually be a dangerous multi-threading scenario), it's probably best if you implement your service using the IntentService class.

The IntentService does the following:

1. Creates a default worker thread that executes all intents delivered to onStartCommand() separate from your application's main thread.

2. Creates a work queue that passes one intent at a time to your onHandleIntent() implementation, so you never have to worry about multi-threading.

3. Stops the service after all start requests have been handled, so you never have to call stopSelf().

4. Provides default implementation of onBind() that returns null.

5. Provides a default implementation of onStartCommand() that sends the intent to the work queue and then to your onHandleIntent() implementation.

All this adds up to the fact that all you need to do is implement onHandleIntent() to do the work provided by the client. (Though, you also need to provide a small constructor for the service.)

Service V/s Thread

A service is simply a component that can run in the background even when the user is not interacting with your application. Thus, you should create a service only if that is what you need.

If you need to perform work outside your main thread, but only while the user is interacting with your application, then you should probably instead create a new thread and not a service. For example, if you want to play some music, but only while your activity is running, you might create a thread in onCreate(), start running it in onStart(), then stop it in onStop(). Also consider using AsyncTask or HandlerThread, instead of the traditional Thread class.

Remember that if you do use a service, it still runs in your application's main thread by default, so you should still create a new thread within the service if it performs intensive or blocking operations.

Caution: A service runs in the main thread of its hosting process—the service does not create its own thread and does not run in a separate process (unless you specify otherwise). This means that, if your service is going to do any CPU intensive work or blocking operations (such as MP3 playback or networking), you should create a new thread within the service to do that work. By using a separate thread, you will reduce the risk of Application Not Responding (ANR) errors and the application's main thread can remain dedicated to user interaction with your activities.

Handling Runtime Changes in Android

Handling Runtime Changes:

Some device configurations can change during runtime (such as screen orientation, keyboard availability, and language). When such a change occurs, Android restarts the running Activity (onDestroy() is called, followed by onCreate()). The restart behavior is designed to help your application adapt to new configurations by automatically reloading your application with alternative resources that match the new device configuration.

To properly handle a restart, it is important that your activity restores its previous state through the normal Activity lifecycle, in which Android calls onSaveInstanceState() before it destroys your activity so that you can save data about the application state. You can then restore the state during onCreate() or onRestoreInstanceState().

To test that your application restarts itself with the application state intact, you should invoke configuration changes (such as changing the screen orientation) while performing various tasks in your application. Your application should be able to restart at any time without loss of user data or state in order to handle events such as configuration changes or when the user receives an incoming phone call and then returns to your application much later after your application process may have been destroyed. To learn how you can restore your activity state, read about the Activity lifecycle.

However, you might encounter a situation in which restarting your application and restoring significant amounts of data can be costly and create a poor user experience. In such a situation, you have two other options:

1. Retain an object during a configuration change
Allow your activity to restart when a configuration changes, but carry a stateful Object to the new instance of your activity.

2. Handle the configuration change yourself
Prevent the system from restarting your activity during certain configuration changes, but receive a callback when the configurations do change, so that you can manually update your activity as necessary.

Retaining an Object During a Configuration Change:

If restarting your activity requires that you recover large sets of data, re-establish a network connection, or perform other intensive operations, then a full restart due to a configuration change might be a slow user experience. Also, it might not be possible for you to completely restore your activity state with the Bundle that the system saves for you with the onSaveInstanceState() callback—it is not designed to carry large objects (such as bitmaps) and the data within it must be serialized then deserialized, which can consume a lot of memory and make the configuration change slow. In such a situation, you can alleviate the burden of reinitializing your activity by retaining a stateful Object when your activity is restarted due to a configuration change.

To retain an object during a runtime configuration change:

1. Override the onRetainNonConfigurationInstance() method to return the object you would like to retain.

2. When your activity is created again, call getLastNonConfigurationInstance() to recover your object.
When the Android system shuts down your activity due to a configuration change, it calls onRetainNonConfigurationInstance() between the onStop() and onDestroy() callbacks. In your implementation of onRetainNonConfigurationInstance(), you can return any Object that you need in order to efficiently restore your state after the configuration change.

A scenario in which this can be valuable is if your application loads a lot of data from the web. If the user changes the orientation of the device and the activity restarts, your application must re-fetch the data, which could be slow. What you can do instead is implement onRetainNonConfigurationInstance() to return an object carrying your data and then retrieve the data when your activity starts again with getLastNonConfigurationInstance(). For example:

public Object onRetainNonConfigurationInstance() {
    final MyDataObject data = collectMyLoadedData();
    return data;

Caution: While you can return any object, you should never pass an object that is tied to the Activity, such as a Drawable, an Adapter, a View or any other object that's associated with a Context. If you do, it will leak all the views and resources of the original activity instance. (Leaking resources means that your application maintains a hold on them and they cannot be garbage-collected, so lots of memory can be lost.)

Then retrieve the data when your activity starts again:

public void onCreate(Bundle savedInstanceState) {

    final MyDataObject data = (MyDataObject) getLastNonConfigurationInstance();
    if (data == null) {
        data = loadMyData();

In this case, getLastNonConfigurationInstance() returns the data saved by onRetainNonConfigurationInstance(). If data is null (which happens when the activity starts due to any reason other than a configuration change) then this code loads the data object from the original source.

Handling the Configuration Change Yourself:

If your application doesn't need to update resources during a specific configuration change and you have a performance limitation that requires you to avoid the activity restart, then you can declare that your activity handles the configuration change itself, which prevents the system from restarting your activity.

Note: Handling the configuration change yourself can make it much more difficult to use alternative resources, because the system does not automatically apply them for you. This technique should be considered a last resort when you must avoid restarts due to a configuration change and is not recommended for most applications.

To declare that your activity handles a configuration change, edit the appropriate element in your manifest file to include the android:configChanges attribute with a value that represents the configuration you want to handle. Possible values are listed in the documentation for the android:configChanges attribute (the most commonly used values are "orientation" to prevent restarts when the screen orientation changes and "keyboardHidden" to prevent restarts when the keyboard availability changes). You can declare multiple configuration values in the attribute by separating them with a pipe | character.

For example, the following manifest code declares an activity that handles both the screen orientation change and keyboard availability change:

activity android:name=".MyActivity"

Now, when one of these configurations change, MyActivity does not restart. Instead, the MyActivity receives a call to onConfigurationChanged(). This method is passed a Configuration object that specifies the new device configuration. By reading fields in the Configuration, you can determine the new configuration and make appropriate changes by updating the resources used in your interface. At the time this method is called, your activity's Resources object is updated to return resources based on the new configuration, so you can easily reset elements of your UI without the system restarting your activity.

Caution: Beginning with Android 3.2 (API level 13), the "screen size" also changes when the device switches between portrait and landscape orientation. Thus, if you want to prevent runtime restarts due to orientation change when developing for API level 13 or higher (as declared by the minSdkVersion and targetSdkVersion attributes), you must include the "screenSize" value in addition to the "orientation" value. That is, you must decalare android:configChanges="orientation|screenSize". However, if your application targets API level 12 or lower, then your activity always handles this configuration change itself (this configuration change does not restart your activity, even when running on an Android 3.2 or higher device).

For example, the following onConfigurationChanged() implementation checks the current device orientation:

public void onConfigurationChanged(Configuration newConfig) {

    // Checks the orientation of the screen
    if (newConfig.orientation == Configuration.ORIENTATION_LANDSCAPE) {
        Toast.makeText(this, "landscape", Toast.LENGTH_SHORT).show();
    } else if (newConfig.orientation == Configuration.ORIENTATION_PORTRAIT){
        Toast.makeText(this, "portrait", Toast.LENGTH_SHORT).show();

The Configuration object represents all of the current configurations, not just the ones that have changed. Most of the time, you won't care exactly how the configuration has changed and can simply re-assign all your resources that provide alternatives to the configuration that you're handling. For example, because the Resources object is now updated, you can reset any ImageViews with setImageResource() and the appropriate resource for the new configuration is used (as described in Providing Resources).

Notice that the values from the Configuration fields are integers that are matched to specific constants from the Configuration class. For documentation about which constants to use with each field, refer to the appropriate field in the Configuration reference.

Remember: When you declare your activity to handle a configuration change, you are responsible for resetting any elements for which you provide alternatives. If you declare your activity to handle the orientation change and have images that should change between landscape and portrait, you must re-assign each resource to each element during onConfigurationChanged().

If you don't need to update your application based on these configuration changes, you can instead not implement onConfigurationChanged(). In which case, all of the resources used before the configuration change are still used and you've only avoided the restart of your activity. However, your application should always be able to shutdown and restart with its previous state intact, so you should not consider this technique an escape from retaining your state during normal activity lifecycle. Not only because there are other configuration changes that you cannot prevent from restarting your application, but also because you should handle events such as when the user leaves your application and it gets destroyed before the user returns to it.

For more about which configuration changes you can handle in your activity, see the android:configChanges documentation and the Configuration class.

AsyncTask Class Overview

AsyncTask Class Overview

AsyncTask enables proper and easy use of the UI thread. This class allows to perform background operations and publish results on the UI thread without having to manipulate threads and/or handlers.

An asynchronous task is defined by a computation that runs on a background thread and whose result is published on the UI thread. An asynchronous task is defined by 3 generic types, called Params, Progress and Result, and 4 steps, called onPreExecute, doInBackground, onProgressUpdate and onPostExecute.


AsyncTask must be subclassed to be used. The subclass will override at least one method (doInBackground(Params...)), and most often will override a second one (onPostExecute(Result).)

Here is an example of subclassing:

private class DownloadFilesTask extends AsyncTask {
     protected Long doInBackground(URL... urls) {
         int count = urls.length;
         long totalSize = 0;
         for (int i = 0; i < count; i++) {
             totalSize += Downloader.downloadFile(urls[i]);
             publishProgress((int) ((i / (float) count) * 100));
         return totalSize;

     protected void onProgressUpdate(Integer... progress) {

     protected void onPostExecute(Long result) {
         showDialog("Downloaded " + result + " bytes");

Once created, a task is executed very simply:

new DownloadFilesTask().execute(url1, url2, url3);

AsyncTask's generic types:

The three types used by an asynchronous task are the following:

1. Params, the type of the parameters sent to the task upon execution.
2. Progress, the type of the progress units published during the background computation.
3. Result, the type of the result of the background computation.

Not all types are always used by an asynchronous task. To mark a type as unused, simply use the type Void:

private class MyTask extends AsyncTask { ... }

The 4 steps:

When an asynchronous task is executed, the task goes through 4 steps:

1. onPreExecute(), invoked on the UI thread immediately after the task is executed. This step is normally used to setup the task, for instance by showing a progress bar in the user interface.

2. doInBackground(Params...), invoked on the background thread immediately after onPreExecute() finishes executing. This step is used to perform background computation that can take a long time. The parameters of the asynchronous task are passed to this step. The result of the computation must be returned by this step and will be passed back to the last step. This step can also use publishProgress(Progress...) to publish one or more units of progress. These values are published on the UI thread, in the onProgressUpdate(Progress...) step.

3. onProgressUpdate(Progress...), invoked on the UI thread after a call to publishProgress(Progress...). The timing of the execution is undefined. This method is used to display any form of progress in the user interface while the background computation is still executing. For instance, it can be used to animate a progress bar or show logs in a text field.

4. onPostExecute(Result), invoked on the UI thread after the background computation finishes. The result of the background computation is passed to this step as a parameter.

Cancelling a task:

A task can be cancelled at any time by invoking cancel(boolean). Invoking this method will cause subsequent calls to isCancelled() to return true. After invoking this method, onCancelled(Object), instead of onPostExecute(Object) will be invoked after doInBackground(Object[]) returns. To ensure that a task is cancelled as quickly as possible, you should always check the return value of isCancelled() periodically from doInBackground(Object[]), if possible (inside a loop for instance.)

Threading rules:

There are a few threading rules that must be followed for this class to work properly:

- The task instance must be created on the UI thread.
- execute(Params...) must be invoked on the UI thread.
- Do not call onPreExecute(), onPostExecute(Result), doInBackground(Params...), onProgressUpdate(Progress...) manually.
- The task can be executed only once (an exception will be thrown if a second execution is attempted.)

Memory observability:

AsyncTask guarantees that all callback calls are synchronized in such a way that the following operations are safe without explicit synchronizations.

- Set member fields in the constructor or onPreExecute(), and refer to them in doInBackground(Params...).
- Set member fields in doInBackground(Params...), and refer to them in onProgressUpdate(Progress...) and onPostExecute(Result).