AsyncTask Callback Methods Udacity English (auto-generated) English

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a sync task is unlike all the other classes that we've used before all the

AsyncTask is unlike all the other classes that we've used before.

other classes like activity or view

All the other classes,

perform their work on the main thread

like activity or view, perform their work on the main thread.

yeah with acing tasks however some parts

>> Yeah, with AsyncTask, however,

of the class are run on the main thread in the foreground and other parts of the

some parts of the class are run on the main thread in the foreground.

tasks are run on a separate thread which

And other parts of the task are run on a separate thread,

is obviously in the background yes

which is obviously in the background.

so async tasks deals with more than one

>> Yes, so AsyncTask deals with more than one thread.

thread so let's walk through this together step by step async task is an

So let's walk through this together step by step.

abstract class you first learned about

AsyncTask is an abstract class.

abstract classes from the last course

You first learned about abstract classes from the last course with Jessica when

with Jessica when you were building the Miwok language app an abstract class

you were building the Miwok language app.

contains some methods that are left

An abstract class contains some methods that are left blank for

blank for us as developers to override and provide our own implementations to

us as developers to override and provide our own implementations.

use an async task we'll need to make a custom subclass we'll call this one

To use an AsyncTask, we'll need to make a custom subclass.

earthquake async task the subclass is

We'll call this one EarthquakeAsyncTask.

required to override the method due in background from our earlier

The subclass is required to override the method doInBackground().

demonstration whatever work you want to

>From our earlier demonstration, whatever work you want to happen in

happen in the background thread should be included in the do and background method once you've got the earthquake

the background thread should be included in the doInBackground() method.

data from the server you probably want

Once you've got the earthquake data from the server,

to update the UI but to update the UI you must be on the main thread not a background thread so how do you

you probably want to update the UI. But to update the UI you must be on the main thread, not a background thread.

communicate between the main and background threads you can override a

So how do you communicate between the main and background threads?

method an async task called on post

You can override a method in AsyncTask called onPostExecute().

execute this method runs on the main thread which is why I highlighted it in

This method runs on the main thread which is why I highlighted it in green.

green this method will be called after the background work has done which is

This method will be called after the background work is done,

why it's called onpostexecute if I

which is why it's called onPostExecute().

reveal more details about the methods we

If I reveal more details about the methods,

can see that the doing background method returns a result that result is passed in as the input to the onpostexecute method if the result of the do and

we can see that the doInBackground() method returns a result. That result is passed in as the input to the onPostExecute() method.

background method is a list of earthquakes from the server then an on

If the result of the doInBackground() method is a list of earthquakes from

post execute that list of data now has

the server, then in onPostExecute() that list of data now has been migrated back

been migrated back to the main thread where we can update the ListView in the

to the main thread where we can update the ListView in the app to

app to display the new data if you

display the new data.

need to there is also an async task method named on pre execute which is

If you need to, there is also an AsyncTask method named OnPreExecut()

called on the main thread before the

which is called on the main thread before the background work is started,

background work has started hence the name on pre execute developers will

hence the name onPreExecute().

override this method if there's

Developers will override this method if there's something that needs to be done

something that needs to be done before preparing to do the background work

before preparing to do the background work.

sometimes the background work can take a really long time some developers may

Sometimes the background work can take a really long time.

want to update the user about the

Some developers may want to update the user about the progress in

progress in the background task for

the background task.

example an app may download a photo from

For example, an app may download a photo from the Internet,

the internet and may display a progress bar in the meantime within the doing

it may display a progress bar in the meantime.

background method we can periodically call the publish progress method and

Within the doInBackground() method, we can periodically call

pass in the new progress value then we

the publishProgress method and pass on the new progress value.

can override the on progress update

Then we can override the onProgressUpdate method which occurs on

method which occurs on the main thread which can be used to update the UI with the new progress values which is 72% in

the main thread, which can be used to update the UI with a new progress value,

this case let's look at the

which is 72% in this case.

documentation for the async task class

Let's look at the documentation for the AsyncTask class.

we can scroll down to the methods

We can scroll down to the methods section and

section and you can see that doing background is listed as being an

you can see that doInBackground is listed as being an abstract method.

abstract method so we are required to implement this method we also recognize

So we are required to implement this method.

other methods like on pre execute on

We also recognize other methods like onPreExecute(),

progress update and on post execute

onProgressUpdate and onPostExecute.

these methods are not abstract so it's

These methods are not abstract, so it's not required for

not required for us to implement these methods scrolling back up we can also

us to implement these methods.

find this code example of a class called

Scrolling back up, we can also find this code example of a class called

download files tasks that subclasses async tasks we'll talk about this

DownloadFilesTask that subclasses AsyncTask.

example in more detail later but the

We'll talk about this example in more detail later.

main gist is that in the doing

But the main gist is that in the doInBackground() method, a file is

background method a file is downloaded which can take a long time so it's done on a background thread

downloaded, which can take a long time, so it's done on a background thread.

note that the input in the doing background method can be a list of items

Note that the input in the doInBackground() method can be a list of

to process which in this case is a list of files to download progress is

items to process, which in this case, is a list of files to download.

reported incrementally and we can update

Progress is reported incrementally and

the screen with the percentage progress

we can update the screen with the percentage progress so far.

so far after the background work is completed we show a dialog with how many bytes of

After the background work is completed, we show a dialogue with how many

information were downloaded successfully

bytes of information were downloaded successfully.

now in this quiz I want you to fill in this table about the most common

Now in this quiz, I want you to fill in this table about the most common

callback methods an async task describe

callback methods in AsyncTask.

what each method is called in your own words then indicate whether

Describe what each method is called in your own words, then indicate whether

method is run on the main thread or on a

the method is run on the main thread or on a background thread.

background thread you can check the link below the quiz if you need help

You can check the link below the quiz if you need help.

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