Only one thread can manipulate the user interface,
e.g., call
setText
to write text in a
TextView.
The thread that does this is called the
UI thread.
It will usually be the main thread,
i.e., the first thread that is created.
To let another thread manipulate the UI indirectly,
we passed
Messages
to and from a
Handler
here.
Another way to let another thread manipulate the UI indirectly
is to create a subclass of the generic class
AsyncTask.
An
AsyncTask
with a
doInBackground
method is like a
Thread
with a
run
method.
The
AsyncTask
eliminates the need for the
Handler
and
Message;
see
Processes
and Threads
and
Painless
Threading.
doInBackground
is executed by the non-main thread.
Every other method of the
AsyncTask,
and in fact every other method of the entire app,
is executed by the main thread.
Do not call
doInBackground,
onProgressUpdate,
or
onPostExecute
yourself.
Allow them to be called automatically.
doInBackground
should check during each loop to see if the non-main thread has been
cancelled,
since the main thread could cancel it
at any time.
The three data types in the
<angle brackets>
of
AsyncTask
must be classes,
not built-in types such as
int
or
double.
All three are used by the method
doInBackground.
The dots
...
execute
method of the
AsyncTask,
or
Void
if there are no arguments.
These arguments are then passed to the
doInBackground
method of the
AsyncTask.doInBackground
passes to
publishProgress.
These arguments are then passed to
onProgressUpdate.doInBackground.
This return value is then passed to
onPostExecute.