Threads, Part II

8/12/2019 Threads, Part II

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Android ProgrammingLecture 20:

Threads, Part II

11/16/2011


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Handler ExampleIn this example, GUI thread

doesnt count button is responsive;

I can trigger interleaved counting

threads


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Other Uses of Handler

Handler also

supports a

postDelayed(

Runnable runnable,

long delayInMillis)

method Allows delay

before executes

Shuttle bus app reads bus id and delay from

saved SharedPreferences, then starts a loop

(below) of updates at the designated rate

loops by spawning itself again


4/18

CityApp Example

In the CityApp, it seems reasonable that we could pushthe network (web service) and parsing componentsinto a separate thread

Should make GUI much more responsive

However, for the CityApp, once the network query hasbeen started, we dont necessarily need to do anythingelse as a user

It would be nice to have some feedback about progressthough Repeated use ofpostmethod in Handler - Fetching data, Parsing

data, messages????


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Runnables Runnable as used in the example are pretty static

we cant parameterize them in meaningful ways

Could arguably create classes that implementRunnable interface, and the classes could beparameterized

On next slide, I take the Runnables from a fewslides back that updated the GUI with

Start/Done messages and make themparameterized so that I can write any message


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Parameterized Runnable Example

Before After


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AsyncTask

Android supports a fairly simple mechanismthat simplifies/hides the process of working

directly with threads, theAsyncTaskclass

(asynchronous task)

One extends the AsyncTask class and

implements several methods

doInBackground do the actual work (required)

onProgressUpdate report progress (optional)

onPostExecute report results (optional)

onPreExecute initialize (optional)


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AsyncTasks

Put your seatbelt on

AsyncTasks are conceptually straight-forward,

but syntactically challenging

Well start with an example and think of itconceptually


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Using AsyncTask

Lets lookat an

example:

counting

to ten

million

again


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Varargs

AsyncTask makes use of varargs variablenumber of arguments to a function

C/C++ printf:

printf(Hello!);

printf(Hello %d, x);printf(Hello %d %d, x, y)

Same function, takes arbitrary number ofparameters as final argument


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VarargsIn Java, write function header as follows:

void f(type parameter)

{ code }

meaning functionfcan take an arbitrary number ofparameters of type type (but only that type, whichis different from C/C++)

Can be passed as a comma separated seriesof values or as an array

Only allowed for last parameter


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AsyncTask

Remember: One extends the AsyncTask classand implements several methods

doInBackground do the actual work (required)

onProgressUpdate report progress (optional) onPostExecute report results (optional)

onPreExecute initialize (optional)


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AsyncTask In defining class, use extends AsyncTaskproviding as comma separated

class parametersthe following in this order: Single class type of input parameter(s) to be used in doInBackground

function

Single class type used to report progress Usually Integer

Single class type used as result value from task, which must match returnvalue type from doInBackground

If your function doesnt need parameters, indicate a type of Void (notVoid)

class MyTask extends AsyncTask means canuse String parameters for doInBackgroundand progress andcompletion reporting will use ints (Integer class)

Write AsyncTask extension as an inner class of Activity class so has

access to Activity GUI components


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Implementing AsyncTask

Writeprotected ReturnTypedoInBackground(ParameterType params)

Not thread-safe (dont change GUI directly)

Must return a value of ReturnType

Can process parameters using an array or iteration

Array:

for (j = 0; j < params.length; j++) { do work }

Iteration:

for (String x: params) { }

If want to update progress, callpublishProgress

method with appropriate arguments

Android passes this off to your onProgressUpdate method


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Implementing AsyncTask

Writeprotected void onProgressUpdate(Typeprogress)

Is thread-safe (can change GUI directly)

Update progress bar???

Writeprotected void onPostExecute(Type

result)

Is thread-safe (can change GUI directly)


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Using AsyncTask

Declare and construct an instance of yourAsyncTask subclass

Call the executemethod on that instance, sending

in a comma separated list of values of the type

that doInBackgroundtakes as parameters

If no parameters to be sent, use (Void[]) null

A few last rules:

The task must be allocated, and executed on the mainGUI thread

Any allocated task can only be run once

Internal methods should not be called directly


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Using AsyncTask

Lets lookat an

example:

counting

to ten

million

again


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Using AsyncTask with Progress Updates

Added onProgressUpdate method and

publishProgress callMultiple counting tasks

executing (2nd, 3rdinterleaved)

Threads, Part II

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