안드로이드 스레드 2/10

2편은 있다가 적을 랬는데... 어씽크 테스크 정리를 해야 할 것 같아서.

뭐 안드로이드 디벨로퍼 사이트 가면 있으니 나도 복습차 좀 적어 본다.

뭐매 그건 그렇고 벌써 api버전이 16까지 나왔다. ㅡㅡ;

이젠 자바 버츄어 머신보다 안드로이드 달빅머신이 더 멋있게 보인다.

난 구글 빠돌이.

http://developer.android.com/reference/android/os/AsyncTask.html

에 가면 다음과 같은 것들이 있다.

public abstract class

AsyncTask 어쓍크 테스크

extends Object

java.lang.Object
↳	android.os.AsyncTask

Class Overview

---

AsyncTask enables proper and easy use of the UI thread. UI쓰레드를 겁나 쉽게 쓰게 해준다.

This class allows to perform background operations and publish results on the UI thread without having to manipulate threads and/or handlers. UI쓰레드 위에서 핸들러나 쓰레드 조종없이 쓸 수 있다. 결국 겁나 쉽다는 뜻.

AsyncTask is designed to be a helper class around Thread and Handler and does not constitute a generic threading framework. AsyncTasks should ideally be used for short operations (a few seconds at the most.) 

중요한 대목이다. 어씽크 테스트는 몇초안에 끝나는 작은 작업들을 위해 쓰여져야 한다는 것이다. 쓰레드나 핸들러 조작으로 만든거니께.

If you need to keep threads running for long periods of time, it is highly recommended you use the various APIs provided by the java.util.concurrent pacakge such as Executor, ThreadPoolExecutor and FutureTask.

뭐 겁나 긴 작업하는 쓰레드를 원하면 위에 표시된 링크를 클릭 자바 1.5부터 추가된 ExecutorService랑 같겠지. 사실 일전에 포스팅 했던 java.lang.Object 면 충분하겠지만.

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 1. onPreExecute, 2. doInBackground, 3. onProgressUpdate and 4. onPostExecute.

순서는 잘 봐둬야 한다.

Developer Guides

For more information about using tasks and threads, read the Processes and Threads developer guide.

Usage

---

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: 이그잼플 다음과 같이 쓰면 된다.1/10에서 포스팅 한 run() 처럼 걍 똑같이 적고 내용 채워주면 구현은 끝.

 private class DownloadFilesTask extends AsyncTask<URL, Integer, Long> {
     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));
             // Escape early if cancel() is called
             if (isCancelled()) break;
         }
         return totalSize;
     }

     protected void onProgressUpdate(Integer... progress) {
         setProgressPercent(progress[0]);
     }

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

Once created, a task is executed very simply:

쓸때는 요렇게 아.... 겁나 쉽다. 포스팅 끗. 나머지는 심심풀이로 읽으시길 바란다.

 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<Void, Void, Void> { ... }
 

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 ofonPostExecute(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 fromdoInBackground(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 AsyncTask class must be loaded on the UI thread. This is done automatically as of JELLY_BEAN.
• 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).

Order of execution

---

When first introduced, AsyncTasks were executed serially on a single background thread. Starting with DONUT, this was changed to a pool of threads allowing multiple tasks to operate in parallel. Starting with HONEYCOMB, tasks are executed on a single thread to avoid common application errors caused by parallel execution.

If you truly want parallel execution, you can invoke executeOnExecutor(java.util.concurrent.Executor, Object[]) with THREAD_POOL_EXECUTOR.

Summary

---

Nested Classes
enum	AsyncTask.Status	Indicates the current status of the task.

Fields
public static final Executor	SERIAL_EXECUTOR	An Executor that executes tasks one at a time in serial order.
public static final Executor	THREAD_POOL_EXECUTOR	An Executor that can be used to execute tasks in parallel.

Public Constructors
	AsyncTask() Creates a new asynchronous task.

Public Methods
final boolean	cancel(boolean mayInterruptIfRunning) Attempts to cancel execution of this task.
static void	execute(Runnable runnable) Convenience version of execute(Object) for use with a simple Runnable object.
final AsyncTask	execute(Params... params) Executes the task with the specified parameters.
final AsyncTask	executeOnExecutor(Executor exec, Params... params) Executes the task with the specified parameters.
final Result	get(long timeout, TimeUnit unit) Waits if necessary for at most the given time for the computation to complete, and then retrieves its result.
final Result	get() Waits if necessary for the computation to complete, and then retrieves its result.
final AsyncTask.Status	getStatus() Returns the current status of this task.
final boolean	isCancelled() Returns true if this task was cancelled before it completed normally.

Protected Methods
abstract Result	doInBackground(Params... params) Override this method to perform a computation on a background thread.
void	onCancelled(Result result) Runs on the UI thread after cancel(boolean) is invoked and doInBackground(Object[]) has finished.
void	onCancelled() Applications should preferably override onCancelled(Object).
void	onPostExecute(Result result) Runs on the UI thread after doInBackground(Params...).
void	onPreExecute() Runs on the UI thread before doInBackground(Params...).
void	onProgressUpdate(Progress... values) Runs on the UI thread after publishProgress(Progress...) is invoked.
final void	publishProgress(Progress... values) This method can be invoked from doInBackground(Params...) to publish updates on the UI thread while the background computation is still running.

[Expand] Inherited Methods
From class java.lang.Object

Fields

---

public static final Executor SERIAL_EXECUTOR

Since: API Level 11

An Executor that executes tasks one at a time in serial order. This serialization is global to a particular process.

public static final Executor THREAD_POOL_EXECUTOR

Since: API Level 11

An Executor that can be used to execute tasks in parallel.

Public Constructors

---

public AsyncTask ()

Since: API Level 3

Creates a new asynchronous task. This constructor must be invoked on the UI thread.

Public Methods

---

public final boolean cancel (boolean mayInterruptIfRunning)

Since: API Level 3

Attempts to cancel execution of this task. This attempt will fail if the task has already completed, already been cancelled, or could not be cancelled for some other reason. If successful, and this task has not started when cancel is called, this task should never run. If the task has already started, then the mayInterruptIfRunning parameter determines whether the thread executing this task should be interrupted in an attempt to stop the task.

Calling this method will result in onCancelled(Object) being invoked on the UI thread after doInBackground(Object[]) returns. Calling this method guarantees that onPostExecute(Object) is never invoked. After invoking this method, you should check the value returned by isCancelled() periodically from doInBackground(Object[]) to finish the task as early as possible.

Parameters

mayInterruptIfRunning	true if the thread executing this task should be interrupted; otherwise, in-progress tasks are allowed to complete.

Returns

• false if the task could not be cancelled, typically because it has already completed normally; true otherwise

See Also

• isCancelled()
• onCancelled(Object)

public static void execute (Runnable runnable)

Since: API Level 11

Convenience version of execute(Object) for use with a simple Runnable object. See execute(Object[]) for more information on the order of execution.

See Also

• execute(Object[])
• executeOnExecutor(java.util.concurrent.Executor, Object[])

public final AsyncTask execute (Params... params)

Since: API Level 3

Executes the task with the specified parameters. The task returns itself (this) so that the caller can keep a reference to it.

Note: this function schedules the task on a queue for a single background thread or pool of threads depending on the platform version. When first introduced, AsyncTasks were executed serially on a single background thread. Starting with DONUT, this was changed to a pool of threads allowing multiple tasks to operate in parallel. Starting HONEYCOMB, tasks are back to being executed on a single thread to avoid common application errors caused by parallel execution. If you truly want parallel execution, you can use the executeOnExecutor(Executor, Params...) version of this method with THREAD_POOL_EXECUTOR; however, see commentary there for warnings on its use.

This method must be invoked on the UI thread.

Parameters

params	The parameters of the task.

Returns

• This instance of AsyncTask.

Throws

IllegalStateException	If getStatus() returns either RUNNING or FINISHED.

See Also

• executeOnExecutor(java.util.concurrent.Executor, Object[])
• execute(Runnable)

public final AsyncTask executeOnExecutor (Executor exec, Params... params)

Since: API Level 11

Executes the task with the specified parameters. The task returns itself (this) so that the caller can keep a reference to it.

This method is typically used with THREAD_POOL_EXECUTOR to allow multiple tasks to run in parallel on a pool of threads managed by AsyncTask, however you can also use your own Executor for custom behavior.

Warning: Allowing multiple tasks to run in parallel from a thread pool is generally not what one wants, because the order of their operation is not defined. For example, if these tasks are used to modify any state in common (such as writing a file due to a button click), there are no guarantees on the order of the modifications. Without careful work it is possible in rare cases for the newer version of the data to be over-written by an older one, leading to obscure data loss and stability issues. Such changes are best executed in serial; to guarantee such work is serialized regardless of platform version you can use this function with SERIAL_EXECUTOR.

This method must be invoked on the UI thread.

Parameters

exec	The executor to use. THREAD_POOL_EXECUTOR is available as a convenient process-wide thread pool for tasks that are loosely coupled.
params	The parameters of the task.

Returns

• This instance of AsyncTask.

Throws

IllegalStateException	If getStatus() returns either RUNNING or FINISHED.

See Also

• execute(Object[])

public final Result get (long timeout, TimeUnit unit)

Since: API Level 3

Waits if necessary for at most the given time for the computation to complete, and then retrieves its result.

Parameters

timeout	Time to wait before cancelling the operation.
unit	The time unit for the timeout.

Returns

• The computed result.

Throws

CancellationException	If the computation was cancelled.
ExecutionException	If the computation threw an exception.
InterruptedException	If the current thread was interrupted while waiting.
TimeoutException	If the wait timed out.

public final Result get ()

Since: API Level 3

Waits if necessary for the computation to complete, and then retrieves its result.

Returns

• The computed result.

Throws

CancellationException	If the computation was cancelled.
ExecutionException	If the computation threw an exception.
InterruptedException	If the current thread was interrupted while waiting.

public final AsyncTask.Status getStatus ()

Since: API Level 3

Returns the current status of this task.

Returns

• The current status.

public final boolean isCancelled ()

Since: API Level 3

Returns true if this task was cancelled before it completed normally. If you are calling cancel(boolean) on the task, the value returned by this method should be checked periodically from doInBackground(Object[]) to end the task as soon as possible.

Returns

• true if task was cancelled before it completed

See Also

• cancel(boolean)

Protected Methods

---

protected abstract Result doInBackground (Params... params)

Since: API Level 3

Override this method to perform a computation on a background thread. The specified parameters are the parameters passed to execute(Params...) by the caller of this task. This method can callpublishProgress(Progress...) to publish updates on the UI thread.

Parameters

params	The parameters of the task.

Returns

• A result, defined by the subclass of this task.

See Also

• onPreExecute()
• onPostExecute(Result)
• publishProgress(Progress...)

protected void onCancelled (Result result)

Since: API Level 11

Runs on the UI thread after cancel(boolean) is invoked and doInBackground(Object[]) has finished.

The default implementation simply invokes onCancelled() and ignores the result. If you write your own implementation, do not call super.onCancelled(result).

Parameters

result	The result, if any, computed in doInBackground(Object[]), can be null

See Also

• cancel(boolean)
• isCancelled()

protected void onCancelled ()

Since: API Level 3

Applications should preferably override onCancelled(Object). This method is invoked by the default implementation of onCancelled(Object).

Runs on the UI thread after cancel(boolean) is invoked and doInBackground(Object[]) has finished.

See Also

• onCancelled(Object)
• cancel(boolean)
• isCancelled()

protected void onPostExecute (Result result)

Since: API Level 3

Runs on the UI thread after doInBackground(Params...). The specified result is the value returned by doInBackground(Params...).

This method won't be invoked if the task was cancelled.

Parameters

result	The result of the operation computed by doInBackground(Params...).

See Also

• onPreExecute()
• doInBackground(Params...)
• onCancelled(Object)

protected void onPreExecute ()

Since: API Level 3

Runs on the UI thread before doInBackground(Params...).

See Also

• onPostExecute(Result)
• doInBackground(Params...)

protected void onProgressUpdate (Progress... values)

Since: API Level 3

Runs on the UI thread after publishProgress(Progress...) is invoked. The specified values are the values passed to publishProgress(Progress...).

Parameters

values	The values indicating progress.

See Also

• publishProgress(Progress...)
• doInBackground(Params...)

protected final void publishProgress (Progress... values)

Since: API Level 3

This method can be invoked from doInBackground(Params...) to publish updates on the UI thread while the background computation is still running. Each call to this method will trigger the execution ofonProgressUpdate(Progress...) on the UI thread. onProgressUpdate(Progress...) will note be called if the task has been canceled.

Parameters

values	The progress values to update the UI with.

See Also

• onProgressUpdate(Progress...)
• doInBackground(Params...)

Except as noted, this content is licensed under Apache 2.0. For details and restrictions, see the Content License.

Android 4.1 r1 - 26 Jul 2012 18:49

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