[Note]Android Threads, Handlers and AsyncTask

先看過Processes and Threads會有比較清楚的概念

當Adnroid Application 啟動後, 系統會建一個主要的thread 稱 "main thread" or "UI thread", 所有的components 皆跑在這個UI thread, system calls 也是透過UI thread dispatched給各個component, ex: onKeyDown, touch event.

UI thread 如因大量運算或等待而blocked, 預設超過5秒ANR(Application Not Responding)就會發生.

且Android UI components 並非thread-safe, 使用上要特別小心.

所以:

1. long time computation使用另外的thread, 不要寫在 UI Thread.

2. 不要在UI thread 之外使用UI component method.

透過Thread, Handler and AsyncTask perform asynchronous processing, 避免UI thread block.

* Threads

Android 提供以下的method, 可在其它的thread 下調用 UI thread.

Activity.runOnUiThread(Runnable)
View.post(Runnable) <-- used in example code.
View.postDelayed(Runnable, long)

或是使用Handler or AsyncTasks class 達到同樣的效果.

* Handler
有2個主要用途:

(1) message scheduling,  post action at specific point.

(2) 將其它thread發出來的action 放入message queue中, 避免race condition.

 

處理message 需要override handleMessage(), 透過sendMessage() or sendEmptyMessage() method.

執行Runnanble則是使用post() method.

一個Activity 只需有一個Handler instance.

* AsyncTask
使用AsyncTask必需繼承它, 且override doInBackground() method.

4 steps:

呼叫execute() 開始執行, 之後onPerExecute()接著自動被呼叫, 通常用來 initial status of task.

接著doInBackground() 被調用, 一般為long time computation時使用.

call publishProgress()後會調用onProgressUpdate(), onProgressUpdate()調用於UI thread, 一般用來update progress bar, UI animate.

publishProgress() trigger時間點是無法預期的.

doInBackground() 結束後會trigger onPostExecute()調用於UI thread, 用來返回結果.

 

* Thread, Handler 在Avtivity 結束後 thread就結束, 但AsyncTack則否.

Ans: 

Internally, AsyncTask uses fixed thread pool.
(See http://developer.android.com/reference/java/util/concurrent/ThreadPoolExecutor.html)

So, even if AsyncTask finished, thread does not die. But thread in thread pool can be killed.

 

Examples

Layout XML file:

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"

    android:layout_width="match_parent"

    android:layout_height="match_parent"

    android:orientation="vertical" >

    <Button

        android:id="@+id/button1"

        android:layout_width="match_parent"

        android:layout_height="wrap_content"

        android:text="@string/run" />

    <ProgressBar

        android:id="@+id/progressBar1"

        style="?android:attr/progressBarStyleHorizontal"

        android:layout_width="match_parent"

        android:layout_height="60dp" />

</LinearLayout>

 

Thread example code:

package com.samchen.samdemos.threads;

import java.io.IOException;

import java.net.URL;

import java.net.URLConnection;

import android.app.Activity;

import android.graphics.Bitmap;

import android.graphics.BitmapFactory;

import android.opengl.GLSurfaceView;

import android.os.Bundle;

import android.os.SystemClock;

import android.util.Log;

import android.view.View;

import android.widget.Button;

import android.widget.ProgressBar;

import android.widget.Toast;

public class DemoThreads extends Activity {

    final boolean isThread = true;

    final String TAG = "DemoThreads";

    private ProgressBar mProgress;

    private int mProgressStatus = 0;

    boolean isRunning = false;

   

    /** Called when the activity is first created. */

    @Override

    public void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        setContentView(R.layout.demothreads);

        Button btnRun = (Button) findViewById(R.id.button1);

        mProgress = (ProgressBar) findViewById(R.id.progressBar1);

        btnRun.setOnClickListener(new Button.OnClickListener() {

            @Override

            public void onClick(View arg0) {

                if(isRunning){

                    Log.d(TAG,"thread is runing");

                    Toast.makeText(getApplicationContext(),

                            getString(R.string.running),

                            Toast.LENGTH_SHORT).show();

                    return;

                }else

                    isRunning = true;

               

                if (isThread) {

                    // thread example

                    new Thread(new Runnable() {

                        public void run() {

                            while (mProgressStatus < 100) {

                                // update the bar status

                                mProgress.post(new Runnable() {

                                    public void run() {

                                        mProgress.setProgress(mProgressStatus);

                                        Log.d(TAG, "update mProgressStatus : "

                                                + mProgressStatus);

                                    }

                                });

                                // do something long

                                SystemClock.sleep(500);

                                mProgressStatus++;

                            }

                        }

                    }).start();

                } else {

                    // non-thread example

                    while (mProgressStatus < 100) {

                        // update the bar status

                        mProgress.post(new Runnable() {

                            public void run() {

                                mProgress.setProgress(mProgressStatus);

                            }

                        });

                        // do something long

                        //SystemClock.sleep(500);

                        //if use SystemClock.sleep, system will be crash.

                        try{

                            Thread.sleep(500);

                        }catch (InterruptedException e){

                            Log.d(TAG,"InterruptedException!!!!!!!");

                            e.printStackTrace();

                        }

                        mProgressStatus++;

                       

                    }

                }

            }

        });

    }

}

Handler example:

package com.samchen.samdemos.threads;

import java.io.IOException;

import java.net.URL;

import java.net.URLConnection;

import android.app.Activity;

import android.graphics.Bitmap;

import android.graphics.BitmapFactory;

import android.opengl.GLSurfaceView;

import android.os.Bundle;

import android.os.Handler;

import android.os.SystemClock;

import android.util.Log;

import android.view.View;

import android.widget.Button;

import android.widget.ProgressBar;

import android.widget.Toast;

public class DemoHandler extends Activity {

    final String TAG = "DemoHandler";

    private ProgressBar mProgress;

    private int mProgressStatus = 0;

    private Handler mHandler;

    boolean isRunning = false;

    /** Called when the activity is first created. */

    @Override

    public void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        setContentView(R.layout.demothreads);

        Button btnRun = (Button) findViewById(R.id.button1);

        mProgress = (ProgressBar) findViewById(R.id.progressBar1);

        mHandler = new Handler();

        btnRun.setOnClickListener(new Button.OnClickListener() {

            @Override

            public void onClick(View arg0) {

                if (!isRunning) {

                    startProgress(mProgress);

                    isRunning = true;

                } else{

                    Log.d(TAG, "thread is runing");

                    Toast.makeText(getApplicationContext(),

                            getString(R.string.running),

                            Toast.LENGTH_SHORT).show();

                }

            }

        });

    }

    public void startProgress(View view) {

        // Do something long

        Runnable runnable = new Runnable() {

            @Override

            public void run() {

                for (mProgressStatus = 0; mProgressStatus <= 100; mProgressStatus++) {

                    //final int value = i;

                    SystemClock.sleep(500);

                   

                    mHandler.post(new Runnable() {

                        @Override

                        public void run() {

                            Log.d(TAG, "update mProgressStatus : "

                                    + mProgressStatus);

                            mProgress.setProgress(mProgressStatus);

                        }

                    });

                }

            }

        };

        new Thread(runnable).start();

    }

}

AsyncTask example:

package com.samchen.samdemos.threads;

import android.app.Activity;

import android.opengl.GLSurfaceView;

import android.os.AsyncTask;

import android.os.Bundle;

import android.os.Handler;

import android.os.SystemClock;

import android.util.Log;

import android.view.View;

import android.widget.Button;

import android.widget.ProgressBar;

import android.widget.Toast;

public class DemoAsyncTask extends Activity {

    final String TAG = "DemoAsyncTask";

    private ProgressBar mProgress;

    private int mProgressStatus = 0;

    boolean isRunning = false;

    private updateTask mTask;

    /** Called when the activity is first created. */

    @Override

    public void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        setContentView(R.layout.demothreads);

        mProgress = (ProgressBar) findViewById(R.id.progressBar1);

        Button btnRun = (Button) findViewById(R.id.button1);

        btnRun.setOnClickListener(new Button.OnClickListener() {

            @Override

            public void onClick(View arg0) {

                if (!isRunning) {

                    mTask = new updateTask();

                    mTask.execute();

                    isRunning = true;

                } else {

                    Log.d(TAG, "thread is runing");

                    Toast.makeText(getApplicationContext(),

                            getString(R.string.running), Toast.LENGTH_SHORT)

                            .show();

                }

            }

        });

    }

    private class updateTask extends AsyncTask<Void, Integer, Void> {

        protected void onPostExecute(Void result) {

           

        }

        protected void onProgressUpdate(Integer... value) {

            Log.d(TAG, "onProgressUpdate : " + value);

            mProgress.setProgress(value[0]);

        }

        protected void onPreExecute() {

            // TODO Auto-generated method stub

            mProgressStatus = 0;

        }

        protected Void doInBackground(Void... params) {

            // TODO Auto-generated method stub

            while (mProgressStatus < 100) {

                Log.d(TAG, "doInBackground mProgressStatus : "

                        + mProgressStatus);

                // do something long

                publishProgress(mProgressStatus);

                SystemClock.sleep(500);

                mProgressStatus++;

            }

            return null;

        }

    }

}

Ref:

http://www.vogella.com/articles/AndroidPerformance/article.html

http://android-er.blogspot.tw/2010/11/progressbar-running-in-asynctask.html

http://stackoverflow.com/questions/10688878/android-how-to-reconnect-to-asynctask-after-ondestroy-and-relaunch-oncreate