Android實戰技巧:多線程AsyncTask

Understanding AsyncTask
AsyncTask是Android 1.5 Cubake加入的用於實現異步操作的一個類,在此之前隻能用Java SE庫中的Thread來實現多線程異步,AsyncTask是Android平臺自己的異步工具,融入瞭Android平臺的特性,讓異步操作更加的安全,方便和實用。實質上它也是對Java SE庫中Thread的一個封裝,加上瞭平臺相關的特性,所以對於所有的多線程異步都強烈推薦使用AsyncTask,因為它考慮,也融入瞭Android平臺的特性,更加的安全和高效。
AsyncTask可以方便的執行異步操作(doInBackground),又能方便的與主線程進行通信,它本身又有良好的封裝性,可以進行取消操作(cancel())。關於AsyncTask的使用,文檔說的很明白,下面直接上實例。

實例
這個實例用AsyncTask到網絡上下載圖片,同時顯示進度,下載完圖片更新UI。

[java]
package com.hilton.effectiveandroid.concurrent; 
 
import java.io.IOException; 
import java.io.InputStream; 
import java.io.OutputStream; 
import java.net.HttpURLConnection; 
import java.net.MalformedURLException; 
import java.net.URL; 
 
import android.app.Activity; 
import android.content.Context; 
import android.graphics.Bitmap; 
import android.graphics.BitmapFactory; 
import android.os.AsyncTask; 
import android.os.Bundle; 
import android.os.SystemClock; 
import android.view.View; 
import android.widget.Button; 
import android.widget.ImageView; 
import android.widget.ProgressBar; 
 
import com.hilton.effectiveandroid.R; 
 
/*
 * AsyncTask cannot be reused, i.e. if you have executed one AsyncTask, you must discard it, you cannot execute it again.
 * If you try to execute an executed AsyncTask, you will get "java.lang.IllegalStateException: Cannot execute task: the task is already running"
 * In this demo, if you click "get the image" button twice at any time, you will receive "IllegalStateException".
 * About cancellation:
 * You can call AsyncTask#cancel() at any time during AsyncTask executing, but the result is onPostExecute() is not called after
 * doInBackground() finishes, which means doInBackground() is not stopped. AsyncTask#isCancelled() returns true after cancel() getting
 * called, so if you want to really cancel the task, i.e. stop doInBackground(), you must check the return value of isCancelled() in
 * doInBackground, when there are loops in doInBackground in particular.
 * This is the same to Java threading, in which is no effective way to stop a running thread, only way to do is set a flag to thread, and check
 * the flag every time in Thread#run(), if flag is set, run() aborts.
 */ 
public class AsyncTaskDemoActivity extends Activity { 
    private static final String ImageUrl = "http://up.aiwalls.com/2012/0515/20120515100913728.jpg"; 
    private ProgressBar mProgressBar; 
    private ImageView mImageView; 
    private Button mGetImage; 
    private Button mAbort; 
     
    @Override 
    public void onCreate(Bundle icicle) { 
    super.onCreate(icicle); 
    setContentView(R.layout.async_task_demo_activity); 
    mProgressBar = (ProgressBar) findViewById(R.id.async_task_progress); 
    mImageView = (ImageView) findViewById(R.id.async_task_displayer); 
    final ImageLoader loader = new ImageLoader(); 
    mGetImage = (Button) findViewById(R.id.async_task_get_image); 
    mGetImage.setOnClickListener(new View.OnClickListener() { 
        public void onClick(View v) { 
        loader.execute(ImageUrl); 
        } 
    }); 
    mAbort = (Button) findViewById(R.id.asyc_task_abort); 
    mAbort.setOnClickListener(new View.OnClickListener() { 
        public void onClick(View v) { 
        loader.cancel(true); 
        } 
    }); 
    mAbort.setEnabled(false); 
    } 
     
    private class ImageLoader extends AsyncTask<String, Integer, Bitmap> { 
    private static final String TAG = "ImageLoader"; 
 
    @Override 
    protected void onPreExecute() { 
        // Initialize progress and image 
        mGetImage.setEnabled(false); 
        mAbort.setEnabled(true); 
        mProgressBar.setVisibility(View.VISIBLE); 
        mProgressBar.setProgress(0); 
        mImageView.setImageResource(R.drawable.icon); 
    } 
     
    @Override 
    protected Bitmap doInBackground(String… url) { 
        /*
         * Fucking ridiculous thing happened here, to use any Internet connections, either via HttpURLConnection
         * or HttpClient, you must declare INTERNET permission in AndroidManifest.xml. Otherwise you will get
         * "UnknownHostException" when connecting or other tcp/ip/http exceptions rather than "SecurityException"
         * which tells you need to declare INTERNET permission.
         */ 
        try { 
        URL u; 
        HttpURLConnection conn = null; 
        InputStream in = null; 
        OutputStream out = null; 
        final String filename = "local_temp_image"; 
        try { 
            u = new URL(url[0]); 
            conn = (HttpURLConnection) u.openConnection(); 
            conn.setDoInput(true); 
            conn.setDoOutput(false); 
            conn.setConnectTimeout(20 * 1000); 
            in = conn.getInputStream(); 
            out = openFileOutput(filename, Context.MODE_PRIVATE); 
            byte[] buf = new byte[8196]; 
            int seg = 0; 
            final long total = conn.getContentLength(); 
            long current = 0; 
            /*
             * Without checking isCancelled(), the loop continues until reading whole image done, i.e. the progress
             * continues go up to 100. But onPostExecute() will not be called.
             * By checking isCancelled(), we can stop immediately, i.e. progress stops immediately when cancel() is called.
             */ 
            while (!isCancelled() && (seg = in.read(buf)) != -1) { 
            out.write(buf, 0, seg); 
            current += seg; 
            int progress = (int) ((float) current / (float) total * 100f); 
            publishProgress(progress); 
            SystemClock.sleep(1000); 
            } 
        } finally { 
            if (conn != null) { 
            conn.disconnect(); 
            } 
            if (in != null) { 
            in.close(); 
            } 
            if (out != null) { 
            out.close(); 
            } 
        } 
        return BitmapFactory.decodeFile(getFileStreamPath(filename).getAbsolutePath()); 
        } catch (MalformedURLException e) { 
        e.printStackTrace(); 
        } catch (IOException e) { 
        e.printStackTrace(); 
        } 
        return null; 
    } 
     
    @Override 
    protected void onProgressUpdate(Integer… progress) { 
        mProgressBar.setProgress(progress[0]); 
    } 
     
    @Override 
    protected void onPostExecute(Bitmap image) { 
        if (image != null) { 
        mImageView.setImageBitmap(image); 
        } 
        mProgressBar.setProgress(100); 
        mProgressBar.setVisibility(View.GONE); 
        mAbort.setEnabled(false); 
    } 
    } 

運行結果
 
先後順序分別是下載前,下載中和下載後
 
 

總結
關於怎麼使用看文檔和這個例子就夠瞭,下面說下,使用時的註意事項:

1. AsyncTask對象不可重復使用,也就是說一個AsyncTask對象隻能execute()一次,否則會有異常拋出"java.lang.IllegalStateException: Cannot execute task: the task is already running"
2. 在doInBackground()中要檢查isCancelled()的返回值,如果你的異步任務是可以取消的話。
cancel()僅僅是給AsyncTask對象設置瞭一個標識位,當調用瞭cancel()後,發生的事情隻有:AsyncTask對象的標識位變瞭,和doInBackground()執行完成後,onPostExecute()不會被回調瞭,而doInBackground()和onProgressUpdate()還是會繼續執行直到doInBackground()結束。所以要在doInBackground()中不斷的檢查isCancellled()的返回值,當其返回true時就停止執行,特別是有循環的時候。如上面的例子,如果把讀取數據的isCancelled()檢查去掉,圖片還是會下載,進度也一直會走,隻是最後圖片不會放到UI上(因為onPostExecute()沒被回調)!
這裡的原因其實很好理解,想想Java SE的Thread吧,是沒有方法將其直接Cacncel掉的,那些線程取消也無非就是給線程設置標識位,然後在run()方法中不斷的檢查標識而已。
3. 如果要在應用程序中使用網絡,一定不要忘記在AndroidManifest中聲明INTERNET權限,否則會報出很詭異的異常信息,比如上面的例子,如果把INTERNET權限拿掉會拋出"UnknownHostException"。剛開始很疑惑,因為模擬器是可以正常上網的,後來Google瞭下才發現原來是沒權限,但是疑問還是沒有消除,既然沒有聲明網絡權限,為什麼不直接提示無網絡權限呢?

對比Java SE的Thread
Thread是非常原始的類,它隻有一個run()方法,一旦開始,無法停止,它僅適合於一個非常獨立的異步任務,也即不需要與主線程交互,對於其他情況,比如需要取消或與主線程交互,都需添加額外的代碼來實現,並且還要註意同步的問題。

而AsyncTask是封裝好瞭的,可以直接拿來用,如果你僅執行獨立的異步任務,可以僅實現doInBackground()。

所以,當有一個非常獨立的任務時,可以考慮使用Thread,其他時候,盡可能的用AsyncTask。

 

摘自 浪人的星空

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