Android基础——异步消息处理

异步消息处理

  • 异步消息处理是什么?
  • 子线程更新UI
  • 处理机制
  • AsyncTask介绍
  • AsyncTask使用

异步消息处理是什么?

异步消息处理解决了子线程进行UI操作的问题,如必须在子线程执行耗时操作,根据返回结果更新响应的UI控件。

子线程更新UI

修改activity_main,当点击按钮时,TextView的内容会改变:




    

修改MainActivity,通过Handler和Message实现在按钮点击事件中开启子线程更新UI:

public class MainActivity extends AppCompatActivity {

    public static final int UPDATE_TEXT = 1;
    private TextView textView;
    private Handler handler = new Handler() {
        public void handleMessage(Message msg) {
            switch (msg.what) {
                case UPDATE_TEXT:
                    textView.setText("hello java");
                    break;
                default:
                    break;
            }
        }
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        textView = findViewById(R.id.unChange_text);
        Button change = findViewById(R.id.change);
        change.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                new Thread(new Runnable() {
                    @Override
                    public void run() {
                        Message message = new Message();
                        message.what = UPDATE_TEXT;
                        handler.sendMessage(message);
                    }
                }).start();
            }
        });
    }

}

处理机制

  • Message:在线程间传递消息,可携带少量信息
  • Handler:利用sendMessage()发送信息,handleMessage()处理信息
  • MessageQueue:存放所有Handler发送的消息,等待被处理,线程独有
  • Looper:通过loop()方法将MessagesQueue中的Message取出传递到handleMessage()方法中,线程独有
    Android基础——异步消息处理_第1张图片

AsyncTask介绍

AsyncTask是基于异步消息处理机制的封装包,方便我们更容易进行子线程和主线程之间的切换。

通过继承AsyncTask,指定其3个泛型参数:

  • Params:传入的参数
  • Progress:用于显示当前进度的类型
  • Result:用于指定返回值类型

重写其4个方法:

  • onPreExecute():在后台任务执行前调用,用于初始化
  • doInBackgroud(Params…):里面的代码会在子线程运行,在这里处理耗时操作,结束后通过return返回结果。方法中不可以进行UI操作,要更新UI调用publisProgress(Params…)传递参数
  • onProgressUpdate(Params…):调用publisProgress(Params…)后,随后就会调用这个方法,其参数为publisProgress(Params…)中的Params,可在此方法中更新UI
  • onPostExecute(Result):当后台任务执行完并通过return返回时,随后就会调用这个方法,其参数为doInBackgroud()返回值,可在此方法中更新UI并进行收尾工作

AsyncTask使用

接下来将实现一个下载例子,添加okhttp依赖:

implementation 'com.squareup.okhttp3:okhttp:4.9.0'

添加权限:


 

新建回调接口用于对下载过程监听:

public interface DownloadListener {
    void onProgress(int progress);
    void onSuccess();
    void onFailed();
    void onPaused();
    void onCanceled();
}

新建DownloadTask继承AsyncTask:

  • doInBackground()处理下载逻辑,需要判断文件是否存在,是否断点续传
  • onProgressUpdate()中更新进度条
  • onPostExecute()对下载结果回调
public class DownloadTask extends AsyncTask {
    public static final int TYPE_SUCCESS = 0;
    public static final int TYPE_FAILED = 1;
    public static final int TYPE_PAUSED = 2;
    public static final int TYPE_CANCELED = 3;

    private DownloadListener listener;
    private boolean isCanceled = false;
    private boolean isPaused = false;
    private  int lastProgress;
    public DownloadTask(DownloadListener listener) {
        this.listener = listener;
    }
    @Override
    protected Integer doInBackground(String... params) {
        InputStream is = null;
        RandomAccessFile saveFile = null;
        File file = null;
        try {
            long downloadedLength = 0;  //记录已下载的文件长度
            String downloadUrl = params[0];
            String fileName = downloadUrl.substring(downloadUrl.lastIndexOf("/"));
            String directory = Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS).getPath();
            file = new File(directory + fileName);
            if(file.exists()){
                downloadedLength = file.length();
            }
            long contentLength = getContentLength(downloadUrl);  //文件总字节
            if(contentLength == 0){
                return TYPE_FAILED;
            }else if(contentLength == downloadedLength){
                return TYPE_SUCCESS;
            }
            OkHttpClient client = new OkHttpClient();
            Request request = new Request.Builder()
                    .addHeader("RANGE", "bytes=" + downloadedLength + "-")
                    .url(downloadUrl)
                    .build();
            Response response = client.newCall(request).execute();
            if(request != null){
                is = response.body().byteStream();
                saveFile = new RandomAccessFile(file, "rw");
                saveFile.seek(downloadedLength);
                byte[] b = new byte[1024];
                int total = 0;
                int len;
                while((len = is.read(b)) != -1){
                    if(isCanceled){
                        return TYPE_CANCELED;
                    }else if(isPaused){
                        return TYPE_PAUSED;
                    }else {
                        total += len;
                        saveFile.write(b, 0, len);
                        int progress = (int) ((total + downloadedLength) * 100 / contentLength);
                        publishProgress(progress);
                    }
                }
                response.body().close();
                return TYPE_SUCCESS;
            }
        }catch (Exception e){
            e.printStackTrace();
        }finally {
            try {
                if(is != null){
                    is.close();
                }
                if(saveFile != null){
                    saveFile.close();
                }
                if(isCanceled && file != null){
                    file.delete();
                }
            }catch (Exception e){
                e.printStackTrace();
            }
        }
        return TYPE_FAILED;
    }
    @Override
    protected void onProgressUpdate(Integer... values) {
        int progress = values[0];
        if(progress > lastProgress){
            listener.onProgress(progress);
            lastProgress = progress;
        }
    }
    @Override
    protected void onPostExecute(Integer status) {
        switch (status){
            case TYPE_SUCCESS:
                listener.onSuccess();
                break;
            case TYPE_FAILED:
                listener.onFailed();
                break;
            case TYPE_PAUSED:
                listener.onPaused();
                break;
            case TYPE_CANCELED:
                listener.onCanceled();
                break;
            default:
                break;
        }
    }

    public void  pauseDownload(){
        isPaused = true;
    }
    public void  cancelDownload(){
        isCanceled = true;
    }
    private long getContentLength(String downloadUrl) throws IOException {
        OkHttpClient client = new OkHttpClient();
        Request request = new Request.Builder()
                .url(downloadUrl)
                .build();
        Response response = client.newCall(request).execute();
        if(response != null && response.isSuccessful()){
            long contentLength = response.body().contentLength();
            response.body().close();
            return contentLength;
        }
        return 0;
    }
}

新建DownloadService继承Service,在回调接口处理通知栏的显示,通过Binder暴露给Activity调用:

public class DownloadService extends Service {
    private DownloadTask downloadTask;

    private int mProgress;
    private String downloadUrl;
    private DownloadListener listener = new DownloadListener() {
        @Override
        public void onProgress(int progress) {
            mProgress=progress;
            getNotificationManager().notify(1, getNotification("Downloading...", progress));
        }

        @Override
        public void onSuccess() {
            downloadTask = null;
            stopForeground(true);
            getNotificationManager().notify(1, getNotification("Download Success", -1));
            Toast.makeText(DownloadService.this, "Download Success", Toast.LENGTH_SHORT).show();
        }

        @Override
        public void onFailed() {
            downloadTask = null;
            stopForeground(true);
            getNotificationManager().notify(1, getNotification("Download Failed", -1));
            Toast.makeText(DownloadService.this, "Download Failed", Toast.LENGTH_SHORT).show();
        }

        @Override
        public void onPaused() {
            downloadTask = null;
            stopForeground(true);
            getNotificationManager().notify(1, getNotification("Download Pausing", mProgress));
            Toast.makeText(DownloadService.this, "Paused", Toast.LENGTH_SHORT).show();
        }

        @Override
        public void onCanceled() {
            downloadTask = null;
            stopForeground(true);
            Toast.makeText(DownloadService.this, "Canceled", Toast.LENGTH_SHORT).show();
        }
    };

    private DownloadBinder mBinder = new DownloadBinder();

    @Override
    public IBinder onBind(Intent intent) {
        return mBinder;
    }

    class DownloadBinder extends Binder {
        public void startDownload(String url){
            if(downloadTask == null){
                downloadUrl = url;
                downloadTask =new DownloadTask(listener);
                downloadTask.execute(downloadUrl);
                startForeground(1, getNotification("Downloading...",0));
                Toast.makeText(DownloadService.this, "Downloading...", Toast.LENGTH_SHORT).show();
            }
        }

        public void pauseDownload(){
            if(downloadTask != null){
                downloadTask.pauseDownload();
            }
        }
        public void cancelDownload(){
            if(downloadTask != null){
                downloadTask.cancelDownload();
            }
            if(downloadUrl != null){
                String fileName = downloadUrl.substring(downloadUrl.lastIndexOf("/"));
                String directory = Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS).getPath();
                File file =new File(directory + fileName);
                if(file.exists()){
                    file.delete();
                }
                getNotificationManager().cancel(1);
                stopForeground(true);
                Toast.makeText(DownloadService.this, "Canceled", Toast.LENGTH_SHORT).show();
            }
        }
    }

    private NotificationManager getNotificationManager() {
        return (NotificationManager) getSystemService(NOTIFICATION_SERVICE);
    }

    private Notification getNotification(String title, int progress) {
        Intent intent = new Intent(this, MainActivity.class);
        PendingIntent pi = PendingIntent.getActivity(this, 0, intent, 0);
        Notification.Builder builder = new Notification.Builder(this);
        builder.setSmallIcon(R.mipmap.ic_launcher);
        builder.setLargeIcon(BitmapFactory.decodeResource(getResources(), R.mipmap.ic_launcher));
        builder.setContentIntent(pi);
        builder.setContentTitle(title);
        if (Build.VERSION.SDK_INT > 26) {
            NotificationChannel notificationChannel = new NotificationChannel("1", "test", NotificationManager.IMPORTANCE_DEFAULT);
            getNotificationManager().createNotificationChannel(notificationChannel);
            builder.setChannelId("1");
        }
        if(progress>0){
            builder.setContentText(progress + "%");
            builder.setProgress(100, progress, false);
        }
        return builder.build();
    }
}

修改MainActivity,获取运行时权限,绑定Service通过按钮进行调用:

public class MainActivity extends AppCompatActivity implements View.OnClickListener {

    private DownloadService.DownloadBinder downloadBinder;
    private ServiceConnection connection = new ServiceConnection() {
        @Override
        public void onServiceConnected(ComponentName componentName, IBinder service) {
            downloadBinder = (DownloadService.DownloadBinder) service;
        }

        @Override
        public void onServiceDisconnected(ComponentName componentName) {

        }
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Button startDownload = (Button) findViewById(R.id.start);
        Button pauseDownload = (Button) findViewById(R.id.pause);
        Button cancelDownload = (Button) findViewById(R.id.cancel);
        startDownload.setOnClickListener(this);
        pauseDownload.setOnClickListener(this);
        cancelDownload.setOnClickListener(this);
        Intent intent = new Intent(this, DownloadService.class);
        startService(intent);
        bindService(intent, connection, BIND_AUTO_CREATE);

        if (ContextCompat.checkSelfPermission(MainActivity.this, Manifest.permission.WRITE_EXTERNAL_STORAGE) != PackageManager.PERMISSION_GRANTED) {
            ActivityCompat.requestPermissions(MainActivity.this, new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE}, 1);
        }
    }

    @Override
    public void onClick(View view) {
        if (downloadBinder == null) {
            return;
        }
        switch (view.getId()) {
            case R.id.start:
                String url = "https://redirector.gvt1.com/edgedl/android/studio/install/2020.3.1.22/android-studio-2020.3.1.22-windows.exe";
                downloadBinder.startDownload(url);
                break;
            case R.id.pause:
                downloadBinder.pauseDownload();
                break;
            case R.id.cancel:
                downloadBinder.cancelDownload();
                break;
            default:
                break;

        }
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
        switch (requestCode) {
            case 1:
                if (grantResults.length > 0 && grantResults[0] != PackageManager.PERMISSION_GRANTED) {
                    Toast.makeText(this, "You deny the permission", Toast.LENGTH_SHORT).show();
                    finish();
                }
                break;
            default:
        }
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        unbindService(connection);

    }
}

效果图:
Android基础——异步消息处理_第2张图片

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