AsyncTask的使用流程

• 创建一个类继承AsyncTask类，并实现相关方法
• 定义一个接口类，来回调数据
• 在活动中调创建自定义Task类，并执行任务

1. 定义接口类，以实现回调数据

public interface AsyncTaskListener {
    //任务执行前
    void onPrepare();
    //更新任务进度
    void onUpdate(Integer... values);
    //完成任务，返回结果
    void onFinish(String result);
    //取消任务
    void onCancel();
    //返回错误消息
    void onError(String msg);
}

2. 创建一个类来继承AsyncTask类，并实现相关方法

//AsyncTask中的类型分别代表<Params,Progress,Result>
public class MyTask extends AsyncTask<String, Integer, String> {
    private AsyncTaskListener listener; //回调接口
    private boolean isDownloadSuccess; //判断是否下载成功

    public MyTask(AsyncTaskListener listener){
        this.listener = listener;
    }

    @Override
    protected void onPreExecute() {
        super.onPreExecute();
        listener.onPrepare();   //任务执行前回调
    }

    @Override
    protected String doInBackground(String... params) {
        InputStream in = null;
        OutputStream out = null;

        /**创建一个文件用来保存下载文件**/
        String filePath = Environment.getExternalStorageDirectory() + "/download/";
        File file = new File(filePath);
        if (!file.exists()){
            file.mkdir();
        }
        File imageFile = new File(filePath + "test.mp3");
        /**创建一个文件用来保存下载文件**/


        try {
            URL url = new URL(params[0]);
            HttpURLConnection connection = (HttpURLConnection) url.openConnection();
            connection.setConnectTimeout(6000);
            connection.setRequestMethod("POST");
            //注意：为了后期获取文件长度
            connection.setRequestProperty("Accept-Encoding", "identity");
            //响应失败，则返回，并回调错误
            if (connection.getResponseCode() != 200){
                Log.d("tag", "connect fail");
                listener.onError("Connect Fail");
                return null;
            }
            
            in = connection.getInputStream();
            out = new FileOutputStream(imageFile);
            //获取到文件长度
            int fileLength = connection.getContentLength();
            int count = 0;
            byte[] buf = new byte[1024];
            int bytesRead;
            while ((bytesRead = in.read(buf)) != -1){
                out.write(buf, 0, bytesRead);
                count += bytesRead;
                //发布更新进度
                publishProgress((int)(((double)count/fileLength)*100), fileLength);
            }
            out.flush();
            //标识下载成功
            isDownloadSuccess = true;
        } catch (IOException e) {
            e.printStackTrace();
            listener.onError(e.getMessage());
        } finally {
            IoUtil.close(in);
            IoUtil.close(out);
        }
        //返回文件路径
        return imageFile.getAbsolutePath();
    }

    @Override
    protected void onProgressUpdate(Integer... values) {
        super.onProgressUpdate(values);
        listener.onUpdate(values);  //回调进度
    }

    @Override
    protected void onPostExecute(String s) {
        super.onPostExecute(s);
        if (isDownloadSuccess){
            listener.onFinish(s);   //当文件下载成功后，才返回结果
        }
    }

    @Override
    protected void onCancelled() {
        super.onCancelled();
        listener.onCancel();
    }
}

3. 创建自定义类，执行任务

public class AsyncTaskActivity extends AppCompatActivity {
    private ProgressDialog progressDialog;  //对话框式进度显示
    private Button btn_download;
    private String url = "http://programmerguru.com/android-tutorial/wp-content/uploads/2014/01/jai_ho.mp3";    //测试url
    private String imageFilePath; //下载保存的文件路径
    private boolean permissionAllowed = false;  //权限设置
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_async_task);
        btn_download = (Button) findViewById(R.id.btn_download);
        btn_download.setOnClickListener(new MyOnClickListener());
        //请求权限
        if (ContextCompat.checkSelfPermission(this, Manifest.permission.WRITE_EXTERNAL_STORAGE) != PackageManager.PERMISSION_GRANTED){
            ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE}, 0);
        } else {
            permissionAllowed = true;
        }
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
        super.onRequestPermissionsResult(requestCode, permissions, grantResults);
        switch (requestCode){
            case 0:{
                if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED){
                    permissionAllowed = true;
                }
            }
        }
    }

    class MyOnClickListener implements View.OnClickListener{

        @Override
        public void onClick(View v) {
            if (permissionAllowed){
                downloadImage();
            }
        }
    }
    
    //下载文件
    private void downloadImage(){
        final MyTask mTask = new MyTask(new AsyncTaskListener() {
            @Override
            public void onPrepare() {
                //开始前，设置进度对话框
                btn_download.setEnabled(false);
                progressDialog = new ProgressDialog(AsyncTaskActivity.this);
                progressDialog.setTitle("下载中");
                progressDialog.show();
            }

            @Override
            public void onUpdate(Integer... values) {
                //更新进度
                String unit = "KB";
                if (values[1] > 1024*1024){
                    values[1] /= 1024;
                    unit = "MB";
                }
                progressDialog.setMessage("共" + String.valueOf(values[1]/1024) + unit + ", 已下载" + String.valueOf(values[0]) + "%");
            }

            @Override
            public void onFinish(String result) {
                //返回结果
                btn_download.setEnabled(true);
                progressDialog.dismiss();
                Toast.makeText(AsyncTaskActivity.this, "下载成功", Toast.LENGTH_LONG).show();
                imageFilePath = result;
                Log.d("tag","imageFilePath:" + imageFilePath);
            }

            @Override
            public void onCancel() {

            }

            @Override
            public void onError(final String error) {
                //发送错误，返回错误，注意：这里是非UI线程内
                runOnUiThread(new Runnable() {
                    @Override
                    public void run() {
                        Toast.makeText(AsyncTaskActivity.this, error, Toast.LENGTH_LONG).show();
                    }
                });
            }
        });
        //执行任务
        mTask.execute(url);
    }
}