Thread线程笔记

UI线程模型(单一线程模型)

  1. 不能阻塞主线程;阻塞超过时限会被判定“应用程序未响应”ANR
  2. 非UI线程不能访问UI工具包
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    textView = findViewById(R.id.textView);

    // Fail: Only the original thread that created a view hierarchy can touch its views.
    new Thread() {
        @Override
        public void run() {
            int count = 0;
            while (count < 1000) {
                textView.setText("当前TextView的值是" + count);
                try {
                    Thread.sleep(100);
                } catch (Exception e) {
                    e.printStackTrace();
                }
                count++;
            }
        }
    }.start();
}

解决方案

  1. 工作线程尝试访问UI线程,并委托UI线程更新UI
  2. 工作线程与UI线程之间通信,让工作线程发送消息给UI线程,让UI线程根据消息更新UI
  3. 使用AsyncTask

方案1:从工作线程访问UI线程

  1. Activity.runOnUiThread(Runnable)
  2. View.post(Runnable)
  3. View.postDelayed(Runnable, long)

View.post(Runnable)

new Thread() {
    @Override
    public void run() {
        count = 0;
        while (count < 1000) {
            textView.post(new Runnable() {
                @Override
                public void run() {
                    textView.setText("当前计数器:" + count);
                }
            });

            try {
                Thread.sleep(100);
            } catch (Exception e) {
                e.printStackTrace();
            }
            count++;
        }
    }
}.start();

方案3:使用AsyncTask

AsyncTask<T param, T progress, T result>

  • param 发送给异步任务所需的参数类型
  • progress 后台运算过程中进度的参数类型
  • result 后台运算的结果类型

不需要的泛型参数可以标记为Void

任务通常经历的4个步骤:

  1. onPreExecute() 执行异步任务之前,运行在UI线程中
  2. doInBackground(Params... ) 执行在后台的长时间操作;运行的结果通过此步骤返回,并被传递到最后一步。此步中可以使用publishProgress(progress... )发布进度
  3. onProgressUpdate(progress... ) 调用publishProgress()之后在UI线程中执行
  4. onPostExecute(result... ) 后台操作结束后在UI线程中执行

上述4步骤中的方法不能手动调用,故AsyncTask实例必须在UI线程中创建,execute()方法必须在UI线程中调用。

示例:下载图片并展示进度

@Override
public void onClick(View view) {
    if (view.getId() == R.id.button) {
        new MyAsyncTask().execute("https://qfstudio.net/i_bg_cool.jpg");
    }
}

public class MyAsyncTask extends AsyncTask<String, Integer, byte[]> {
    @Override
    protected void onPreExecute() {
        progress.setProgress(0);
    }

    @Override
    protected byte[] doInBackground(String... strings) {
        ByteArrayOutputStream out = new ByteArrayOutputStream();
        try {
            URL url = new URL(strings[0]);
            HttpURLConnection con = (HttpURLConnection) url.openConnection();
            Log.i("Code", new Integer(con.getResponseCode()).toString());
            if (con.getResponseCode() == 200) {
                InputStream in = con.getInputStream();
                long length = con.getContentLength();
                int current = 0, temp;
                byte[] buff = new byte[1024];
                while ((temp = in.read(buff)) != -1) {
                    current += temp;
                    int progress = (int)(current/(double)length * 100);
                    publishProgress(progress);
                    out.write(buff, 0, temp);
                    out.flush();
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
        return out.toByteArray();
    }

    @Override
    protected void onProgressUpdate(Integer... values) {
        progress.setProgress(values[0]);
    }

    @Override
    protected void onPostExecute(byte[] result) {
        if (result != null && result.length != 0) {
            Bitmap bm = BitmapFactory.decodeByteArray(result, 0, result.length);
            imageView.setImageBitmap(bm);
        } else {
            Toast.makeText(MainActivity.this, "网络连接出现了问题", Toast.LENGTH_LONG).show();
        }
    }
}

示例:取消异步任务

调用cancel(boolean)来取消异步任务:调用此方法后isCanceled()返回true,并且异步任务结束后运行onCancel(Object)而不是onPostExecute()

应该在doInBackground()中检查isCanceled()的值。

@Override
public void onClick(View view) {
    if (!onGoing) {
        task = new MyAsyncTask();
        task.execute("https://qfstudio.net/i_bg_cool.jpg");
        onGoing = true;
    } else {
        task.cancel(true);
        onGoing = false;
    }
}

public class MyAsyncTask extends AsyncTask<String, Integer, byte[]> {
    @Override
    protected void onPreExecute() {
        progress.setProgress(0);
    }

    @Override
    protected byte[] doInBackground(String... strings) {
        ByteArrayOutputStream out = new ByteArrayOutputStream();
        try {
            URL url = new URL(strings[0]);
            HttpURLConnection con = (HttpURLConnection) url.openConnection();
            Log.i("Code", new Integer(con.getResponseCode()).toString());
            if (con.getResponseCode() == 200) {
                InputStream in = con.getInputStream();
                long length = con.getContentLength();
                int current = 0, temp;
                byte[] buff = new byte[1024];
                while ((temp = in.read(buff)) != -1) {
                    current += temp;
                    int progress = (int)(current/(double)length * 100);
                    publishProgress(progress);
                    out.write(buff, 0, temp);
                    out.flush();

                    if (isCancelled()) break;
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
        return out.toByteArray();
    }

    @Override
    protected void onProgressUpdate(Integer... values) {
        progress.setProgress(values[0]);
    }

    @Override
    protected void onCancelled() {
        Toast.makeText(MainActivity.this, "已取消", Toast.LENGTH_LONG).show();
    }

    @Override
    protected void onPostExecute(byte[] result) {
        if (result != null && result.length != 0) {
            Bitmap bm = BitmapFactory.decodeByteArray(result, 0, result.length);
            imageView.setImageBitmap(bm);
        } else {
            Toast.makeText(MainActivity.this, "网络连接出现了问题", Toast.LENGTH_LONG).show();
        }
    }
}

最后更新: 2019年11月28日