Android progressBar setVisibility() 不工作 [英] Android progressBar setVisibility() not working

问题描述

有人可以帮助我理解为什么 progressBar.setVisibility() 在使用 CountDownTimer 时有效，但在进行 async 下载时无效?事实上，即使第二个 onPostExecute() 显示，async 下载的第一个 Toast 甚至没有显示.下面是一个有效的，或者更确切地说是不工作的演示.非常感谢.

Can someone please help me understand why progressBar.setVisibility() works when using a CountDownTimer but not when doing async download? In fact, the first Toast on async download does not even show even though the second one onPostExecute() does. Below is a working, or rather NOT working, demo. Thanks a lot.

我的MainActivity:

public class MainActivity extends AppCompatActivity {

    ProgressBar progressBar;

    int[] items = { 12982418, 12998698, 12993549, 12995125, 12987537, 12993021, 12991986, 13008408, 12983417, 12986060, 12998395, 12985644, 13014731, 12986433, 12985074, 12994455, 12994262, 12986759, 13011932, 13005211, 12993521, 12987900, 12992623, 12981736, 12986649, 12991524, 13000035, 12989278, 13013868, 13009417, 13013327, 12981605, 12985768, 13000158, 13015035, 13002596, 13015944, 12997893, 12999767, 13010949, 12996835, 13013517, 13006555, 13013143, 13010016, 13005792, 13016948, 13007235, 12998343, 12987102 };
    int counter;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        progressBar = (ProgressBar) findViewById(R.id.progressBar);
        progressBar.setIndeterminate(true);
        progressBar.setKeepScreenOn(true);
        progressBar.setVisibility(View.INVISIBLE);
    }

    public void actionTimer(View view) {
        Toast.makeText(this, "starting progressBar - CountDownTimer", Toast.LENGTH_SHORT).show();
        progressBar.setVisibility(View.VISIBLE);
        new CountDownTimer(5000, 500) {
            @Override
            public void onTick(long millisUntilFinished) {
            }
            @Override
            public void onFinish() {
                progressBar.setVisibility(View.INVISIBLE);
                Toast.makeText(getApplicationContext(), "timer done", Toast.LENGTH_SHORT).show();
            }
        }.start();
    }

    public void actionJson(View view) {
        Toast.makeText(this, "starting progressBar - json fetch", Toast.LENGTH_SHORT).show();
        progressBar.setVisibility(View.VISIBLE);

        String urlTemplate = "https://hacker-news.firebaseio.com/v0/item/___ITEM_NUMBER___.json?print=pretty";
        counter = 0;

        for (int i = 0; i < items.length; i++) {
            String url = urlTemplate.replaceAll("___ITEM_NUMBER___", String.valueOf(items[i]));
            //Log.i("thisDoesNotWork", url);

            DownloadJson downloadJson = new DownloadJson();
            try {
                downloadJson.execute(url).get();
            } catch (InterruptedException e) {
                e.printStackTrace();
            } catch (ExecutionException e) {
                e.printStackTrace();
            }
        }
    }

    public class DownloadJson extends AsyncTask<String, Void, String> {

        @Override
        protected String doInBackground(String... params) {
            Log.i("DownloadJson", "url=" + params[0]);

            URL url;
            HttpURLConnection urlConnection = null;

            try {
                url = new URL(params[0]);
                urlConnection = (HttpURLConnection) url.openConnection();
                InputStream inputStream = urlConnection.getInputStream();

                BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream));
                StringBuilder resultBuilder = new StringBuilder();
                String line;
                while ((line = reader.readLine()) != null) {
                    resultBuilder.append(line).append("
");
                }
                //Log.i("DownloadJson", "resultBuilder.length()=" + resultBuilder.length());
                return resultBuilder.toString();

            } catch (Exception e) {
                e.printStackTrace();
                return "{"status" : "Hacker News json download failed"}";
            }
        }

        @Override
        public void onPostExecute(String s) {
            super.onPostExecute(s);
            counter += 1;
            if (counter == items.length) {
                progressBar.setVisibility(View.INVISIBLE);
                Toast.makeText(getApplicationContext(), "download done", Toast.LENGTH_SHORT).show();
            }
        }
    }
}

我的XML:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/activity_main"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.plaudev.progressbar.MainActivity">

    <LinearLayout
        android:orientation="horizontal"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:gravity="center_vertical">

        <Button
            android:text="@string/timer"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignParentTop="true"
            android:layout_centerHorizontal="true"
            android:id="@+id/buttonTimer"
            android:layout_weight="1"
            android:onClick="actionTimer"
            android:layout_margin="25dp" />

        <Button
            android:text="@string/json"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:id="@+id/buttonJson"
            android:layout_weight="1"
            android:onClick="actionJson"
            android:layout_margin="25dp" />
    </LinearLayout>

    <ProgressBar
        style="?android:attr/progressBarStyleLarge"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerVertical="true"
        android:layout_centerHorizontal="true"
        android:id="@+id/progressBar" />
</RelativeLayout>

推荐答案

答案是将整个 for 循环放在 onClick actionJson() 到一个工作线程中如下此处注明:

The answer is to put the whole for loop in the onClick actionJson() into a worker thread as follows as noted here:

public void actionJson(View view) {
        Toast.makeText(this, "starting progressBar - json fetch", Toast.LENGTH_SHORT).show();
        progressBar.setVisibility(View.VISIBLE);

        final String urlTemplate = "https://hacker-news.firebaseio.com/v0/item/___ITEM_NUMBER___.json?print=pretty";
        counter = 0;

        // simply put it in a worker thread does the job
        new Thread(new Runnable() {
            public void run() {
                for (int i = 0; i < items.length; i++) {
                    String url = urlTemplate.replaceAll("___ITEM_NUMBER___", String.valueOf(items[i]));
                    //Log.i("actionJson", url);
                    DownloadJson downloadJson = new DownloadJson();
                    try {
                        downloadJson.execute(url).get();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    } catch (ExecutionException e) {
                        e.printStackTrace();
                    }
                }
            }
        }).start();
    }

因此，即使最初定义的 for 循环确实单独生成了一大堆 asyncTasks，它仍然以某种方式阻塞了主线程.

So even though the for loop as originally defined does spawn a whole bunch of asyncTasks individually, it is still somehow blocking the main thread.

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