如何在android中的不同活动中使用带有进度条的异步任务

Question

我在这个项目中使用了 elasticView 进度条库。当我直接在单个Activity 和AsyncTask 中使用它时，它工作正常，但是当我创建新的Java 类以从MainActivity 下载和访问时。它显示类未找到异常。这是我的完整代码和错误日志。

---

主活动

---

public class MainActivity extends AppCompatActivity {
    Button b1;

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

        b1=(Button)findViewById(R.id.button);
        b1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                download d=new download();
                d.startdownload();
            }
        });
    }

---

下载.java

---

import android.content.Context;
import android.os.AsyncTask;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Toast;

import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.HttpURLConnection;
import java.net.URL;

import it.michelelacorte.elasticprogressbar.ElasticDownloadView;

public class download extends AppCompatActivity {
    ElasticDownloadView mElasticDownloadView;

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

        mElasticDownloadView = (ElasticDownloadView) findViewById(R.id.elastic_download_view);
        mElasticDownloadView.setVisibility(View.VISIBLE);

        startdownload();
    }

    public void startdownload() {

        final DownloadTask downloadTask = new DownloadTask(download.this);
        mElasticDownloadView.setVisibility(View.GONE);
        downloadTask.execute("http://farm1.static.flickr.com/114/298125983_0e4bf66782_b.jpg");
    }

    private class DownloadTask extends AsyncTask<String, Integer, String> {
        private Context context;

        public DownloadTask(Context context) {
            this.context = context;
        }

        @Override
        protected String doInBackground(String... sUrl) {


            InputStream input = null;
            OutputStream output = null;
            HttpURLConnection connection = null;
            try {
                URL url = new URL(sUrl[0]);
                connection = (HttpURLConnection) url.openConnection();
                connection.connect();
                if (connection.getResponseCode() != HttpURLConnection.HTTP_OK) {
                    return "Server returned HTTP " + connection.getResponseCode()
                            + " " + connection.getResponseMessage();
                }

                int fileLength = connection.getContentLength();
                input = connection.getInputStream();
                output = new FileOutputStream("/sdcard/some_photo_from_gdansk_poland.jpg");

                byte data[] = new byte[4096];
                long total = 0;
                int count;
                while ((count = input.read(data)) != -1) {

                    if (isCancelled()) {
                        input.close();
                        return null;
                    }
                    total += count;

                    if (fileLength > 0) {
                        int status = ((int) (total * 100 / fileLength));
                        publishProgress(status);
                    }
                    output.write(data, 0, count);
                }
            } catch (Exception e) {
                return e.toString();
            } finally {
                try {
                    if (output != null)
                        output.close();
                    if (input != null)
                        input.close();
                } catch (IOException ignored) {
                }

                if (connection != null)
                    connection.disconnect();
            }
            return null;

        }

        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            mElasticDownloadView.startIntro();
        }

        @Override
        protected void onProgressUpdate(Integer... progress) {
            super.onProgressUpdate(progress);
            mElasticDownloadView.setProgress(10);
        }

        @Override
        protected void onPostExecute(String result) {
            mElasticDownloadView.success();
            if (result != null)
                Toast.makeText(context, "Download error: " + result, Toast.LENGTH_LONG).show();
        }
    }
}

---

activity_main.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: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="net.logicalsteps.download_notif.MainActivity">

        <include layout="@layout/elastic"/>

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="start download"
            android:id="@+id/button"
            android:layout_alignParentTop="true"
            android:layout_centerHorizontal="true"
            android:layout_marginTop="31dp" />
    </RelativeLayout>

---

elastic.xml

---

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"

    android:layout_width="wrap_content"
    android:layout_height="100dp"
    android:layout_alignParentBottom="true"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true">

    <it.michelelacorte.elasticprogressbar.ElasticDownloadView
        android:id="@+id/elastic_download_view"
        android:layout_width="match_parent"
        android:layout_height="90dp"
        android:layout_alignParentBottom="true"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"
        android:layout_marginBottom="27dp" />



</RelativeLayout>

---

错误日志：

---

E/AndroidRuntime: FATAL EXCEPTION: main 
Process: net.logicalsteps.download_notif, PID: 23132  
java.lang.NullPointerException: Attempt to invoke virtual method 'void it.michelelacorte.elasticprogressbar.ElasticDownloadView.setVisibility(int)' on a null object referenc
        at net.logicalsteps.download_notif.download.startdownload(download.java:41)
        at net.logicalsteps.download_notif.MainActivity$1.onClick(MainActivity.java:39)
        at android.view.View.performClick(View.java:5162)
        at android.view.View$PerformClick.run(View.java:20873)
        at android.os.Handler.handleCallback(Handler.java:739)
        at android.os.Handler.dispatchMessage(Handler.java:95)                                      at android.os.Looper.loop(Looper.java:145)                                         at android.app.ActivityThread.main(ActivityThread.java:5835)                                         at java.lang.reflect.Method.invoke(Native Method)                                         at java.lang.reflect.Method.invoke(Method.java:372)                                              at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1388                                       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1183)

Answer 1

你需要从另一个Activity开始一个Intent，否则，你的类将被初始化，但Activity不会启动它的生命周期（onCreate方法等不会被调用） ：

    b1.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent downloadIntent = new Intent(v.getContext(), download.class);
            startActivity(downloadIntent);
        }
    });

如果您不想打开一个新的Activity，您可以将您的AsyncTask 移动到您的MainActivity，当它下载时，您可以使用您的ProgressBar 显示一个自定义AlertDialog。您可以在此here 上找到更多信息。

Answer 2

你只需要看看这里

 download d=new download();
 d.startdownload();

您调用了 startdownload() 方法，但直到那时 onCreate 的 download Activity 未被调用，因此无法初始化 mElasticDownloadView。因此，也可以在 startdownload 方法中或任何您感觉舒适的地方初始化它，或者在 onClick() 方法中启动 Activity download。

Answer 3

错误原因

发生这种情况的原因是，当系统启动download 活动时，它会执行设置mElasticDownloadView = ... 的download.onCreate。但是当你在onClick 中创建它时，你不会调用onCreate 所以mElasticDownloadView 不会被初始化。

使用活动

现在在onClick 中创建download Activity 的实例：

download d=new download();
d.startdownload();

这不是创建活动的常用方式。如果您这样做，活动将不会从布局 XML 文件中扩展其所有 UI，因此也没有进度条。如果你想显示活动，你应该使用：

Intent intent = new Intent(MainActivity.this, download.class);
startActivity(intent);

现在，如果你想向 Activity 传递消息，你可以通过 Intent 传递它：

intent.putExtra(EXTRA_MESSAGE, message);

然后在download.onStart你可以检索到这条消息getIntent().getStringExtra(...)。

但是，无论如何，在您的download.onCreate 中您调用startdownload，因此您不需要进行消息传递。

在此处了解更多信息：https://developer.android.com/training/basics/firstapp/starting-activity.html