使用 AsyncTask 下载许多图像并将它们发布到 ImageView

Question

我坐着，试图用Android做一些练习。我今天的观点是制作一个简单的应用程序，它将下载数据（来自 URL 的图像）并在布局的 ImageView 控件中显示它们。我在网上看到了一些例子，并完成了我的应用程序。一切似乎都很好，但是当我按下按钮时，我开始了它的工作，但随后失败显示错误：空指针 9 错误读取文件）。这是我的代码：

package com.example.htmlcontent;
import java.io.BufferedInputStream;
import java.io.InputStream;
import java.net.URL;
import java.util.ArrayList;
import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.drawable.BitmapDrawable;
import android.graphics.drawable.Drawable;
import android.os.AsyncTask;
import android.os.Bundle;
import android.widget.Button;
import android.widget.ImageView;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;

    public class MainActivity extends Activity {
        private ImageView mImageView;
        private ImageView mImageView2;
        public Button button;
        public static ArrayList<Drawable> drawable;
        public static String[] URLs = {"http://zitterman.com/wp-content/uploads/2013/07/19194927_1371972212.jpg","http://i.imgur.com/CQzlM.jpg"};
        /** Called when the activity is first created. */
        @Override
        public void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);

            mImageView = (ImageView) findViewById(R.id.test_image);
            mImageView2 = (ImageView) findViewById(R.id.test_image2);
            button = (Button) findViewById(R.id.download1);
            button.setOnClickListener(new OnClickListener() {
                @Override
                public void onClick(View v) {
                    new DownloadImage().execute();
                }
            });
        }

        /**
         * Simple functin to set a Drawable to the image View
         * @param drawable
         */
        @SuppressWarnings("deprecation")
        private void setImage()
        {
            if(drawable.get(0) == null)
            {
                System.out.println("DRAWABLE JEST NULL");
            }
            mImageView.setBackgroundDrawable(drawable.get(0));
            mImageView2.setBackgroundDrawable(drawable.get(1));
        }
        public class DownloadImage extends AsyncTask<Void, Void, Void> {

            /**
             * Called after the image has been downloaded
             * -> this calls a function on the main thread again
             */
            protected void onPostExecute(Drawable image)
            {
                setImage();
            }
            protected void onPreExecute()
            {
                Log.i("333333", "Uruchamiam WATEK SCIAGANIA ASYNCTASKIEM PLIKU Z NETA");
            }
            @Override
            protected Void doInBackground(Void... params) {
                downloadImage();
                return null;
            }
            /**
             * Actually download the Image from the _url
             * @param _url
             * @return
             */
            @SuppressWarnings("deprecation")
            private void downloadImage()
            {
                //Prepare to download image
                URL url;        
                InputStream in;
                BufferedInputStream buf;
                //BufferedInputStream buf;
                for(int i = 0; i<URLs.length; i++)
                {
                    try {
                    url = new URL(URLs[i]);
                    in = url.openStream();
                    // Read the inputstream 
                    buf = new BufferedInputStream(in);
                    // Convert the BufferedInputStream to a Bitmap
                    Bitmap bMap = BitmapFactory.decodeStream(buf);
                    if (in != null) {
                        in.close();
                    }
                    if (buf != null) {
                        buf.close();
                    }
                     drawable.add(new BitmapDrawable(bMap));
                } catch (Exception e) {
                    Log.e("Error reading file", e.toString());
                }
                }
            }

        }
    }

和我的XML文件布局：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    >
    <Button
        android:id="@+id/download1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Button" />
<TextView
    android:layout_width="102dp"
    android:layout_height="wrap_content"
    android:text="hello" />
    <ImageView
        android:id="@+id/test_image"
        android:layout_width="match_parent"
        android:layout_height="200dp"
        android:background="@drawable/ic_launcher" />
    <ImageView
        android:id="@+id/test_image2"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:background="@drawable/ic_launcher" />
</LinearLayout>

正如您在代码中看到的那样，我已经制作了可绘制列表的数组列表。代码中没有错误。只有那个空指针。

Answer 1

我认为这是因为您忘记初始化可绘制对象。更改为：

public static ArrayList<Drawable> drawable = new ArrayList<Drawable>();

接下来，因为您的异步任务是<Void, Void, Void>.您的帖子执行应如下所示：

    @Override
    protected void onPostExecute(Void aVoid) {
        setImage();
    }

泛型类型<A,B,C>对应于不同方法的参数和返回类型。您应该在此处阅读更多相关信息：https://stackoverflow.com/a/6053673/827110

（为了完整起见）您还需要AndroidManifest.xml添加中的互联网权限（就在<application..之前）：

<uses-permission android:name="android.permission.INTERNET" />

Answer 2

将异步任务更改为

 public class DownloadImage extends AsyncTask<Void, Void,  ArrayList<Drawable>> {

        /**
         * Called after the image has been downloaded
         * -> this calls a function on the main thread again
         */
        protected void onPostExecute( ArrayList<Drawable> drawable)
        {
            setImage(drawable);
        }
        protected void onPreExecute()
        {
            Log.i("333333", "Uruchamiam WATEK SCIAGANIA ASYNCTASKIEM PLIKU Z NETA");
        }
        @Override
        protected  ArrayList<Drawable> doInBackground(Void... params) {
            downloadImage();
            return drawable;
        }


 private void setImage(ArrayList<Drawable> drawable)
    {
        if(drawable.get(0) == null)
        {
            System.out.println("DRAWABLE JEST NULL");
        }
        mImageView.setBackgroundDrawable(drawable.get(0));
        mImageView2.setBackgroundDrawable(drawable.get(1));
    }

Answer 3

好的，我已经按照你说的纠正了它。是的，我一开始忘了初始化这个数组列表，但你比我快;所以

public class MainActivity extends Activity {
        private ImageView mImageView;
        private ImageView mImageView2;
        public Button button;
        public static ArrayList<Drawable> drawable = new ArrayList<Drawable>();
        public static String[] URLs = {"http://zitterman.com/wp-content/uploads/2013/07/19194927_1371972212.jpg","http://i.imgur.com/CQzlM.jpg"};
        /** Called when the activity is first created. */
        @Override
        public void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);

            mImageView = (ImageView) findViewById(R.id.test_image);
            mImageView2 = (ImageView) findViewById(R.id.test_image2);
            button = (Button) findViewById(R.id.download1);
            button.setOnClickListener(new OnClickListener() {
                @Override
                public void onClick(View v) {
                    new DownloadImage().execute();
                }
            });
        }

        /**
         * Simple functin to set a Drawable to the image View
         * @param drawable
         */
        @SuppressWarnings("deprecation")
        private void setImage( ArrayList<Drawable> drawable)
        {
            if(drawable.get(0) == null)
            {
                System.out.println("DRAWABLE JEST NULL");
            }
            mImageView.setBackgroundDrawable(drawable.get(0));
            mImageView2.setBackgroundDrawable(drawable.get(1));
        }
        public class DownloadImage extends AsyncTask<Void, Void, ArrayList<Drawable>> {
            /**
             * Called after the image has been downloaded
             * -> this calls a function on the main thread again
             */
            protected void onPostExecute( ArrayList<Drawable> drawable)
            {
                setImage(drawable);
            }
            protected void onPreExecute()
            {
                Log.i("333333", "Uruchamiam WATEK SCIAGANIA ASYNCTASKIEM PLIKU Z NETA");
            }
            @Override
            protected ArrayList<Drawable> doInBackground(Void... params) {
                downloadImage();
                return null;
            }
            /**
             * Actually download the Image from the _url
             * @param _url
             * @return
             */
            @SuppressWarnings("deprecation")
            private void downloadImage()
            {
                //Prepare to download image
                URL url;        
                InputStream in;
                BufferedInputStream buf;
                //BufferedInputStream buf;
                for(int i = 0; i<URLs.length; i++)
                {
                    try {
                    url = new URL(URLs[i]);
                    in = url.openStream();
                    // Read the inputstream 
                    buf = new BufferedInputStream(in);
                    // Convert the BufferedInputStream to a Bitmap
                    Bitmap bMap = BitmapFactory.decodeStream(buf);
                    if (in != null) {
                        in.close();
                    }
                    if (buf != null) {
                        buf.close();
                    }
                     drawable.add(new BitmapDrawable(bMap));
                } catch (Exception e) {
                    Log.e("Error reading file", e.toString());
                }
                }
            }

        }
    }

现在，尝试下载图像时会崩溃。我不完全明白为什么我应该将 ArrayList 作为第三个参数....？为什么我在存储图像的位置声明静态数组？OnPostExecute应该只调用函数，这将使其余的工作。