java - 使用 AsyncTask 下载许多图像并将它们发布到 ImageView

Question

我坐下来尝试用 Android 做一些练习。我今天的重点是制作简单的应用程序，它将下载数据(来自 URL 的图像)并将它们显示在布局中的 ImageView 控件中。我在网上看到了一些示例并完成了我的应用程序。一切似乎都很好，但当我按下按钮时，我开始了它的工作，但随后无法显示错误:NULL POINTER 9error reading file)。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 ArrayList 中看到的那样，我制作了可绘制列表。代码没有错误。只有 NULL POINTER。

Answer 1

我认为这是因为您忘记了初始化可绘制对象。更改为:

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

下一步，因为你的 AsyncTask 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>对应不同方法的参数和返回类型。您应该在这里阅读更多相关信息:https://stackoverflow.com/a/6053673/827110

(为了完整起见)您还需要在您的 AndroidManifest.xml 中获得互联网许可添加(就在 <application.. 之前):

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