安卓——AsyncTask初学（AsyncTask作为独立类和内部类实现图片下载）

￡神魔★判官ぃ 2022-05-29 11:22 119阅读 0赞

异步任务：

安卓的单线程模型

耗时的任务放在其他线程

AsyncTask

在子线程中更新UI

封装、简化异步操作

![70][]

![70 1][]

目的：

![70 2][]

图片转化成流：基本的网络操作

![70 3][]

![70 4][]

上面是慕课网的AsyncTask的讲解([https://www.imooc.com/learn/377][https_www.imooc.com_learn_377])，老师把继承了AsyncTask的子类写做内部类，下面我来展示一下独立类的写法

AsyncTask作为独立类

我要感谢这篇博客[https://blog.csdn.net/shadow066/article/details/17380399][https_blog.csdn.net_shadow066_article_details_17380399],它用了接口的回调实现

![70 5][]![70 6][]

activity\_main.xml

<?xml version="1.0" encoding="utf-8"?>
    <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:app="http://schemas.android.com/apk/res-auto"
        xmlns:tools="http://schemas.android.com/tools"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical"
        tools:context="com.example.lenovo.androidasynctask.MainActivity">
    
    
        <ImageView
            android:layout_weight="100"
            android:layout_width="fill_parent"
            android:layout_height="fill_parent"
            android:id="@+id/image"
            />
    
        <Button
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:id="@+id/button"
            android:text="下载图片"
            />
    </LinearLayout>

然后是一个接口，这个很重要

TaskListener.java

package com.example.lenovo.androidasynctask;
    
    import android.graphics.Bitmap;
    
    /**
     * Created by lenovo on 2018/3/22.
     */
    
    public interface TaskListener {
        public void onCompletedListener(Bitmap result);
    }

MainActivity.java

在按钮点击事件中new一个Mytask类，加上构造参数并执行我们图片的目标地址。

然后在里面实现一下接口的抽象方法onCompletedListener()方法,这个方法的参数就是下载的图片以bitmap形式存储的。然后在方法里给ImageView填充图片。

package com.example.lenovo.androidasynctask;
    
    import android.app.Activity;
    import android.app.ProgressDialog;
    import android.graphics.Bitmap;
    import android.os.Bundle;
    import android.view.View;
    import android.widget.Button;
    import android.widget.ImageView;
    
    public class MainActivity extends Activity {
    
        private ProgressDialog dia;
        String picture = "http://b.hiphotos.baidu.com/image/pic/item/908fa0ec08fa513d17b6a2ea386d55fbb2fbd9e2.jpg";
        private Button button;
        private ImageView imageView;
        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);
    
            button=(Button)findViewById(R.id.button);
            imageView=(ImageView)findViewById(R.id.image);
    
    
    
            dia=new ProgressDialog(this);
    
    
            button.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    imageView.setImageBitmap(null);
                    new Mytask(dia,MainActivity.this,new TaskListener(){
    
                        @Override
                        public void onCompletedListener(Bitmap result) {
                            imageView.setImageBitmap(result);
                        }
                    }).execute(picture);
                }
            });
        }
    }

最后是继承了AsyncTask的独立类Mytask

这个类的构造方法有三个参数（ProgressDialog,Context,TaskListener），第三个参数就是我们可爱的接口

当任务执行完后，在onPostExecute()方法中调用TaskListener接口中的onCompletedListenr()方法，放心我们会在MainActivity的点击事件实现这个方法的。

package com.example.lenovo.androidasynctask;
    
    
    import java.io.ByteArrayOutputStream;
    import java.io.IOException;
    import java.io.InputStream;
    
    import org.apache.http.HttpResponse;
    import org.apache.http.client.HttpClient;
    import org.apache.http.client.methods.HttpGet;
    import org.apache.http.impl.client.DefaultHttpClient;
    
    import android.content.Context;
    import android.os.AsyncTask;
    import android.app.ProgressDialog;
    import android.graphics.Bitmap;
    import android.graphics.BitmapFactory;
    import android.widget.ImageView;
    import android.widget.ProgressBar;
    
    import com.example.*;
    
    /**
     * Created by lenovo on 2018/3/22.
     */
    
    public class Mytask extends AsyncTask<String ,Integer,Bitmap> {
    
        private Context context;
        private TaskListener taskListener;
        private ProgressDialog dia;
    
        //MyTask的构造方法
        public Mytask(ProgressDialog dias,Context context, TaskListener listener) {
            super();
            dia=dias;
            this.context = context;
            this.taskListener = listener;
        }
    
        //在doInBackground()方法执行前调用，主要执行一些初始化的工作，在UI线程中执行
        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            dia.setTitle("提示信息");
            dia.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
            dia.setCancelable(false);
            dia.show();
        }
    
        //干活的代码写在这里
        @Override
        protected Bitmap doInBackground(String... strings) {
            Bitmap bitmap=null;
            ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
            InputStream inputStream=null;
    
            try{
             HttpClient httpClient = new DefaultHttpClient();
             HttpGet httpGet = new HttpGet(strings[0]);
             HttpResponse httpResponse = httpClient.execute(httpGet);
             if(httpResponse.getStatusLine().getStatusCode()==200){
                 inputStream = httpResponse.getEntity().getContent();
                 //先获得文件的总长度
    
                 long file_length = httpResponse.getEntity().getContentLength();
                 int len=0;
                 byte [] data =new byte[1024];
                 int total_length=0;
                 int value=0;
                 while((len = inputStream.read(data))!=-1){
                     total_length+=len;
                     value=(int)((total_length/(float)file_length)*100);
                     //调用updata函数，跟新进度
                     publishProgress(value);
                     outputStream.write(data,0,len);
                 }
                 byte[] result =outputStream.toByteArray();
                 bitmap= BitmapFactory.decodeByteArray(result,0,result.length);
             }
            }
            catch (Exception e){
                    e.printStackTrace();
            }finally {
                if(inputStream!=null){
                    try{
                        inputStream.close();
                    }catch (IOException e){
                        e.printStackTrace();
                    }
                }
            }
            return bitmap;
        }
    
        //更新进度条
        //在任务执行过程中调用publishProgress()方法报告进度，此方法被调用，并在UI县城中执行
        @Override
        protected void onProgressUpdate(Integer... values) {
            super.onProgressUpdate(values);
            dia.setProgress(values[0]);
        }
    
        //在工作完成后次方法被调用，在UI线程中执行
        @Override
        protected void onPostExecute(Bitmap result) {
            super.onPostExecute(result);
            dia.dismiss();
            taskListener.onCompletedListener(result);
        }
    
        @Override
        protected void onCancelled(Bitmap result) {
            super.onCancelled(result);
        }
    }

这期间还遇到了几个小插曲

![70 7][]

上图总的doInbackground()方法中那些http的类，我也不知道用的哪个jar包，总之在上图右边的libs我把要用的都加上了，有想要的在评论里留下邮箱哦。

然后这么多jar包在一块就变异出错了，出错的话看这俩博客

1.解决了jar包里重复文件的问题

[https://www.cnblogs.com/skyeblogs/p/8514187.html][https_www.cnblogs.com_skyeblogs_p_8514187.html]

2.解决了

Error:Execution failed for task ':app:transformResourcesWithMergeJavaResForDebug'.

> More than one file was found with OS independent path 'META-INFXXX'

[https://blog.csdn.net/pyfysf/article/details/78486201][https_blog.csdn.net_pyfysf_article_details_78486201]

[70]: /images/20220529/645b575ccccc45daa16e6c7451710e28.png
[70 1]: /images/20220529/0da873f353c94d9e9587dd618a8cc476.png
[70 2]: /images/20220529/e4b0ebc582ec40aa852e8074a0bc896a.png
[70 3]: /images/20220529/bf10ac63602e4c768ae086ca5df35ce3.png
[70 4]: /images/20220529/c5dc6c6fac264b55b3e7f9cd72e186f1.png
[https_www.imooc.com_learn_377]: https://www.imooc.com/learn/377
[https_blog.csdn.net_shadow066_article_details_17380399]: https://blog.csdn.net/shadow066/article/details/17380399
[70 5]: https://img-blog.csdn.net/20180322215656387?watermark/2/text/aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl8zODE5OTc3MA==/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70
[70 6]: /images/20220529/ce72ca1a7ff2425f9e492b71c8502be8.png
[70 7]: /images/20220529/9e36174ddaa24a26882bc7ccf4671d0a.png
[https_www.cnblogs.com_skyeblogs_p_8514187.html]: https://www.cnblogs.com/skyeblogs/p/8514187.html
[https_blog.csdn.net_pyfysf_article_details_78486201]: https://blog.csdn.net/pyfysf/article/details/78486201