AsyncTask学习笔记

AsyncTask介绍

AsyncTask是一个轻量级的处理线程更新Android UI 的方式，AsyncTack是个抽象类，使用时应该继承它，它定义了3个泛型

 1. Params：启动任务执行时输入的参数
 2. Progress：后台任务完成时的数据类型
 3. Result：后台任务结束时返回的数据类型

AsyncTask使用

 1. 为上面三个泛型指定具体数据类型
 2. 必须实现doInBackground（）方法
 3. 根据业务来实现不同的方法：onProgressUpdate  onPreExecute  onPostExecute方法
 4. 调用AnsyncTask子类实例

注：必须在UI线程中创建AsyncTask的实例
必须在UI线程中调用AsyncTask的excute方法
每个AsyncTask只能执行一次，否则会报错

AsyncTask方法调用顺序

 1. 首先调用onPreExecute方法，一般是初始化各种控件
 2. 执行onInbBackground执行目的任务
 3. 任务不断调用publishProgress更新进度，也就相当于调用了onProgressUpdate方法
 4. 任务完成调用onPostExecute并且把返回值传递给onPostExecute

AsyncTask实例

介绍：使用AsyncTask来下载并且显示到界面

java代码

package com.phone.hty.myapplication;

import android.app.ProgressDialog;
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.Button;
import android.widget.TextView;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLConnection;

public class MainActivity extends AppCompatActivity {

    private TextView mMteTextView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mMteTextView = (TextView) findViewById(R.id.textView);
        Button button = (Button) findViewById(R.id.myButton);
        assert button != null;
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                MyAsyncStack myAsyncStack = new MyAsyncStack(MainActivity.this);
                try {
                    myAsyncStack.execute(new URL("http://110.65.7.96:8080/hty.txt"));
                } catch (MalformedURLException e) {
                    e.printStackTrace();
                }
            }
        });


    }
    class MyAsyncStack extends AsyncTask<URL, Integer, String> {
        ProgressDialog progressDialog;
        int length;
        Context mContext;


        public MyAsyncStack(Context context) {
            mContext = context;

        }

        /** * 描述：AsyncStack * 参数：URL * 形参： * 时间：2016/3/2714:18 */

        @Override
        protected String doInBackground(URL... params) {
            StringBuilder stringBuilder = new StringBuilder();
            try {
                URLConnection connection = params[0].openConnection();
                BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(
                        connection.getInputStream(), "utf-8"
                ));
                String line = null;
                while ((line = bufferedReader.readLine()) != null) {
                    stringBuilder.append(line + "\n");
                    length++;
                    publishProgress(length);
                }return stringBuilder.toString();


            } catch (IOException e) {
                e.printStackTrace();
            }


            return null;
        }

        /** * Runs on the UI thread before {@link #doInBackground}. * * @see #onPostExecute * @see #doInBackground */
        @Override
        protected void onPreExecute() {
            progressDialog = new ProgressDialog(mContext);
            progressDialog.setTitle("人物进行中");
            progressDialog.setMessage("请等待");
            progressDialog.setCancelable(false);
            progressDialog.setMax(202);
            progressDialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
            progressDialog.setIndeterminate(false);
            progressDialog.show();


        }

        /** * Runs on the UI thread after {@link #publishProgress} is invoked. * The specified values are the values passed to {@link #publishProgress}. * * @param values The values indicating progress. * @see #publishProgress * @see #doInBackground */
        @Override
        protected void onProgressUpdate(Integer... values) {
            mMteTextView.setText("已经读取了" + values[0] + "行");
            progressDialog.setProgress(values[0]);
        }

        /** * <p>Runs on the UI thread after {@link #doInBackground}. The * specified result is the value returned by {@link #doInBackground}.</p> * <p/> * <p>This method won't be invoked if the task was cancelled.</p> * * @param s The result of the operation computed by {@link #doInBackground}. * @see #onPreExecute * @see #doInBackground * @see #onCancelled(Object) */
        @Override
        protected void onPostExecute(String s) {
            mMteTextView.setText(s);
            progressDialog.dismiss();
        }
    }
}