AsyncTask详解 并实现实例-模拟下载

API中的内容：
简介
Class Overview
AsyncTask enables proper and easy use of the UI thread. This class allows to perform background operations and publish results on the UI thread without having to manipulate threads and/or handlers.
AsyncTask是更简单的UI线程实现方式。这个类允许后台操作，并允许发布结果到UI线程。而不需要去操作thread和handler
AsyncTask is designed to be a helper class around Thread and Handler
 and does not constitute a generic threading framework. AsyncTasks should ideally be used for short operations (a few seconds at the most.) If you need to keep threads running for long periods of time, it is highly recommended you use the various APIs provided by the java.util.concurrent package such as Executor, ThreadPoolExecutor and FutureTask.

AsyncTask实现原理还是Thread和handler，不过谷歌包装好了（那就用呗）。
An asynchronous task is defined by a computation that runs on a background thread and
 whose result is published on the UI thread. An asynchronous task is defined by 3 generic types, called Params, Progress and Result, and 4 steps, called onPreExecute, doInBackground, onProgressUpdate and onPostExecute.
AsyncTask的三个泛型分别对应的是Params, Progress and Result

还有四个步骤：见名思义吧
The 4 steps
When an asynchronous task is executed, the task goes through 4 steps:     
onPreExecute(), invoked on the UI thread before the task     is executed. This step is normally used to setup the task, for instance by     showing a progress bar in the user interface.     
doInBackground(Params...), invoked on the background thread     immediately after onPreExecute() finishes executing. This step is used     to perform background computation that can take a long time. The parameters     of the asynchronous task are passed to this step. The result of the computation must     be returned by this step and will be passed back to the last step. This step     can also use publishProgress(Progress...) to publish one or more units     of progress. These values are published on the UI thread, in the     onProgressUpdate(Progress...) step.
onProgressUpdate(Progress...), invoked on the UI thread after a     call to publishProgress(Progress...). The timing of the execution is     undefined. This method is used to display any form of progress in the user     interface while the background computation is still executing. For instance,     it can be used to animate a progress bar or show logs in a text field.     
onPostExecute(Result), invoked on the UI thread after the background     computation finishes. The result of the background computation is passed to     this step as a parameter. 
千万别忘了在主线程中执行！！！
Once created, a task is executed very simply:
new DownloadFilesTask().execute(url1, url2, url3);

实例：模拟下载（练手加看个大概）

布局是这个样子的

<LinearLayout 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:orientation="vertical"
    tools:context=".MainActivity">

    <ProgressBar
        android:id="@+id/progressbar"
        style="?android:attr/progressBarStyleHorizontal"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:progress="44"
        />

    <Button
        android:id="@+id/button_down"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="开始下载" />

</LinearLayout>
AsyncTask  实际上就是google帮我们封装好的线程UI同步类。通过过修饰handler和Thread类实现：代码如下

public class MainActivity extends AppCompatActivity {

    private Button mButtonDown;
    private ProgressBar mProgressBar;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mButtonDown = (Button) findViewById(R.id.button_down);
        mProgressBar = (ProgressBar) findViewById(R.id.progressbar);
        mButtonDown.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                new DownloadFilesTask().execute();
            }
        });
    }

    private class DownloadFilesTask extends AsyncTask<String, String, String> {
        private int count=0;

        /**
         * @param params   execute（）中传入的参数
         * @return       传给onPostExecute的参数
         * doInBackground不能与UI进行交互，但可以传递给onProgressUpdate，onPostExecute方法进行操作
         */
        @Override
        protected String doInBackground(String... params) {
            while (count<101){
                count++;
                publishProgress(""+count);
                try {
                    Thread.sleep(200);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            return "下载完成";
        }

        /**
         * 在doInBackground结束时执行
         * @param s
         */
        @Override
        protected void onPostExecute(String s) {
            super.onPostExecute(s);
            mButtonDown.setText(s);
        }

        /**
         * 在doInBackground中使用publishProgress(""+count)给onProgressUpdate传递参数，进行UI的更新
         * @param values
         */
        @Override
        protected void onProgressUpdate(String... values) {
            super.onProgressUpdate(values);
            int count = Integer.parseInt(values[0]);
            mProgressBar.setProgress(count);
        }
    }
}