制作下载器

利用近期学习的知识做一个下载器，输入网址即可下载的下载器。
首先复习上节课的知识。
多线程的那些事：
->何为线程，多线程？
->为什么要使用多线程？
帮助提高效率，耗时操作并行。多线程可防止一条线卡死。
->ANR（Application Not Responding）
->什么时候使用多线程？
->Android当中的多线程。
总结来说：线程就相当于做饭的过程，OnCreate一步一步按顺序执行：摘菜->洗菜->切菜->做菜->吃菜。 电饭锅相当于另外一个线程，帮做米饭，同时你吃菜，电饭锅会在蒸熟米饭后语音或其他方式提醒。
如何使用多线程？
->Main/UI Thread和Worker Thread。
->Main/UI Thread和Worker Thread之间的通信。
->Thread/Runnable.
->线程安全
->Activity.runOnUiThread(Runnable)
->View.post(Runnable)
->View.post Delayed(Runnable,long)
->Handler
->Async Task
->同步代码块（Synchronized block）.
下载器的制作方法：
在activity_main.xml文件中做好下载器的下载界面：
下载按钮：

输入网址的文本框：

进度条：

进度更新显示：

输入网址提示：

在MainActivity.java文件中：

public class MainActivity extends Activity implements View.OnClickListener{
    public static final String GEEK_BAND = "RENHONG";
    private Handler mHandler = new DownloadHandler(this);
    private TextView mTextView;
    private Button mDownloadButton;
    private ProgressBar mProgressBar;
    private EditText mEditText;
    public TextView getmTextView() {
        return mTextView;
    }
    public ProgressBar getmProgressBar() {
        return mProgressBar;
    }
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mTextView = (TextView) findViewById(R.id.text_view);
        mEditText=(EditText) findViewById(R.id.EditText);
        mDownloadButton = (Button) findViewById(R.id.download_button);
        mProgressBar = (ProgressBar) findViewById(R.id.progressBar);
        mDownloadButton.setOnClickListener(this);
    }
    @Override
    public void onClick(View v) {
        switch (v.getId()){
            case R.id.download_button:
                String apkurl=mEditText.getText().toString();
                new TestTask().execute(apkurl);
                break;
        }
    }
    class TestTask extends AsyncTask{
        @Override
        protected void onPreExecute() {
            super.onPreExecute();
        }
        @Override
        protected String doInBackground(String...prams) {
            try {
                URL url = new URL(prams[0]);
                URLConnection urlConnection = url.openConnection();
                InputStream inputStream = urlConnection.getInputStream();
                int contentLength = urlConnection.getContentLength();
 // 要下载的文件的大小
                String downloadFoldersName = Environment.getExternalStorageDirectory() + File.separator + GEEK_BAND + File.separator;
                File file = new File(downloadFoldersName);
                if(!file.exists()){
                    file.mkdir();
                }
                String fileName = downloadFoldersName + "test.apk";
                File apkFile = new File(fileName);
                if(apkFile.exists()){
                    apkFile.delete();
                }
                int downloadSize = 0;
                byte[] bytes = new byte[1024];
                int length = 0;
                OutputStream outputStream = new FileOutputStream(fileName);
                while ((length = inputStream.read(bytes)) != -1){
                    outputStream.write(bytes, 0, length);
                    downloadSize += length;
                    int progress = downloadSize * 100/ contentLength;
                    Message message = mHandler.obtainMessage();
                    message.obj = progress;
                    message.what = 0;
                    mHandler.sendMessage(message);
                }
                inputStream.close();
                outputStream.close();
            }
       catch (IOException e) {
                e.printStackTrace();
            }
            return null;
        }
        @Override
        protected void onPostExecute(String s) {
            super.onPostExecute(s);
        }
    }
    public static class DownloadHandler extends Handler{
    public final WeakReferencemActivity;
    public DownloadHandler(MainActivity activity){
        mActivity = new WeakReference<>(activity);
    }
        public void handleMessage(Message msg){
            super.handleMessage(msg);
            MainActivity activity = mActivity.get();
            switch (msg.what){
                case 0:
                    int progress = (int)msg.obj;
                    activity.getmProgressBar().setProgress(progress);
                    activity.getmTextView().setText("已下载："+progress);
                    if (progress == 100){
                        Toast.makeText(activity,"下载完成",Toast.LENGTH_SHORT).show();
                    }
                    break;
            }
        }
    }
}

在Mainifest.xml文件中注册网络：