<RelativeLayout 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"
  tools:context="com.dragon.android.textbar.MainActivity$PlaceholderFragment" >

  <!-- prigressBar 进度条 -->
  <!-- progress 当前进度 -->
  <!-- indeterminate 不明确的  默认false -->
  <ProgressBar
    android:id="@+id/progressBar1"
    style="?android:attr/progressBarStyleHorizontal"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_centerInParent="true"
    android:max="100"
    android:progress="0"
    android:indeterminate="true"/>

  <Button
    android:id="@+id/button1"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentTop="true"
    android:layout_centerHorizontal="true"
    android:onClick="startLoad"
    android:layout_marginTop="86dp"
    android:background="#009FEE"
    android:text="@string/start"
    android:textColor="#ffffff" />

  <TextView
    android:id="@+id/textView1"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_above="@+id/progressBar1"
    android:background="@null"
    android:layout_alignParentLeft="true" />
  
</RelativeLayout>

<?xml version="1.0" encoding="utf-8"?>
<resources>

  <string name="app_name">hwdownload</string>
  <string name="hello_world">Hello world!</string>
  <string name="action_settings">Settings</string>
  <string name="start">开始</string>
  <string name="stop">暂停</string>
  <string name="contin">继续</string>

</resources>

package com.dragon.android.textbar;

import java.io.IOException;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.ProgressBar;
import android.widget.TextView;
import android.widget.Toast;

/**
 * 只有创建一个 View 的线程才可以改变这个 View 的UI!!! 主线程也叫 UI 线程
 */
public class MainActivity extends Activity {
  private ProgressBar progressBar1;
  private Button button1;
  private TextView textView1;

  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.fragment_main);

    progressBar1 = (ProgressBar) findViewById(R.id.progressBar1);
    button1 = (Button) findViewById(R.id.button1);
    textView1 = (TextView) findViewById(R.id.textView1);

  }

  public void startLoad(View view) {
     String text = (String) button1.getText();
    // 设置按钮内容 ----并没有用...
    button1.setText(text.equals(getResources().getString(R.string.start)) ? R.string.stop
        : (text.equals(getResources().getString(R.string.stop)) ? R.string.contin
            : R.string.stop));
    progressBar1.setIndeterminate(false);

    new Thread(new Runnable() {
      private int percent;

      @Override
      public void run() {
        try {
          // 打开 URL 必须在子线程
          URL url = new URL(
              "http://b.zol-img.com.cn/sjbizhi/images/9/540x960/1472549276394.jpg");
          HttpURLConnection conn = (HttpURLConnection) url.openConnection();
          // conn.setRequestMethod("GET");
          // conn.setReadTimeout(5000);
          // conn.setConnectTimeout(5000);

          int contentLength = conn.getContentLength();

          if (conn.getResponseCode() == HttpURLConnection.HTTP_OK) {
            InputStream is = conn.getInputStream();

            byte[] buffer = new byte[1024];
            int len = -1;
            int sum = 0;
            while ((len = is.read(buffer)) != -1) {
              sum += len;
              // 注意强转方式，防止一直为0
              percent = (int) (100.0 * sum / contentLength);
              // 在主线程上运行的子线程
              runOnUiThread(new Runnable() {

                @Override
                public void run() {
                  progressBar1.setProgress(percent);
                  textView1.setText(percent + "%");
                  if (percent == progressBar1.getMax()) {
                    Toast.makeText(MainActivity.this,
                        "下载完成!", Toast.LENGTH_SHORT)
                        .show();
                  }
                }
              });
            }
            is.close();
            conn.disconnect();
          }
        } catch (IOException e) {
          e.printStackTrace();
        }
      }
    }).start();
  }
}