在Android4.0之后谷歌强制要求连接网络不能在主线程进行访问，所以要开辟子线程来访问网络但是子线程中无法更新ui
所以使用Handler实现从子线程快速跳回主线程的操作。
1）Handler 和runOnUiThread()的使用
可以进行ui的更新
final Bitmap bitmap = BitmapFactory.decodeFile(file.getAbsolutePath());
//runOnUiThread 不管在哪里调用 action 也就是Runnable()的run()方法中的语句都在运行在UI线程
runOnUiThread(new Runnable() {
@Override
public void run() {
iv.setImageBitmap(bitmap);
}
});2)图片查看器案例 （如何使用图片缓存）
public class MainActivity extends AppCompatActivity {
private EditText et_path;
private ImageView iv;
private Handler handler = new Handler() {
@Override
public void handleMessage(Message msg) {
Bitmap bitmap = (Bitmap) msg.obj;
iv.setImageBitmap(bitmap);
}
};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
et_path = findViewById(R.id.et_path);
iv = findViewById(R.id.iv);
}
//[2]给按钮设置点击事件 查看指定路径图片
public void click(View v) {
new Thread() {
public void run() {
File file = new File(getCacheDir(), "test.png");
if (file.exists() && file.length() > 0) {
//使用缓存图片
System.out.println("使用缓存图片");
Bitmap cachebitmap = BitmapFactory.decodeFile(file.getAbsolutePath());
//把cachebitmap显示到iv上
Message msg = Message.obtain();
msg.obj = cachebitmap;
handler.sendMessage(msg);
} else {
//第一次访问联网获取数据
System.out.println("第一次联网获取图片");
try {
//[2.1] 获取指定路径
String path = et_path.getText().toString().trim();
//[2.2] 获取URL对象
URL url = new URL(path);
//[2.3]获取HttpURLConnection对象
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
//[2.4]设置请求的方式
conn.setRequestMethod("GET");
//[2.5]设置超时时间
conn.setConnectTimeout(5000);
//[2.6]获取服务器返回的状态码
int code = conn.getResponseCode();
if (code == 200) {
//[2.7]获取图片数据 以流的形式返还
InputStream in = conn.getInputStream();
//[2.7.1] 缓存图片 谷歌提供了缓存目录
FileOutputStream fos = new FileOutputStream(file);
int len = -1;
byte[] buffer = new byte[1024];
while ((len = in.read(buffer)) != -1) {
fos.write(buffer, 0, len);
}
fos.close();
in.close();
//[2.8]通过位图工厂把按文件路径获取 Bitmap
Bitmap bitmap = BitmapFactory.decodeFile(file.getAbsolutePath());
//使用 Message的静态方法Message.obtain()获取Message对象可以减少创建对象提高效率
Message msg = Message.obtain();
msg.obj = bitmap;
handler.sendMessage(msg);
}
} catch (Exception e) {
e.printStackTrace();
}
}
}
}.start();
}
}3) 网页源码查看器案例
public class MainActivity extends AppCompatActivity {
private EditText et_path;
private TextView tv_result;
private final int REQUESTUCESS = 0;
private final int REQUESTNOTFOUND = 1;
private final int REQUESTEXCEPTION = 2;
//在主线程中定义一个Handler
private Handler handler = new Handler() {
@Override
//这个方法在主线程中执行所以就可以在主线程中更新ui了
public void handleMessage(Message msg) {
//分类识别消息
switch (msg.what) {
case REQUESTUCESS:
//将接收到的Message转化成字符串显示在TextView上
String content = (String) msg.obj;
tv_result.setText(content);
break;
case REQUESTNOTFOUND:
Toast.makeText(getApplicationContext(), "无法连接到指定网址", Toast.LENGTH_LONG).show();
break;
case REQUESTEXCEPTION:
Toast.makeText(getApplicationContext(), "服务器忙", Toast.LENGTH_LONG).show();
break;
}
}
};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
et_path = findViewById(R.id.et_path);
tv_result = findViewById(R.id.tv_result);
}
public void click(View v) {
//[2.0]创建一个子线程
new Thread() {
public void run() {
try {
//[2.1]获取访问路径
String path = et_path.getText().toString().trim();
//[2.2]获取URL
URL url = new URL(path);
//[2.3]获取HttpURLConnection的实例 用于发送或接收数据
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
//[2.4]设置发送get请求
conn.setRequestMethod("GET");//GET要求大写可以不用写默认为HttpURLConnectionget请求
//[2.5]设置请求超时时间
conn.setConnectTimeout(5000);
//[2.6]获取服务器返回的状态码
int code = conn.getResponseCode();
//[2.7]对状态码code进行判断
if (code == 200) {
//[2.8]获取服务器返回的的数据 返回的为一个流
InputStream in = conn.getInputStream();
//[2.9] 使用定义好的工具类 把流转化成字符串
String content = StreamTools.readStram(in);
//[2.9.0] 创建Message
Message msg = new Message();
msg.what = REQUESTUCESS;//标明是哪个消息
msg.obj = content;
//2.9.1拿着我们创建的handler（助手）在子线程里对系统说要更新ui
//发送了一条消息（msg）携带数据这句话执行完我们重写的handleMessage(Message msg)就会执行
handler.sendMessage(msg);
//[2.9.1] 把数据显示到TextView上
//tv_result.setText(content);
} else {
Message msg = new Message();
msg.what = REQUESTNOTFOUND;//标明是哪个消息
handler.sendMessage(msg);
}
} catch (Exception e) {
e.printStackTrace();
Message msg = new Message();
msg.what = REQUESTEXCEPTION;//标明是哪个消息
handler.sendMessage(msg);
}
}
}.start();
}
}
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