Volley学习-浅析

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Volley

最近有用到，看官网学习了下
官网
https://developer.android.google.cn/training/volley/
1. 在主线程中传递数据 ，所以可以方法中直接操作ui控件
2. 网络请求代码，使用 HttpURLConnection实现的

1.简单请求操作

1.1所需权限

<uses-permission android:name="android.permission.INTERNET" />

1.2简单操作

final  TextView mTextView =  (TextView) findViewById(R.id.text);  
// ...  // Instantiate the RequestQueue.  
RequestQueue queue =  Volley.newRequestQueue(this);  
String url ="http://www.google.com";  // Request a string response from the provided URL.  
StringRequest stringRequest =  new  StringRequest(Request.Method.GET, url,  new  Response.Listener<String>()  {  @Override  public  void onResponse(String response)  {  // Display the first 500 characters of the response string. mTextView.setText("Response is: "+ response.substring(0,500));  }  
},  new  Response.ErrorListener()  {  @Override  public  void onErrorResponse(VolleyError error)  { mTextView.setText("That didn't work!");  }  
});  // Add the request to the RequestQueue.  
queue.add(stringRequest);

1.3取消网络请求

1. Define your tag and add it to your requests.

public  static  final  String TAG =  "MyTag";  
StringRequest stringRequest;  // Assume this exists.  
RequestQueue mRequestQueue;  // Assume this exists.  // Set the tag on the request.  
stringRequest.setTag(TAG);  // Add the request to the RequestQueue.  
mRequestQueue.add(stringRequest);

1. In your activity’s onStop() method, cancel all requests that have this tag.

@Override  
protected  void onStop ()  {  super.onStop();  if  (mRequestQueue !=  null)  { mRequestQueue.cancelAll(TAG);  }  
}

2.设置RequestQueue

2.1网络和缓存两大点

RequestQueue需要两件事来完成它的工作：一个用来执行请求的传输的网络 BasicNetwork，以及一个用来处理缓存的缓存DiskBasedCache。
BasicNetwork代码是HttpURLConnection实现的

RequestQueue mRequestQueue;
// Instantiate the cache
Cache cache = new DiskBasedCache(getCacheDir(), 1024 * 1024); // 1MB cap
// Set up the network to use HttpURLConnection as the HTTP client.
Network network = new BasicNetwork(new HurlStack());
// Instantiate the RequestQueue with the cache and network.
mRequestQueue = new RequestQueue(cache, network);
// Start the queue
mRequestQueue.start();
String url = "http://www.example.com";
// Formulate the request and handle the response.
StringRequest stringRequest = new StringRequest(Request.Method.GET, url,new Response.Listener<String>(){@Overridepublic void onResponse(String response){// Do something with the response}
},
new Response.ErrorListener(){@Overridepublic void onErrorResponse(VolleyError error){// Handle error}
});
// Add the request to the RequestQueue.
mRequestQueue.add(stringRequest);
// ...

2.2使用单例

如果你的应用经常使用网络，最好设置一个请求队列的单一实例。
1. 在Application.oncreate（）中设置RequestQueue，这种官方不推荐
2. 推荐的方法是实现一个封装RequestQueue和其他功能的单例类
注意：RequestQueue必须用Application context实例化，而不是activity context
以图片显示为例：

public class MySingleton {private static MySingleton mInstance;private RequestQueue mRequestQueue;private ImageLoader mImageLoader;private static Context mCtx;private MySingleton(Context context) {mCtx = context;mRequestQueue = getRequestQueue();mImageLoader = new ImageLoader(mRequestQueue, ImageLoader.ImageCache() {private final LruCache<String, Bitmap> = new LruCache<String, Bitmap>(20);@Overridepublic Bitmap getBitmap(String url) {return cache.get(url);}@Overridepublic void putBitmap(String url, Bitmap bitmap) {cache.put(url, bitmap);}});}public static synchronized MySingleton getInstance(Context context) {if (mInstance == null) {mInstance = new MySingleton(context);}return mInstance;}public RequestQueue getRequestQueue() {if (mRequestQueue == null) {// getApplicationContext() is key, it keeps you from leaking the// Activity or BroadcastReceiver if someone passes one in.mRequestQueue = Volley.newRequestQueue(mCtx.getApplicationContext());}return mRequestQueue;}public <T> void addToRequestQueue(Request<T> req) {getRequestQueue().add(req);}public ImageLoader getImageLoader() {return mImageLoader;}
}

以下是使用案例：

// Get a RequestQueue  
RequestQueue queue =  MySingleton.getInstance(this.getApplicationContext()). getRequestQueue();   
// ...    
// Add a request (in this example, called stringRequest) to your RequestQueue.  
MySingleton.getInstance(this).addToRequestQueue(stringRequest);

3.标准使用

• StringRequest指定url，结果以string串形式返回
• JsonObjectRequest and JsonArrayRequest (both subclasses ofJsonRequest). 指定url，结果以json或json array形式返回

原声 访问，返回json对象

String url =  "http://my-json-feed";  JsonObjectRequest jsonObjectRequest =  new  JsonObjectRequest  (Request.Method.GET, url,  null,  new  Response.Listener<JSONObject>()  {  @Override  public  void onResponse(JSONObject response)  { mTextView.setText("Response: "  + response.toString());  }  
},  new  Response.ErrorListener()  {  @Override  public  void onErrorResponse(VolleyError error)  {  // TODO: Handle error  }  
});  // Access the RequestQueue through your singleton class.  
MySingleton.getInstance(this).addToRequestQueue(jsonObjectRequest);

4.实现自定义的请求

4.1自定义请求

对于需要实现定制请求的情况，需要做以下工作：
- 继承Requst类， 表示期望返回的类型，详细可以参考StringRequest和ImageRequest
- 实现抽象方法parseNetworkResponse（）和deliverResponse（），下面将对此进行更详细的描述

4.1.1parseNetworkResponse

响应服务器返回，并信息进行对应解析、转换（如字符串、图像或JSON）

@Override
protected Response<T> parseNetworkResponse(NetworkResponse response) {try {String json = new String(response.data,HttpHeaderParser.parseCharset(response.headers));return Response.success(gson.fromJson(json, clazz),HttpHeaderParser.parseCacheHeaders(response));}// handle errors
// ...
}

注意：
parseNetworkResponse的参数NetworkResponse包含byte[], HTTP status code, 和响应头
必须返回一个 Response对象，成功时为你需要的响应对象，失败时为失败信息

如果协议中有非标准的缓冲语义，你可以这样做：

return  Response.success(myDecodedObject,HttpHeaderParser.parseCacheHeaders(response));

Volley调用parseNetworkResponse在工作线程中，这个可以确保耗时操作不会阻塞主线程（ui线程）

4.1.2deliverResponse

Volley calls you back on the main thread with the object you returned inparseNetworkResponse(). Most requests invoke a callback interface here, for example:
这块不是太理解，等分析源码了，再完善吧！

protected  void deliverResponse(T response)  { listener.onResponse(response);
}

5. Example: GsonRequest

Gson搞的一个案例

public class GsonRequest<T> extends Request<T> {private final Gson gson = new Gson();private final Class<T> clazz;private final Map<String, String> headers;private final Listener<T> listener;/*** Make a GET request and return a parsed object from JSON.** @param url URL of the request to make* @param clazz Relevant class object, for Gson's reflection* @param headers Map of request headers*/public GsonRequest(String url, Class<T> clazz, Map<String, String> headers,Listener<T> listener, ErrorListener errorListener) {super(Method.GET, url, errorListener);this.clazz = clazz;this.headers = headers;this.listener = listener;}@Overridepublic Map<String, String> getHeaders() throws AuthFailureError {return headers != null ? headers : super.getHeaders();}@Overrideprotected void deliverResponse(T response) {listener.onResponse(response);}@Overrideprotected Response<T> parseNetworkResponse(NetworkResponse response) {try {String json = new String(response.data,HttpHeaderParser.parseCharset(response.headers));return Response.success(gson.fromJson(json, clazz),HttpHeaderParser.parseCacheHeaders(response));} catch (UnsupportedEncodingException e) {return Response.error(new ParseError(e));} catch (JsonSyntaxException e) {return Response.error(new ParseError(e));}}
}

原理图

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