OKHttp Retrofit 可以離線使用快取資料嗎？

Retrofit 與 OKHttp 可以在離線時使用快取資料嗎？

本題探討了使用 Retrofit 和 OKHttp 來快取 HTTP 回應用於離線存取。問題中提供的程式碼遵循常見方法，但無法離線檢索快取的回應表示需要一些額外的配置。

Edit for Retrofit 2.x

對於 Retrofit 2.x，離線快取的首選方法是使用 OkHttp 攔截器。

1. 建立一個攔截器：

   <code class="java">private static final Interceptor REWRITE_CACHE_CONTROL_INTERCEPTOR = new Interceptor() {
       @Override public Response intercept(Chain chain) throws IOException {
           Response originalResponse = chain.proceed(chain.request());
           if (Utils.isNetworkAvailable(context)) {
               int maxAge = 60; // read from cache for 1 minute
               return originalResponse.newBuilder()
                       .header("Cache-Control", "public, max-age=" + maxAge)
                       .build();
           } else {
               int maxStale = 60 * 60 * 24 * 28; // tolerate 4-weeks stale
               return originalResponse.newBuilder()
                       .header("Cache-Control", "public, only-if-cached, max-stale=" + maxStale)
                       .build();
           }
       }
   };</code>

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2. 設定客戶端：

   <code class="java">OkHttpClient client = new OkHttpClient();
   client.networkInterceptors().add(REWRITE_CACHE_CONTROL_INTERCEPTOR);
   
   //setup cache
   File httpCacheDirectory = new File(context.getCacheDir(), "responses");
   int cacheSize = 10 * 1024 * 1024; // 10 MiB
   Cache cache = new Cache(httpCacheDirectory, cacheSize);
   
   //add cache to the client
   client.setCache(cache);</code>

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3. 設定客戶端：

   <code class="java">Retrofit retrofit = new Retrofit.Builder()
           .baseUrl(BASE_URL)
           .client(client)
           .addConverterFactory(GsonConverterFactory.create())
           .build();</code>

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<code class="java">File httpCacheDirectory = new File(context.getCacheDir(), "responses");

Cache cache = null;
try {
   cache = new Cache(httpCacheDirectory, 10 * 1024 * 1024);
} catch (IOException e) {
   Log.e("OKHttp", "Could not create http cache", e);
}

OkHttpClient okHttpClient = new OkHttpClient();
if (cache != null) {
   okHttpClient.setCache(cache);
}</code>

OKHttp 2.0.x（原始答案）

原始答案

<code class="java">RestAdapter.Builder builder= new RestAdapter.Builder()
   .setRequestInterceptor(new RequestInterceptor() {
        @Override
        public void intercept(RequestFacade request) {
            request.addHeader("Accept", "application/json;versions=1");
            if (MyApplicationUtils.isNetworkAvailable(context)) {
                int maxAge = 60; // read from cache for 1 minute
                request.addHeader("Cache-Control", "public, max-age=" + maxAge);
            } else {
                int maxStale = 60 * 60 * 24 * 28; // tolerate 4-weeks stale
                request.addHeader("Cache-Control", 
                    "public, only-if-cached, max-stale=" + maxStale);
            }
        }
});</code>

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