ResourceLoadingDelegate class to allow common code to load files.

chrome/common/extensions/file_loading_delegate.h

+// Copyright (c) 2012 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CHROME_COMMON_EXTENSIONS_FILE_LOADING_DELEGATE_H_
+#define CHROME_COMMON_EXTENSIONS_FILE_LOADING_DELEGATE_H_
+
+#include "base/callback.h"
+
+class ExtensionResource;
+
+namespace extensions {
+
+// Use this in common code to load extension resources asynchronously.  In
+// process types that can't load resources, pointers to this type may be NULL.
+class ResourceLoadingDelegate {
+ public:
+  virtual ~FileLoadingDelegate() {}
+
+  // Loads the file represented by |resource|, parses/transforms it using
+  // |transform_on_bg_thread|, and notifies the caller when everything's done
+  // using |on_loaded| or |on_failure|.  Must be called on the UI thread.
+  //
+  // |transform_on_bg_thread| must be thread-safe since it may be called on a
+  // non-UI thread.  Usually this means it should be stateless.
+  //
+  // |on_loaded| and |on_failure| should generally be bound to a WeakPtr to deal
+  // with loads that finish after the caller is destroyed.
+  //
+  // |on_failure| may be null, in which case failure will be ignored.
+  template<typename IntermediateType>
+  StartLoad(const ExtensionResource& resource,
+            base::Callback<IntermediateType(const std::string&)>
+            transform_on_bg_thread,
+            base::Callback<void(const IntermediateType&)> on_loaded,
+            base::Callback<void()> on_failure);
+
+ protected:
+  class LoadInfo {
+   public:
+    virtual ~LoadInfo() {}
+    // Call this on a background thread
+    virtual void TransformOnBgThread(const std::string& file_data) = 0;
+    virtual void OnLoaded() = 0;
+  };
+
+ private:
+  // Subclass implementations must load the file referred to by |resource| as a
+  // string of bytes, call load_info->TransformOnBgThread(file_contents), and
+  // then call load_info->OnLoaded().  TransformOnBgThread should be called on a
+  // thread that can afford long-running operations, and OnLoaded must be called
+  // on the UI thread.
+  virtual StartLoadImpl(const ExtensionResource& resource,
+                        scoped_ptr<LoadInfo> load_info) = 0;
+};
+
+template<typename IntermediateType>
+ResourceLoadingDelegate::StartLoad(
+    const ExtensionResource& resource,
+    base::Callback<IntermediateType(const std::string&)>
+    transform_on_bg_thread,
+    base::Callback<void(const IntermediateType&)> on_loaded) {
+  // Implement LoadInfo for this particular intermediate type:
+  class LoadInfoImpl : public LoadInfo {
+   public:
+    LoadInfoImpl(base::Callback<IntermediateType(const std::string&)>
+                 transform_on_bg_thread,
+                 base::Callback<void(const IntermediateType&)> on_loaded)
+        : transform_on_bg_thread_(transform_on_bg_thread),
+          on_loaded_(on_loaded) {
+    }
+    virtual void TransformOnBgThread(const std::string& file_data) {
+      intermediate_data_ = transform_on_bg_thread_(file_data);
+    }
+    virtual void OnLoaded() {
+      on_loaded_(intermediate_data_);
+    }
+  };
+
+  // Call the subclass's implementation function.
+  StartLoadImpl(resource, scoped_ptr<LoadInfo>(
+      new LoadInfoImpl(transform_on_bg_thread, on_loaded)));
+}
+
+}  // namespace extensions
+
+#endif  // CHROME_COMMON_EXTENSIONS_FILE_LOADING_DELEGATE_H_