Introduce IO thread for background tasks

Currently we run some tasks on the UI thread that shouldn't block the frame,
such as decoding images. This patch introduces a background thread on which we
can run these tasks.

sky/engine/core/script/dart_controller.cc

@@ -54,7 +54,7 @@ void CallHandleMessage(base::WeakPtr<DartState> dart_state) {
 
 void MessageNotifyCallback(Dart_Isolate dest_isolate) {
   DCHECK(Platform::current());
-  Platform::current()->mainThreadTaskRunner()->PostTask(FROM_HERE,
+  Platform::current()->GetUITaskRunner()->PostTask(FROM_HERE,
       base::Bind(&CallHandleMessage, DartState::From(dest_isolate)->GetWeakPtr()));
 }
 

sky/engine/platform/SharedBuffer.cpp

@@ -126,7 +126,7 @@ static void didCreateSharedBuffer(SharedBuffer* buffer)
     MutexLocker locker(statsMutex());
     liveBuffers().add(buffer);
 
-    Platform::current()->mainThreadTaskRunner()->PostTask(FROM_HERE, base::Bind(&printStats));
+    Platform::current()->GetUITaskRunner()->PostTask(FROM_HERE, base::Bind(&printStats));
 }
 
 static void willDestroySharedBuffer(SharedBuffer* buffer)

sky/engine/public/platform/Platform.h

@@ -170,8 +170,8 @@ public:
     // Returns a value such as "en-US".
     virtual WebString defaultLocale() { return WebString(); }
 
-    virtual base::SingleThreadTaskRunner* mainThreadTaskRunner() { return 0; }
-
+    virtual base::SingleThreadTaskRunner* GetUITaskRunner() { return 0; }
+    virtual base::SingleThreadTaskRunner* GetIOTaskRunner() { return 0; }
 
     // Vibration -----------------------------------------------------------
 

sky/shell/shell.cc

@@ -62,9 +62,15 @@ Shell::Shell(scoped_ptr<ServiceProviderContext> service_provider_context)
 
   gpu_thread_.reset(new base::Thread("gpu_thread"));
   gpu_thread_->StartWithOptions(options);
+  gpu_task_runner_ = gpu_thread_->message_loop()->task_runner();
 
   ui_thread_.reset(new base::Thread("ui_thread"));
   ui_thread_->StartWithOptions(options);
+  ui_task_runner_ = ui_thread_->message_loop()->task_runner();
+
+  io_thread_.reset(new base::Thread("io_thread"));
+  io_thread_->StartWithOptions(options);
+  io_task_runner_ = io_thread_->message_loop()->task_runner();
 
   ui_task_runner()->PostTask(FROM_HERE, base::Bind(&Engine::Init));
 }

sky/shell/shell.h

@@ -25,12 +25,16 @@ class Shell {
       scoped_ptr<ServiceProviderContext> service_provider_context);
   static Shell& Shared();
 
-  scoped_refptr<base::SingleThreadTaskRunner> gpu_task_runner() const {
-    return gpu_thread_->message_loop()->task_runner();
+  base::SingleThreadTaskRunner* gpu_task_runner() const {
+    return gpu_task_runner_.get();
   }
 
-  scoped_refptr<base::SingleThreadTaskRunner> ui_task_runner() const {
-    return ui_thread_->message_loop()->task_runner();
+  base::SingleThreadTaskRunner* ui_task_runner() const {
+    return ui_task_runner_.get();
+  }
+
+  base::SingleThreadTaskRunner* io_task_runner() const {
+    return io_task_runner_.get();
   }
 
   ServiceProviderContext* service_provider_context() const {
@@ -47,6 +51,12 @@ class Shell {
 
   scoped_ptr<base::Thread> gpu_thread_;
   scoped_ptr<base::Thread> ui_thread_;
+  scoped_ptr<base::Thread> io_thread_;
+
+  scoped_refptr<base::SingleThreadTaskRunner> gpu_task_runner_;
+  scoped_refptr<base::SingleThreadTaskRunner> ui_task_runner_;
+  scoped_refptr<base::SingleThreadTaskRunner> io_task_runner_;
+
   scoped_ptr<ServiceProviderContext> service_provider_context_;
   TracingController tracing_controller_;
 

sky/shell/ui/platform_impl.cc

@@ -4,11 +4,12 @@
 
 #include "sky/shell/ui/platform_impl.h"
 
+#include "sky/shell/shell.h"
+
 namespace sky {
 namespace shell {
 
-PlatformImpl::PlatformImpl()
-    : main_thread_task_runner_(base::MessageLoop::current()->task_runner()) {
+PlatformImpl::PlatformImpl() {
 }
 
 PlatformImpl::~PlatformImpl() {
@@ -18,8 +19,12 @@ blink::WebString PlatformImpl::defaultLocale() {
   return blink::WebString::fromUTF8("en-US");
 }
 
-base::SingleThreadTaskRunner* PlatformImpl::mainThreadTaskRunner() {
-  return main_thread_task_runner_.get();
+base::SingleThreadTaskRunner* PlatformImpl::GetUITaskRunner() {
+  return Shell::Shared().ui_task_runner();
+}
+
+base::SingleThreadTaskRunner* PlatformImpl::GetIOTaskRunner() {
+  return Shell::Shared().io_task_runner();
 }
 
 }  // namespace shell

sky/shell/ui/platform_impl.h

@@ -18,11 +18,11 @@ class PlatformImpl : public blink::Platform {
 
   // blink::Platform methods:
   blink::WebString defaultLocale() override;
-  base::SingleThreadTaskRunner* mainThreadTaskRunner() override;
 
- private:
-  scoped_refptr<base::SingleThreadTaskRunner> main_thread_task_runner_;
+  base::SingleThreadTaskRunner* GetUITaskRunner() override;
+  base::SingleThreadTaskRunner* GetIOTaskRunner() override;
 
+ private:
   DISALLOW_COPY_AND_ASSIGN(PlatformImpl);
 };