Use associated interface on DownloadedTempFile

content/browser/loader/downloaded_temp_file_impl.cc

Index: content/browser/loader/downloaded_temp_file_impl.cc
diff --git a/content/browser/loader/downloaded_temp_file_impl.cc b/content/browser/loader/downloaded_temp_file_impl.cc
index 3109f9026886721bb7f96332713b018d1d0df39a..7bc0cd9c8557c95cb6f4ceded0e01b3b8fc98d64 100644
--- a/content/browser/loader/downloaded_temp_file_impl.cc
+++ b/content/browser/loader/downloaded_temp_file_impl.cc
@@ -5,16 +5,33 @@
 #include "content/browser/loader/downloaded_temp_file_impl.h"
 
 #include "content/browser/loader/resource_dispatcher_host_impl.h"
+#include "mojo/public/cpp/bindings/associated_group.h"
+#include "mojo/public/cpp/bindings/strong_associated_binding.h"
 #include "mojo/public/cpp/bindings/strong_binding.h"
 
 namespace content {
 
 // static
-mojo::InterfacePtr<mojom::DownloadedTempFile> DownloadedTempFileImpl::Create(
+mojom::DownloadedTempFileAssociatedPtrInfo DownloadedTempFileImpl::Create(
+    mojo::AssociatedGroup* associated_group,
+    int child_id,
+    int request_id) {
+  mojo::AssociatedInterfacePtrInfo<mojom::DownloadedTempFile> ptr_info;
+  mojo::AssociatedInterfaceRequest<mojom::DownloadedTempFile> request;
+  associated_group->CreateAssociatedInterface(
+      mojo::AssociatedGroup::WILL_PASS_PTR, &ptr_info, &request);
+  mojo::MakeStrongAssociatedBinding(
+      base::MakeUnique<DownloadedTempFileImpl>(child_id, request_id),
+      std::move(request));

[mmenke, 2016/12/15 16:48:19]

Sorry for all the round trips, but I really have no clue what this does.  We
have a "associated_group" (Which I guess is a group of things that share a pipe)
from the ResourceHandler, and want to make a mojo object using that pipe on the
shared interface.

So the we create the interface on the pipe...But then what is a PtrInfo?  What
is an interface request?  What does making a strong binding not return anything?

I'm having trouble mapping this code to what it's actually doing.  The
method/class descriptions don't provide much help.  I'm pushing back on this
largely because I find this word soup a common problem with mojo setup, and this
seems a much more confusing soup than usual.

[mmenke, 2016/12/15 16:51:08]

And if we want Mojo to be the way for everyone to do things, including people
not directly involved in mojo development, I think code this confusing really
needs to either be better explained, renamed, or otherwise cleaned up to be more
intuitive

[tzik, 2017/01/05 10:47:56]

Hmm, I don't have good explanation that.
Yuzhu: Could you help us to understand the difference of AssociatedInterfacePtr
and AssociatedInterfacePtr*Info*? Why do we need to use PtrInfo instead of Ptr
on "associated" keyword in .mojom?
Since we don't have suitable helper function for the instance set up, I'm using
the interface request as the raw set up sequence.
I think we should add a helper to hide that as a part of implementation detail.

[yzshen1, 2017/01/13 15:55:03]

AssociatedInterfacePtrInfo is the "handle" that could be used to construct an
AssociatedInterfacePtr.

AssociatedInterfacePtr is the proxy object that you can set up to make method
calls. It is sending request and monitoring response using that "handle".

The reason why we map a mojom associated interface parameter to AIPtrInfo
instead of AIPtr: because it doesn't make sense to construct an AIPtr when you
want to pass this associated interface to the remote side.
I think it is unavoidable that mojo users have to learn about the mojo
terminology. :) 

There are documents about Mojo here:
https://www.chromium.org/developers/design-documents/mojo

In particular, we have document about associated interfaces here:
https://www.chromium.org/developers/design-documents/mojo/associated-interfaces

[mmenke, 2017/01/13 17:48:33]

I agree with this, which is why I was shocked to find all these classes and
objects undocumented or poorly documented in the code.

+  return ptr_info;
+}
+
+// static
+mojom::DownloadedTempFilePtr DownloadedTempFileImpl::CreateForTesting(
     int child_id,
     int request_id) {
   mojo::InterfacePtr<mojom::DownloadedTempFile> ptr;
-  auto binding = mojo::MakeStrongBinding(
+  mojo::MakeStrongBinding(
       base::MakeUnique<DownloadedTempFileImpl>(child_id, request_id),
       mojo::GetProxy(&ptr));
   return ptr;