media::BindToLoop() is born, with example uses to slim down WebMediaPlayerProxy.

media/base/bind_to_loop.h

+// This file was GENERATED by command:
+//     pump.py bind_to_loop.h.pump
+// DO NOT EDIT BY HAND!!!
+
+
+// Copyright (c) 2012 The Chromium Authors. All rights reserved.

[Ami GONE FROM CHROMIUM, 2012/08/16 20:25:53]

Now with tests!

+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef MEDIA_BASE_BIND_TO_LOOP_H_
+#define MEDIA_BASE_BIND_TO_LOOP_H_
+
+#include "base/bind.h"
+#include "base/message_loop_proxy.h"
+
+// This is a helper utility for base::Bind()ing callbacks on to particular
+// MessageLoops.  A typical use is when |a| (of class |A|) wants to hand a
+// callback such as base::Bind(&A::AMethod, a) to |b|, but needs to ensure that
+// when |b| executes the callback, it does so on a particular MessageLoop.
+//
+// Typical usage: request to be called back on the current thread:
+// other->StartAsyncProcessAndCallMeBack(
+//    media::BindToLoop(MessageLoopProxy::current(),
+//                      base::Bind(&MyClass::MyMethod, this)));
+
+namespace media {
+
+static void TrampolineRun(

[awong, 2012/08/16 17:19:21]

Should these be in an internal namespace?

Also, should these be out-of-lined?

[Ami GONE FROM CHROMIUM, 2012/08/16 20:25:53]

The "static"s are leftover from prototyping.  Gone.

[Ami GONE FROM CHROMIUM, 2012/08/16 20:25:53]

You mean put TrampolineRun in media::internal but leave BindToLoop in media?  I
didn't think it was worth it when there were $ARITY BTL's, but now that there's
only one using the struct trick, done.
Which "these"?  The arity>0 cases are templatized, so must appear in the .h. 
Moving just the arity==0 cases out-of-line doesn't seem worthwhile to me.

+    scoped_refptr<base::MessageLoopProxy> loop, const base::Closure& cb) {

[awong, 2012/08/16 17:19:21]

Can |loop| be a const ref?

[Ami GONE FROM CHROMIUM, 2012/08/16 20:25:53]

Done.

+  loop->PostTask(FROM_HERE, base::Bind(cb));
+}
+
+static base::Closure BindToLoop(
+    scoped_refptr<base::MessageLoopProxy> loop, const base::Closure& cb) {
+  return base::Bind(&TrampolineRun, loop, cb);
+}
+
+template<typename A1>
+static void TrampolineRun(
+    scoped_refptr<base::MessageLoopProxy> loop,
+    const base::Callback<void(A1)>& cb,
+    A1 a1) {

[awong, 2012/08/16 17:19:21]

We're not unwrapping A1 into a ForwardType like Callback does.  It also doesn't
use CallbackForward() to pass the actual |a1| into the later Bind().

I think this might break some things such as
  - array types
  - move-only types
  - non-const reference

I'm guessing you may not care, but can we verify and call it out in the comment
as an expected "won't work?"

[Ami GONE FROM CHROMIUM, 2012/08/16 20:25:53]

I'd rather things worked across the board, and I was able to make scoped_ptr
(move-only) work by adding CallbackForward.

As discussed offline, ForwardType doesn't work here, necessitating arg copy and
excluding arrays (added comment).

You mentioned non-const refs; aren't those banned from Bind() in the first
place?

+  loop->PostTask(FROM_HERE, base::Bind(cb, a1));
+}
+
+template<typename A1>
+static base::Callback<void(A1)> BindToLoop(
+    scoped_refptr<base::MessageLoopProxy> loop,
+    const base::Callback<void(A1)>& cb) {
+  return base::Bind(&TrampolineRun<A1>, loop, cb);
+}
+
+template<typename A1, typename A2>
+static void TrampolineRun(
+    scoped_refptr<base::MessageLoopProxy> loop,
+    const base::Callback<void(A1, A2)>& cb,
+    A1 a1, A2 a2) {
+  loop->PostTask(FROM_HERE, base::Bind(cb, a1, a2));
+}
+
+template<typename A1, typename A2>
+static base::Callback<void(A1, A2)> BindToLoop(
+    scoped_refptr<base::MessageLoopProxy> loop,
+    const base::Callback<void(A1, A2)>& cb) {
+  return base::Bind(&TrampolineRun<A1, A2>, loop, cb);
+}
+
+template<typename A1, typename A2, typename A3>
+static void TrampolineRun(

[awong, 2012/08/16 17:19:21]

If you use the funciton-signature trick to make TrampolineRun a single-argument
template, I think you can remove all the specializations of BindToLoop.  You'd
have to embed TrampileRun into a class or struct though.

template <typename T> struct Trampoline;
template <typename A1, typename A2>
struct TrampolineHelper<void(A1, A2)> {
  Callback<void(A1, A2)> Do(
     scoped_refptr<base::MessageLoopProxy> loop,
     const base::Callback<void(A1, A2)>& cb,
     A1 a1, A2 a2)
}

and then just one

template <typename T>
static base::Callback<T> BindToLoop(
    scoped_refptr<base::MessageLoopProxy> loop,
    const base::Callback<T>& cb) {
  return base::Bind(&TrampolineHelper::Do, loop, cb);
}

[Ami GONE FROM CHROMIUM, 2012/08/16 20:25:53]

Nice, done.

+    scoped_refptr<base::MessageLoopProxy> loop,
+    const base::Callback<void(A1, A2, A3)>& cb,
+    A1 a1, A2 a2, A3 a3) {
+  loop->PostTask(FROM_HERE, base::Bind(cb, a1, a2, a3));
+}
+
+template<typename A1, typename A2, typename A3>
+static base::Callback<void(A1, A2, A3)> BindToLoop(
+    scoped_refptr<base::MessageLoopProxy> loop,
+    const base::Callback<void(A1, A2, A3)>& cb) {
+  return base::Bind(&TrampolineRun<A1, A2, A3>, loop, cb);
+}
+
+template<typename A1, typename A2, typename A3, typename A4>
+static void TrampolineRun(
+    scoped_refptr<base::MessageLoopProxy> loop,
+    const base::Callback<void(A1, A2, A3, A4)>& cb,
+    A1 a1, A2 a2, A3 a3, A4 a4) {
+  loop->PostTask(FROM_HERE, base::Bind(cb, a1, a2, a3, a4));
+}
+
+template<typename A1, typename A2, typename A3, typename A4>
+static base::Callback<void(A1, A2, A3, A4)> BindToLoop(
+    scoped_refptr<base::MessageLoopProxy> loop,
+    const base::Callback<void(A1, A2, A3, A4)>& cb) {
+  return base::Bind(&TrampolineRun<A1, A2, A3, A4>, loop, cb);
+}
+
+template<typename A1, typename A2, typename A3, typename A4, typename A5>
+static void TrampolineRun(
+    scoped_refptr<base::MessageLoopProxy> loop,
+    const base::Callback<void(A1, A2, A3, A4, A5)>& cb,
+    A1 a1, A2 a2, A3 a3, A4 a4, A5 a5) {
+  loop->PostTask(FROM_HERE, base::Bind(cb, a1, a2, a3, a4, a5));
+}
+
+template<typename A1, typename A2, typename A3, typename A4, typename A5>
+static base::Callback<void(A1, A2, A3, A4, A5)> BindToLoop(
+    scoped_refptr<base::MessageLoopProxy> loop,
+    const base::Callback<void(A1, A2, A3, A4, A5)>& cb) {
+  return base::Bind(&TrampolineRun<A1, A2, A3, A4, A5>, loop, cb);
+}
+
+template<typename A1, typename A2, typename A3, typename A4, typename A5,
+    typename A6>
+static void TrampolineRun(
+    scoped_refptr<base::MessageLoopProxy> loop,
+    const base::Callback<void(A1, A2, A3, A4, A5, A6)>& cb,
+    A1 a1, A2 a2, A3 a3, A4 a4, A5 a5, A6 a6) {
+  loop->PostTask(FROM_HERE, base::Bind(cb, a1, a2, a3, a4, a5, a6));
+}
+
+template<typename A1, typename A2, typename A3, typename A4, typename A5,
+    typename A6>
+static base::Callback<void(A1, A2, A3, A4, A5, A6)> BindToLoop(
+    scoped_refptr<base::MessageLoopProxy> loop,
+    const base::Callback<void(A1, A2, A3, A4, A5, A6)>& cb) {
+  return base::Bind(&TrampolineRun<A1, A2, A3, A4, A5, A6>, loop, cb);
+}
+
+template<typename A1, typename A2, typename A3, typename A4, typename A5,
+    typename A6, typename A7>
+static void TrampolineRun(
+    scoped_refptr<base::MessageLoopProxy> loop,
+    const base::Callback<void(A1, A2, A3, A4, A5, A6, A7)>& cb,
+    A1 a1, A2 a2, A3 a3, A4 a4, A5 a5, A6 a6, A7 a7) {
+  loop->PostTask(FROM_HERE, base::Bind(cb, a1, a2, a3, a4, a5, a6, a7));
+}
+
+template<typename A1, typename A2, typename A3, typename A4, typename A5,
+    typename A6, typename A7>
+static base::Callback<void(A1, A2, A3, A4, A5, A6, A7)> BindToLoop(
+    scoped_refptr<base::MessageLoopProxy> loop,
+    const base::Callback<void(A1, A2, A3, A4, A5, A6, A7)>& cb) {
+  return base::Bind(&TrampolineRun<A1, A2, A3, A4, A5, A6, A7>, loop, cb);
+}
+
+}  // namespace media
+
+#endif  // MEDIA_BASE_BIND_TO_LOOP_H_