[ios] Convert FlutterPlatformViewsController to smart pointer

[iskakaushik]

Prior to this change FlutterPlatformViewsController was passed around as a raw pointer. This change makes it so that this is instead shared as a shared_ptr. This also makes the current unique_ptr hold in FlutterEngine.mm to be a shared one.

[gaaclarke]

Is this getting called at least once per frame? If so it may be desirable to pass back a const reference to allow any copying of the shared_ptr.

[gaaclarke]

Another big win here, nice.

[gaaclarke]

Nice!

[gaaclarke]

LGTM with a nit that I'd avoid copying the shared_ptr so much by using const references like i've mentioned in your other PR. Especially if this stuff is happening at least once a frame.

Also, a warning, you might be introducing a retain cycle. I'd run on device to make sure that FlutterPlatformViewsController gets deleted when you delete an engine. I don't know if FlutterPlatformViewsController, AccessibilityBridge, FlutterEngine, ExternalViewEmbedder form a cycle. But it's complicated enough that it might be.

[iskakaushik]

I tried making it a const reference initially, but then ran into a problem where we pass in nullptr all the way down to the ios_external_view_embedder.mm. Also the platformViewsController object is created once per engine and all the components that use it also are created in engine or shell initialization.

About the retain cycle, good point, I will take a look and see if it's a problem.

[gaaclarke]

I tried making it a const reference initially, but then ran into a problem where we pass in nullptr all the way down to the ios_external_view_embedder.mm

That isn't a problem. There is an implicit constructor for shared_ptr that will create an rvalue whose reference will get passed on the stack, try this out:

#include <memory>
#include <iostream>

namespace {
  void foo(const std::shared_ptr<int>& ptr ) {
    std::cout << ptr.get() << std::endl;
  }
}

int main() {
  foo(nullptr);
}

[iskakaushik]

@gaaclarke makes sense. I've moved them to be const references.

[iskakaushik]

I am still investigating to see if there is a retain cycle, and also looking to see if platformViewsController on FlutterEngine can be called multiple times per frame.

[iskakaushik]

platformViewsController is not called multiple times per frame but I saw no harm in making it return a reference instead, so changed it. Also I ran the app and killed the engine and saw that the platformViewsController indeed got destroyed.