This patch allows embedders to split the Flutter layer tree into multiple
chunks. These chunks are meant to be composed one on top of another. This gives
embedders a chance to interleave their own contents between these chunks.
The Flutter embedder API already provides hooks for the specification of
textures for the Flutter engine to compose within its own hierarchy (for camera
feeds, video, etc..). However, not all embedders can render the contents of such
sources into textures the Flutter engine can accept. Moreover, this composition
model may have overheads that are non-trivial for certain use cases. In such
cases, the embedder may choose to specify multiple render target for Flutter to
render into instead of just one.
The use of this API allows embedders to perform composition very similar to the
iOS embedder. This composition model is used on that platform for the embedding
of UIKit view such and web view and map views within the Flutter hierarchy.
However, do note that iOS also has threading configurations that are currently
not available to custom embedders.
The embedder API updates in this patch are ABI stable and existing embedders
will continue to work are normal. For embedders that want to enable this
composition mode, the API is designed to make it easy to opt into the same in an
incremental manner.
Rendering of contents into the “root” rendering surface remains unchanged.
However, now the application can push “platform views” via a scene builder.
These platform views need to handled by a FlutterCompositor specified in a new
field at the end of the FlutterProjectArgs struct.
When a new platform view in introduced within the layer tree, the compositor
will ask the embedder to create a new render target for that platform view.
Render targets can currently be OpenGL framebuffers, OpenGL textures or software
buffers. The type of the render target returned by the embedder must be
compatible with the root render surface. That is, if the root render surface is
an OpenGL framebuffer, the render target for each platform view must either be a
texture or a framebuffer in the same OpenGL context. New render target types as
well as root renderers for newer APIs like Metal & Vulkan can and will be added
in the future. The addition of these APIs will be done in an ABI & API stable
manner.
As Flutter renders frames, it gives the embedder a callback with information
about the position of the various platform views in the effective hierarchy.
The embedder is then meant to put the contents of the render targets that it
setup and had previously given to the engine onto the screen (of course
interleaving the contents of the platform views).
Unit-tests have been added that test not only the structure and properties of
layer hierarchy given to the compositor, but also the contents of the texels
rendered by a test compositor using both the OpenGL and software rendering
backends.
Fixes b/132812775
Fixesflutter/flutter#35410
Behavior (visual) changes should be very minor. Things that are to be expected:
* A few things were not color managed correctly by the transform canvas (color emoji, some color filters). Those will be handled correctly with the tagged surfaces (although we're always transforming to sRGB, so nothing should change until we target a wider gamut).
* Image filtering will happen in the source color space, rather than the destination. Very minor.
* The transform canvas did caching of images in the destination color space. Now, the conversion happens at draw time. If there are performance issues, images can be pre-converted to the destination with makeColorSpace().
Moved the frame buffer specific logic from IOSGLContext to IOSGLRenderTarget.
use recording canvases for overlays
Support platform view overlays with gl rendering.
This also changes the overlay canvases (for both software and gl
rendering) be recording canvases, and only rasterize them after
finishing the paint traversal.
We are currently clearing the offscreen surface before rasterizing, but
as we draw the image snapshot of the offscreen surface into the onscreen
surface transparent pixels are blended with the current contents of the onscreen surface instead of replacing them. This is
particularly noticeable when embedding platform views.
For flow to manipulate the embedded UIViews during the paint traversal
it needs some hook in PaintContext.
This PR introduces a ViewEmbeder interface that is implemented by the
iOS PlatformViewsController and plumbs it into PaintContext.
The ViewEmbedder interface is mainly a place holder at this point, as
this PR is focused on just the plumbing.
This causes Skia to fall back to the ES2 shading language when the GL
driver doesn't clearly communicate that it supports the ES3 external
image extension. Doing so ensures that video playback (which relies on
external images) works on many more devices.
* Support multiple shells in a single process.
The Flutter Engine currently works by initializing a singleton shell
instance. This shell has to be created on the platform thread. The shell
is responsible for creating the 3 main threads used by Flutter (UI, IO,
GPU) as well as initializing the Dart VM. The shell, references to task
runners of the main threads as well as all snapshots used for VM
initialization are stored in singleton objects. The Flutter shell only
creates the threads, rasterizers, contexts, etc. to fully support a
single Flutter application. Current support for multiple Flutter
applications is achieved by making multiple applications share the same
resources (via the platform views mechanism).
This scheme has the following limitations:
* The shell is a singleton and there is no way to tear it down. Once you
run a Flutter application in a process, all resources managed by it
will remain referenced till process termination.
* The threads on which the shell performs its operations are all
singletons. These threads are never torn down and multiple Flutter
applications (if present) have to compete with one another on these
threads.
* Resources referenced by the Dart VM are leaked because the VM isn't
shutdown even when there are no more Flutter views.
* The shell as a target does not compile on Fuchsia. The Fuchsia content
handler uses specific dependencies of the shell to rebuild all the
shell dependencies on its own. This leads to differences in frame
scheduling, VM setup, service protocol endpoint setup, tracing, etc..
Fuchsia is very much a second class citizen in this world.
* Since threads and message loops are managed by the engine, the engine
has to know about threading and platform message loop interop on each
supported platform.
Specific updates in this patch:
* The shell is no longer a singleton and the embedder holds the unique
reference to the shell.
* Shell setup and teardown is deterministic.
* Threads are no longer managed by the shell. Instead, the shell is
given a task runner configuration by the embedder.
* Since the shell does not own its threads, the embedder can control
threads and the message loops operating on these threads. The shell is
only given references to the task runners that execute tasks on these
threads.
* The shell only needs task runner references. These references can be
to the same task runner. So, if the embedder thinks that a particular
Flutter application would not need all the threads, it can pass
references to the same task runner. This effectively makes Flutter
application run in single threaded mode. There are some places in the
shell that make synchronous calls, these sites have been updated to
ensure that they don’t deadlock.
* The test runner and the headless Dart code runner are now Flutter
applications that are effectively single threaded (since they don’t
have rendering concerns of big-boy Flutter application).
* The embedder has to guarantee that the threads and outlive the shell.
It is easy for the embedder to make that guarantee because shell
termination is deterministic.
* The embedder can create as many shell as it wants. Typically it
creates a shell per Flutter application with its own task runner
configuration. Most embedders obtain these task runners from threads
dedicated to the shell. But, it is entirely possible that the embedder
can obtain these task runners from a thread pool.
* There can only be one Dart VM in the process. The numerous shell
interact with one another to manage the VM lifecycle. Once the last
shell goes away, the VM does as well and hence all resources
associated with the VM are collected.
* The shell as a target can now compile and run on Fuchsia. The current
content handler has been removed from the Flutter engine source tree
and a new implementation has been written that uses the new shell
target.
* Isolate management has been significantly overhauled. There are no
owning references to Dart isolates within the shell. The VM owns the
only strong reference to the Dart isolate. The isolate that has window
bindings is now called the root isolate. Child isolates can now be
created from the root isolate and their bindings and thread
configurations are now inherited from the root isolate.
* Terminating the shell terminates its root isolates as well as all the
isolates spawned by this isolate. This is necessary be shell shutdown
is deterministic and the embedder is free to collect the threads on
which the isolates execute their tasks (and listen for mircrotasks
flushes on).
* Launching the root isolate is now significantly overhauled. The shell
side (non-owning) reference to an isolate is now a little state
machine and illegal state transitions should be impossible (barring
construction issues). This is the only way to manage Dart isolates in
the shell (the shell does not use the C API is dart_api.h anymore).
* Once an isolate is launched, it must be prepared (and hence move to
the ready phase) by associating a snapshot with the same. This
snapshot can either be a precompiled snapshot, kernel snapshot, script
snapshot or source file. Depending on the kind of data specified as a
snapshot as well as the capabilities of the VM running in the process,
isolate preparation can fail preparation with the right message.
* Asset management has been significantly overhauled. All asset
resolution goes through an abstract asset resolver interface. An asset
manager implements this interface and manages one or more child asset
resolvers. These asset resolvers typically resolve assets from
directories, ZIP files (legacy FLX assets if provided), APK bundles,
FDIO namespaces, etc…
* Each launch of the shell requires a separate and fully configured
asset resolver. This is necessary because launching isolates for the
engine may require resolving snapshots as assets from the asset
resolver. Asset resolvers can be shared by multiple launch instances
in multiple shells and need to be thread safe.
* References to the command line object have been removed from the
shell. Instead, the shell only takes a settings object that may be
configured from the command line. This makes it easy for embedders and
platforms that don’t have a command line (Fuchsia) to configure the
shell. Consequently, there is only one spot where the various switches
are read from the command line (by the embedder and not the shell) to
form the settings object.
* All platform now respect the log tag (this was done only by Android
till now) and each shell instance have its own log tag. This makes
logs from multiple Flutter application in the same process (mainly
Fuchsia) more easily decipherable.
* The per shell IO task runner now has a new component that is
unfortunately named the IOManager. This component manages the IO
GrContext (used for asynchronous texture uploads) that cooperates with
the GrContext on the GPU task runner associated with the shell. The
IOManager is also responsible for flushing tasks that collect Skia
objects that reference GPU resources during deterministic shell
shutdown.
* The embedder now has to be careful to only enable Blink on a single
instance of the shell. Launching the legacy text layout and rendering
engine multiple times is will trip assertions. The entirety of this
runtime has been separated out into a separate object and can be
removed in one go when the migration to libtxt is complete.
* There is a new test target for the various C++ objects that the shell
uses to interact with the Dart VM (the shell no longer use the C API
in dart_api.h). This allows engine developers to test VM/Isolate
initialization and teardown without having the setup a full shell
instance.
* There is a new test target for the testing a single shell instances
without having to configure and launch an entire VM and associated
root isolate.
* Mac, Linux & Windows used to have different target that created the
flutter_tester referenced by the tool. This has now been converted
into a single target that compiles on all platforms.
* WeakPointers vended by the fml::WeakPtrFactory(notice the difference
between the same class in the fxl namespace) add threading checks on
each use. This is enabled by getting rid of the “re-origination”
feature of the WeakPtrFactory in the fxl namespace. The side effect of
this is that all non-thread safe components have to be created, used
and destroyed on the same thread. Numerous thread safety issues were
caught by this extra assertion and have now been fixed.
* Glossary of components that are only safe on a specific thread (and
have the fml variants of the WeakPtrFactory):
* Platform Thread: Shell
* UI Thread: Engine, RuntimeDelegate, DartIsolate, Animator
* GPU Thread: Rasterizer, Surface
* IO Thread: IOManager
This patch was reviewed in smaller chunks in the following pull
requests. All comments from the pulls requests has been incorporated
into this patch:
* flutter/assets: https://github.com/flutter/engine/pull/4829
* flutter/common: https://github.com/flutter/engine/pull/4830
* flutter/content_handler: https://github.com/flutter/engine/pull/4831
* flutter/flow: https://github.com/flutter/engine/pull/4832
* flutter/fml: https://github.com/flutter/engine/pull/4833
* flutter/lib/snapshot: https://github.com/flutter/engine/pull/4834
* flutter/lib/ui: https://github.com/flutter/engine/pull/4835
* flutter/runtime: https://github.com/flutter/engine/pull/4836
* flutter/shell: https://github.com/flutter/engine/pull/4837
* flutter/synchronization: https://github.com/flutter/engine/pull/4838
* flutter/testing: https://github.com/flutter/engine/pull/4839
Fixesflutter/flutter#13018
When linear blending was disabled, we started rendering directly to FBO0 again. We can't attach stencil there, and the profile graph code triggers a path that (by default) uses it. This option forces us to use alternate rendering methods.
Note that the graph rendering code is constructing a fairly complex path. It would probably be much faster to render as a simpler series of drawRect calls for each box (which would get batched inside Skia).
This retains gamut correction (adjusting colors for screens with different capabilities), but does all blending and interpolation with sRGB-encoded values. That matches the behavior expected by most users, as well as the behavior of nearly all other systems. It also greatly simplifies the EGL code.
A future Skia change will make this behavior more of a first-class citizen, so some of these implementation details will change again, but the behavior will not. The bulk of this change (elimination of complication from the GL surface code) is permanent - it's just the SkColorSpaceXformCanvas that will be replaced.
