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typedef MessageBoxDart = int function(
int hWnd,
Pointer<Utf16> lpText,
Pointer<Utf16> lpCaption,
int uType,
);
void exampleFfi() {
final user32 = DynamicLibrary.open('user32.dll');
final messageBox =
user32.lookupFunction<MessageBoxNative, MessageBoxDart>('MessageBoxW');
final result = messageBox(
0, // no owner window
'test message'.toNativeUtf16(), // message
'window caption'.toNativeUtf16(), // window title
0, // OK button only
);
}
<code_end>
<topic_end>
<topic_start>
rendering native controls in a flutter app
because flutter content is drawn to a texture and its widget tree is entirely
internal, there’s no place for something like an android view to exist within
flutter’s internal model or render interleaved within flutter widgets. that’s a
problem for developers that would like to include existing platform components
in their flutter apps, such as a browser control.
flutter solves this by introducing platform view widgets
(androidview
and UiKitView)
that let you embed this kind of content on each platform. platform views can be
integrated with other flutter content3. each of
these widgets acts as an intermediary to the underlying operating system. for
example, on android, AndroidView serves three primary functions:
inevitably, there is a certain amount of overhead associated with this
synchronization. in general, therefore, this approach is best suited for complex
controls like google maps where reimplementing in flutter isn’t practical.
typically, a flutter app instantiates these widgets in a build() method based
on a platform test. as an example, from the
google_maps_flutter plugin:
communicating with the native code underlying the AndroidView or UiKitView
typically occurs using the platform channels mechanism, as previously described.
at present, platform views aren’t available for desktop platforms, but this is
not an architectural limitation; support might be added in the future.
<topic_end>
<topic_start>
hosting flutter content in a parent app
the converse of the preceding scenario is embedding a flutter widget in an
existing android or iOS app. as described in an earlier section, a newly created
flutter app running on a mobile device is hosted in an android activity or iOS
UIViewController. flutter content can be embedded into an existing android or
iOS app using the same embedding API.
the flutter module template is designed for easy embedding; you can either embed
it as a source dependency into an existing gradle or xcode build definition, or
you can compile it into an android archive or iOS framework binary for use
without requiring every developer to have flutter installed.
the flutter engine takes a short while to initialize, because it needs to load
flutter shared libraries, initialize the dart runtime, create and run a dart
isolate, and attach a rendering surface to the UI. to minimize any UI delays
when presenting flutter content, it’s best to initialize the flutter engine
during the overall app initialization sequence, or at least ahead of the first
flutter screen, so that users don’t experience a sudden pause while the first
flutter code is loaded. in addition, separating the flutter engine allows it to
be reused across multiple flutter screens and share the memory overhead involved
with loading the necessary libraries.
more information about how flutter is loaded into an existing android or iOS app
can be found at the load sequence, performance and memory
topic.
<topic_end>
<topic_start>
flutter web support
while the general architectural concepts apply to all platforms that flutter
supports, there are some unique characteristics of flutter’s web support that
are worthy of comment.
dart has been compiling to JavaScript for as long as the language has existed,
with a toolchain optimized for both development and production purposes. many
important apps compile from dart to JavaScript and run in production today,
including the advertiser tooling for google ads.
because the flutter framework is written in dart, compiling it to JavaScript was
relatively straightforward.
however, the flutter engine, written in c++,
is designed to interface with the
underlying operating system rather than a web browser.
a different approach is therefore required.
on the web, flutter provides a reimplementation of the
engine on top of standard browser APIs.
we currently have two options for
rendering flutter content on the web: HTML and WebGL.
in HTML mode, flutter uses HTML, CSS, canvas, and SVG.
to render to WebGL, flutter uses a version of skia
compiled to WebAssembly called
CanvasKit.
while HTML mode offers the best code size characteristics,
CanvasKit provides the fastest path to the
browser’s graphics stack,
and offers somewhat higher graphical fidelity with the
native mobile targets4.
the web version of the architectural layer diagram is as follows:
perhaps the most notable difference compared to other platforms on which flutter
runs is that there is no need for flutter to provide a dart runtime. instead,