Engine Support for Dynamic View Resizing #173610

LouiseHsu · 2025-08-12T08:33:56Z

Part of #169147 (iOS) and #149033 (Android). Implementation PRs coming up!

This enables support for dynamic view resizing in non-web embedders. While this PR enables support, it does not implement support.

Related PRs:
dart:ui: flutter/engine#48090
framework: #138648
framework reland: #140918
Web: flutter/engine#50271

Pre-launch Checklist

I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
I read the [Tree Hygiene] wiki page, which explains my responsibilities.
I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
I signed the [CLA].
I listed at least one issue that this PR fixes in the description above.
I updated/added relevant documentation (doc comments with ///).
I added new tests to check the change I am making, or this PR is [test-exempt].
I followed the [breaking change policy] and added [Data Driven Fixes] where supported.
All existing and new tests are passing.

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Code Review

This pull request introduces the necessary engine-side support for dynamic view resizing. The changes correctly plumb through view constraints from the embedder to the framework. My review identified a functional issue where new constraint parameters are ignored in hooks.dart, a documentation error in platform_dispatcher.dart, a minor typo, and a C++ style guide violation in shell.h. Addressing these points will improve the correctness and quality of the implementation.

engine/src/flutter/lib/ui/platform_dispatcher.dart

engine/src/flutter/shell/common/shell.h

LouiseHsu · 2025-08-12T21:50:35Z

This PR still needs tests btw.

mboetger · 2025-08-12T21:57:34Z

engine/src/flutter/lib/ui/window.dart

-      size?.width ?? physicalSize.width,
-      size?.height ?? physicalSize.height,
+      size?.width ?? _viewConfiguration.viewConstraints.maxWidth,
+      size?.height ?? _viewConfiguration.viewConstraints.maxHeight,


In my testing this didn't need to change.

interesting. let me try that

I believe because the size is calculated from constraints in view.dart and then passed to the render function here

mboetger · 2025-08-12T21:59:48Z

engine/src/flutter/shell/common/shell.cc

@@ -1080,7 +1079,6 @@ void Shell::OnPlatformViewSetViewportMetrics(int64_t view_id,

  if (metrics.device_pixel_ratio <= 0 || metrics.physical_width <= 0 ||
      metrics.physical_height <= 0) {


This will need to be updated if we are passing 0 as width or height. Should probably consider the constraints:

if (metrics.device_pixel_ratio <= 0) { return; } if (metrics.physical_width == 0 && !(metrics.min_width_constraint > 0 || metrics.max_width_constraint > 0)) { FML_LOG(ERROR) << "Ignoreing invalid viewport metrics width!"; return; } if (metrics.physical_height == 0 && !(metrics.min_height_constraint > 0 || metrics.max_height_constraint > 0)) { FML_LOG(ERROR) << "Ignoreing invalid viewport metrics height!"; return; }

Why are we passing 0 as width or height? That's not immediately obvious to me

If width/height is just set to WRAP_CONTENT for Android it will be 0. I'm still working on the best way to force it to be a certain initial width/height there had to be at least a minimum height of 1 pixel - so it may not end up being any issue, but initial testing I definitely had to add a check for the min/max constraints.

ah good point.

engine/src/flutter/lib/ui/window.dart

engine/src/flutter/lib/ui/window/viewport_metrics.cc

loic-sharma · 2025-08-12T22:11:56Z

engine/src/flutter/shell/common/shell.cc

+    bool has_given_constraints = (metrics.min_width_constraint != 0.0 ||
+                                  metrics.max_width_constraint != 0.0 ||
+                                  metrics.min_height_constraint != 0.0 ||
+                                  metrics.max_height_constraint != 0.0);
+
+    if (has_given_constraints) {
+      expected_frame_constraints_[view_id] = {
+          .min_width = metrics.min_width_constraint,
+          .max_width = metrics.max_width_constraint,
+          .min_height = metrics.min_height_constraint,
+          .max_height = metrics.max_height_constraint,
+
+      };
+    } else {
+      expected_frame_constraints_[view_id] = {
+          .min_width = metrics.physical_width,
+          .max_width = metrics.physical_width,
+          .min_height = metrics.physical_height,
+          .max_height = metrics.physical_height,
+      };
+    }


If the embedder does not support content-sized views, it should pass tight constraints. If so, can we simplify this logic to just use the constraints? (Feel free to push back on this feedback if I'm missing something!)

Suggested change

bool has_given_constraints = (metrics.min_width_constraint != 0.0 ||

metrics.max_width_constraint != 0.0 ||

metrics.min_height_constraint != 0.0 ||

metrics.max_height_constraint != 0.0);

if (has_given_constraints) {

expected_frame_constraints_[view_id] = {

.min_width = metrics.min_width_constraint,

.max_width = metrics.max_width_constraint,

.min_height = metrics.min_height_constraint,

.max_height = metrics.max_height_constraint,

};

} else {

expected_frame_constraints_[view_id] = {

.min_width = metrics.physical_width,

.max_width = metrics.physical_width,

.min_height = metrics.physical_height,

.max_height = metrics.physical_height,

};

}

expected_frame_constraints_[view_id] = {

.min_width = metrics.min_width_constraint,

.max_width = metrics.max_width_constraint,

.min_height = metrics.min_height_constraint,

.max_height = metrics.max_height_constraint,

};

loic-sharma · 2025-08-12T22:14:59Z

engine/src/flutter/shell/common/shell.cc

  if (metrics.device_pixel_ratio <= 0 || metrics.physical_width <= 0 ||
      metrics.physical_height <= 0) {


Could we add a check that the size is within the constraints?

loic-sharma · 2025-08-12T22:22:00Z

engine/src/flutter/shell/common/shell.h

+  double max_width = 0.0;
+  double min_height = 0.0;
+  double max_height = 0.0;
+};


FYI, //shell/platform/common has:

flutter/engine/src/flutter/shell/platform/common/geometry.h

Lines 86 to 103 in f6bbf76

class BoxConstraints {

public:

BoxConstraints() = default;

BoxConstraints(const std::optional<Size>& smallest,

const std::optional<Size>& biggest)

: smallest_(smallest.value_or(Size(0, 0))),

biggest_(

biggest.value_or(Size(std::numeric_limits<double>::infinity(),

std::numeric_limits<double>::infinity()))) {}

BoxConstraints(const BoxConstraints& other) = default;

Size biggest() const { return biggest_; }

Size smallest() const { return smallest_; }

private:

Size smallest_ = Size(0, 0);

Size biggest_ = Size(std::numeric_limits<double>::infinity(),

std::numeric_limits<double>::infinity());

};

I'd consider moving that down to //fml/geometry.h or something

engine/src/flutter/shell/common/shell.h

mboetger · 2025-08-12T22:29:20Z

engine/src/flutter/lib/ui/window/viewport_metrics.h

If you change the constructor, you'll need to change where it's called - platform_view_android_jni_impl.cc (even though I know you didn't want to change platform code). Otherwise, you'll need to make a new constructor.

LouiseHsu added 4 commits August 12, 2025 01:21

init

d07aee8

remove accidental bundled change

62a07aa

remove accidental changes

647d622

format

9d49640

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Engine Support for Dynamic View Resizing #173610

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		@@ -1080,7 +1079,6 @@ void Shell::OnPlatformViewSetViewportMetrics(int64_t view_id,

		if (metrics.device_pixel_ratio <= 0 \|\| metrics.physical_width <= 0 \|\|
		metrics.physical_height <= 0) {

	class BoxConstraints {
	public:
	BoxConstraints() = default;
	BoxConstraints(const std::optional<Size>& smallest,
	const std::optional<Size>& biggest)
	: smallest_(smallest.value_or(Size(0, 0))),
	biggest_(
	biggest.value_or(Size(std::numeric_limits<double>::infinity(),
	std::numeric_limits<double>::infinity()))) {}
	BoxConstraints(const BoxConstraints& other) = default;
	Size biggest() const { return biggest_; }
	Size smallest() const { return smallest_; }

	private:
	Size smallest_ = Size(0, 0);
	Size biggest_ = Size(std::numeric_limits<double>::infinity(),
	std::numeric_limits<double>::infinity());
	};

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LouiseHsu commented Aug 12, 2025 •

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