fix: InteractiveViewer failing to render when alignment is set

[ndonkoHenri]

Problem

InteractiveViewer passed its alignment property to Flutter's InteractiveViewer(alignment:) unparsed. Control.get<T> infers T from the parameter and returns the wire value, an {x, y} map, as is, so any alignment failed the cast in build():

type '_Map<dynamic, dynamic>' is not a subtype of type 'Alignment?'

Flutter then replaced the viewer with its error widget, a grey box in release builds, and the message only reached Python as page.on_error. JS web builds omit that implicit cast (dart2js -O4), so there the map reached Transform and the viewer painted nothing, raising a NoSuchMethodError for every frame or pointer event that touched it. 0.28.x parsed the value with parseAlignment; the parser was lost in the 1.0 rewrite, so this has been broken since 0.80.0.

Fixes #6744

Changes

• Dart: read alignment with getAlignment.
• Docstring: alignment was described as the position of the content. Flutter passes it to Transform.alignment, so it is the origin the content is scaled around and never moves the content. The docstring now says so and points to Container.alignment for positioning, and notes that gestures and the pan() limits assume a top-left origin.
• Changelog entry.

Not in this PR: making alignment position the content instead (by wrapping it in Align), and hardening Control.get<T> so a mistyped value fails with the control and property name. #6684 was the same cast bug in GestureDetector.

Testing

• A widget test that builds the viewer from a MessagePack-shaped Control tree reproduced the cast error on main and passed with the fix. It also checked that the parsed value reaches Flutter's InteractiveViewer and that nothing is passed when alignment is unset. The existing packages/flet tests pass.
• The issue's sample: the published 1.0.1 macOS client showed the grey box and the cast error above, and a JS web build of main an empty viewer. A web client built from this branch renders the content with no page.on_error reports.
• The docstring's note, measured with the fix in a 900×650 viewer with alignment=Alignment.CENTER: one wheel tick moves the point under the pointer by about (-47, -34) px, and at 2.5× the pan limits are off by (-675, -487) px; with no alignment both are exact. zoom(2) scales around the center, but pan() then stops short of one edge.
• flutter analyze, dart format and ruff.

Test code

import flet as ft


def main(page: ft.Page):
    page.add(
        ft.InteractiveViewer(
            content=ft.Text("InteractiveViewer child", size=40, color=ft.Colors.RED),
            width=900,
            height=650,
            alignment=ft.Alignment(0, 0),
        )
    )


ft.run(main)

Summary by Sourcery

Fix InteractiveViewer alignment handling and improve diagnostics and documentation for reused desktop clients and the audio example.

Bug Fixes:

• Fix InteractiveViewer rendering failures when its alignment property is set by correctly converting the alignment value before passing it to Flutter.

Enhancements:

• Clarify that InteractiveViewer.alignment controls the transformation origin rather than content placement, including the implications for gestures and panning.

Documentation:

• Document the warning shown when a desktop client from a previous build is used and how to switch back to the standard client.

Chores:

• Add a warning identifying desktop clients reused from platform build directories.
• Clarify the audio playback example metadata to describe local asset playback and provide setup guidance.

[sourcery-ai]

Hey - I've reviewed your changes and they look great!

---

Sourcery is free for open source - if you like our reviews please consider sharing them ✨

• X
• Mastodon
• LinkedIn
• Facebook

[cloudflare-workers-and-pages]

Deploying flet-website-v2 with    Cloudflare Pages

Latest commit:	0ff1ca0
Status:	✅  Deploy successful!
Preview URL:	https://530e609e.flet-website-v2.pages.dev
Branch Preview URL:	https://fix-interactive-viewer-align.flet-website-v2.pages.dev

View logs

[Copilot]

Copilot review overview

🟡 Changes recommended

The regression test described in the PR is absent from the changes.

Get a fresh assessment by requesting another Copilot review.

Review effort: Balanced
Findings: 1

Open (1)

• Add widget test covering MessagePack alignment map and unset cases · New

What changed in this PR

Fixes InteractiveViewer rendering failures by parsing alignment into Flutter’s expected type and clarifying its transformation semantics.

Changes:

• Parse wire-format alignment values with getAlignment.
• Clarify alignment behavior and limitations.
• Add a 1.0.2 changelog entry.

File	Description
CHANGELOG.md	Records the bug fix.
packages/​flet/​lib/​src/​controls/​interactive_viewer.dart	Parses alignment values correctly.
sdk/​python/​packages/​flet/​src/​flet/​controls/​core/​interactive_viewer.py	Documents alignment semantics.

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.