Image Insertion

How we added the ability to insert images via keyboard or copy/paste into Flutter

Android

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Relevant Flutter Issue

One of the most crucial requirements for a chat app is to allow users to insert rich content directly from their keyboard. On Android, this can be things such as stickers, GIFs, bitmoji, and even AI-based context-aware items (e.g. the keyboard offering to paste a recent screenshot). This functionality is sorely missing in Flutter, and we were forced to modify the Flutter Engine and Framework ourselves to implement it.

The basic content commit API is described at https://developer.android.com/guide/topics/text/image-keyboard. The Java code required to interface with Flutter's `TextInputClient` was added in the Flutter Engine.

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PR to Flutter Engine

Our PR to the Flutter Engine has been merged! The code essentially attaches the TextInputChannel to the contentCommit API, and sends the file data through the PlatformChannel back to the Flutter Framework.

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PR to Flutter Framework

Our PR to the Flutter Framework is currently pending review. The framework code organizes the data received from the PlatformChannel into a Dart class, and then this data can be manipulated however the developer chooses.

Web

Flutter currently does not have any way to detect image content when a paste action is performed on a text field. We implemented this with the great interoperability that Dart and JS have. We created a simple function that uses the JS package to call JS code from Dart:

@JS()
import 'package:js/js.dart';
import 'package:js/js_util.dart';

// Call invokes JavaScript `getPastedImage()`.
@JS('getPastedImage')
external dynamic getClipboardData();

Future<dynamic> getPastedImageWeb() {
  return promiseToFuture(getClipboardData());
}

This is called when we detect a CTRL-V event on the text field using a RawKeyboardListener. The JS function unfortunately only works in Chrome, but it uses the browser Clipboard APIs to get any media content:

function checkClipboard() {
  return new Promise(resolve => {
    navigator.permissions.query({ name: "clipboard-read" }).then((result) => {
      // If permission to read the clipboard is granted or if the user will
      // be prompted to allow it, we proceed.
      if (result.state == "granted" || result.state == "prompt") {
        navigator.clipboard.read().then((data) => {
          for (let i = 0; i < data.length; i++) {
            if (!data[i].types.includes("image/png")) {
              resolve(null);
            } else {
              data[i].getType("image/png").then((blob) => {
                resolve(blob);
              });
            }
          }
        });
      } else {
        resolve(null);
      }
    });
  });
}

async function getPastedImage() {
  const result = await checkClipboard();
  return result;
}

This function is placed inside the web/index.html file as a <script>.

Desktop

pasteboard | Flutter PackageDart packages

Pasteboard Package

We use the pasteboard package on Desktop to get the most recent item (image) from the clipboard. This is also called when we detect a CTRL-V event on the text field using a RawKeyboardListener.