Compresses image as native plugin (Obj-C/Kotlin)

This library can works on Android and iOS.

• flutter_image_compress
  • Why don't you use dart to do it
  • Usage
  • About common params
    • minWidth and minHeight
    • rotate
    • autoCorrectionAngle
    • quality
    • format
    • keepExif
  • Result
    • About List<int> and Uint8List
  • Android
  • iOS
  • Troubleshooting or common error
    • Compressing returns null
    • Android build error
    • About EXIF information
      • About Exif handle code

Q：Dart already has image compression libraries. Why use native?

A：For unknown reasons, image compression in Dart language is not efficient, even in release version. Using isolate does not solve the problem.

dependencies:
  flutter_image_compress: ^0.6.3

import 'package:flutter_image_compress/flutter_image_compress.dart';

Use as:

See full example

There are several ways to use the library api.

  // 1. compress file and get a List<int>
  Future<List<int>> testCompressFile(File file) async {
    var result = await FlutterImageCompress.compressWithFile(
      file.absolute.path,
      minWidth: 2300,
      minHeight: 1500,
      quality: 94,
      rotate: 90,
    );
    print(file.lengthSync());
    print(result.length);
    return result;
  }

  // 2. compress file and get file.
  Future<File> testCompressAndGetFile(File file, String targetPath) async {
    var result = await FlutterImageCompress.compressAndGetFile(
        file.absolute.path, targetPath,
        quality: 88,
        rotate: 180,
      );

    print(file.lengthSync());
    print(result.lengthSync());

    return result;
  }

  // 3. compress asset and get List<int>.
  Future<List<int>> testCompressAsset(String assetName) async {
    var list = await FlutterImageCompress.compressAssetImage(
      assetName,
      minHeight: 1920,
      minWidth: 1080,
      quality: 96,
      rotate: 180,
    );

    return list;
  }

  // 4. compress List<int> and get another List<int>.
  Future<List<int>> testComporessList(List<int> list) async {
    var result = await FlutterImageCompress.compressWithList(
      list,
      minHeight: 1920,
      minWidth: 1080,
      quality: 96,
      rotate: 135,
    );
    print(list.length);
    print(result.length);
    return result;
  }

minWidth and minHeight are constraints on image scaling.

For example, a 4000*2000 image, minWidth set to 1920, minHeight set to 1080, the calculation is as follows:

// Using dart as an example, the actual implementation is Kotlin or OC.
import 'dart:math' as math;

void main() {
  var scale = calcScale(
    srcWidth: 4000,
    srcHeight: 2000,
    minWidth: 1920,
    minHeight: 1080,
  );

  print("scale = $scale"); // scale = 1.8518518518518519
  print("target width = ${4000 / scale}, height = ${2000 / scale}"); // target width = 2160.0, height = 1080.0
}

double calcScale({
  double srcWidth,
  double srcHeight,
  double minWidth,
  double minHeight,
}) {
  var scaleW = srcWidth / minWidth;
  var scaleH = srcHeight / minHeight;

  var scale = math.max(1.0, math.min(scaleW, scaleH));

  return scale;
}

If your image width is smaller than minWidth or height samller than minHeight, scale will be 1, that is, the size will not change.

If you need to rotate the picture, use this parameter.

This property only exists in the version after 0.5.0.

And for historical reasons, there may be conflicts with rotate attributes, which need to be self-corrected.

Modify rotate to 0 or autoCorrectionAngle to false.

Quality of target image.

If format is png, the param will be ignored in iOS.

Supports jpeg or png, default is jpeg.

If this parameter is true, EXIF information is saved in the compressed result.

Attention should be paid to the following points:

1. Default value is false.
2. Even if set to true, the direction attribute is not included.
3. Only support jpg format, PNG format does not support.

The result of returning a List collection will not have null, but will always be an empty array.

The returned file may be null. In addition, please decide for yourself whether the file exists.

You may need to convert List<int> to Uint8List to display images.

To use Uint8List, you need import package to your code like this:

final image = Uint8List.fromList(imageList)
ImageProvider provider = MemoryImage(Uint8List.fromList(imageList));

Usage in Image Widget:

List<int> image = await testCompressFile(file);
ImageProvider provider = MemoryImage(Uint8List.fromList(image));

Image(
  image: provider ?? AssetImage("img/img.jpg"),
),

Write to file usage:

void writeToFile(List<int> image, String filePath) {
  final file = File(filePath);
  file.writeAsBytes(image, flush: true, mode: FileMode.write);
}

You may need to update Kotlin to version 1.2.71(Recommend 1.3.21) or higher.

No problems currently found.

Sometimes, compressing will return null. You should check if you can read/write the file, and the parent folder of the target file must exist.

For example, use the path_provider plugin to access some application folders, and use a permission plugin to request permission to access SD cards on Android/iOS.

Caused by: org.gradle.internal.event.ListenerNotificationException: Failed to notify project evaluation listener.
        at org.gradle.internal.event.AbstractBroadcastDispatch.dispatch(AbstractBroadcastDispatch.java:86)
        ...
Caused by: java.lang.AbstractMethodError
        at org.jetbrains.kotlin.gradle.plugin.KotlinPluginKt.resolveSubpluginArtifacts(KotlinPlugin.kt:776)
        ...

See flutter/flutter/issues#21473

You need to upgrade your Kotlin version to 1.2.71+(recommended 1.3.31).

If Flutter supports more platforms (Windows, Mac, Linux, etc) in the future and you use this library, propose an issue or PR!

Using this library, EXIF information will be removed by default.

EXIF information can be retained by setting keepExif to true, but not direction information.

The iOS code was copied from dvkch/SYPictureMetadata, LICENSE

The android code was copied from flutter/plugin/image_picker and edit some. (BSD 3 style)