Sain (サイン)

A Compose Multiplatform library for capturing and exporting signatures as ImageBitmap with customizable options. Perfect
for electronic signature, legal documents and more.

See the project's website for documentation.

Sample Usage

var imageBitmap by remember {
    mutableStateOf<ImageBitmap?>(null)
}

Sain(
    signatureHeight = 250.dp,
    signaturePadColor = Color.White,
    signatureBorderStroke = BorderStroke(
        width = .5.dp,
        color = MaterialTheme.colorScheme.onSurface,
    ),
    signaturePadShape = RoundedCornerShape(8.dp),
    onComplete = { signatureBitmap ->
        if (signatureBitmap != null) {
            imageBitmap = signatureBitmap
        } else {
            println("Signature is empty")
        }
    },
) { action ->
    Row(
        modifier = Modifier
            .padding(top = 16.dp)
            .fillMaxWidth(),
        horizontalArrangement = Arrangement.spacedBy(16.dp),
    ) {
        Button(
            modifier = Modifier.weight(1f),
            onClick = {
                imageBitmap = null
                action(SignatureAction.CLEAR)
            },
        ) {
            Text("Clear")
        }
        Button(
            modifier = Modifier.weight(1f),
            onClick = {
                action(SignatureAction.COMPLETE)
            },
        ) {
            Text("Complete")
        }
    }
}

Base64 Encoding for ImageBitmap

To make it easier to store or transfer ImageBitmap across platforms in a format that is platform-independent, we provide a utility to convert ImageBitmap to a Base64 string. This functionality is useful in scenarios like signature storage or image uploads.

Add the following lines to your commonMain, androidMain, and iosMain modules to implement this feature.

Implementation

commonMain

In commonMain, define the expect function:

expect fun ImageBitmap.toBase64(): String

androidMain

In androidMain, implement the actual function:

actual fun ImageBitmap.toBase64(): String {
    val bitmap = this.asAndroidBitmap()
    val byteArrayOutputStream = ByteArrayOutputStream()
    bitmap.compress(Bitmap.CompressFormat.PNG, 100, byteArrayOutputStream)
    val byteArray = byteArrayOutputStream.toByteArray()
    return Base64.encodeToString(byteArray, Base64.NO_WRAP)
}

Note: The asAndroidBitmap() extension function is from androidx.compose.ui.graphics.

iosMain

In iosMain, implement the actual function:

@OptIn(ExperimentalForeignApi::class)
fun ImageBitmap.toUIImage(): UIImage? {
    val width = this.width
    val height = this.height
    val buffer = IntArray(width * height)

    this.readPixels(buffer)

    val colorSpace = CGColorSpaceCreateDeviceRGB()
    val context = CGBitmapContextCreate(
        data = buffer.refTo(0),
        width = width.toULong(),
        height = height.toULong(),
        bitsPerComponent = 8u,
        bytesPerRow = (4 * width).toULong(),
        space = colorSpace,
        bitmapInfo = CGImageAlphaInfo.kCGImageAlphaPremultipliedLast.value
    )

    val cgImage = CGBitmapContextCreateImage(context)
    return cgImage?.let { UIImage.imageWithCGImage(it) }
}

actual fun ImageBitmap.toBase64(): String {
    val uiImage = this.toUIImage()
    val jpegData = uiImage?.let { UIImageJPEGRepresentation(it, 0.5) }
        ?: return ""
    return jpegData.base64Encoding()
}

Usage

To use this function in your multiplatform project:

val base64String = imageBitmap.toBase64()
// Store or transfer the `base64String` as needed

License

Copyright 2023 Joel Kanyi

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

   http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.