how to save bitmap to android gallery

There were several different ways to do it before API 29 (Android Q) but all of them involved one or a few APIs that are deprecated with Q. In 2019, here’s a way to do it that is both backward and forward compatible:

(And since it is 2019 so I will write in Kotlin)

    /// @param folderName can be your app's name
    private fun saveImage(bitmap: Bitmap, context: Context, folderName: String) {
        if (android.os.Build.VERSION.SDK_INT >= 29) {
            val values = contentValues()
            values.put(MediaStore.Images.Media.RELATIVE_PATH, "Pictures/" + folderName)
            values.put(MediaStore.Images.Media.IS_PENDING, true)
            // RELATIVE_PATH and IS_PENDING are introduced in API 29.

            val uri: Uri? = context.contentResolver.insert(MediaStore.Images.Media.EXTERNAL_CONTENT_URI, values)
            if (uri != null) {
                saveImageToStream(bitmap, context.contentResolver.openOutputStream(uri))
                values.put(MediaStore.Images.Media.IS_PENDING, false)
                context.contentResolver.update(uri, values, null, null)
            }
        } else {
            val directory = File(Environment.getExternalStorageDirectory().toString() + separator + folderName)
            // getExternalStorageDirectory is deprecated in API 29

            if (!directory.exists()) {
                directory.mkdirs()
            }
            val fileName = System.currentTimeMillis().toString() + ".png"
            val file = File(directory, fileName)
            saveImageToStream(bitmap, FileOutputStream(file))
            if (file.absolutePath != null) {
                val values = contentValues()
                values.put(MediaStore.Images.Media.DATA, file.absolutePath)
                // .DATA is deprecated in API 29
                context.contentResolver.insert(MediaStore.Images.Media.EXTERNAL_CONTENT_URI, values)
            }
        }
    }

    private fun contentValues() : ContentValues {
        val values = ContentValues()
        values.put(MediaStore.Images.Media.MIME_TYPE, "image/png")
        values.put(MediaStore.Images.Media.DATE_ADDED, System.currentTimeMillis() / 1000);
        values.put(MediaStore.Images.Media.DATE_TAKEN, System.currentTimeMillis());
        return values
    }

    private fun saveImageToStream(bitmap: Bitmap, outputStream: OutputStream?) {
        if (outputStream != null) {
            try {
                bitmap.compress(Bitmap.CompressFormat.PNG, 100, outputStream)
                outputStream.close()
            } catch (e: Exception) {
                e.printStackTrace()
            }
        }
    }

Also, before calling this, you need to have WRITE_EXTERNAL_STORAGE first.