TextViewのmaxLengthのサロゲートペア対応(絵文字に対応した文字数制限)

import android.text.InputFilter;
import android.text.Spanned;

/**
 * サロゲートペア対応版のLengthFilter
 */
public class InputLengthFilter implements InputFilter {
    private final int mMax;

    public InputLengthFilter(int max) {
        mMax = max;
    }

    public CharSequence filter(CharSequence source, int start, int end, Spanned dest,
                               int dstart, int dend) {
        int realLen = dest.toString().codePointCount(0, dest.length());
        int keep = mMax - (realLen - (dend - dstart));
        if (keep <= 0) {
            return "";
        } else if (keep >= end - start) {
            return null; // keep original
        } else {
            keep += start;
            if (Character.isHighSurrogate(source.charAt(keep - 1))) {
                return source.subSequence(start, keep + 1);
            }
            return source.subSequence(start, keep);
        }
    }

    public int getMax() {
        return mMax;
    }
}