set emoji text BackgroundColorSpan not work

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use system can set BackgroundColorSpan(),but no working on self.What is the solution.

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here is my code

`
public CharSequence buildSpannableStringBackground(CharSequence content, CharSequence keyword) {
if (TextUtils.isEmpty(content)) {
return new SpannableStringBuilder("");
}

    if (TextUtils.isEmpty(keyword)) {
        return content;
    }
    SpannableStringBuilder spannableString = new SpannableStringBuilder(content);
    String lowerContent = content.toString().toLowerCase();
    String lowerKeyword = keyword.toString().toLowerCase();
    int index = lowerContent.indexOf(lowerKeyword);
    if (index != -1) {
        int color = getApplicationContext().getResources().getColor(R.color.yc_color_FFFA00);
        while (index != -1) {
            spannableString.setSpan(new BackgroundColorSpan(color), index, index + lowerKeyword.length(), Spanned.SPAN_EXCLUSIVE_EXCLUSIVE);
            index = lowerContent.indexOf(lowerKeyword, index + 1);//从这个索引往后开始第一个出现的位置
        }
    }
    return spannableString;
}

`

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