TagWatcher.java

package com.askmirrormirror.android.presentation.common;

import android.support.annotation.ColorInt;
import android.text.Editable;
import android.text.Spanned;
import android.text.TextWatcher;
import android.text.style.CharacterStyle;
import android.text.style.ForegroundColorSpan;

import java.util.ArrayList;
import java.util.List;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

/**
 * Simple TextWatcher to highlight all tags.
 * Default trigger is '#'. But you can provide your own set.
 *
 * Created by:
 * @author Dmytro Gorodnytskyi
 *         on 12-Dec-16.
 */

public class TagWatcher implements TextWatcher {

    private static char mTrigger = '#';

    private boolean mCheckText;
    private int mColor;
    private List<CharacterStyle> mSpans;
    private Pattern mPattern;
    private char[] mExternalTriggers;

    public TagWatcher(@ColorInt int highlightColor) {
        mSpans = new ArrayList<>();
        mColor = highlightColor;
        mPattern = Pattern.compile("#\\w+");
    }

    public TagWatcher(@ColorInt int highlightColor, char[] triggers) {
        mSpans = new ArrayList<>();
        mColor = highlightColor;
        mPattern = Pattern.compile(buildPattern(triggers));
        mExternalTriggers = triggers;
    }

    @Override
    public void beforeTextChanged(CharSequence s, int start, int count, int after) { }

    @Override
    public void onTextChanged(CharSequence s, int start, int before, int count) {
        for (int i = 0; i < s.length(); i++) {
            if (mExternalTriggers == null && s.charAt(i) == mTrigger) {
                mCheckText = true;
                return;
            }
            else if (mExternalTriggers != null) {
                for (char trigger : mExternalTriggers) {
                    if (s.charAt(i) == trigger) {
                        mCheckText = true;
                        return;
                    }
                }
            }
        }
    }

    @Override
    public void afterTextChanged(Editable s) {
        if (mCheckText) {
            mCheckText = false;
            clearSpans(s);
            addSpans(s);
        }
    }

    private String buildPattern(char[] triggers) {
        StringBuilder str = new StringBuilder("(");
        for (int i = 0; i < triggers.length; i++) {
            str.append(triggers[i]);
            if (i < triggers.length - 1) {
                str.append('|');
            }
        }
        str.append(")\\w+");
        return str.toString();
    }

    private void clearSpans(Editable s) {
        for (CharacterStyle span : mSpans) {
            s.removeSpan(span);
        }
        mSpans.clear();
    }

    private void addSpans(Editable s) {
        Matcher matcher = mPattern.matcher(s);
        while (matcher.find()) {
            ForegroundColorSpan span = new ForegroundColorSpan(mColor);
            s.setSpan(span, matcher.start(), matcher.end(), Spanned.SPAN_EXCLUSIVE_EXCLUSIVE);
            mSpans.add(span);
        }
    }
}