RTEditor/src/main/java/com/onegravity/rteditor/utils/io/output/StringBuilderWriter.java
@Override
    public Writer append(CharSequence value, int start, int end) {
        builder.append(value, start, end);
        return this;
    }
@Override
    public Writer append(CharSequence value, int start, int end) {
        builder.append(value, start, end);
        return this;
    }
Append a portion of a character sequence to the {@link StringBuilder}.

@param value The character to append
@param start The index of the first character
@param end The index of the last character + 1
@return This writer instance