debug_toolbar工作原理

#toolbar的中间件的响应处理函数，会调用到panel.generate_stats(request, response)
def process_response(self, request, response):
        toolbar = self.__class__.debug_toolbars.pop(threading.current_thread().ident, None)
        if not toolbar:#DebugToolbar对象，每请求对应一个，它可以存储各panel的数据
            return response

        # Run process_response methods of panels like Django middleware.
        for panel in reversed(toolbar.enabled_panels):
            new_response = panel.process_response(request, response)
            if new_response:
                response = new_response

        # Deactivate instrumentation ie. monkey-unpatch. This must run
        # regardless of the response. Keep 'return' clauses below.
        # (NB: Django's model for middleware doesn't guarantee anything.)
        for panel in reversed(toolbar.enabled_panels):#禁止调试模板
            panel.disable_instrumentation()

        # Check for responses where the toolbar can't be inserted.
        content_encoding = response.get('Content-Encoding', '')
        content_type = response.get('Content-Type', '').split(';')[0]
        if any((getattr(response, 'streaming', False),
                'gzip' in content_encoding,
                content_type not in _HTML_TYPES)):
            return response

        # Collapse the toolbar by default if SHOW_COLLAPSED is set.
        if toolbar.config['SHOW_COLLAPSED'] and 'djdt' not in request.COOKIES:
            response.set_cookie('djdt', 'hide', 864000)

        # Insert the toolbar in the response.
        content = force_text(response.content, encoding=settings.DEFAULT_CHARSET)
        insert_before = dt_settings.CONFIG['INSERT_BEFORE']#'/body'
        try:                    # Python >= 2.7
            pattern = re.escape(insert_before)
            bits = re.split(pattern, content, flags=re.IGNORECASE)
        except TypeError:       # Python < 2.7
            pattern = '(.+?)(%s|$)' % re.escape(insert_before)
            matches = re.findall(pattern, content, flags=re.DOTALL | re.IGNORECASE)
            bits = [m[0] for m in matches if m[1] == insert_before]
            # When the body ends with a newline, there's two trailing groups.
            bits.append(''.join(m[0] for m in matches if m[1] == ''))
        if len(bits) > 1:
            # When the toolbar will be inserted for sure, generate the stats.
            for panel in reversed(toolbar.enabled_panels):
                panel.generate_stats(request, response)#调用panel生成统计数据

            bits[-2] += toolbar.render_toolbar()#bits[-2]为倒数第二个，即/body前面的内容
            response.content = insert_before.join(bits)
            if response.get('Content-Length', None):
                response['Content-Length'] = len(response.content)
        return response

timerpanel时间统计的面板：

def generate_stats(self, request, response):
        stats = {}
        if hasattr(self, '_start_time'):
            stats['total_time'] = (time.time() - self._start_time) * 1000#总共花费的时间
        if hasattr(self, '_start_rusage'):
            self._end_rusage = resource.getrusage(resource.RUSAGE_SELF)
            stats['utime'] = 1000 * self._elapsed_ru('ru_utime')
            stats['stime'] = 1000 * self._elapsed_ru('ru_stime')
            stats['total'] = stats['utime'] + stats['stime']
            stats['vcsw'] = self._elapsed_ru('ru_nvcsw')
            stats['ivcsw'] = self._elapsed_ru('ru_nivcsw')
            stats['minflt'] = self._elapsed_ru('ru_minflt')
            stats['majflt'] = self._elapsed_ru('ru_majflt')
            # these are documented as not meaningful under Linux.  If you're running BSD
            # feel free to enable them, and add any others that I hadn't gotten to before
            # I noticed that I was getting nothing but zeroes and that the docs agreed. :-(
            #
            #        stats['blkin'] = self._elapsed_ru('ru_inblock')
            #        stats['blkout'] = self._elapsed_ru('ru_oublock')
            #        stats['swap'] = self._elapsed_ru('ru_nswap')
            #        stats['rss'] = self._end_rusage.ru_maxrss
            #        stats['srss'] = self._end_rusage.ru_ixrss
            #        stats['urss'] = self._end_rusage.ru_idrss
            #        stats['usrss'] = self._end_rusage.ru_isrss

        self.record_stats(stats)#调用记录统计函数。

#panel的记录函数
def record_stats(self, stats):
        """
        Store data gathered by the panel. ``stats`` is a :class:`dict`.

        Each call to ``record_stats`` updates the statistics dictionary.
        """
        self.toolbar.stats.setdefault(self.panel_id, {}).update(stats)#更新后，每个请求后的数据统计