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class WSGIHandler(base.BaseHandler):
    request_class = WSGIRequest

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.load_middleware()

    def __call__(self, environ, start_response):
        set_script_prefix(get_script_name(environ))
        signals.request_started.send(sender=self.__class__, environ=environ)
        # WSGIRequest对象 -> HttpRequest类
        # def echo(request, *args, **kwargs):  
        #    data = request.GET
        #    ret_value = data.get('name', 'world')
        #    return HttpResponse(ret_value, content_type='text/plain')
        # 在 app/view/echo 中, 传入的 request 即为当前封装的request_class对象
        request = self.request_class(environ)
        # HttpResponse 类
        response = self.get_response(request)

        response._handler_class = self.__class__

        status = "%d %s" % (response.status_code, response.reason_phrase)
        response_headers = [
            *response.items(),
            *(("Set-Cookie", c.output(header="")) for c in response.cookies.values()),
        ]
        start_response(status, response_headers)
        if getattr(response, "file_to_stream", None) is not None and environ.get(
            "wsgi.file_wrapper"
        ):
            # If `wsgi.file_wrapper` is used the WSGI server does not call
            # .close on the response, but on the file wrapper. Patch it to use
            # response.close instead which takes care of closing all files.
            response.file_to_stream.close = response.close
            response = environ["wsgi.file_wrapper"](
                response.file_to_stream, response.block_size
            )
        return response

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    def get_response(self, request):
        """Return an HttpResponse object for the given HttpRequest."""
        # Setup default url resolver for this thread
        set_urlconf(settings.ROOT_URLCONF)
        # self._middleware_chain() 是处理的主要逻辑, 包含中间件和自己的视图层处理逻辑
        response = self._middleware_chain(request)
        response._resource_closers.append(request.close)
        if response.status_code >= 400:
            log_response(
                "%s: %s",
                response.reason_phrase,
                request.path,
                response=response,
                request=request,
            )
        return response

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class BaseHandler:
    _view_middleware = None
    _template_response_middleware = None
    _exception_middleware = None
    _middleware_chain = None

    def load_middleware(self, is_async=False):
        """
        Populate middleware lists from settings.MIDDLEWARE.

        Must be called after the environment is fixed (see __call__ in subclasses).
        """
        self._view_middleware = []
        self._template_response_middleware = []
        self._exception_middleware = []

        # 关键代码
        get_response = self._get_response_async if is_async else self._get_response
        handler = convert_exception_to_response(get_response)
        handler_is_async = is_async
        for middleware_path in reversed(settings.MIDDLEWARE):
            middleware = import_string(middleware_path)
            middleware_can_sync = getattr(middleware, "sync_capable", True)
            middleware_can_async = getattr(middleware, "async_capable", False)
            if not middleware_can_sync and not middleware_can_async:
                raise RuntimeError(
                    "Middleware %s must have at least one of "
                    "sync_capable/async_capable set to True." % middleware_path
                )
            elif not handler_is_async and middleware_can_sync:
                middleware_is_async = False
            else:
                middleware_is_async = middleware_can_async
            try:
                # Adapt handler, if needed.
                adapted_handler = self.adapt_method_mode(
                    middleware_is_async,
                    handler,
                    handler_is_async,
                    debug=settings.DEBUG,
                    name="middleware %s" % middleware_path,
                )
                mw_instance = middleware(adapted_handler)
            except MiddlewareNotUsed as exc:
                if settings.DEBUG:
                    if str(exc):
                        logger.debug("MiddlewareNotUsed(%r): %s", middleware_path, exc)
                    else:
                        logger.debug("MiddlewareNotUsed: %r", middleware_path)
                continue
            else:
                handler = adapted_handler

            if mw_instance is None:
                raise ImproperlyConfigured(
                    "Middleware factory %s returned None." % middleware_path
                )

            if hasattr(mw_instance, "process_view"):
                self._view_middleware.insert(
                    0,
                    self.adapt_method_mode(is_async, mw_instance.process_view),
                )
            if hasattr(mw_instance, "process_template_response"):
                self._template_response_middleware.append(
                    self.adapt_method_mode(
                        is_async, mw_instance.process_template_response
                    ),
                )
            if hasattr(mw_instance, "process_exception"):
                # The exception-handling stack is still always synchronous for
                # now, so adapt that way.
                self._exception_middleware.append(
                    self.adapt_method_mode(False, mw_instance.process_exception),
                )

            handler = convert_exception_to_response(mw_instance)
            handler_is_async = middleware_is_async

        # Adapt the top of the stack, if needed.
        handler = self.adapt_method_mode(is_async, handler, handler_is_async)
        # We only assign to this when initialization is complete as it is used
        # as a flag for initialization being complete.
        self._middleware_chain = handler

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class WSGIHandler(base.BaseHandler):
    request_class = WSGIRequest

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.load_middleware()