* 简单请求和非简单请求的区别？

 简单请求：一次请求
 非简单请求：两次请求，在发送数据之前会先发一次请求用于做“预检”，只有“预检”通过后才再发送一次请求用于数据传输。
* 关于“预检”

- 请求方式：OPTIONS
- “预检”其实做检查，检查如果通过则允许传输数据，检查不通过则不再发送真正想要发送的消息
- 如何“预检”
  => 如果复杂请求是PUT等请求，则服务端需要设置允许某请求，否则“预检”不通过
  Access-Control-Request-Method
  => 如果复杂请求设置了请求头，则服务端需要设置允许某请求头，否则“预检”不通过
  Access-Control-Request-Headers

def test(request):
 import json
 obj=HttpResponse(json.dumps({'name':'lqz'}))
 # obj['Access-Control-Allow-Origin']='*'
 obj['Access-Control-Allow-Origin']='http://127.0.0.1:8004'
 return obj

from django.utils.deprecation import MiddlewareMixin
class CorsMiddleWare(MiddlewareMixin):
 def process_response(self,request,response):
  if request.method=="OPTIONS":
   #可以加*
   response["Access-Control-Allow-Headers"]="Content-Type"
  response["Access-Control-Allow-Origin"] = "http://localhost:8080"
  return response

pip install django-cors-headers

INSTALLED_APPS = [
 'django.contrib.admin',
 'django.contrib.auth',
 'django.contrib.contenttypes',
 'django.contrib.sessions',
 'django.contrib.messages',
 'django.contrib.staticfiles', 
 'corsheaders',
]
MIDDLEWARE_CLASSES = [
 'django.middleware.security.SecurityMiddleware',
 'django.contrib.sessions.middleware.SessionMiddleware', 
 'corsheaders.middleware.CorsMiddleware', 
 'django.middleware.common.CommonMiddleware',
 'django.middleware.csrf.CsrfViewMiddleware',
 'django.contrib.auth.middleware.AuthenticationMiddleware',
 'django.contrib.auth.middleware.SessionAuthenticationMiddleware',
 'django.contrib.messages.middleware.MessageMiddleware',
 'django.middleware.clickjacking.XFrameOptionsMiddleware',
]

CORS_ORIGIN_WHITELIST = (
 '127.0.0.1:8080',
 'localhost:8080',
)
CORS_ALLOW_CREDENTIALS = True # 指明在跨域访问中，后端是否支持对cookie的操作。

CORS_ALLOW_METHODS = (
 'DELETE',
 'GET',
 'OPTIONS',
 'PATCH',
 'POST',
 'PUT',
 'VIEW',
)
CORS_ALLOW_HEADERS = (
 'XMLHttpRequest',
 'X_FILENAME',
 'accept-encoding',
 'authorization',
 'content-type',
 'dnt',
 'origin',
 'user-agent',
 'x-csrftoken',
 'x-requested-with',
 'Pragma',
)