## CORS (跨域資源共享) 中間件
CORS (Cross-origin resource sharing) 中間件實現了 [CORS](http://www.w3.org/TR/cors) 的標準。CORS為Web服務器提供跨域訪問控制，從而實現安全的跨域數據傳輸。
*用法*
```go
e.Use(middleware.CORS())
```
### 自定義配置
*用法*
```go
e := echo.New()
e.Use(middleware.CORSWithConfig(middleware.CORSConfig{
AllowOrigins: []string{"https://labstack.com", "https://labstack.net"},
AllowHeaders: []string{echo.HeaderOrigin, echo.HeaderContentType, echo.HeaderAccept},
}))
```
### 配置
```go
// CORSConfig defines the config for CORS middleware.
CORSConfig struct {
// Skipper defines a function to skip middleware.
Skipper Skipper
// AllowOrigin defines a list of origins that may access the resource.
// Optional. Default value []string{"*"}.
AllowOrigins []string `json:"allow_origins"`
// AllowMethods defines a list methods allowed when accessing the resource.
// This is used in response to a preflight request.
// Optional. Default value DefaultCORSConfig.AllowMethods.
AllowMethods []string `json:"allow_methods"`
// AllowHeaders defines a list of request headers that can be used when
// making the actual request. This in response to a preflight request.
// Optional. Default value []string{}.
AllowHeaders []string `json:"allow_headers"`
// AllowCredentials indicates whether or not the response to the request
// can be exposed when the credentials flag is true. When used as part of
// a response to a preflight request, this indicates whether or not the
// actual request can be made using credentials.
// Optional. Default value false.
AllowCredentials bool `json:"allow_credentials"`
// ExposeHeaders defines a whitelist headers that clients are allowed to
// access.
// Optional. Default value []string{}.
ExposeHeaders []string `json:"expose_headers"`
// MaxAge indicates how long (in seconds) the results of a preflight request
// can be cached.
// Optional. Default value 0.
MaxAge int `json:"max_age"`
}
```
*默認配置*
```go
DefaultCORSConfig = CORSConfig{
Skipper: defaultSkipper,
AllowOrigins: []string{"*"},
AllowMethods: []string{echo.GET, echo.HEAD, echo.PUT, echo.PATCH, echo.POST, echo.DELETE},
}
```
