Set up a web development configuration file

By Sydney Bao

Flutter web includes a development server that defaults to serving your application in the localhost domain using HTTP on a randomly assigned port. While command-line arguments offer a quick way to modify the server's behavior, this document focuses on a more structured approach: defining your server's behavior through a centralized web_dev_config.yaml file. This configuration file allows you to customize server settings&emdash;host, port, HTTPS settings, and proxy rules&emdash;ensuring a consistent development environment.

Add a web_dev_config.yaml file to the root directory of your Flutter project. If you haven't set up a Flutter project, visit Building a web application with Flutter to get started.

You can define the host, port, and HTTPS settings for your development server.

server:
  host: "0.0.0.0" # Defines the binding address <string>
  port: 8080 # Specifies the port <int> for the development server
  https:
    cert-path: "/path/to/cert.pem" # Path <string> to your TLS certificate
    cert-key-path: "/path/to/key.pem" # Path <string> to TLS certificate key

You can also inject custom HTTP headers into the development server's responses.

server:
  headers:
    - name: "X-Custom-Header" # Name <string> of the HTTP header
      value: "MyValue" # Value <string> of the HTTP header
    - name: "Cache-Control"
      value: "no-cache, no-store, must-revalidate"

Requests are matched in order from the web_dev_config.yaml file.

Use the prefix field for simple path prefix matching.

server:
  proxy:
    - target: "http://localhost:5000/" # Base URL <string> of your backend
      prefix: "/users/" # Path <string>
    - target: "http://localhost:3000/"
      prefix: "/data/"
      replace: "/report/" # Replacement <string> of path in redirected URL (optional)
    - target: "http://localhost:4000/"
      prefix: "/products/"
      replace: ""

Explanation:

• A request to /users/names is forwarded to http://localhost:5000/users/names.
• A request to /data/2023/ is forwarded to http://localhost:3000/report/2023 because replace: “/report/” replaces the /data/ prefix.
• A request to /products/item/123 is forwarded to http://localhost:4000/item/123 because replace: "" removes the /products/ prefix by replacing it with an empty string.

You can also use the regex field for more flexible and complex matching.

server:
  proxy:
    - target: "http://localhost:5000/"
      regex: "/users/(\d+)/$" # Path <string> matches requests like /users/123/
    - target: "http://localhost:4000/"
      regex: "^/api/(v\d+)/(.*)" # Matches requests like /api/v1/users
      replace: "/$2?apiVersion=$1" # Allows capture groups (optional)

Explanation:

• A request to /users/123/ matches the first rule exactly, so it is forwarded to http://localhost:5000/users/123/.
• A request to /api/v1/users/profile/ starts with the second rule path so it is forwarded to http://localhost:4000/users/profile/?apiVersion=v1.

Remember the order of precedence for settings:

1. Command-line arguments (such as --web-hostname, --web-port)
2. web_dev_config.yaml settings
3. Built-in default values