React Native Expo应用无法与Flask服务器建立连接的问题求助

Hey there, let's break down what's going wrong here and fix that network error step by step!

1. Missing Protocol & Wrong Localhost Address #

Your fetch URL 127.0.0.1:5000/post has two issues:

• It's missing the http:// protocol header, which is required for fetch to recognize it as a valid network address.
• In Expo/React Native simulators, 127.0.0.1 points to the simulator itself, not your computer running the Flask server.

Fix this by:

• Using your computer's local network IP (like http://192.168.1.100:5000/post) if you're testing with Expo Go on a physical phone.
• Using http://localhost:5000/post or your computer's IP if testing on a simulator.

2. Incorrect POST Request Format #

You're passing a raw object as the body, but fetch won't automatically convert it to JSON, and you haven't set the correct request headers. Update your frontend code like this:

fetch("http://你的电脑局域网IP:5000/post", {
  method: 'POST',
  headers: {
    'Content-Type': 'application/json',
  },
  body: JSON.stringify({ 'base64': photoData["base64"] }),
})
.then(response => response.json())
.then(result => {
  console.log("******************\nRESULT");
  console.log(result);
})
.catch(err => {
  console.log(err);
})

3. Backend is Reading from URL Params, Not POST Body #

Your Flask code uses request.args['base64'], which pulls data from URL query parameters (like /post?base64=xxx), but your frontend sends data in the POST JSON body. Update your backend to read the JSON payload instead:

from flask import request, jsonify
import base64

@app.route('/post', methods=['POST']) # Use uppercase POST for consistency
def webhook():
    data = request.get_json()
    file = data['base64']
    rv = base64.b64encode(str.encode(file)) # Convert to bytes
    rv = rv.decode('ascii') # Convert back to string
    return jsonify(rv) # Return properly formatted JSON response

Extra Checks to Ensure Connection #

• Run your Flask server with app.run(host='0.0.0.0', port=5000) so it listens on all network interfaces (not just 127.0.0.1).
• Make sure your computer's firewall isn't blocking incoming traffic on port 5000.
• If using Expo Go on a phone, confirm it's connected to the same WiFi network as your computer.

内容的提问来源于stack exchange，提问作者Екатерина Яцкив