FutureBuilder 和相机

Question

我是 Flutter 的新手，但我仍在为 Future 的一些方面而苦恼。 我有一个底部导航栏，页面之间的切换是通过 PageView 完成的。 问题是当我切换到我的“照片”页面时，它会永远显示一个循环进度指示器。我有点迷茫，老实说我不知道​​我做错了什么，看起来未来的建设者永远不会得到价值。 只是为了提供更多信息：如果我进行热刷新，相机开始工作并出现图片。

这是 PhotoScreen 页面：

import 'dart:async';
import 'dart:io';

import 'package:camera/camera.dart';
import 'package:flutter/material.dart';

// A screen that allows users to take a picture using a given camera.
class PhotoScreen extends StatefulWidget {
  const PhotoScreen({
    Key key,
  }) : super(key: key);

  @override
  PhotoScreenState createState() => PhotoScreenState();
}

class PhotoScreenState extends State<PhotoScreen> {
  CameraDescription camera;
  CameraController _controller;
  Future<void> _initializeControllerFuture;

  @override
  void initState() {
    super.initState();
    // Obtain a list of the available cameras on the device.
    inizializza();
  }

  @override
  void dispose() {
    // Dispose of the controller when the widget is disposed.
    _controller.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('Take a picture')),
      // Wait until the controller is initialized before displaying the
      // camera preview. Use a FutureBuilder to display a loading spinner
      // until the controller has finished initializing.
      body: FutureBuilder<void>(
        future: _initializeControllerFuture,
        builder: (context, snapshot) {
          if (snapshot.connectionState == ConnectionState.done) {
            // If the Future is complete, display the preview.
            return CameraPreview(_controller);
          } else {
            // Otherwise, display a loading indicator.
            return Center(child: CircularProgressIndicator());
          }
        },
      ),
      floatingActionButton: FloatingActionButton(
        child: Icon(Icons.camera_alt),
        // Provide an onPressed callback.
        onPressed: () async {
          // Take the Picture in a try / catch block. If anything goes wrong,
          // catch the error.
          try {
            // Ensure that the camera is initialized.
            await _initializeControllerFuture;

            // Attempt to take a picture and get the file `image`
            // where it was saved.
            final image = await _controller.takePicture();
          } catch (e) {
            // If an error occurs, log the error to the console.
            print(e);
          }
        },
      ),
    );
  }

  void inizializza() async {
    final cameras = await availableCameras();

    // Get a specific camera from the list of available cameras.
    final firstCamera = cameras.first;
    // To display the current output from the Camera,
    // create a CameraController.
    _controller = CameraController(
      // Get a specific camera from the list of available cameras.
      firstCamera,
      // Define the resolution to use.
      ResolutionPreset.medium,
    );

    // Next, initialize the controller. This returns a Future.
    _initializeControllerFuture = _controller.initialize();
  }
}

Answer 1

inizializza 方法实际上是异步的，您在 initState() 中执行它，但事实并非如此。 我认为你的 inizializza 方法应该是这样的：

Future<void> inizializza() async {
    final cameras = await availableCameras();

    // Get a specific camera from the list of available cameras.
    final firstCamera = cameras.first;
    // To display the current output from the Camera,
    // create a CameraController.
    _controller = CameraController(
      // Get a specific camera from the list of available cameras.
      firstCamera,
      // Define the resolution to use.
      ResolutionPreset.medium,
    );

    // Next, initialize the controller. This returns a Future.
    await _controller.initialize();
  }

这就是FutureBuilder要接收的Future，而不是在initState()中执行它

FutureBuilder<void>(
  future: inizializza(),
  ...
)