Flutter开发setState能否在build中直接调用详解

import 'package:flutter/material.dart';
class TestPage extends StatefulWidget {
  const TestPage({super.key});
  @override
  State<TestPage> createState() => _State();
}
class _State extends State<TestPage> {
  int _count = 0;
  @override
  Widget build(BuildContext context) {
    setState(() {
      _count++;
    });
    return Scaffold(
      appBar: AppBar(
        title: const Text('测试页面'),
      ),
      body: Center(
        child: Text(
          '$_count',
          style: const TextStyle(fontSize: 24),
        ),
      ),
    );
  }
}

class _State extends State<TestPage> {
  int _count = 0;
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('测试页面'),
      ),
      body: Center(
        child: Builder(
          builder: (context) {
            setState(() {
              _count++;
            });
            return Text(
              '$_count',
              style: const TextStyle(fontSize: 24),
            );
          }
        ),
      ),
    );
  }
}

void markNeedsBuild() {
  // ...
  // 前半部分，断言重新 build 是否满足上面说的前提。
  assert(() {
    if (owner!._debugBuilding) {
      assert(owner!._debugCurrentBuildTarget != null);
      assert(owner!._debugStateLocked);
      // _debugIsInScope() 用来判断是否满足前提条件。
      if (_debugIsInScope(owner!._debugCurrentBuildTarget!)) {
        return true;
      }
      if (!_debugAllowIgnoredCallsToMarkNeedsBuild) {
        final List<DiagnosticsNode> information = <DiagnosticsNode>[
          ErrorSummary('setState() or markNeedsBuild() called during build.'),
          // ...
        ];
        // ...
      }
      // ...
    }());
  // ...
}

bool _debugIsInScope(Element target) {
  Element? current = this;
  while (current != null) {
    if (target == current) {
      return true;
    }
    current = current._parent;
  }
  return false;
}

void markNeedsBuild() {
  // ...
  // 前半部分是断言。
  if (dirty) {
    return;
  }
  _dirty = true;
  owner!.scheduleBuildFor(this);
}