Dart Flutter SlideTransition 从 Offset OFF SCREEN 开始

您可能希望try 在initState中使用addPostFrameCallback方法.

    @override
    void initState() {
      super.initState();
      WidgetsBinding.instance.addPostFrameCallback((_){
         // Schedule code execution once after the frame has rendered
         print(MediaQuery.of(context).size.toString());
      });
    }

Flutter Api Docs Link个

OR您也可以使用Future来执行以下操作:

    @override
    void initState() {
      super.initState();
      new Future.delayed(Duration.zero, () {
          // Schedule a zero-delay future to be executed
          print(MediaQuery.of(context).size.toString());
      });
    }

希望这有帮助.

UPDATED个

这有点像是unusual种方式，但它真的做到了你需要的东西.

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatefulWidget {
  @override
  State<StatefulWidget> createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> with SingleTickerProviderStateMixin {
  Animation<Offset> animation;
  AnimationController animationController;

  @override
  void initState() {
    super.initState();

    animationController = AnimationController(
      vsync: this,
      duration: Duration(seconds: 2),
    );
    animation = Tween<Offset>(
      begin: Offset(0.0, 1.0),
      end: Offset(0.0, 0.0),
    ).animate(CurvedAnimation(
      parent: animationController,
      curve: Curves.fastLinearToSlowEaseIn,
    ));

    Future<void>.delayed(Duration(seconds: 1), () {
      animationController.forward();
    });
  }

  @override
  void dispose() {
    // Don't forget to dispose the animation controller on class destruction
    animationController.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Stack(
        alignment: Alignment.center,
        fit: StackFit.expand,
        children: <Widget>[
          SlideTransition(
            position: animation,
            child: Column(
              mainAxisAlignment: MainAxisAlignment.center,
              children: <Widget>[
                CircleAvatar(
                  backgroundImage: NetworkImage(
                    'https://pbs.twimg.com/media/DpeOMc3XgAIMyx_.jpg',
                  ),
                  radius: 50.0,
                ),
              ],
            ),
          ),
        ],
      ),
    );
  }
}

它是这样工作的:

1.我们创建了一个包含Stack个项目的列表，其中包含展开其中任何子项的选项.

2.将要显示的每个slide包装为Column，子项居中对齐.该列将占用堆栈大小的2.%.

3.将动画的开始Offset设置为Offset(0.0, 1.0).

请记住，偏移量中的dx和dy不是像素或类似的东西，而是Widget's宽度或高度的比率.例如:如果你的widget's width is 100.0，你把0.25 as dx，它将result in moving your child to the right by 25.0 points.

因此，将偏移量设置为(0.0，1.0)会将列从屏幕移到底部100%的高度(这是在Flutter 中工作的页面过渡的数量).

4美元.在延迟1秒后将柱动画恢复到其原始位置.

SECOND UPDATE个

此代码根据屏幕大小和小部件大小计算偏移量. 我不知道有没有更好的方法来做这件事.

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Page(),
    );
  }
}

class Page extends StatefulWidget {
  @override
  State<StatefulWidget> createState() => _PageState();
}

class _PageState extends State<Page> with SingleTickerProviderStateMixin {
  // Set the initial position to something that will be offscreen for sure
  Tween<Offset> tween = Tween<Offset>(
    begin: Offset(0.0, 10000.0),
    end: Offset(0.0, 0.0),
  );
  Animation<Offset> animation;
  AnimationController animationController;

  GlobalKey _widgetKey = GlobalKey();

  @override
  void initState() {
    super.initState();

    // initialize animation controller and the animation itself
    animationController = AnimationController(
      vsync: this,
      duration: Duration(seconds: 2),
    );
    animation = tween.animate(animationController);

    Future<void>.delayed(Duration(seconds: 1), () {
      // Get the screen size
      final Size screenSize = MediaQuery.of(context).size;
      // Get render box of the widget
      final RenderBox widgetRenderBox =
          _widgetKey.currentContext.findRenderObject();
      // Get widget's size
      final Size widgetSize = widgetRenderBox.size;

      // Calculate the dy offset.
      // We divide the screen height by 2 because the initial position of the widget is centered.
      // Ceil the value, so we get a position that is a bit lower the bottom edge of the screen.
      final double offset = (screenSize.height / 2 / widgetSize.height).ceilToDouble();

      // Re-set the tween and animation
      tween = Tween<Offset>(
        begin: Offset(0.0, offset),
        end: Offset(0.0, 0.0),
      );
      animation = tween.animate(animationController);

      // Call set state to re-render the widget with the new position.
      this.setState((){
        // Animate it.
        animationController.forward();
      });
    });
  }

  @override
  void dispose() {
    // Don't forget to dispose the animation controller on class destruction
    animationController.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Stack(
      alignment: Alignment.center,
      fit: StackFit.loose,
      children: <Widget>[
        SlideTransition(
          position: animation,
          child: CircleAvatar(
            key: _widgetKey,
            backgroundImage: NetworkImage(
              'https://pbs.twimg.com/media/DpeOMc3XgAIMyx_.jpg',
            ),
            radius: 50.0,
          ),
        ),
      ],
    );
  }
}