first Flutter application

today , Let's read it together Flutter Code after project initialization ！

Create an

• adopt AS Create a new Flutter engineering , We will get a default counter application example

• Project dart The code is mainly in lib/main.dart in

class MyApp extends StatelessWidget {@overrideWidget build(BuildContext context) {return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,),
      home: MyHomePage(title: 'Flutter Demo Home Page'),);}
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key? key, required this.title}) : super(key: key);
  final String title;
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text('You have pushed the button this many times:'),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.headline4,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}

The code analysis

1. Import package

import 'package:flutter/material.dart';

Some used later widget It's from here , So it can be understood as a UI Component library .

2. Application entry

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

• It is the same as most languages ,Flutter Or say dart yes main Function is the entry of the application .main Invocation in function runApp Method , Its function is to start Flutter application .runApp It accepts a Widget Parameters

• How to understand this Widget What about parameters? ？

  • Just think of it as another name for a component

  • Or for this entrance , Is and App.jsx,App.vue A very similar thing

• main The function uses js Anonymous function in , This abbreviation is more natural

3. Application structure

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      // apply name   
      title: 'Flutter Demo', 
      theme: ThemeData(
        // Blue theme   
        primarySwatch: Colors.blue,
      ),
      // Application home routing   
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

• MyApp Class representative Flutter application , It inherited StatelessWidget class , that , As a subclass, it is a StatelessWidget Class

  • So to put it simply , be-all Widget Are divided into two categories

    • StatelessWidget

    • StatefulWidget

  • In fact, that is React Controlled and uncontrolled components in

    • It's transparent at once, isn't it

  • that , If analogy Vue

    • about Vue2

      • Pass in props To control is to be controlled

      • Changes in internal data can be understood as controlled , It doesn't need to be like React Go to setState Or call useStata To update

    • about Vue3

      • Internal data use ref and reactive The package is a controlled component

• stay Flutter in , Almost all widget, Including some css The styles of are all based on widget In the form of

  • I feel that the style is not pure css Come on

  • But think about it , In this way, the code reads friendly to novices

  • For beginners ,Flutter The style control of should be easier to understand

• Flutter When building a page , Will call the build Method ,widget The main job is to provide a build() Method

  • This method is how to build this widge Components

• MaterialApp yes Material library A common foundation provided in Widget, It allows you to set the name of the application , The theme , Language , Home page and route list etc .

• home It's the home page of the app , It is also a what ？

  • widget

home page

1. First time to know Widget

class MyHomePage extends StatefulWidget {
  MyHomePage({Key? key, required this.title}) : super(key: key);
  final String title;
  
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
 ...
}

• Then continue to look up , We App Home page home This widget That is to say MyHomePage It is a controlled component ！

• Let's talk about the two in detail

  • 1. Controlled , Uncontrolled , A stateful , No state, no more , Simply put, is it responsive , Can you change

  • 2. a key ：Stateful widget It consists of at least two classes :

    • StatefulWidget class

    • State class ,StateWidget Class itself is immutable , however State The state held in class is widget Changes may occur in the life cycle

2._MyHomePageState Class is MyHomePage Class corresponding state class .

• A strange thing

  • MyHomePage Not in class build Method , Everyone mentioned above widget There is one. build The method seems different

  • This method was moved to \_MyHomePageState In the method

  • Now let's take a look at the state class

3.State class

a._MyHomePageState Class parsing

below , Let's come to Kangkang, which includes those things ！

• State of component

For example, this initialization item is a counter , So the state is a count

int _counter = 0; // Used to record the total number of button clicks 

• Set the auto increment function of the State

void _incrementCounter() {
    setState(() {
        _counter++;
    });
}

When the button is clicked , Just call this function , Changing the state will use setState Method , This and React The way in which the aggregation of class components changes state is very similar

• hear Flutter This method is optimized , There is no need to modify every widget, The source code has not been understood

• structure UI Interface build Method

  • When MyHomePage First created ,\_MyHomePageState Classes will also be created , When initialization is complete ,Flutter The framework will call widget Of build Method to build widget Trees

    • This and cssdom Trees , Render a tree

Widget build(BuildContext context) {
  return Scaffold(
    appBar: AppBar(
      title: Text(widget.title),
    ),
    body: Center(
      child: Column(
        mainAxisAlignment: MainAxisAlignment.center,
        children: <Widget>[
          Text('You have pushed the button this many times:'),
          Text(
            '$_counter',
            style: Theme.of(context).textTheme.headline4,
          ),
        ],
      ),
    ),
    floatingActionButton: FloatingActionButton(
      onPressed: _incrementCounter,
      tooltip: 'Increment',
      child: Icon(Icons.add),
    ), 
  );
}

• Scaffold yes Material library Page scaffolding provided in , It provides a default navigation bar , Title and include home screen widget Trees Of body attribute . By default, all routes are through Scaffold establish

• body Component tree of

  • It contains a Center Components , It can move sub components to the center of the screen

    • It's really css Semantically

  • Center Self assembly is a Column Components , It can arrange all self components in order along the vertical direction of the screen

    • Child components a.Text, Show fixed text

    • Child components b.Text, Show \_counter The number of States

• floatingActionButton It's the band in the bottom right corner of the page + The button , its onPressed Property to accept a callback function , Namely onClick, ha-ha

• So the logic is like this

  • Click button , Call auto increment function

  • setState Cause page changes rebuild Reach a state ！！！

b. Why? build Methods on the State What about China? ？

• Divide and rule ,state Autonomy improves efficiency

  • State access is convenient

  • If you put it in StatefulWidget, That needs to be done state Pass in , It becomes troublesome

    • Or the state State disclosure , But this is very danger 了

• Come gently , Walk softly

  • If build Put it in there , It will bring trouble to inheritance

    • There will be multiple build

    • One mountain cannot tolerate two tigers , Can fight