最近几年来，地理信息系统无论是在理论上还是应用上都处在一个飞速发展的阶段。 GIS被应用于多个领域的建模和决策支持，如城市管理、区划、环境整治等等，地理信息成为信息时代重要的组成部分之一； “数字地球”概念的提出，更进一步推动了作为其技术支撑的GIS的发展。 与此同时，一些学者致力于相关的理论研究，如空间感知、空间数据误差、空间关系的形式化等等。 这恰好说明了地理信息系统作为应用技术和学科的两个方面，并且这两个方面构成了相互促进的发展过程。
React treats components as a state machine (State Machines). Achieve different states by interacting with the user, and then render the UI to keep the user interface and data consistent.
In React, you only need to update the state and then according to the new state re-render the user interface (do not manipulate DOM).
The following example creates a name that extends to React.Component . The ES6 class of the render() method used in the this.state to modify the current time.
Add a class constructor to initialize the state Next, we will have Clock set its own timer and update it every second. In applications with many components, it is important to release the resources consumed by the components at the time of destruction. Whenever Similarly, whenever We can declare special methods on the component class to run some code when the component is mounted or unloaded: Instance resolution: Executes after the component is output to DOM Code execution order: When React then calls When The browser calls every second Once Neither the parent component nor the child component can know whether a component is stateful or stateless, and they should not care whether a component is defined as a function or a class. This is why the state is often referred to as local or encapsulation. Exceptfor the component that owns and sets it, other components are not accessible. In the following example This is often referred to as top-down or unidirectional data flow. Any stateis always owned by certain components, and any data or UI exported from that state can only affect components below the tree. If you imagine a component tree as a waterfall of attributes, the state of each component is like an additional water source, which is connected at anypoint, but also flows down. To show that all components are truly isolated, we can create an App component that renders three Each of the above examples In We can use stateless components in stateful components, or we can use stateful components in stateless components. this.state class components should always use the props call the underlying constructor. 3.6.1. React instance ¶
classClockextendsReact.Component{constructor(props){super(props);this.state={date:newDate()};}render(){return( );}}ReactDOM.render(Hello,world!
Now it's {this.state.date.toLocaleTimeString()}.
Add a lifecycle method to a class ¶
Clock component is loaded into the DOM we all want to generate a timer, which is in the React is called mounting. Clock generated this. DOM when it is removed, we also want to clear the timer, which is in the React is called uninstalling. 3.6.2. React instance ¶
classClockextendsReact.Component{constructor(props){super(props);this.state={date:newDate()};}componentDidMount(){this.timerID=setInterval(()=>this.tick(),1000);}componentWillUnmount(){clearInterval(this.timerID);}tick(){this.setState({date:newDate()});}render(){return( );}}ReactDOM.render(Hello,world!
Now it's{this.state.date.toLocaleTimeString()}.
componentDidMount() and componentWillUnmount() methods are called lifecycle hooks. componentDidMount() , Hook,we can set a timer on this hook. this.timerID for the ID of the timer, we can use the componentWillUnmount() uninstall the timer in the hook.
ReactDOM.render() react calls the Clock the constructor of the component. Due to Clock need to display the current time, so use an object that contains the current time toinitialize the this.state . We will update this status later. Clock component render() method. This is React knowing what should be displayed on the screen, and then React updates DOM to match Clock the rendered output of the. Clock the output of the is inserted into the DOM, React calls componentDidMount() life cycle hooks. In which Clock component requires the browser to set a timer that is called once per second tick() . tick() method. In which Clock Ccomponent is called by using an object containing the current time setState() to schedule UI updates. By calling the setState() react knows that the state has changed and calls the render() method to determine what should be displayed on the screen. This time, render() In the method this.state.date will be different, so the rendered outputwill contain the update time and update the DOM accordingly. Clock the component is removed from the DOM and React calls the componentWillUnmount() . This hook function, the timer will also be cleared. Data flows from top to bottom ¶
FormattedDate the component will receive the date value, and do not know that it comes from Clock status, or from Clock or enter it manually: 3.6.3. React instance ¶
functionFormattedDate(props){returnNow it's{props.date.toLocaleTimeString()}.
;}classClockextendsReact.Component{constructor(props){super(props);this.state={date:newDate()};}componentDidMount(){this.timerID=setInterval(()=>this.tick(),1000);}componentWillUnmount(){clearInterval(this.timerID);}tick(){this.setState({date:newDate()});}render(){return( Hello,world!
Clock : 3.6.4. React instance ¶
functionFormattedDate(props){returnNow it's{props.date.toLocaleTimeString()}.
;}classClockextendsReact.Component{constructor(props){super(props);this.state={date:newDate()};}componentDidMount(){this.timerID=setInterval(()=>this.tick(),1000);}componentWillUnmount(){clearInterval(this.timerID);}tick(){this.setState({date:newDate()});}render(){return( Hello,world!
Clock components set up their own timers and update them independently. React an application, whether a component is stateful or stateless is considered to be the implementation details of a component that may change over time.