constructor() : The format of the method is as follows:
constructor(props)
Before the React component is mounted, its constructor is called constructor() .
React.Component : When a subclass implements a constructor, it should be called before other statements super(props) .
When the following example creates a component, React calls the constructor: In React, constructors are used only in the following two cases: By giving Bind an instance for the event handler. If you don’t initialize In Can only be specified directly in the constructor
In recent years, Geographic Information Systems (GIS) have undergone rapid development in both theoretical and practical dimensions. GIS has been widely applied for modeling and decision-making support across various fields such as urban management, regional planning, and environmental remediation, establishing geographic information as a vital component of the information era. The introduction of the “Digital Earth” concept has further accelerated the advancement of GIS, which serves as its technical foundation. Concurrently, scholars have been dedicated to theoretical research in areas like spatial cognition, spatial data uncertainty, and the formalization of spatial relationships. This reflects the dual nature of GIS as both an applied technology and an academic discipline, with the two aspects forming a mutually reinforcing cycle of progress. 3.13.1. Example ¶
classHeaderextendsReact.Component{constructor(props){super(props);this.state={favoritewebsite:"runoob"};}render(){return(<h1>My favorite websites{this.state.favoritewebsite}h1>);}}ReactDOM.render(<Header/>,document.getElementById('root'));
this.state assign an object to initialize the interior state . state or without method binding, you do not need to implement a constructor for the React component. constructor() do not call the setState() method. If your component needs to use the internal state directly in the constructor for this.state initial assignment state : constructor(props) {
super(props);
// Do not call here this.setState()
this.state = { counter: 0 };
this.handleClick = this.handleClick.bind(this);
}
this.state assign a value. If you need to assign values in other methods, you should use the this.setState() replace.
Principles, Technologies, and Methods of Geographic Information Systems
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