Calling functions after state change occurs in reactjs

问题

My question is this. I have two components. First component is an image cropper. Second component is the one that i should display the cropped image.

The problem i'm facing is i can pass the cropped image to my second component but i have to press the button that crops the image and pass to the second component, twice. On the second click only my image is passing to the second component. But i can display the cropped image in the first component only by one click. I think it is happening because in reactjs state changes are not occurring immediately. So how can i fix this.

My approach was to create a prop function in the 1st component as this.props.croppedImage(this.state.preview.img); here this.state.preview.img is the cropped image. And in the 2nd component i'm getting the cropped image by calling the prop function.

My code

1st component (cropper)

class CropperTest extends React.Component {

    constructor(props) {

        super(props);

        this.state = {
            name: "beautiful",
            scale: 1,
            preview: null,
       }

        this.handleSave = this.handleSave.bind(this);

    }

    handleSave = () => {

        const img = this.editor.getImageScaledToCanvas().toDataURL();

        this.setState({
            preview: {
                img,
                scale: this.state.scale,
            }
        })

        this.props.croppedImage(this.state.preview.img);

    }

    setEditorRef = (editor) => {
        this.editor = editor
    }

    render() {
        return (
            <div>
                <div className="overlay"></div>

                <div className="crop_div">
                    <AvatarEditor
                        image={this.props.cropImage}
                        ref={this.setEditorRef}
                        width={450}
                        height={450}
                        border={50}
                        color={[255, 255, 255, 0.6]} // RGBA
                        scale={this.state.scale}
                        rotate={0}
                    />
                </div>



                <div className="zoom_slider text_align_center">
                    <input className="crop_btn" type='button' onClick={this.handleSave} value='Save'/>

                </div>

            </div>
        )
    }
}

export default CropperTest;

2nd component

Here i'm basically doing the following.

  <CropperTest  croppedImage = {this.getCroppedImg}/>

    getCroppedImg(img){

            alert("Perfect Storm");
            this.setState({

                previewImg:img

            })

        }

回答1:

I think it is happening because in reactjs state changes are not occurring immediately. So how can i fix this?

From the React#setState,

setState(updater, [callback])

setState() enqueues changes to the component state. The setState doesn't immediately update the state. setState() does not always immediately update the component. It may batch or defer the update until later. This makes reading this.state right after calling setState() a potential pitfall.Instead, use componentDidUpdate or a setState callback (setState(updater, callback))

Call the this.props.croppedImage in the setState callback.You will get updated values of component state. In your case its this.state.preview

this.setState({
  preview: {
    img,
    scale: this.state.scale,
  }
}, () => {
  this.props.croppedImage(this.state.preview.img);
})

来源：https://stackoverflow.com/questions/45992682/calling-functions-after-state-change-occurs-in-reactjs

标签

javascript

reactjs

prop