[ant design] form-为何-getFieldValue-在-getFieldDecorator-中可以取到值，但是在getFieldDecorator-之前就不行呢？

const { getFieldDecorator, getFieldsError, getFieldError, isFieldTouched, getFieldValue, getFieldsValue } = this.props.form

getFieldValue('fieldKey') // TODO 1. 为何此时为 undefined？？
const uploadButton = (
    <div>
        <Icon type="plus" />
        <div className="ant-upload-text">Upload</div>
    </div>
)

return getFieldDecorator('fieldKey', {
    valuePropName: 'fileList',
    getValueFromEvent: this.normFile,
    initialValue: [
        {
            uid: "-1",
            name: "xxx.png",
            status: "done",
            url: "https://zos.alipayobjects.com/rmsportal/jkjgkEfvpUPVyRjUImniVslZfWPnJuuZ.png"
        }
    ]
})(
    <Upload
        action="/v1/img/upload"
        listType="picture-card"
        showUploadList={true}
        headers={headers}
        // onChange={info => {
        //     this.handleChange(info, k)
        // }}
    >
        {
            // TODO 2. 为何此时是可以取到值的呢？getFieldDecorator 的执行机机制是什么呢？
            (getFieldValue('fieldKey') || []).length === 1 ? null : uploadButton
        }

    </Upload>
)