本篇參考https://www.salesforcelwc.in/2019/10/lookup-in-lwc.html，感謝前人種樹。

我們做lightning的時候經常會遇到Look up 或者MD欄位在頁面搜尋展示的需求，在標準的頁面很常見，而且很好看。但是很遺憾的是在自定義元件中還沒有現成的標準組件去搞定。下面介紹兩種方式去實現展示lookup / MD欄位的元件樣式。

 一.  record-edit-form搭配 lightning-input-field 曲線救國

標準功能也不是100%的不近人情，還是給出一個workaround的方案去實現，實現的方式為在後臺宣告一個Lookup / MD的元件，然後使用lightning-input-field去實現。此元件會根據欄位的型別去自動轉換成其相應的樣式進行展示，效果很像classic中的apex:inputField或者lightning aura中的lightning:inputField。使用lightning-record-edit-form來指定某個表的LDS，使用lightning-input-field進行效果展示，然後提交的操作時阻止預設的submit操作並且在event detail中獲取到我們選擇的Look up/MD對應的ID即可。demo中在Account上新建一個欄位Test_User__c，型別為Look up (User).

 testLookUpForLwc.html:使用LDS設定object api name為Account，這樣下面就可以通過lightning-input-field針對Account的欄位的型別動態展示相關的樣式

<template>
    <lightning-record-edit-form
        object-api-name='Account'
        onsubmit={handleSubmit}
    >
        <lightning-input-field field-name="Test_User__c"></lightning-input-field>
        <lightning-button type="submit" label="get test user id" variant="brand"> 
        </lightning-button>
    </lightning-record-edit-form>
</template>

testLookUpForLwc.js：針對submit事件首先組織提交免得生成不需要的記錄，然後通過event.detail.fields.Test_User__c便可以獲取到所選擇的Test_User__c的ID。

import { LightningElement,track } from 'lwc';

export default class TestLookUpForLwc extends LightningElement {
    handleSubmit(event) {
        event.preventDefault();
        console.log(JSON.stringify(event.detail.fields.Test_User__c));
    }
}

結果展示：選擇一個使用者以後，點選get test user id便可以獲取到當前選擇的user的id。

 二.自定義元件實現

 上面的方式好是好，但是此種寫法沒法更改相關的label資訊，國內專案可能新建立個欄位進行translation也可以實現，後臺進行匹配也可以，但是對日專案可能管理嚴格，所以需要考慮自定義元件實現。自定義元件的實現的原理相對簡單，難得是UI的構建，好在前輩有畫好的功能直接使用，對上面的連結中的程式碼進行簡單的修改即可使用。

customLookUpForLwc.html：展示UI，上面是一個lightning-pill / lightning-input，通過isValue來判斷當前是輸入框還是展示pill，下面是列表。當列表選擇以後觸發事件父進行處理。

<template>
    <div>
        <div class="slds-form-element">
            <div class="slds-form-element__control">
                <div class="slds-combobox_container">
                    <div id="box" class={boxClass} aria-expanded="true" aria-haspopup="listbox" role="combobox">
                        {searchLabel}
                        <div class="slds-combobox__form-element slds-input-has-icon slds-input-has-icon_right" role="none">
                            <template if:true={isValue}>
                                <div id="lookup-pill" class="slds-pill-container">
                                    <lightning-pill class="pillSize" label={valueObj} name={valueObj} onremove={handleRemovePill}>
                                        <lightning-icon icon-name={iconName} alternative-text="acc" ></lightning-icon>
                                    </lightning-pill>
                                </div>
                            </template>
                            <template if:false={isValue}>
                                <div class="slds-p-top_none">
                                    <lightning-input class={inputClass} type="search" id="input" value={searchTerm}
                                        onclick={handleClick}  onchange={onChange}
                                        variant="label-hidden" autocomplete="off" placeholder="Search..." label='account search'>
                                    </lightning-input>
                                </div>
                            </template>
                        </div>
                        <div id="listbox-id-1" class="slds-dropdown slds-dropdown_length-with-icon-7 slds-dropdown_fluid" role="listbox">
                            <ul class="slds-listbox slds-listbox_vertical" role="presentation">
                                <template for:each={options} for:item="item">
                                    <li key={item.Id} onclick={onSelect} data-id={item.Id} role="presentation">
                                        <span class="slds-lookup__item-action slds-lookup__item-action--label" role="option">
                                            <lightning-icon class="slds-icon slds-icon--small slds-icon-text-default" icon-name={iconName} alternative-text={objName} size="small"></lightning-icon>
                                            <span class="slds-truncate">{item.Name}</span>
                                        </span>
                                    </li>
                                </template>
                            </ul>
                        </div>
                    </div>
                </div>
            </div>
        </div>
    </div>

</template>

customLookUpForLwc.js

/* eslint-disable no-console */
/* eslint-disable @lwc/lwc/no-async-operation */

import lookUp from '@salesforce/apex/CustomLookUpForLwcController.lookUp';
import { getObjectInfo } from 'lightning/uiObjectInfoApi';
import { getRecord } from 'lightning/uiRecordApi';
import { api, LightningElement, track, wire } from 'lwc';

export default class CustomLookUpForLwc extends LightningElement {
    //store object record id
    @api valueId;
    //record API name
    @api objName;
    //record icon name,see Lightning Design System to choose
    @api iconName;

    @api filter = '';
    //unique key used to mark the unique component. several component use this component need to mapping
    @api uniqueKey;
    //used to set the field to fetch.eg: ['Account.Name'] means we need to search account name field as filter
    @api fields;

    //search label show in lookup component
    @api searchLabel;

    @track searchTerm;
    //record name value
    @track valueObj;
    //record href
    @track href;
    //fetch result
    @track options;
    //is available value to show in lightning-pill
    @track isValue = false;

    @track blurTimeout;

    //css
    @track boxClass = 'slds-combobox slds-dropdown-trigger slds-dropdown-trigger_click slds-has-focus';
    @track inputClass = '';

    @wire(lookUp, {searchTerm : '$searchTerm', myObject : '$objName', filter : '$filter'})
    wiredRecords({ error, data }) {
        if (data) {
            this.record = data;
            this.error = undefined;
            this.options = this.record;
            console.log("common this.options", JSON.stringify(this.options));
        } else if (error) {
            this.error = error;
            this.record = undefined;
            console.log("wire.error",this.error);
        }
    }

    //To get preselected or selected record
    @wire(getRecord, { recordId: '$valueId', fields: '$fields' })
    wiredOptions({ error, data }) {
        if (data) {
            console.log('execute1');
            this.record = data;
            this.error = undefined;
            this.valueObj = this.record.fields.Name.value;
            this.href = '/'+this.record.id;
            this.isValue = true;
            console.log("this.href", this.href);
            console.log("this.record", JSON.stringify(this.record));
        } else if (error) {
            console.log('execute2');
            this.error = error;
            this.record = undefined;
            console.log("this.error", this.error);
        }
    }

    handleClick() {
        console.log("In handleClick");

        this.searchTerm = '';
        this.inputClass = 'slds-has-focus';
        this.boxClass = 'slds-combobox slds-dropdown-trigger slds-dropdown-trigger_click slds-has-focus slds-is-open';
    }

    onSelect(event) {
        console.log("In onSelect");
        let ele = event.currentTarget;
        let selectedId = ele.dataset.id;
        console.log("selectedId", selectedId);
        //As a best practise sending selected value to parent and inreturn parent sends the value to @api valueId
        let key = this.uniqueKey;
        const valueSelectedEvent = new CustomEvent('valueselect', {
            detail: { selectedId, key },
        });
        this.dispatchEvent(valueSelectedEvent);

        // if(this.blurTimeout) {
        //     clearTimeout(this.blurTimeout);
        // }
        console.log(this.isValue);
        this.boxClass = 'slds-combobox slds-dropdown-trigger slds-dropdown-trigger_click slds-has-focus';
    }

    onChange(event) {
        console.log("In onChange");
        this.searchTerm = event.target.value;
        console.log("searchTerm",this.searchTerm);
    }

    handleRemovePill() {
        console.log("In handleRemovePill");
        this.isValue = false;
        let selectedId = '';
        let key = this.uniqueKey;
        const valueSelectedEvent = new CustomEvent('valueselect', {
            detail: { selectedId, key },
        });
        this.dispatchEvent(valueSelectedEvent);
    }

}

testLookUpForLwc.html：引入元件，設定幾個必填的引數，其中icon-name賦值可以選擇連結中的以下內容進行查詢https://lightningdesignsystem.com/icons/。使用accountId用來獲取前臺元件傳遞過來的ID。

<template>
    <c-custom-look-up-for-lwc
        unique-key={item.Id} 
        value-id={accountId} 
        obj-name="Account" 
        icon-name="standard:account" 
        search-label="Search Account" 
        onvalueselect={handleSelection}
        search-label
        fields={item.fields}>
    </c-custom-look-up-for-lwc>
</template>

testLookUpForLwc.js：handleSelection用來獲取accountId

import { LightningElement,track } from 'lwc';

export default class TestLookUpForLwc extends LightningElement {
    @track item = {
        id:'xxx',
        fields:['Account.Name']
    };

    @track accountId;

    handleSelection(event) {
        console.log(event.detail.selectedId);
        this.accountId = event.detail.selectedId;
    }

}

效果展示：

 總結：篇中通過兩種方式實現lookup功能及樣式的實現，如果第一種能搞定強烈推薦使用第一種，因為標準的功能穩定性以及效率會好很多，如果第一種搞定不了可以考慮自定義。lwc不易，且開發且珍