如何检测 ion-content 是否有滚动条？

Question

我想在 ion-content 上有或没有滚动条时隐藏或显示元素。更具体地说，我想在没有滚动条时显示一个按钮（以在列表中加载更多项目）并在有滚动条的地方隐藏它（因此更多项目的加载由 ion-infinite-scroll 完成）。

我的 Ionic 应用程序也将部署到桌面，因此大屏幕用户最初不会看到滚动条，因此不会触发 ion-infinite-scroll。

这是一个展示该问题的演示：

home.page.html

<ion-header>
  <ion-toolbar>
    <ion-title>
      Ionic header
    </ion-title>
  </ion-toolbar>
</ion-header>

<ion-content>
  <div class="ion-padding">
    <p *ngFor="let item of itemList">{{ item }}</p>

    <!-- How to hide this button when ion-content has a scrollbar? -->
    <!-- *ngIf="???" -->
    <ion-button (click)="incrementItemList(5)">Load more items</ion-button>
  </div>

  <ion-infinite-scroll (ionInfinite)="loadMoreItems($event)">
    <ion-infinite-scroll-content loadingSpinner="crescent"></ion-infinite-scroll-content>
  </ion-infinite-scroll>
</ion-content>

<ion-footer>
  <ion-toolbar>
    <ion-title>
      Ionic footer
    </ion-title>
  </ion-toolbar>
</ion-footer>

home.page.ts

import { Component } from '@angular/core';

@Component({
  selector: 'app-home',
  templateUrl: 'home.page.html',
  styleUrls: ['home.page.scss'],
})
export class HomePage {

  itemList: string[] = [];

  constructor() {}

  ionViewWillEnter() {
    this.incrementItemList(5);
  }

  incrementItemList(times: number) {
    for (let i = 1; i <= times; i++) {
      this.itemList.push(`Lorem ipsum dolor sit amet consectetur adipisicing elit. Quia placeat nam sapiente iusto eligendi`);
    }
  }

  loadMoreItems(event: any) {
    setTimeout(() => {
      this.incrementItemList(15);
      event.target.complete();
    }, 1000);
  }

}

我正在使用 Ionic 4.5.0 + Angular。

我尝试过使用getScrollElement、scrollHeight、clientHeight、offsetHeight，但没有成功。

有什么想法吗？

Answer 1

更新：我睡在上面，然后意识到我完全错了。

这是一个工作示例。它会检查三个位置以确保不会错过出现的滚动条：

1. 页面加载时
2. 添加新内容时
3. 当用户滚动时

在我的测试环境中，两次单击添加按钮时会出现在屏幕高度的位置，因此该按钮并不总是消失。我添加了ionScrollEnd 检查以提供额外的捕获方法。

您可以删除散布在代码周围的console.log，我将其保留以帮助您了解发生了什么。

示例页面：

<ion-header>
  <ion-toolbar>
    <ion-title>
      Scrollbar Test
    </ion-title>
  </ion-toolbar>
</ion-header>

<ion-content [scrollEvents]="true" (ionScrollEnd)="onScrollEnd()">
  <div class="ion-padding">
    <p *ngFor="let item of itemList">{{ item }}</p>

    <ion-button *ngIf="!hasScrollbar" (click)="addMoreItemsButtonClick(5)">Load more items</ion-button>
  </div>

  <ion-infinite-scroll (ionInfinite)="loadMoreItems($event)">
    <ion-infinite-scroll-content loadingSpinner="crescent"></ion-infinite-scroll-content>
  </ion-infinite-scroll>
</ion-content>

<ion-footer>
  <ion-toolbar>
    <ion-title>
      Ionic footer
    </ion-title>
  </ion-toolbar>
</ion-footer>

示例代码：

import { Component, OnInit, ViewChild, ElementRef } from '@angular/core';
import { IonContent } from '@ionic/angular';

@Component({
  selector: 'app-scrollheight',
  templateUrl: './scrollheight.page.html',
  styleUrls: ['./scrollheight.page.scss'],
})
export class ScrollheightPage implements OnInit {

  public hasScrollbar: Boolean = false;

  @ViewChild(IonContent) private content: IonContent;

  itemList: string[] = [];

  constructor() { }

  ngOnInit() {
    // check at startup
    this.checkForScrollbar();
  }

  ionViewWillEnter() {
    this.incrementItemList(5);
  }

  addMoreItemsButtonClick(quantity: number) {
    this.incrementItemList(quantity);

    // check after pushing to the list
    this.checkForScrollbar();
  }

  onScrollEnd() {
    // check after scrolling
    this.checkForScrollbar();
  }

  incrementItemList(times: number) {
    for (let i = 1; i <= times; i++) {
      this.itemList.push(`Lorem ipsum dolor sit amet consectetur adipisicing elit. Quia placeat nam sapiente iusto eligendi`);
    }
  }

  loadMoreItems(event: any) {
    setTimeout(() => {
      this.incrementItemList(15);

      event.target.complete();
    }, 1000);
  }

  checkForScrollbar() {
    this.content.getScrollElement().then((scrollElement) => {
      console.log("checking for scroll bar");
      console.log({scrollElement});
      console.log({scrollHeight: scrollElement.scrollHeight});
      console.log({clientHeight: scrollElement.clientHeight});
      this.hasScrollbar = (scrollElement.scrollHeight > scrollElement.clientHeight);
      console.log({hasScrollBar: this.hasScrollbar});
    });
  }
}

Answer 2

在 rtpHarry 的帖子（谢谢！）的帮助下，我终于为这个用例想出了一个合适的解决方案：

home.page.html

<ion-content>
  <div class="ion-padding">
    <p *ngFor="let item of itemList">{{ item }}</p>

    <!--
      '*ngIf' removes the button from the DOM and changes the size of 'ion-content' which
      is problematic in some scenarios so I toggle the visibility CSS property instead
    -->
    <ion-button [style.visibility]="hasScrollbar ? 'hidden' : 'visible'" (click)="incrementItemList(5)">Load more items</ion-button>
  </div>

  <ion-infinite-scroll (ionInfinite)="loadMoreItems($event)">
    <ion-infinite-scroll-content loadingSpinner="crescent"></ion-infinite-scroll-content>
  </ion-infinite-scroll>
</ion-content>

home.page.ts

import { Component, ViewChild, HostListener } from '@angular/core';
import { IonContent } from '@ionic/angular';

@Component({
  selector: 'app-home',
  templateUrl: 'home.page.html',
  styleUrls: ['home.page.scss'],
})
export class HomePage {

  hasScrollbar = false;

  itemList: string[] = [];

  @ViewChild(IonContent, {static: false}) private content: IonContent;

  // checks if there's a scrollbar when the user resizes the window or zooms in/out
  @HostListener('window:resize', ['$event'])
  onResize() {
    this.checkForScrollbar();
  }

  constructor() {}

  ionViewWillEnter() {
    this.incrementItemList(5);
  }

  incrementItemList(times: number) {
    for (let i = 1; i <= times; i++) {
      this.itemList.push(`Lorem ipsum dolor sit amet consectetur adipisicing elit. Quia placeat nam sapiente iusto eligendi`);
    }

    this.checkForScrollbar();
  }

  loadMoreItems(event: any) {
    setTimeout(() => {
      this.incrementItemList(15);
      event.target.complete();
    }, 1000);
  }

  async checkForScrollbar() {
    const scrollElement = await this.content.getScrollElement();
    this.hasScrollbar = scrollElement.scrollHeight > scrollElement.clientHeight;
  }

}

Answer 3

您能否通过从@angular/core 导入来使用“NgZone” 示例代码在这里。

.ts

import { Component, OnInit, ViewChild, NgZone } from '@angular/core';
@Component({
  selector: 'app-selector',
  templateUrl: './app.page.html',
  styleUrls: ['./app.page.scss'],
})
export class App implements OnInit {
@ViewChild("listingContent") listingContent;
public hasScrollbar: Boolean = false;
public itemList:Array<any> = [];
    constructor(private zone: NgZone) {}

    scrollHandler(event) {
        this.zone.run(()=>{
            if (event.detail.scrollTop > (document.documentElement.clientHeight + 1)) {
                this.hasScrollbar = true;
            } else {
                this.hasScrollbar = false;
            }
        });
    }
}

.html

<ion-header no-border></ion-header>
<ion-content #listingContent padding-top (ionScroll)="scrollHandler($event)" scroll-events="true">
<div class="ion-padding">
    <p *ngFor="let item of itemList">{{ item }}</p>

    <ion-button *ngIf="!hasScrollbar" (click)="addMoreItemsButtonClick(5)">Load more items</ion-button>
  </div>

</ion-content>

希望这会有所帮助..