[英]Vue 3 Composition API - Object is possibly 'null'
我正在使用Vue 3 Composition api和Typescript來使用HammerJS package實現捏合縮放。
我正在嘗試按照 CodePen JavaScript中編寫的一個工作示例: https://codepen.io/bakho/details/GBzvbB在 Vue 中實施。
然而,當我試圖讓它在我的 Vue 應用程序中工作時遇到了一些問題,我不確定如何解決它。
以下是以下錯誤:
// Object is possibly 'null'.
imageContainer.value.offsetWidth;
// Object is possibly 'null'.
imageContainer.value.appendChild(displayImage);
// Object is possibly 'null'.
imageContainer.value.addEventListener...
這是完整的源代碼:
<template>
<h1>Image Zoom</h1>
<div class="imageContainer" ref="imageContainer"></div>
</template>
<script lang="ts">
import Hammer from "hammerjs";
import { defineComponent } from "vue";
import { ref } from 'vue';
export default defineComponent({
setup() {
const imageUrl = "https://source.unsplash.com/random";
const imageContainer = ref(null)
let minScale = 1; let maxScale = 4; let imageWidth : any; let imageHeight : any; let containerWidth : any;
let containerHeight : any; let displayImageX = 0; let displayImageY = 0; let displayImageScale = 1;
let displayDefaultWidth : any; let displayDefaultHeight
let rangeX = 0; let rangeMaxX = 0; let rangeMinX = 0;
let rangeY = 0; let rangeMaxY = 0; let rangeMinY = 0;
// let displayImageRangeY = 0;
let displayImageCurrentX = 0;
let displayImageCurrentY = 0;
let displayImageCurrentScale = 1;
function resizeContainer() {
containerWidth = imageContainer.value.offsetWidth;
containerHeight = imageContainer.value.offsetHeight;
if (displayDefaultWidth !== undefined && displayDefaultHeight !== undefined) {
displayDefaultWidth = displayImage.offsetWidth;
displayDefaultHeight = displayImage.offsetHeight;
updateRange();
displayImageCurrentX = clamp(displayImageX, rangeMinX, rangeMaxX);
displayImageCurrentY = clamp(displayImageY, rangeMinY, rangeMaxY);
updateDisplayImage(
displayImageCurrentX,
displayImageCurrentY,
displayImageCurrentScale
);
}
}
resizeContainer();
function clamp(value, min, max) {
return Math.min(Math.max(min, value), max);
}
function clampScale(newScale) {
return clamp(newScale, minScale, maxScale);
}
const displayImage = new Image();
displayImage.src = imageUrl;
displayImage.onload = function(){
imageWidth = displayImage.width;
imageHeight = displayImage.height;
imageContainer.value.appendChild(displayImage);
displayImage.addEventListener('mousedown', e => e.preventDefault(), false);
displayDefaultWidth = displayImage.offsetWidth;
displayDefaultHeight = displayImage.offsetHeight;
rangeX = Math.max(0, displayDefaultWidth - containerWidth);
rangeY = Math.max(0, displayDefaultHeight - containerHeight);
}
imageContainer.value.addEventListener('wheel', e => {
displayImageScale = displayImageCurrentScale = clampScale(displayImageScale + (e.wheelDelta / 800));
updateRange();
displayImageCurrentX = clamp(displayImageCurrentX, rangeMinX, rangeMaxX)
displayImageCurrentY = clamp(displayImageCurrentY, rangeMinY, rangeMaxY)
updateDisplayImage(displayImageCurrentX, displayImageCurrentY, displayImageScale);
}, false);
function updateDisplayImage(x, y, scale) {
const transform = 'translateX(' + x + 'px) translateY(' + y + 'px) translateZ(0px) scale(' + scale + ',' + scale + ')';
displayImage.style.transform = transform;
displayImage.style.webkitTransform = transform;
displayImage.style.transform = transform;
}
function updateRange() {
rangeX = Math.max(0, Math.round(displayDefaultWidth * displayImageCurrentScale) - containerWidth);
rangeY = Math.max(0, Math.round(displayDefaultHeight * displayImageCurrentScale) - containerHeight);
rangeMaxX = Math.round(rangeX / 2);
rangeMinX = 0 - rangeMaxX;
rangeMaxY = Math.round(rangeY / 2);
rangeMinY = 0 - rangeMaxY;
}
const hammertime = new Hammer(imageContainer);
hammertime.get('pinch').set({ enable: true });
hammertime.get('pan').set({ direction: Hammer.DIRECTION_ALL });
hammertime.on('pan', ev => {
displayImageCurrentX = clamp(displayImageX + ev.deltaX, rangeMinX, rangeMaxX);
displayImageCurrentY = clamp(displayImageY + ev.deltaY, rangeMinY, rangeMaxY);
updateDisplayImage(displayImageCurrentX, displayImageCurrentY, displayImageScale);
});
hammertime.on('pinch pinchmove', ev => {
displayImageCurrentScale = clampScale(ev.scale * displayImageScale);
updateRange();
displayImageCurrentX = clamp(displayImageX + ev.deltaX, rangeMinX, rangeMaxX);
displayImageCurrentY = clamp(displayImageY + ev.deltaY, rangeMinY, rangeMaxY);
updateDisplayImage(displayImageCurrentX, displayImageCurrentY, displayImageCurrentScale);
});
hammertime.on('panend pancancel pinchend pinchcancel', () => {
displayImageScale = displayImageCurrentScale;
displayImageX = displayImageCurrentX;
displayImageY = displayImageCurrentY;
});
return {};
},
});
</script>
<style>
.imageContainer {
width: 96%;
height: 96%;
max-width: 800px;
max-height: 600px;
position: absolute;
overflow: hidden;
top: 0;
right: 0;
bottom: 0;
left: 0;
margin: auto;
background: #2b2b2c;
display: flex;
flex-direction: column;
align-items: center;
justify-content: center;
}
.imageContainer > img {
display: block;
max-width: 100%;
max-height: 100%;
cursor: move;
touch-action: none;
}
</style>
誰能告訴我出了什么問題,為什么會導致這個Object is possibly 'null'
所以這是該錯誤的原因:
const imageContainer = ref(null)
-> 您將值設置為null
和 TypeScript 警告您訪問該 object 的屬性可能會引發錯誤,因為初始值為null
並且因為您正在嘗試在模板中使用ref
元素可以存在,也可以不存在。
const imageContainer = document.querySelector('.imageContainer')
-> 您正在查詢存在或不存在的 HTML 元素,這意味着您還可以獲得null
作為值,而 TypeScript 再次警告您可以訪問該元素的屬性object 可以拋出錯誤
解決方案:
setup
function 返回imageContainer
變量,以便 Vue 將其與模板中的ref
綁定,並且可以在onMounted
function 內部訪問該變量,因為它是一個ref
,這意味着它尚未安裝到 DOM 中。 在 Vue.js 組件生命周期中created
和mounted
掛鈎之前調用setup
,您無權訪問那里的任何 DOM 內容。import { ref, onMounted } from 'vue'
export default {
setup() {
const imageContainer = ref(null)
onMounted(() => {
// here you access imageContainer variable
})
return {
imageContainer
}
}
}
這是一個 Typescript 錯誤。 問題是您沒有處理變量為 null 的可能性。您需要做的就是正確定義類型並處理 null 的情況。 閱讀 Vue 文檔中的Typing ref
以了解我的意思。
以下是您的案例的一些示例:
// Define the ref as an HTMLElement or null
const imageContainer = ref<HTMLElement | null>(null);
...
// Set the width/height based on the imageContainer, or use 0 if it is null.
containerWidth = imageContainer.value?.offsetWidth ?? 0;
containerHeight = imageContainer.value?.offsetHeight ?? 0;
...
// Only call appendChild/addEventListener if the ref is not null.
imageContainer.value?.appendChild(displayImage);
如果在出現錯誤的地方應用此方法,則應該可以解決問題。
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