@HostBinding()可以為指令的宿主元素添加類、樣式、屬性等。

使用@HostBinding的一個例子：

import { Directive, HostBinding, HostListener } from '@angular/core';@Directive({selector: '[appRainbow]' }) export class RainbowDirective{possibleColors = ['darksalmon', 'hotpink', 'lightskyblue', 'goldenrod', 'peachpuff','mediumspringgreen', 'cornflowerblue', 'blanchedalmond', 'lightslategrey'];@HostBinding('style.color') color: string;@HostBinding('style.borderColor') borderColor: string;@HostListener('keydown') onKeydown(){const colorPick = Math.floor(Math.random() * this.possibleColors.length);console.log('Jerry colorPick: ' + colorPick);this.color = this.borderColor = this.possibleColors[colorPick];} }

通過這行語句：

@HostBinding('style.color') color: string;

我們將directive內(nèi)部的color屬性值同host元素的css樣式style.color建立了綁定關(guān)系。這樣，用TypeScript修改directive的color屬性值，會自動更新host元素的css樣式值。

視圖的更新檢測入口：core.js里的tick()函數(shù)：

位于ApplicationRef內(nèi)：

setHostBindingsByExecutingExpandoInstructions(tView, lView);

/*** Update a style binding on an element with the provided value.** If the style value is falsy then it will be removed from the element* (or assigned a different value depending if there are any styles placed* on the element with `styleMap` or any static styles that are* present from when the element was created with `styling`).** Note that the styling element is updated as part of `stylingApply`.** \@codeGenApi* @param {?} prop A valid CSS property.* @param {?} value New value to write (`null` or an empty string to remove).* @param {?=} suffix Optional suffix. Used with scalar values to add unit such as `px`.* Note that when a suffix is provided then the underlying sanitizer will* be ignored.** Note that this will apply the provided style value to the host element if this function is called* within a host binding function.** @return {?}*/ function ??styleProp(prop, value, suffix) {checkStylingProperty(prop, value, suffix, false);return ??styleProp; }

/*** Common code between `??classProp` and `??styleProp`.** @param {?} prop property name.* @param {?} value binding value.* @param {?} suffixOrSanitizer suffix or sanitization function* @param {?} isClassBased `true` if `class` change (`false` if `style`)* @return {?}*/ function checkStylingProperty(prop, value, suffixOrSanitizer, isClassBased) {/** @type {?} */const lView = getLView();/** @type {?} */const tView = getTView();// Styling instructions use 2 slots per binding.// 1. one for the value / TStylingKey// 2. one for the intermittent-value / TStylingRange/** @type {?} */const bindingIndex = incrementBindingIndex(2);if (tView.firstUpdatePass) {stylingFirstUpdatePass(tView, prop, bindingIndex, isClassBased);}if (value !== NO_CHANGE && bindingUpdated(lView, bindingIndex, value)) {// This is a work around. Once PR#34480 lands the sanitizer is passed explicitly and this line// can be removed./** @type {?} */let styleSanitizer;if (suffixOrSanitizer == null) {if (styleSanitizer = getCurrentStyleSanitizer()) {suffixOrSanitizer = (/** @type {?} */ (styleSanitizer));}}/** @type {?} */const tNode = (/** @type {?} */ (tView.data[getSelectedIndex() + HEADER_OFFSET]));updateStyling(tView, tNode, lView, lView[RENDERER], prop, lView[bindingIndex + 1] = normalizeAndApplySuffixOrSanitizer(value, suffixOrSanitizer), isClassBased, bindingIndex);} }

/*** Update a simple (property name) styling.** This function takes `prop` and updates the DOM to that value. The function takes the binding* value as well as binding priority into consideration to determine which value should be written* to DOM. (For example it may be determined that there is a higher priority overwrite which blocks* the DOM write, or if the value goes to `undefined` a lower priority overwrite may be consulted.)** @param {?} tView Associated `TView.data` contains the linked list of binding priorities.* @param {?} tNode `TNode` where the binding is located.* @param {?} lView `LView` contains the values associated with other styling binding at this `TNode`.* @param {?} renderer Renderer to use if any updates.* @param {?} prop Either style property name or a class name.* @param {?} value Either style value for `prop` or `true`/`false` if `prop` is class.* @param {?} isClassBased `true` if `class` (`false` if `style`)* @param {?} bindingIndex Binding index of the binding.* @return {?}*/ function updateStyling(tView, tNode, lView, renderer, prop, value, isClassBased, bindingIndex) {if (tNode.type !== 3 /* Element */) {// It is possible to have styling on non-elements (such as ng-container).// This is rare, but it does happen. In such a case, just ignore the binding.return;}/** @type {?} */const tData = tView.data;/** @type {?} */const tRange = (/** @type {?} */ (tData[bindingIndex + 1]));/** @type {?} */const higherPriorityValue = getTStylingRangeNextDuplicate(tRange) ?findStylingValue(tData, tNode, lView, prop, getTStylingRangeNext(tRange), isClassBased) :undefined;if (!isStylingValuePresent(higherPriorityValue)) {// We don't have a next duplicate, or we did not find a duplicate value.if (!isStylingValuePresent(value)) {// We should delete current value or restore to lower priority value.if (getTStylingRangePrevDuplicate(tRange)) {// We have a possible prev duplicate, let's retrieve it.value = findStylingValue(tData, null, lView, prop, bindingIndex, isClassBased);}}/** @type {?} */const rNode = (/** @type {?} */ (getNativeByIndex(getSelectedIndex(), lView)));applyStyling(renderer, isClassBased, rNode, prop, value);} }

最重要的platform-browser.js里的setStyle函數(shù)：這是HTML element的樣式改變最后執(zhí)行的語句：

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