Angular 4.x NgIf

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NgIf 指令作用

ngIf 指令用于根据表达式的值,在指定位置渲染 thenelse 模板的内容

  • then 模板除非绑定到不同的值,否则默认是 ngIf 指令关联的内联模板。

  • else 模板除非绑定对应的值,否则默认是 null。

NgIf 指令语法

简单形式

<!--语法糖-->
<div *ngIf="condition">...</div>
<!--Angular 2.x中使用template-->
<ng-template [ngIf]="condition"><div>...</div></ng-template>

使用else块

<div *ngIf="condition; else elseBlock">...</div>
<ng-template #elseBlock>...</ng-template>

使用then和else块

<div *ngIf="condition; then thenBlock else elseBlock"></div>
<ng-template #thenBlock>...</ng-template>
<ng-template #elseBlock>...</ng-template>

使用as语法

<div *ngIf="condition as value; else elseBlock">{{value}}</div>
<ng-template #elseBlock>...</ng-template>

NgIf 使用示例

@Component({
  selector: 'ng-if-then-else',template: `
    <button (click)="show = !show">{{show ? 'hide' : 'show'}}</button>
    <button (click)="switchPrimary()">Switch Primary</button>
        show = {{show}}
    <br>
    <div *ngIf="show; then thenBlock; else elseBlock">this is ignored</div>
    <ng-template #primaryBlock>Primary text to show</ng-template>
    <ng-template #secondaryBlock>Secondary text to show</ng-template>
    <ng-template #elseBlock>Alternate text while primary text is hidden</ng-template>
 `
})
class NgIfThenElse implements OnInit {
  thenBlock: TemplateRef<any> = null;
  show: boolean = true;
  
  @ViewChild('primaryBlock')
  primaryBlock: TemplateRef<any> = null;
  @ViewChild('secondaryBlock')
  secondaryBlock: TemplateRef<any> = null;
  
  switchPrimary() {
    this.thenBlock = this.thenBlock === this.primaryBlock ? 
      this.secondaryBlock : this.primaryBlock;
  }
  
  ngOnInit() { 
      this.thenBlock = this.primaryBlock;
  }
}

基础知识

TemplateRef

TemplateRef 实例用于表示模板对象,TemplateRef 抽象类的定义如下:

// angular\packages\core\src\linker\template_ref.ts
export abstract class TemplateRef<C> {
  abstract get elementRef(): ElementRef;
  abstract createEmbeddedView(context: C): EmbeddedViewRef<C>;
}

ViewContainerRef

ViewContainerRef 实例提供了 createEmbeddedView() 方法,该方法接收 TemplateRef 对象作为参数,并将模板中的内容作为容器 (comment 元素) 的兄弟元素,插入到页面中。

NgIfContext

NgIfContext 实例用于表示 NgIf 上下文。

// angular\packages\common\src\directives\ng_if.ts
export class NgIfContext {
  public $implicit: any = null;
  public ngIf: any = null;
}

若想进一步了解 TemplateRefViewContainerRef 的相关内容,请参考以下文章

NgIf 源码分析

NgIf 指令定义

@Directive({
   selector: '[ngIf]' // 属性选择器 - <ng-template [ngIf]="condition">
})

NgIf 类私有属性及构造函数

export class NgIf {
  // 创建NgIfContext上下文
  private _context: NgIfContext = new NgIfContext();
  // 表示then模板对象
  private _thenTemplateRef: TemplateRef<NgIfContext>|null = null;
  // 表示else模板对象
  private _elseTemplateRef: TemplateRef<NgIfContext>|null = null;

  // 表示根据then模板创建的EmbeddedViewRef视图
  private _thenViewRef: EmbeddedViewRef<NgIfContext>|null = null;
  // 表示根据else模板创建的EmbeddedViewRef视图
  private _elseViewRef: EmbeddedViewRef<NgIfContext>|null = null;

  constructor(
    private _viewContainer: ViewContainerRef,templateRef: TemplateRef<NgIfContext>) {
      this._thenTemplateRef = templateRef; // then模板的默认值为ngIf指令关联的内联模板
  }
}

NgIf 类输入属性

@Input()
set ngIf(condition: any) {
    this._context.$implicit = this._context.ngIf = condition;
    this._updateView(); // 更新视图
}

@Input()
set ngIfThen(templateRef: TemplateRef<NgIfContext>) {
    this._thenTemplateRef = templateRef;
    this._thenViewRef = null;  // 清除之前创建的视图
    this._updateView();
}

@Input()
set ngIfElse(templateRef: TemplateRef<NgIfContext>) {
    this._elseTemplateRef = templateRef;
    this._elseViewRef = null;  // 清除之前创建的视图
    this._updateView();
}

_updateView() 私有方法

// 更新视图
private _updateView() {
  // this._context.$implicit = this._context.ngIf = condition
  // 若condition表达式的值为truthy
  if (this._context.$implicit) {
  // 若_thenViewRef为null且_thenTemplateRef存在,则创建_thenViewRef内嵌视图
      if (!this._thenViewRef) {
        this._viewContainer.clear();
        this._elseViewRef = null;
        if (this._thenTemplateRef) {
          this._thenViewRef =
              this._viewContainer.createEmbeddedView(this._thenTemplateRef,this._context);
        }
      }
   } else { // condition表达式的值为falsy
     // 若_elseViewRef为null且_elseTemplateRef存在,则创建_elseViewRef内嵌视图
      if (!this._elseViewRef) {
        this._viewContainer.clear();
        this._thenViewRef = null;
        if (this._elseTemplateRef) {
          this._elseViewRef =
              this._viewContainer.createEmbeddedView(this._elseTemplateRef,this._context);
        }
      }
   }
}

ngIf 指令的源码相对比较简单,最核心的是 _updateView() 方法。而该方法中最重要的功能就是如何基于模板对象创建内嵌视图。接下来我们来分析一下 ViewContainerRef 对象的 createEmbeddedView() 方法

ViewContainerRef - createEmbeddedView()

方法签名

// angular\packages\core\src\linker\view_container_ref.ts
export abstract class ViewContainerRef {
   /**
   * 基于TemplateRef对象创建Embedded View(内嵌视图),然后根据`index`指定的值,插入到容器中。  
   * 如果没有指定`index`的值,新创建的视图将作为容器中的最后一个视图插入。
   */ 
  abstract createEmbeddedView<C>(
     templateRef: TemplateRef<C>,context?: C,index?: number):
      EmbeddedViewRef<C>;
}

方法实现

// angular\packages\core\src\view\refs.ts
class ViewContainerRef_ implements ViewContainerData {
   // ...
   createEmbeddedView<C>(
      templateRef: TemplateRef<C>,index?: number):
      EmbeddedViewRef<C> {
        // 调用TemplateRef对象createEmbeddedView()方法创建EmbeddedViewRef对象
        const viewRef = templateRef.createEmbeddedView(context || <any>{});
          // 根据指定的index值,插入到视图容器中
        this.insert(viewRef,index);
        return viewRef;
  }
}

// ViewContainerData接口继承于ViewContainerRef抽象类
export interface ViewContainerData extends ViewContainerRef {
  _embeddedViews: ViewData[];
}

export interface ViewData {
  def: ViewDefinition;
  root: RootData;
  renderer: Renderer2;
  parentNodeDef: NodeDef|null;
  parent: ViewData|null;
  viewContainerParent: ViewData|null;
  component: any;
  context: any;
  nodes: {[key: number]: NodeData};
  state: ViewState;
  oldValues: any[];
  disposables: DisposableFn[]|null;
}

通过观察 ViewContainerRef_ 类中的 createEmbeddedView() 方法,我们发现该方法内部是调用 TemplateRef 对象的 createEmbeddedView() 方法来创建内嵌视图。因此接下来我们再来分析一下 TemplateRef 对象的 createEmbeddedView() 方法

TemplateRef - createEmbeddedView()

方法签名

// angular\packages\core\src\linker\template_ref.ts
export abstract class TemplateRef<C> {
  abstract createEmbeddedView(context: C): EmbeddedViewRef<C>;
}

方法实现

// angular\packages\core\src\view\refs.ts
class TemplateRef_ extends TemplateRef<any> implements TemplateData {
  // ...
  createEmbeddedView(context: any): EmbeddedViewRef<any> {
    return new ViewRef_(Services.createEmbeddedView(
        this._parentView,this._def,this._def.element !.template !,context));
  }
}

export interface TemplateData extends TemplateRef<any> {
  _projectedViews: ViewData[];
}

看完上面的源码,毫无疑问接下来我们要继续分析 Services 对象中的 createEmbeddedView() 方法

Services - createEmbeddedView()

Services 对象定义

// angular\packages\core\src\view\types.ts
export const Services: Services = {
  setCurrentNode: undefined !,createRootView: undefined !,createEmbeddedView: undefined !,createComponentView: undefined !,createNgModuleRef: undefined !,overrideProvider: undefined !,clearProviderOverrides: undefined !,checkAndUpdateView: undefined !,checkNoChangesView: undefined !,destroyView: undefined !,resolveDep: undefined !,createDebugContext: undefined !,handleEvent: undefined !,updateDirectives: undefined !,updateRenderer: undefined !,dirtyParentQueries: undefined !,};

Services 对象初始化

// angular\packages\core\src\view\services.ts
export function initServicesIfNeeded() {
  if (initialized) {
    return;
  }
  initialized = true;
  const services = isDevMode() ? createDebugServices() : createProdServices();
  Services.setCurrentNode = services.setCurrentNode;
  Services.createRootView = services.createRootView;
  Services.createEmbeddedView = services.createEmbeddedView;
  Services.createComponentView = services.createComponentView;
  Services.createNgModuleRef = services.createNgModuleRef;
  Services.overrideProvider = services.overrideProvider;
  Services.clearProviderOverrides = services.clearProviderOverrides;
  Services.checkAndUpdateView = services.checkAndUpdateView;
  Services.checkNoChangesView = services.checkNoChangesView;
  Services.destroyView = services.destroyView;
  Services.resolveDep = resolveDep;
  Services.createDebugContext = services.createDebugContext;
  Services.handleEvent = services.handleEvent;
  Services.updateDirectives = services.updateDirectives;
  Services.updateRenderer = services.updateRenderer;
  Services.dirtyParentQueries = dirtyParentQueries;
}

initServicesIfNeeded() 方法中,会根据当前所处的模式,创建不同的 Services 对象。接下来 我们直接来看一下 createProdServices() 方法

function createProdServices() {
  return {
    setCurrentNode: () => {},createRootView: createProdRootView,createEmbeddedView: createEmbeddedView // 省略了其它方法
}

createEmbeddedView() 方法

// angular\packages\core\src\view\view.ts
export function createEmbeddedView(
    parent: ViewData,anchorDef: NodeDef,viewDef: ViewDefinition,context?: any): ViewData {
  // embedded views are seen as siblings to the anchor,so we need
  // to get the parent of the anchor and use it as parentIndex.
  // 创建ViewData对象
  const view = createView(parent.root,parent.renderer,parent,anchorDef,viewDef);
  // 初始化ViewData对象-设置component及context属性的值
  initView(view,parent.component,context);
  // 创建视图中的节点,即设置view.nodes数组的属性值
  // const nodes = view.nodes; for(...) { ...; nodes[i] = nodeData; }
  createViewNodes(view);
  return view;
}

此时发现如果完整分析所有的方法,会涉及太多的内容。源码分析就到此结束,有兴趣的读者请自行阅读源码哈(请各位读者见谅)。接下来我们来总结一下 createEmbeddedView() 方法调用流程:

ViewContainerRef_ -> createEmbeddedView()
   => TemplateRef_  -> createEmbeddedView()
    => Services -> createEmbeddedView()
     => Call createEmbeddedView()
原文链接:https://www.f2er.com/angularjs/147243.html

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