A variable declared inside of an *ngIf cannot be used outside of the
*ngIf

https://github.com/angular/angular/issues/6179#issuecomment-233374700

Only the opposite way (i.e. declare a variable inside of *ngIf and use
it outside of *ngIf) is not working,and won’t work by design.

https://github.com/angular/angular/issues/6179#issuecomment-233579605

为什么会这样？

1)没有* ngIf

我们来看看这个模板

<h2 myHighlight #tRefVar="myHighlight">tRefVar is {{tRefVar.foo}}</h2>
<div>tRefVar is {{tRefVar?.foo}}</div>

angular将为此创建以下viewDefinition：

function View_App_0(_l) {
  return jit_viewDef1(0,[(_l()(),jit_textDef2(null,['\n      '])),(_l()(),jit_elementDef3(0,null,2,'h2',[['myHighlight','']],null)),jit_directiveDef4(16384,[['tRefVar',4]],jit_HighlightDirective5,[jit_ElementRef6],null),['tRefVar is ',''])),1,'div',[],['\n  ']))],function(_ck,_v) {
        var currVal_0 = jit_nodeValue7(_v,2).foo;
        _ck(_v,3,currVal_0);
        var currVal_1 = ((jit_nodeValue7(_v,2) == null)? null: jit_nodeValue7(_v,2).foo);
        _ck(_v,6,currVal_1);
      });
}

这里没有嵌入式视图.一体化View_App_0.我们可以在这里看到我们的表达式{{tRefVar？.foo}}

var currVal_1 = ((jit_nodeValue7(_v,2).foo);

它从索引为2的节点获取值

jit_directiveDef4(16384,'']))

在同一视图中声明的

2)使用* ngIf

然后让我们按如下方式更改模板

<h2 *ngIf="true" myHighlight #tRefVar="myHighlight">tRefVar is {{tRefVar.foo}}</h2>
<div>tRefVar is {{tRefVar?.foo}}</div>

输出将如下

function View_App_1(_l) {
  return jit_viewDef1(0,jit_elementDef2(0,jit_directiveDef3(16384,jit_HighlightDirective4,[jit_ElementRef5],jit_textDef6(null,'']))],1).foo;
        _ck(_v,currVal_0);
      });
}
function View_App_0(_l) {
  return jit_viewDef1(0,['\n'])),jit_anchorDef8(16777216,View_App_1)),jit_NgIf9,[jit_ViewContainerRef10,jit_TemplateRef11],{ngIf:[0,'ngIf']},_v) {
        var currVal_0 = true;
        _ck(_v,currVal_0);
      },_v) {
        var _co = _v.component;
        var currVal_1 = ((_co.tRefVar == null)? null: _co.tRefVar.foo);
        _ck(_v,5,currVal_1);
      });
}

Angular创建了嵌入式视图View_App_1,分别为View_App_0.我们的表达式{{tRefVar？.foo}}已经变成了

var currVal_1 = ((_co.tRefVar == null)? null: _co.tRefVar.foo);

它只是因为组件属性,因为没有节点会在View_App_0中引用此模板变量.它已进入嵌入式视图View_App_1

var currVal_0 = jit_nodeValue7(_v,1).foo;

所以我们不能引用在嵌入视图之外的嵌入视图中声明的模板变量.

怎么解决？

1)使用[hidden]或css类而不是* ngIf的可见性标志

2)在嵌入视图中将表达式移动到声明tRefVar的位置

<ng-container *ngIf="true">
  <h2 myHighlight #tRefVar="myHighlight">tRefVar is {{tRefVar.foo}}</h2>
  <div>tRefVar is {{tRefVar?.foo}}</div>
</ng-container>

3)使用@ViewChild,因为它将代表组件属性.或者使用@ViewChildren