在Angular 8中使用mat-tree和NgRx的解决方法可以通过以下步骤实现：

1. 安装所需的依赖项：

npm install @ngrx/store @ngrx/effects --save
npm install @angular/material --save

2. 在app.module.ts文件中导入和配置所需的模块和服务：

import { MatTreeModule } from '@angular/material/tree';
import { StoreModule } from '@ngrx/store';
import { EffectsModule } from '@ngrx/effects';
import { TreeEffects } from './tree.effects';
import { treeReducer } from './tree.reducer';

@NgModule({
  imports: [
    MatTreeModule,
    StoreModule.forRoot({ tree: treeReducer }),
    EffectsModule.forRoot([TreeEffects])
  ],
  ...
})
export class AppModule { }

3. 创建一个tree.actions.ts文件来定义树的操作：

import { createAction, props } from '@ngrx/store';

export const loadTree = createAction('[Tree] Load Tree');
export const treeLoaded = createAction('[Tree] Tree Loaded', props<{ treeData: any[] }>());
export const selectNode = createAction('[Tree] Select Node', props<{ node: any }>());

4. 创建一个tree.effects.ts文件来处理树的副作用：

import { Injectable } from '@angular/core';
import { Actions, createEffect, ofType } from '@ngrx/effects';
import { Store } from '@ngrx/store';
import { tap, switchMap, map } from 'rxjs/operators';
import { loadTree, treeLoaded } from './tree.actions';

@Injectable()
export class TreeEffects {
  loadTree$ = createEffect(() =>
    this.actions$.pipe(
      ofType(loadTree),
      switchMap(() => this.treeService.getTreeData()),
      map(treeData => treeLoaded({ treeData })),
    )
  );

  constructor(
    private actions$: Actions,
    private store: Store,
    private treeService: TreeService
  ) { }
}

5. 创建一个tree.reducer.ts文件来管理树的状态：

import { createReducer, on } from '@ngrx/store';
import { loadTree, treeLoaded, selectNode } from './tree.actions';

export interface TreeState {
  treeData: any[];
  selectedNode: any;
}

export const initialState: TreeState = {
  treeData: null,
  selectedNode: null
};

export const treeReducer = createReducer(
  initialState,
  on(treeLoaded, (state, { treeData }) => ({ ...state, treeData })),
  on(selectNode, (state, { node }) => ({ ...state, selectedNode: node }))
);

6. 在组件中使用mat-tree和NgRx：

import { Component, OnInit } from '@angular/core';
import { Store, select } from '@ngrx/store';
import { loadTree, selectNode } from './tree.actions';

@Component({
  selector: 'app-tree',
  template: `
    
      
        
        {{ node.name }}
      
      
        
          {{ treeControl.isExpanded(node) ? 'expand_more' : 'chevron_right' }}
        
        {{ node.name }}
        
      
    
  `
})
export class TreeComponent implements OnInit {
  treeData$ = this.store.pipe(select(state => state.tree.treeData));

  constructor(private store: Store) { }

  ngOnInit() {
    this.store.dispatch(loadTree());
  }

  hasChild(_: number, node: any) {
    return !!node.children && node.children.length > 0;
  }
}

以上是使用mat-tree和NgRx的解决方法的示例代码。