custom filter in mat-table

I found the solution here.

It's necessary to rewrite filterPredicate, and just use it as usual, filterPredicate needs to return true when filter passes and false when it doesn't

export interface Element {
 name: string;
 position: number;
 weight: number;
 symbol: string;
}


dataSource = new MatTableDataSource(ELEMENT_DATA);
/* configure filter */
this.dataSource.filterPredicate = 
  (data: Element, filter: string) => data.name.indexOf(filter) != -1;


applyFilter(filterValue: string) {
   filterValue = filterValue.trim(); // Remove whitespace
   filterValue = filterValue.toLowerCase(); // MatTableDataSource defaults to lowercase matches
   this.dataSource.filter = filterValue;
 }

Be aware that all fields of the class displayed in table rows, are subject to filtering, even if you do not display that field as a column.

export class City {
  id: number;//is not displayed in mat table
  code: string;
  name: string;
  country:Country;
}

Any filter for the dataSource of city table, also applies to id column, which generally we do not display to the end user.

//this filter will also apply to id column
this.cityDataSource.filter = filterValue;

Don't forget to apply .trim().toLowerCase() on your data or you may encounter unexpected results. See my example below:

this.dataSource.filterPredicate = (data:
  {name: string}, filterValue: string) =>
  data.name.trim().toLowerCase().indexOf(filterValue) !== -1;

applyFilter(filterValue: string) {
    this.dataSource.filter = filterValue.trim().toLowerCase();
}

when using parent-child componenet (or service with say observable) you must always in the component containing the MatTableDataSource set an "applyFilter" function (name not matters)

in the table component i added @Input's for the filter string value as a FormControl, and sent a filter function (filterfn)

  @Input() data: any[];
  @Input() filterControl: FormControl;
  @Input() filterfn: (data: any, filter: string) => boolean;

in the init

 ngOnInit(): void {
    this.dataSource.filterPredicate = this.filterfn
    this.dataSource.data = this.data;
    this.initExpandedDefaultValues();
    //my "applyFilter"
    this.filterControl.valueChanges.subscribe(searchValue => this.dataSource.filter = searchValue)
  }

in parent html

<div *ngIf="users">
    <expended-table [data]="users"
        [filterfn]="filterFunction"
        [filterControl]="search">
    </expended-table>
</div>

in parent component

public users:User[]
  search:FormControl = new FormControl()
  public usersFiltered:User[]

  filterFunction(u: User, searchValue: string) : boolean{
    if (searchValue) {
      let v = searchValue.trim().toLowerCase()
      if (
        u.firstName.toLowerCase().includes(v) || 
        u.lastName.toLowerCase().includes(v) || 
        //bla bla bla more tests
        u.permission.toLowerCase().includes(v)  
      )
      { return true } 
      else { return false}
    } //end if searchValue
    else 
    {
      return true
    }
  }