What is the best way to bind an enum to a dropdown menu in Angular 4?

Currently, I am facing an issue with data binding in a dropdown menu using an Enum data structure in Typescript. The object's category field is not being selected by default. Is there a better solution to overcome this problem?

export enum CategoryEnum {
      EXAMPLECAT1 = 1,
      EXAMPLECAT2 = 2,
      EXAMPLECAT3 = 3,
      EXAMPLECAT4 = 4
    }

@Component({
  selector: 'category',
  template: `
    <label for="appCategory">Example Category: </label>
    <select class="form-control" id="exampleCategory" required 
            (change)="parseValue($event.target.value)"
            (ngModelChange)="changeSelectedType($event)">
      <option *ngFor="let category of categoryTypes"
              [value]="category">{{category.type}}</option>
    </select>
  `
})

export class CategoryComponent {

  private selectedCategoryType: CategoryEnum
  private categoryTypes;

  constructor(){
    this.categoryTypes = CategoryMapping
  }

  parseValue(value : string) {
    this.selectedCategoryType = AppCategoryEnum[value];
  }

  //Logging: Change Selected Product type callback
  private changeSelectedType(event: any) {
    console.log(event); //object, depends on ngValue
    console.log(this.selectedCategoryType); //object, depends on ngValue
  }
}

To map the enum data type to labels, the following code snippet can be used:

export const CategoryMapping = [
  { value: ExampleCategoryEnum.EXAMPLECAT1, type: 'Example 1' },
  { value: ExampleCategoryEnum.EXAMPLECAT2, type: 'Example 2'},
  { value: ExampleCategoryEnum.EXAMPLECAT3, type: 'Example 3'},
  { value: ExampleCategoryEnum.EXAMPLECAT4, type: 'Example 4'}
];

Answer №1

In order to simplify the binding process, I utilized ngModel. Another property was created to bind with the select element and retrieve the value as CategoryEnum.

component

// For demonstration purposes only, changed to public
public get selectedCategoryType(): CategoryEnum {
  return this.selectedValue ? this.selectedValue.value : null; 
}
private categoryTypes;

public selectedValue: any;

constructor() {
  this.categoryTypes = CategoryMapping;
  this.selectedValue = this.categoryTypes[2]; // set default value
}

template

<select class="form-control" id="exampleCategory" required [(ngModel)]="selectedValue">
  <option></option>
  <option *ngFor="let category of categoryTypes"[ngValue]="category">{{category.type}}</option>
</select>
<br>

Selected Category Type: {{selectedCategoryType | json}}

If you need to pass non-primitive types (such as objects) as options, use [ngValue] as mentioned by @trichetriche.

Check out the demo here

Answer №2

To start, you should create a function that can convert an enum into an object array.

convertEnumToKeyValue(t:any):{value:any,label:any}[]
{
    let keys=Object.keys(t);
    let valueIndex=(keys.length/2);
    let keyArray:{value:any,label:any}[]= [];
    for(let i=0;i<valueIndex;i++)
    {
        keyArray.push({value:keys[i],label:keys[i+valueIndex]});
    }
    return keyArray;
}

Next, you will need to call this function with your enum values like so:

enum Colors 
{
   Red=1,
   Green=2,
   Blue=3
}

Finally, make the following call:

colorTypes= convertEnumToKeyValue(Colors)

You can now use the colorTypes array with ngFor directive in your select element.

Answer №3

A simple way to customize your selection is by setting the value attribute to "1":

<select ... [value]="1">

In this example, we are using the default category enum value of 1.

Instead of using the entire object for the option value, you can simply use category.value:

<option ... [value]="category.value" >

Make sure to update your .ts file to accommodate this change.

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