# Focusing on the First Invalid Field in a DevExtreme Tab Panel Component: A Hack Explained

# Introduction

In this blog post, we will explore a code snippet that addresses the issue of focusing on the first invalid field within a tab control component of the DevExtreme library in an Angular application. The code provides a workaround to handle validation errors and ensures that the user's attention is immediately drawn to the first field that requires correction.

# Code Explanation

Let's dissect the code step by step to understand how it achieves the desired behavior.

```typescript
focusOnInvalidTabAndField() {
  const brokenRules: any = this.validationGroup.instance.validate().brokenRules;
  if (brokenRules && brokenRules.length > 0) {
    // Find the closest dx-item.
    const closestTab = brokenRules[0].validator._$element[0].closest('.dx-multiview-item-content');
```

1. The function `focusOnInvalidTabAndField()` is a custom method responsible for focusing on the first invalid field within the tab panel.
    
2. The `validate()` method is invoked on the `validationGroup.instance`, which presumably refers to the instance of the validation group associated with the tab panel component. It returns an object containing the broken validation rules.
    
3. The `brokenRules` variable is assigned the `brokenRules` property of the validation result, allowing us to access the list of broken rules.
    

```typescript
    if (closestTab) {
      // Get the tab index of the closest tab panel.
      const tabIndex = closestTab.dataset.tabItemIndex;
```

1. The code checks if the `closestTab` exists. If an invalid field is found, the `closestTab` refers to the DOM element associated with the tab panel that contains the invalid field.
    
2. The `tabIndex` variable is assigned the value of the `tabItemIndex` attribute from the `dataset` property of the `closestTab` element. This attribute presumably stores the index of the tab panel.
    

```typescript
      // Select the found panel.
      this.selectedTabIndex = tabIndex;
      if (this.model.main.Headerbutton) {
        this.selectedHeaderButtonTabIndex = tabIndex;
      }
```

1. The code updates the `selectedTabIndex` property to the value stored in `tabIndex`, which effectively changes the selected tab panel to the one containing the first invalid field.
    
2. Additionally, if the condition `this.model.main.Headerbutton` evaluates to true, it updates the `selectedHeaderButtonTabIndex` property with the same value. This step might be specific to the application's logic and can be ignored if not applicable.
    

```typescript
      // Focus the first error field.
      brokenRules[0].validator.focus();
    }
  }
}
```

1. Finally, the `focus()` method is invoked on the `validator` property of the first item in the `brokenRules` array. This method triggers the focus on the first invalid field, bringing it into the user's immediate attention.
    

# Conclusion

The provided code snippet demonstrates a practical solution to focus on the first invalid field within a DevExtreme tab panel component. By utilizing the broken validation rules and leveraging the DOM manipulation capabilities, the code effectively selects the tab panel containing the invalid field and brings it into view, allowing the user to easily identify and address the validation error.

Please note that this code snippet assumes the presence of specific properties and elements (`validationGroup`, `instance`, `model.main.Headerbutton`, etc.) in the surrounding codebase, and it may require adjustments to fit different scenarios.

# Full Code Snippet

```javascript
focusOnInvalidTabAndField() {
    const brokenRules: any = this.validationGroup.instance.validate().brokenRules;
    if (brokenRules && brokenRules.length > 0) {
      // Find the closest dx-item.
      const closesetTab = brokenRules[0].validator._$element[0].closest('.dx-multiview-item-content');
      if (closesetTab) {
        // Get the tab index of the closest tab panel.
        const tabIndex = closesetTab.dataset.tabItemIndex;
        // Select the found panel.
        this.selectedTabIndex = tabIndex;
        if (this.model.main.Headerbutton) {
          this.selectedHeaderButtonTabIndex = tabIndex;
        }
        // Focus the first error field.
        brokenRules[0].validator.focus();
      }
    }
  }
```