Introduction
In this blog post, we will discuss a common issue encountered when working with Angular Material’s mat-table component. Specifically, we will explore the problem of inconsistent row indexing when adding and removing rows dynamically. We’ll provide step-by-step instructions to reproduce the issue and offer a solution to resolve it. If you’ve been struggling with this problem, you’ve come to the right place!
The Issue
When using mat-table in an Angular application, adding and deleting rows in specific sequences can lead to unexpected behavior. Let’s outline the steps to reproduce this issue:
- Add a new row and set the value for Sunday to 1.
- Add another row and set the value for Sunday to 2.
- Delete the first row.
- At this point, there should be only one row left with Sunday=2.
- Now, add another new row.
Expected Behavior
The new row should be added, and the value of the first row’s Sunday should still be 2.
Actual Behavior
The new row is added, but the value of the first row is reset to 0. It appears as if two new rows have been created instead of one.
Investigation and Solution
To investigate this issue, we’ve created a stripped-down demo application that isolates the problem. You can find the demo on GitHub (link provided). The issue persists in Angular 8 with Angular Material 8 and also in Angular 7.
Upon closer examination, we discovered that the problem lies in how the table binds to the underlying data source. While the underlying array contains the expected values, the table does not bind correctly in certain scenarios.
The solution to this issue is to provide a trackBy function, as recommended in the Angular Material documentation. This function helps Angular track items in the table more accurately.
To implement the solution, follow these steps:
- In your component’s TypeScript file, add the following trackBy function:
code
trackByFn(index, item) {
return index;
}
- In your component’s HTML markup, update the mat-table element to include the trackBy function:
code
<mat-table [dataSource] ="tmpTimesheet" [trackBy] ="trackByFn">
By specifying the trackBy function, Angular will use the index as a unique identifier for each row. This ensures that the row indexing remains consistent even when adding or removing rows dynamically.
Conclusion
In this blog post, we explored the issue of inconsistent row indexing in Angular Material’s mat-table component. We provided step-by-step instructions to reproduce the problem and offered a solution by implementing a trackBy function. By following these guidelines, you can ensure that your mat-table behaves as expected, maintaining consistent row indexing throughout dynamic operations.
We hope this article has been helpful in addressing this common issue. If you have any questions or further insights, feel free to leave a comment below.
