Angular Data GridValue Getters

Normally columns are configured with field attributes, so the column knows what field to take values from in the data. Instead of providing field it is possible to provide a valueGetter instead. A Value Getter is a function that gets called allowing values to be pulled from literally anywhere, including executing any expressions you wish along the way.

You should use colDef.field most of the time. Use value getters when retrieving the data requires more logic, including executing your own expressions (similar to what a spreadsheet would do).

// example value getter, adds two fields together
colDef.valueGetter = params => {
    return params.data.firstName + params.data.lastName;
}

Example Value Getters

The example below demonstrates valueGetter. The following can be noted from the demo:

• Columns A and B are simple columns using field

• Value Getters are used in all subsequent columns as follows:

  • Column '#' prints the row number, taken from the Row Node.
  • Column 'A+B' adds A and B.
  • Column 'A * 1000' multiplies A by 1000.
  • Column 'B * 137' multiplies B by 137.
  • Column 'Random' doesn't take any value from the data, rather it returns a random value.
  • Column 'Chain' takes the value 'A+B' and works on it further, thus chaining value getters.
  • Column 'Const' returns back the same value for each column.

Header Value Getters

Use headerValueGetter instead of colDef.headerName to allow dynamic header names.

The parameters for headerValueGetter differ from standard valueGetter as follows:

• Only one of column or columnGroup will be present, depending on whether it's a column or a column group.
• Parameter location allows you to have different column names depending on where the column is appearing, eg you might want to have a different name when the column is in the column drop zone or the toolbar.

See the Column Tool Panel Example for an example of headerValueGetter used in different locations, where you can change the header name depending on where the name appears.

Filter Value Getters

By default, the values supplied to the filter are retrieved from the data based on the field attribute. This can be overridden by providing a filterValueGetter in the Column Definition as shown below. This is similar to using a normal Value Getter, but is specific to the filter.

Value Cache

The value cache is used for the results of value getters. If you are not using value getters, then you do not need the value cache.

Each time the grid requires a value from a value getter, the value getter is executed. For most use cases, this will not be an issue, as value getters will execute quickly and not have any noticeable performance implications for your application. However sometimes you might implement time-intensive tasks in your value getters. If this is the case, then you can opt to turn on the value cache to store the results of the value getters.

When the value cache is turned on, each time a value getter is executed, its result is stored in the value cache. If the data in the grid has not changed since the last time the value getter was used, then the value is retrieved from the cache instead of executing the value getter again.

This value cache is for advanced users who have time-consuming value getters and want to speed up their applications by introducing a cache to reduce the number of times value getters get executed.

Example: Value Cache

Below shows a grid demonstrating the value cache. The column on the right has a value getter that has a console.log() statement. This will allow us to see exactly when the value getter is called. So it is best to open this example in a new tab and open up the development console. From the example, the following can be noted:

• When the grid is initially loaded, the value getter is called over 100 times. This is for the following reasons:
  • The aggregation requires each value for the group total.
  • The DOM requires each value that is displayed (because of scrolling, not all are displayed)
• As you scroll up and down the grid, the value getters are executed, as the DOM needs values for rendering.
• As you open and close groups, the value getters are executed, as the DOM needs values for rendering.
• Now turn the value cache on by selecting the radio button at the top. The grid gets reset and then works with the value cache on. Notice in the console that the value getter gets executed exactly 100 times, once for each row. Even through the value getter result is used in two places (aggregation and rendering the row), the value getter is only called once. Even scrolling and opening / closing the groups does not result in the value getter getting executed again.

Value Cache Properties

The following are the grid properties related to the value cache.

Expiring the Value Cache

The value cache starts empty. Then as value getters execute, their results are stored and the value cache fills up. Certain events trigger the value cache to be emptied. This is called expiring the value cache. Once expired, every value getter will get executed again next time the value is needed. The events that cause the value cache to expire are the following:

• New row data is set into the grid via setRowData(newData) API or changing the rowData framework bound property*.
• New columns are set into the grid via setColumnDefs() API or changing the columnDefs framework bound property*.
• Data is modified using the rowNode.setData(data) or rowNode.setDataValue(col,value) node API methods.
• A value is modified in the grid using the grid's UI editing feature, e.g. the user double clicks a cell and enters a new value.
• The expireValueCache() grid API method gets called by the application.

Example: Expiring through Editing

The first example above didn't have any editing, so there was no concern for expiring the value cache. This example introduces grid editing. The example differs from the last in the following ways:

• Value Cache is on.
• Editing is enabled.
• There are only 10 rows, so it's easier to count the number of cells.
• There is another column 'Total x10' which also uses a value getter and references the original 'Total' column, thus calling this value getter results in an additional call to the original value getter.

As before, we focus on the value getter of the 'Total' column and can see how many times it gets called via the console. The following can be noted:

• The values in the Total column are used many times as follows:
  • For inserting the value into the DOM (i.e. what's visible in the cell).
  • For calculating the group total for the total column (ie the figure 870,400 is a sum of the 5 values below it).
  • For calculating the 'Total x 10' column, as that value getter also references the values in the 'Total' column.
• Despite the values getting used multiple times, each value getter is executed exactly once. This can be observed by opening the development console and observing the log message the value getter prints.
• If you close and then re-open a group, the value getters are not re-executed, even though the values are needed to re-create the DOM elements that represent the cells.
• Hitting 'Refresh Cells' will refresh all the cells, but again the value getters will not get re-executed.
• Hitting 'Invalidate Cache' and then 'Refresh Cells' will result in the value getters getting re-executed, as the cell refresh operation requires the values and the cache was invalidated. You will notice invalidating and then refreshing doesn't do anything noticeable to the grid, the data is the same, the only hint that anything happened is the value getter's console messages.
• Changing any value in the grid, either editing via the UI directly or hitting the 'Change One Value' button, will result in the value cache getting cleared and all cells getting refreshed (where change detection then updates any changes cells and only changed cells).

Setting to Never Expire

Above details how the cache operates under normal operations. In essence, the value cache gets expired whenever the grid is aware of a change to the underlying data. This strategy is the default strategy for the cache. If you want to persist the cache when data updates, then set the grid property valueCacheNeverExpires=true.

If you have valueCacheNeverExpires=true, then the only event that will expire the value cache will be the api.expireValueCache() method. Use this if your data is changing, but you don't want to execute value getters again, or you want to control exactly when the value cache is expired.

Example: Never Expire

This example is again almost identical to the example above. The difference here is the value cache is turned on but to never invalidate. Note the following:

• When the grid initialises, there are 10 value getter calls. The values are getting cached.

• After you edit a cell, either through the UI or through the API by pressing 'Update One Value', the value getters are not called again, so the Total and Total x 10 columns are not correctly refreshed. The grid already executed the value getters for this column, it will not do it again, it will instead take values from the value cache.

• To get the total column to update after edits, press 'Expire Value Cache' (calls grid API expireValueCache()) and then press 'Aggregate Data & Refresh Cells' (calls grid API refreshClientSideRowModel('aggregate') followed by grid API refreshCells()).

  The call refreshClientSideRowModel('aggregate') is required as aggregations use Value Getters, thus the aggregations at group level (that is the two total column at the two group rows) need aggregation to be re-run for their values to be updated.

  The call refreshCells() is required to update the UI, which in turn also calls Value Getters.