Extend DataFilter implementation

[olicooper]

I would like to propose some updates to the DataFilter implementation. The changes should be non-breaking as the base functionality remains the same, with an additional IsActive attribute along with some additional useful methods.
I have made a number of changes but my main motivation is to know which filters are currently 'active' (being used within a disposable scope). Having access to information about the filters is also very useful.

Because DataFilter is ISingletonDependency the cached filters are kept in memory after their first use. The IsEnabled property isn't a reliable source because the default IsEnabled value can be changed per filter with AbpDataFilterOptions. I tried making the DataFilter Scoped; but the scope can change within a request so the filters reset.

I have added various tests to ensure these work within all common scenarios and I've added documentation to guide developers for the correct use of each method.

Initially, I intend to use this in conjunction with the abp-queryfilter-test project I am working on (mentioned here) - where you can the proposed changes in use.

Changes include:

• Add new methods (GetOrAddFilter, IsActive) and add ReadOnlyFilters property to DataFilter
• Make DataFilter extensible (virtual methods etc.)
• Add ability to set the DefaultFilterState in AbpDataFilterOptions
• Add test suite
• Improve method XML descriptions

[hikalkan]

Thanks @olicooper for this PR.

Having access to information about the filters is also very useful.

Can you please give more info for the use cases? You can show some usage examples and explain the reason. I didn't understand how it is supposed to be used and what are the benefit of such an active concept.

[olicooper]

Yeah sure I will try 😃. The reason I created this PR was partly to facilitate the a proposed change in how data filters are applied in ABP (I created a ticket on the issue here). But essentially, without IsActive you can't see which cached filters are applied for a given query due to the Singleton nature of DataFilters and you can't make DataFilters scoped because nested scopes are created within each request.

Official filters are ISoftDelete and IMultiTenant, but the filtering system allows for anything to be specified, for example I have also created IPassivable (filter for active entities) and ITemplatable (filter for entities marked as templates). The main reason for needing IsActive is to perform conditional logic based on the filter value from within a nested method call or query interceptor. One example could be when filtering for template records, I also require that I search for non-tenant records. But I think it is best to see it in action so here is a real example using the filters to determine if I should modify the EF query before sending it to the database. I didn't know which filters were active within the DbContext because the query is being intercepted rather than being directly called by the AppService, so without IsActive I wouldn't be able to modify the query accordingly.

See AbpFilterAppendingExpressionVisitor.cs - you can also check out the whole demo if you'd like 😀

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