FilterKit is a component written in Swift that let you validate or filter an object, based on a set of properties listed in a dictionary. It's inspired by the filter element in the MapBox Style Specifications. Compatibility is ensured with iOS, macOS, tvOS, watchOS and Linux. There's also a small bridge that enables full (for now) Objective-C interoperability.

Rationale

This chapter has been copied and slightly adapted from the original MapBox filter documentation.

A filter selects specific features from a dictionary of properties. A filter is specified as an array of one of the following forms:

Existential Filters

• ["has", key] feature[key] exists
• ["!has", key] feature[key] does not exist

Comparison Filters

• ["==", key, value] equality: feature[key] = value
• ["!=", key, value] inequality: feature[key] ≠ value
• [">", key, value] greater than: feature[key] > value
• [">=", key, value] greater than or equal: feature[key] ≥ value
• ["<", key, value] less than: feature[key] < value
• ["<=", key, value] less than or equal: feature[key] ≤ value

Set Membership Filters

• ["in", key, v0, ..., vn] set inclusion: feature[key] ∈ {v0, ..., vn}
• ["!in", key, v0, ..., vn] set exclusion: feature[key] ∉ {v0, ..., vn}

Combining Filters

• ["all", f0, ..., fn] logical AND: f0 ∧ ... ∧ fn
• ["any", f0, ..., fn] logical OR: f0 ∨ ... ∨ fn
• ["none", f0, ..., fn] logical NOR: ¬f0 ∧ ... ∧ ¬fn

A key must be a string that identifies a feature property.

A value (and v0, ..., vn for set operators) must be a String, Int, Double or Bool to compare the property value against.

Set membership filters are a compact and efficient way to test whether a field matches any of multiple values.

The comparison and set membership filters implement strictly-typed comparisons; for example, all of the following evaluate to false: 0 < "1", 2 == "2", "true" in [true, false].

The "all", "any", and "none" filter operators are used to create compound filters. The values f0, ..., fn must be filter expressions themselves.

The following filter requires that the road property is equal to either "street_major", "street_minor" or "street_limited".

["in", "road", "street_major", "street_minor", "street_limited"]

The combining filter "all" takes the three other filters that follow it and requires all of them to be true for a feature to be included: a feature must have a road equal to "street_limited", its admin_level must be greater than or equal to 3, and paved must not be false nor 2 . You could change the combining filter to "any" to allow features matching any of those criteria to be included.

[
    "all",
    ["==", "road", "street_limited"],
    [">=", "admin_level", 3],
    ["!in", "paved", false, 2]
]

Usage

First of all import FilterKit module into your Swift class with import FilterKit or Objective-C one with @import FilterKit;.

Then instantiate the Filter object with the desired properties expressed as a Dictionary. On that instance call the compile() function providing the Array containing the filter.

do {
    let result = try Filter(properties: ["foo":"bar"]).compile(["all", ["==", "foo", "bar"]])
    print("Result: \(result)")
} catch let error {
    print(error.localizedDescription)
}

Or in Objective-C:

FILFilter *filter = [[FILFilter alloc] initWithProperties:@{@"foo": @"bar"}];
NSError *error;
FILFilterResult *result = [filter compileWithFilters:@[@"all", @[@"==", @"foo", @"bar"]] error:&error];
if(error != nil) {
    NSLog(@"%@", error);
}
NSLog(@"Result: %d", result.valid);

If you need guidance to parse a JSON file, take a look at the parseFixture(_ named: String) function contained in the FilterKitTests.swift file, included in the Tests/FilterKitTests subfolder.

Edge Cases

Due to differences in the compiler some conditions are returning different results based on Swift version or platform. Those are probably edge cases and shouldn't cause any misbehaviour, but you should be aware that they exist.

Those are the one identified so far:

• 1.09 == true is false only if Swift ≥ 4.
• 1 == 1.0 is false on Linux and true on Apple's Platforms.
• 1 == true is false on Linux and true on Apple's Platforms.

Installation on Apple's Platforms

Manually

Add the FilterKit Xcode project to your own. Then add the FilterKit-PLATFORM framework as desired to the embedded binaries of your app's target.

Installation with Swift Package Manager

In your existing Package.swift file add the following line as member of the dependencies array.

.Package(url: "https://github.com/dimensiosrl/FilterKit.git", majorVersion: 1),

The complete version should appear like this:

import PackageDescription

let package = Package(
    name: "Your App Name",
    targets: [],
    dependencies: [
        .Package(url: "https://github.com/dimensiosrl/FilterKit.git", majorVersion: 1),
        .Package(url: "https://github.com/example/another-dependancy.git", majorVersion: 2),
        ]
)

Tests

The test suite is shared between platforms and targets and resides in the Tests folder. In the Fixtures sub-subfolder there are many json files representing different scenarios that will be parsed and tested against.

In order to run the tests on macOS you can use Xcode or use this command from the Terminal:

xcodebuild -project FilterKit.xcodeproj -scheme "FilterKit macOS" -enableCodeCoverage YES clean build test

If you want to test on Linux and have Docker installed, just run:

docker-compose up

It will pull the latest ibmcom/swift-ubuntu image, mount the project folder as /FilterKit and execute swift package clean; swift test.

In both cases the output will appear directly on the Terminal.

Acknowledgements

• DIMENSION S.r.l..
• Matteo Gavagnin – @macteo.

License

FilterKit is released under the MIT license. See LICENSE for details.