如何对ViewModel进行单元测试

使用MVVM架构来开发APP的好处之一就是便于测试业务逻辑，由于ViewModel不涉及界面，就不用考虑Mock对象了，测试起来也方便的多。

对ViewModel进行单元测试的一般流程

创建ViewModel -> 把输入数据交给ViewModel -> ViewModel把输入数据进行业务逻辑处理，加工成输出数据 -> 测试ViewModel的输出数据是否正确

进行单元测试前需做一些配置，可参考：
http://liuduoios.github.io/2015/10/24/Swift-2-%E4%B8%AD%E7%9A%84%E5%8D%95%E5%85%83%E6%B5%8B%E8%AF%95%E5%85%B3%E9%94%AE%E5%AD%97-testable/

下面拿一个简单的例子来说明

APP启动后界面如下：

这个例子中含有两个ViewModel类，一个是列表的ListViewModel，其中含有根据接口返回数据来计算Cell行数的业务逻辑，内容如下：
这里数据绑定和变化监听用了开源库SwiftBondhttps://github.com/SwiftBond/Bond

import UIKit
import Bond

class ListViewModel: NSObject {

    var numberOfRows: Int {
        return self.items.value?.count ?? 0
    }
    
    var items: Observable<[Item]?> = Observable(nil)
    
    func requestData() {
        let configuration = NSURLSessionConfiguration.defaultSessionConfiguration()
        let session = NSURLSession(configuration: configuration)
        session.dataTaskWithURL(NSURL(string: "http://www.google.com")!) { (data, response, error) -> Void in
            let jsonStr = String(data: data!, encoding: NSUTF8StringEncoding)
            let jsonData = jsonStr?.dataUsingEncoding(NSUTF8StringEncoding)
            do {
                var items = [Item]()
                
                let dic = try NSJSONSerialization.JSONObjectWithData(jsonData!, options: NSJSONReadingOptions(rawValue: 0)) as! [String: AnyObject]
                let itemDics = dic["data"] as! [[String: String]]
                for itemDic in itemDics {
                    items.append(Item(dic: itemDic))
                }
                
                self.items.next(items)
            } catch {
                
            }
        }.resume()
    }
    
    func cellViewModelAtIndexPath(indexPath: NSIndexPath) -> CellViewModel? {
        guard let items = items.value else { return nil }
        if indexPath.row < items.count {
            return CellViewModel(item: items[indexPath.row])
        } else {
            return nil
        }
    }
    
}

另一个是Cell的CellViewModel，其中含有把title变为大写的业务逻辑，内容如下：

import UIKit

class CellViewModel: NSObject {
    
    init(item: Item) {
        super.init()
        self.item = item
    }

    var item: Item?
    
    var title: String? {
        return item?.title?.uppercaseString
    }
    var subtitle: String? {
        return item?.subtitle
    }
    
}

测试`ListViewModel`

这里设计到网络请求数据，因此要用到一个stub开源库，这里使用了Mockingjay。
对于stub开源库的选择：
如果你的项目是纯Swift的，推荐使用Mockingjayhttps://github.com/kylef/Mockingjay
如果你的项目是Objective-C或者OC和Swift混编的，推荐使用OHHTTPStubshttps://github.com/AliSoftware/OHHTTPStubs

import XCTest
import Mockingjay
import Bond
@testable import ViewModelUnitTest

class ListViewModelTests: XCTestCase {
    
    override func setUp() {
        super.setUp()
        // Put setup code here. This method is called before the invocation of each test method in the class.
    }
    
    override func tearDown() {
        // Put teardown code here. This method is called after the invocation of each test method in the class.
        super.tearDown()
    }
    
    func testNumberOfRows() {
        let path = NSBundle(forClass: self.dynamicType).pathForResource("Data", ofType: "json")!
        let data = NSData(contentsOfFile: path)!
        stub(everything, builder: jsonData(data))
        
        let expectation = expectationWithDescription("get items data")
        
        let viewModel = ListViewModel()
        viewModel.items.observeNew({ (items) -> () in
            expectation.fulfill()
        })
        viewModel.requestData()
        
        waitForExpectationsWithTimeout(10) { (error) -> Void in
            if error != nil { XCTFail("time out") }
            XCTAssertEqual(viewModel.numberOfRows, 2, "行数等于2")
        }
    }
    
}

测试`CellViewModel`

不涉及网络请求，直接对业务逻辑进行测试就好了。

import XCTest
@testable import ViewModelUnitTest

class CellViewModelTests: XCTestCase {
    
    override func setUp() {
        super.setUp()
        // Put setup code here. This method is called before the invocation of each test method in the class.
    }
    
    override func tearDown() {
        // Put teardown code here. This method is called after the invocation of each test method in the class.
        super.tearDown()
    }
    
    func testTitle() {
        let item = Item(dic: ["name": "abc", "subname": "def"])
        let viewModel = CellViewModel(item: item)
        XCTAssertEqual(viewModel.title, "ABC", "title是否变为大写")
    }
    
}

对ViewModel进行单元测试的一般流程

下面拿一个简单的例子来说明

测试ListViewModel

测试CellViewModel

测试`ListViewModel`

测试`CellViewModel`