来自：https://github.com/onevcat/Kingfisher
任务计数模式
func testDownloadMultipleImages() {
let expectation = self.expectation(description: "wait for all downloading finish")
let group = DispatchGroup()
for URLString in testKeys {
if let url = URL(string: URLString) {
group.enter()
_ = stubRequest("GET", URLString).andReturn(200)?.withBody(testImageData)
downloader.downloadImage(with: url, options: nil, progressBlock: { (receivedSize, totalSize) -> Void in
}, completionHandler: { (image, error, imageURL, data) -> Void in
XCTAssert(image != nil, "Download should be able to finished for URL: (String(describing: imageURL)).")
group.leave()
})
}
}
group.notify(queue: .main, execute: expectation.fulfill)
waitForExpectations(timeout: 5, handler: nil)
}