我终于弄明白了。我无法相信这是多么困难。难怪苹果没有发布任何关于此的样本。这是我的代码:
// This is based on "Customized Loading in WKWebView" WWDC video (near the end of the
// video) at https://developer.apple.com/videos/play/wwdc2017/220 and A LOT of trial
// and error to figure out how to push work to background thread.
//
// To better understand how WKURLSchemeTask (and internally WebURLSchemeTask) works
// you can refer to the source code of WebURLSchemeTask at
// https://github.com/WebKit/WebKit/blob/main/Source/WebKit/UIProcess/WebURLSchemeTask.cpp
//
// Looking at that source code you can see that a call to any of the internals of
// WebURLSchemeTask (which is made through WKURLSchemeTask) is expected to be on the
// main thread, as you can see by the ASSERT(RunLoop::isMain()) statements at the
// beginning of pretty much every function and property getters. I'm not sure why Apple
// has decided to do these on the main thread since that would result in a blocked UI
// thread if we need to return large responses/files. At the very least they should have
// allowed for calls to come back on any thread and internally pass them to the main
// thread so that developers wouldn't have to write thread-synchronization code over and
// over every time they want to use WKURLSchemeHandler.
//
// The solution to pushing things off main thread is rather cumbersome. We need to call
// into DispatchQueue.global(qos: .background).async {...} but also manually ensure that
// everything is synchronized between the main and bg thread. We also manually need to
// keep track of the stopped tasks b/c a WKURLSchemeTask does not have any properties that
// we could query to see if it has stopped. If we respond to a WKURLSchemeTask that has
// stopped then an unmanaged exception is thrown which Swift cannot catch and the entire
// app will crash.
public class MyURLSchemeHandler: NSObject, WKURLSchemeHandler {
private var stoppedTaskURLs: [URLRequest] = []
public func webView(_ webView: WKWebView, start urlSchemeTask: WKURLSchemeTask) {
let request = urlSchemeTask.request
guard let requestUrl = request.url else { return }
DispatchQueue.global(qos: .background).async { [weak self] in
guard let strongSelf = self, requestUrl.scheme == "my-video-url-scheme" else {
return
}
let filePath = requestUrl.absoluteString
if let fileHandle = FileHandle(forReadingAtPath: filePath) {
// video files can be very large in size, so read them in chuncks.
let chunkSize = 1024 * 1024 // 1Mb
let response = URLResponse(url: requestUrl,
mimeType: "video/mp4",
expectedContentLength: chunkSize,
textEncodingName: nil)
strongSelf.postResponse(to: urlSchemeTask, response: response)
var data = fileHandle.readData(ofLength: chunkSize) // get the first chunk
while (!data.isEmpty && !strongSelf.hasTaskStopped(urlSchemeTask)) {
strongSelf.postResponse(to: urlSchemeTask, data: data)
data = fileHandle.readData(ofLength: chunkSize) // get the next chunk
}
fileHandle.closeFile()
strongSelf.postFinished(to: urlSchemeTask)
} else {
strongSelf.postFailed(
to: urlSchemeTask,
error: NSError(domain: "Failed to fetch resource",
code: 0,
userInfo: nil))
}
// remove the task from the list of stopped tasks (if it is there)
// since we're done with it anyway
strongSelf.stoppedTaskURLs = strongSelf.stoppedTaskURLs.filter{$0 != request}
}
}
public func webView(_ webView: WKWebView, stop urlSchemeTask: WKURLSchemeTask) {
if (!self.hasTaskStopped(urlSchemeTask)) {
self.stoppedTaskURLs.append(urlSchemeTask.request)
}
}
private func hasTaskStopped(_ urlSchemeTask: WKURLSchemeTask) -> Bool {
return self.stoppedTaskURLs.contains{$0 == urlSchemeTask.request}
}
private func postResponse(to urlSchemeTask: WKURLSchemeTask, response: URLResponse) {
post(to: urlSchemeTask, action: {urlSchemeTask.didReceive(response)})
}
private func postResponse(to urlSchemeTask: WKURLSchemeTask, data: Data) {
post(to: urlSchemeTask, action: {urlSchemeTask.didReceive(data)})
}
private func postFinished(to urlSchemeTask: WKURLSchemeTask) {
post(to: urlSchemeTask, action: {urlSchemeTask.didFinish()})
}
private func postFailed(to urlSchemeTask: WKURLSchemeTask, error: NSError) {
post(to: urlSchemeTask, action: {urlSchemeTask.didFailWithError(error)})
}
private func post(to urlSchemeTask: WKURLSchemeTask, action: @escaping () -> Void) {
let group = DispatchGroup()
group.enter()
DispatchQueue.main.async { [weak self] in
if (self?.hasTaskStopped(urlSchemeTask) == false) {
action()
}
group.leave()
}
group.wait()
}
}