I am using a nsurlssesion based wrapper class written in objective-c in my swift project. Everthing is working except I am not able to undertand how the closure works in swift.
In my swift viewcontroller:
DownloadManager().downloadFile(forURL: url, progressBlock: { (progress) -> () in
print("current progress is \(progress)")
}, completionBlock: { (completion) in
print("is completed : \(completion)")
}, enableBackgroundMode: false)
In my Downloadmanager class (which is objc based) whenever nsurlssession delegate is called this happens:
dispatch_async(dispatch_get_main_queue(), ^(void) {
if(download.progressBlock){
download.progressBlock(progress); //exception when progressblock is nil
}
});
Download object have a block type property called progressBlock :
typedef void(^TWRDownloadProgressBlock)(CGFloat progress);
Output:
current progress is 0.0908259372799894
current progress is 0.272477811839968
current progress is 0.363303749119957
current progress is 0.454129686399947
current progress is 0.544955623679936
current progress is 0.635781560959925
current progress is 0.726607498239915
current progress is 0.817433435519904
current progress is 1.0
Flow of the code:
NSURlsession calls it's delegate method.
DowloadManager updates Download object's progress variable.
Somehow in my viewcontroller I can see the updated value of the progress variable inside the closure.
Question:
How is point-3 happening?
Is this the standard way closure behaves in swift? Specifically, I would like to know if the closure is automatically called everytime Dowload object changes?
According to The Swift Guide:
Closures are self-contained blocks of functionality that can be passed around and used in your code. Closures in Swift are similar to blocks in C and Objective-C and to lambdas in other programming languages.
Closures can capture and store references to any constants and variables from the context in which they are defined.
In this case, a closure is defined within the view controller, then passed
around and called later.
The flow would be as follows:
Make a new instance of TWRDownloadManager() and call its downloadFile method.
In this call, pass the progressBlock: ((CGFloat) -> Swift.Void) closure (it takes CGFloat as a parameter and returns nothing).
Down the line, this block is what gets assigned to TWRDownloadObject().progressBlock
Further down the line, NSURLSession Delegate method calls download.progressBlock(progress), which simply calls the closure from #2 and passes the CGFloat value as its parameter.
Here is quick Playground example:
import Foundation
import PlaygroundSupport
import TWRDownloadManager
PlaygroundPage.current.needsIndefiniteExecution = true
let downloadManager = TWRDownloadManager()
let url = "http://download.thinkbroadband.com/200MB.zip"
let progressClosure: (CGFloat) -> () = { (progress) in
print("current progress is \(progress)")
}
let completionClosure: (Bool) -> () = { (completion) in
print("is completed : \(completion)")
}
downloadManager.downloadFile(
forURL: url,
progressBlock: progressClosure,
completionBlock: completionClosure,
enableBackgroundMode: false
)
Related
I'm trying to implement downloading timeline data in a widget and so I created a background URLSession with a corresponding data task to download the JSON:
let session = URLSession(
configuration: .background(withIdentifier: identifier),
delegate: self,
delegateQueue: nil
)
let request = URLRequest(url: ...)
session.dataTask(with: request).resume()
On my widget I then added the onBackgroundURLSessionEvents to store the completion handler, as per the Apple docs:
.onBackgroundURLSessionEvents { identifier in
return SessionCache.shared.isValid(for: identifier)
} _: { identifier, completion in
let data = SessionCache.shared.sessionData(for: identifier)
data.sessionCompletion = completion
}
However, neither the urlSessionDidFinishEvents(forBackgroundURLSession:) nor the onBackgroundURLSessionEvents methods are called. When the network download completes, it just calls the normal urlSession(_:task:didCompleteWithError:) method.
I'm clearly missing something here, but I'm just not seeing what.
Apple's documentation isn't 100% clear on this, but you need to use URLSession.downloadTask instead of dataTask for background sessions. URLSessionDataTasks deliver bytes to those specific delegate methods in your in-memory process. Background download & upload tasks are handed off to nsurlsessiond and only delivered back to your app when they are fully resolved.
Here is how I implement an inline callback assignment in Objective-C:
self.beforeAdjustViews = ^ (UIView* btnView)
{
// do something
};
How do I implement the same thing in Swift 5?
You can either assign directly
self.beforeAdjustViews = { (button) in
// Write Some Code
}
or create a reference variable to this call back - usually better when you have a lot of properties or trying to keep you code simple and beautiful
let beforAdjustViewsCallBack: (btnView: UIView) -> Void = { button in
// Write Some Code
}
Also keep in mind that the same rules for retain cycles are applied, So if you don't want to keep a strong reference of the self inside the block, you will need to use either [weak self] or [unowoned self] based on your use case.
You can try
self.beforeAdjustViews = { [weak self] (btnView) in
///
}
I'm developing an iOS app in Swift4 with an Object-C framework called 'YapDatabase'. There is an Object-C function with a block like this in class 'YapDatabaseConnection':
- (void)readWithBlock:(void (^)(YapDatabaseReadTransaction *transaction))block;
I use the function in this way:
static func readNovelIds() -> [String]? {
let account = XFAccountManager.share().account
var events: [XFNovelClickEvent]?
OTRDatabaseManager.shared.readOnlyDatabaseConnection?.read({ (transaction) in
events = XFNovelClickEvent.allNovelClickEvents(accountId: account.uniqueId, transaction: transaction)
})
guard let clickEvents = events else {
return nil
}
let readNovelsIds = clickEvents.map {
$0.bookId ?? ""
}
return readNovelsIds
}
I thought the closure will be executed immediately after the 'events' parameter declared. In fact, the closure doesn't be executed before result returns. To search the reason, I open the file named 'YapDatabaseConnection.h(Interface)' generated by Xcode (with cmd+shift+o), found the function has been translate to Swift in this way:
open func read(_ block: #escaping (YapDatabaseReadTransaction) -> Void)
So, how do I use this function in a #noescap way?
As the caller, you can't change when the closure is executed. That's up to the read() function. If you control that function, you'll need to modify it to call the closure immediately. If you don't control it, then you can't modify how it behaves.
You can convert an asynchronous call into a synchronous call using a DispatchGroup as described in Waiting until the task finishes. However, you can't make a database call on the main queue; you risk crashing the app. As a general rule, you should just use async calls in this case (i.e. make readNovelIds also be asynchronous and take a completion handler).
The reason why Xcode bridged the objective-c block as #escaping is because the block may be executed after the function return.
Since you don’t own YapDatabase, you couldn’t modify the source code to make it non-escaped, so you may wanna make your readNovelIds function takes a closure as parameter and pass the return value through closure.
static func readNovelIds(resultHandler: #escaping ([String]?) -> ()) {
let account = XFAccountManager.share().account
var events: [XFNovelClickEvent]?
OTRDatabaseManager.shared.readOnlyDatabaseConnection?.read({ (transaction) in
events = XFNovelClickEvent.allNovelClickEvents(accountId: account.uniqueId, transaction: transaction)
if let clickEvents = events {
let readNovelsIds = clickEvents.map {
$0.bookId ?? ""
}
resultHandler(readNovelsIds)
}
resultHandler(nil)
})
}
If the method is in fact synchronous (i.e. it will not allow the block to escape its context), the Objective C header method should be decorated with NS_NOESCAPE. Looking at the documentation (which does say it is synchronous), and the implementation, it should be annotated that way.
- (void)readWithBlock:(void (NS_NOESCAPE ^)(YapDatabaseReadTransaction *transaction))block;
That, I believe, should allow the Swift interface importer to add the #noescaping declaration. You should probably file a bug request on the YapDatabase project; they can change it there.
So far tvOS supports two ways to make tv apps, TVML and UIKit, and there is no official mentions about how to mix up things to make a TVML (that is basically XML) User Interface with the native counter part for the app logic and I/O (like playback, streaming, iCloud persistence, etc).
So, which is the best solution to mix TVML and UIKit in a new tvOS app?
In the following I have tried a solution following code snippets adapted from Apple Forums and related questions about JavaScriptCore to ObjC/Swift binding.
This is a simple wrapper class in your Swift project.
import UIKit
import TVMLKit
#objc protocol MyJSClass : JSExport {
func getItem(key:String) -> String?
func setItem(key:String, data:String)
}
class MyClass: NSObject, MyJSClass {
func getItem(key: String) -> String? {
return "String value"
}
func setItem(key: String, data: String) {
print("Set key:\(key) value:\(data)")
}
}
where the delegate must conform a TVApplicationControllerDelegate:
typealias TVApplicationDelegate = AppDelegate
extension TVApplicationDelegate : TVApplicationControllerDelegate {
func appController(appController: TVApplicationController, evaluateAppJavaScriptInContext jsContext: JSContext) {
let myClass: MyClass = MyClass();
jsContext.setObject(myClass, forKeyedSubscript: "objectwrapper");
}
func appController(appController: TVApplicationController, didFailWithError error: NSError) {
let title = "Error Launching Application"
let message = error.localizedDescription
let alertController = UIAlertController(title: title, message: message, preferredStyle:.Alert ) self.appController?.navigationController.presentViewController(alertController, animated: true, completion: { () -> Void in
})
}
func appController(appController: TVApplicationController, didStopWithOptions options: [String : AnyObject]?) {
}
func appController(appController: TVApplicationController, didFinishLaunchingWithOptions options: [String : AnyObject]?) {
}
}
At this point the javascript is very simple like. Take a look at the methods with named parameters, you will need to change the javascript counter part method name:
App.onLaunch = function(options) {
var text = objectwrapper.getItem()
// keep an eye here, the method name it changes when you have named parameters, you need camel case for parameters:
objectwrapper.setItemData("test", "value")
}
App. onExit = function() {
console.log('App finished');
}
Now, supposed that you have a very complex js interface to export like
#protocol MXMJSProtocol<JSExport>
- (void)boot:(JSValue *)status network:(JSValue*)network user:(JSValue*)c3;
- (NSString*)getVersion;
#end
#interface MXMJSObject : NSObject<MXMJSProtocol>
#end
#implementation MXMJSObject
- (NSString*)getVersion {
return #"0.0.1";
}
you can do like
JSExportAs(boot,
- (void)boot:(JSValue *)status network:(JSValue*)network user:(JSValue*)c3 );
At this point in the JS Counter part you will not do the camel case:
objectwrapper.bootNetworkUser(statusChanged,networkChanged,userChanged)
but you are going to do:
objectwrapper.boot(statusChanged,networkChanged,userChanged)
Finally, look at this interface again:
- (void)boot:(JSValue *)status network:(JSValue*)network user:(JSValue*)c3;
The value JSValue* passed in. is a way to pass completion handlers between ObjC/Swift and JavaScriptCore. At this point in the native code you do all call with arguments:
dispatch_async(dispatch_get_main_queue(), ^{
NSNumber *state = [NSNumber numberWithInteger:status];
[networkChanged.context[#"setTimeout"]
callWithArguments:#[networkChanged, #0, state]];
});
In my findings, I have seen that the MainThread will hang if you do not dispatch on the main thread and async. So I will call the javascript "setTimeout" call that calls the completion handler callback.
So the approach I have used here is:
Use JSExportAs to take car of methods with named parameters and avoid to camel case javascript counterparts like callMyParam1Param2Param3
Use JSValue as parameter to get rid of completion handlers. Use callWithArguments on the native side. Use javascript functions on the JS side;
dispatch_async for completion handlers, possibly calling a setTimeout 0-delayed in the JavaScript side, to avoid the UI to freeze.
[UPDATE]
I have updated this question in order to be more clear. I'm finding a technical solution for bridging TVML and UIKit in order to
Understand the best programming model with JavaScriptCode
Have the right bridge from JavaScriptCore to ObjectiveC and
viceversa
Have the best performances when calling JavaScriptCode from Objective-C
This WWDC Video explains how to communicate between JavaScript and Obj-C
Here is how I communicate from Swift to JavaScript:
//when pushAlertInJS() is called, pushAlert(title, description) will be called in JavaScript.
func pushAlertInJS(){
//allows us to access the javascript context
appController!.evaluateInJavaScriptContext({(evaluation: JSContext) -> Void in
//get a handle on the "pushAlert" method that you've implemented in JavaScript
let pushAlert = evaluation.objectForKeyedSubscript("pushAlert")
//Call your JavaScript method with an array of arguments
pushAlert.callWithArguments(["Login Failed", "Incorrect Username or Password"])
}, completion: {(Bool) -> Void in
//evaluation block finished running
})
}
Here is how I communicate from JavaScript to Swift (it requires some setup in Swift):
//call this method once after setting up your appController.
func createSwiftPrint(){
//allows us to access the javascript context
appController?.evaluateInJavaScriptContext({(evaluation: JSContext) -> Void in
//this is the block that will be called when javascript calls swiftPrint(str)
let swiftPrintBlock : #convention(block) (String) -> Void = {
(str : String) -> Void in
//prints the string passed in from javascript
print(str)
}
//this creates a function in the javascript context called "swiftPrint".
//calling swiftPrint(str) in javascript will call the block we created above.
evaluation.setObject(unsafeBitCast(swiftPrintBlock, AnyObject.self), forKeyedSubscript: "swiftPrint" as (NSCopying & NSObjectProtocol)?)
}, completion: {(Bool) -> Void in
//evaluation block finished running
})
}
[UPDATE] For those of you who would like to know what "pushAlert" would look like on the javascript side, I'll share an example implemented in application.js
var pushAlert = function(title, description){
var alert = createAlert(title, description);
alert.addEventListener("select", Presenter.load.bind(Presenter));
navigationDocument.pushDocument(alert);
}
// This convenience funnction returns an alert template, which can be used to present errors to the user.
var createAlert = function(title, description) {
var alertString = `<?xml version="1.0" encoding="UTF-8" ?>
<document>
<alertTemplate>
<title>${title}</title>
<description>${description}</description>
</alertTemplate>
</document>`
var parser = new DOMParser();
var alertDoc = parser.parseFromString(alertString, "application/xml");
return alertDoc
}
You sparked an idea that worked...almost. Once you have displayed a native view, there is no straightforward method as-of-yet to push an TVML-based view onto the navigation stack. What I have done at this time is:
let appDelegate = UIApplication.sharedApplication().delegate as! AppDelegate
appDelegate.appController?.navigationController.popViewControllerAnimated(true)
dispatch_async(dispatch_get_main_queue()) {
tvmlContext!.evaluateScript("showTVMLView()")
}
...then on the JavaScript side:
function showTVMLView() {setTimeout(function(){_showTVMLView();}, 100);}
function _showTVMLView() {//push the next document onto the stack}
This seems to be the cleanest way to move execution off the main thread and onto the JSVirtualMachine thread and avoid the UI lockup. Notice that I had to pop at the very least the current native view controller, as it was getting sent a deadly selector otherwise.
I'm using Swift to make a game in SpriteKit.
In Objective-C I could use the following method:
(void)enumerateChildNodesWithName:(NSString *)name usingBlock:(void (^)(SKNode *node, BOOL *stop))block
to perform actions on that *node, but I can't get this function working in Swift. Basically, I don't know how to reference that node in Swift.
This is the code I'm using, but I'm having trouble with the "usingBlock:" part. I've tried many things for many hours, but have not succeeded. Help please!
func spawnEnemy() -> () {
let enemy = SKSpriteNode(imageNamed: "enemy")
enemy.name = "enemy"
enemy.position = CGPointMake(100, 100)
self.addChild(enemy)
}
func checkCollisions() -> () {
self.enumerateChildNodesWithName("enemy", usingBlock: ((SKNode!, CMutablePointer<ObjCBool>) -> Void)?)
}
For now, don't trust autocomplete to insert the code you need — it drops in signatures from the "header", but a block signature is not the same as the declaration you need when inserting your own closure for a block parameter.
The formal way to write a closure would be to replicate the signature inside braces, adding local parameter names and using the in keyword to mark the start of the closure body:
self.enumerateChildNodesWithName("enemy", usingBlock: {
(node: SKNode!, stop: UnsafeMutablePointer <ObjCBool>) -> Void in
// do something with node or stop
})
But Swift's type inference means you don't have to write that much. Instead, you can just name the parameters, because their type (as well as the closure's return type) is known:
self.enumerateChildNodesWithName("enemy", usingBlock: {
node, stop in
// do something with node or stop
})
You can also use trailing closure syntax:
self.enumerateChildNodesWithName("enemy") {
node, stop in
// do something with node or stop
}
(You can even drop the local parameter names and refer to parameters by position — e.g. $0 for node — but here isn't a great place to do that because it makes your code far less readable. It's best to reserve $0 and friends for closures where it's blindingly obvious what the parameters are, like the closures you use with map and sort.)
See Closures in The Swift Programming Language for further explanation.
Also, because stop is an UnsafeMutablePointer, the syntax for using it is a bit different than in ObjC: set stop.memory = true to break out of enumeration.