ios


download image in tableView(_:, cellForRowAt:)


when i segue to a viewController with tableView, the tableviewcell immediately send a request to the server using the method fetchUserAvatar(avatarName: handler: (String) -> Void). and this method returns a url that link to the image. download it and cache the image cacheImage is an object of NSCache<NSString, UIImage>. this object cacheImage was initalised in previous view controller and being assigned from pervious to this viewContoller using prepare(for segue: UIStoryboardSegue, sender: Any?). when this viewController shows up, I can't see the image in the cell. but i pop the viewController out and segue to this viewController with tableView again. the image will show. I think(I guess) because
the images weren't all fully downloaded yet. so, i can't see the images. but if i pop out the viewController and load an object of viewController and the viewController gets the images from cache. therefore, the images could be shown.
I want to know how to avoid this problem? thanks.
func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
let cell = tableView.dequeueReusableCell(withIdentifier: "MessageCell", for: indexPath) as! MessageCell
let row = indexPath.row
cell.content.text = messages[row].content
cell.date.text = messages[row].createdDateStrInLocal
cell.messageOwner.text = messages[row].user
if let avatar = cacheImage.object(forKey: messages[row].user as NSString){
cell.profileImageView.image = avatar
} else {
fetchUserAvatar(avatarName: messages[row].user, handler: { [unowned self] urlStr in
if let url = URL(string: urlStr), let data = try? Data(contentsOf: url), let avatar = UIImage(data: data){
self.cacheImage.setObject(avatar, forKey: self.messages[row].user as NSString)
cell.profileImageView.image = avatar
}
})
}
return cell
}
fileprivate func fetchUserAvatar(avatarName: String, handler: #escaping (String) -> Void){
guard !avatarName.isEmpty, let user = self.user, !user.isEmpty else { return }
let url = URL(string: self.url + "/userAvatarURL")
var request = URLRequest(url: url!)
let body = "username=" + user + "&avatarName=" + avatarName
request.httpMethod = "POST"
request.setValue("application/x-www-form-urlencoded; charset=utf-8", forHTTPHeaderField: "Content-Type")
request.httpBody = body.data(using: .utf8)
defaultSession.dataTask(with: request as URLRequest){ data, response, error in
DispatchQueue.main.async {
if let httpResponse = response as? HTTPURLResponse {
if 200...299 ~= httpResponse.statusCode {
print("statusCode: \(httpResponse.statusCode)")
if let urlStr = String(data: data!, encoding: String.Encoding.utf8), urlStr != "NULL" {
handler(urlStr)
}
} else {
print("statusCode: \(httpResponse.statusCode)")
if let unwrappedData = String(data: data!, encoding: String.Encoding.utf8) {
print("POST: \(unwrappedData)")
self.warning(title: "Fail", message: unwrappedData, buttonTitle: "OK", style: .default)
} else {
self.warning(title: "Fail", message: "unknown error.", buttonTitle: "OK", style: .default)
}
}
} else if let error = error {
print("Error: \(error)")
}
}
}.resume()
}
I modified it, and move the download code in viewdidload and reload the tableview, the result is the same.
Does your image view have a fixed size, or are you taking advantage of the intrinsic size? Based upon your description, I'd assume the latter. And updating the cache and reloading the cell inside fetchUserAvatar completion handler should resolve that problem.
But you have two issues here:
You should really use dataTask to retrieve the image, not Data(contentsOf:) because the former is asynchronous and the latter is synchronous. And you never want to do synchronous calls on the main queue. At best, the smoothness of your scrolling will be adversely affected by this synchronous network call. At worst, you risk having the watch dog process kill your app if the network request is slowed down for any reason and you block the main thread at the wrong time.
Personally, I'd have fetchUserAvatar do this second asynchronous request asynchronously and change the closure to return the UIImage rather than the URL as a String.
Perhaps something like:
fileprivate func fetchUserAvatar(avatarName: String, handler: #escaping (UIImage?) -> Void){
guard !avatarName.isEmpty, let user = self.user, !user.isEmpty else {
handler(nil)
return
}
let url = URL(string: self.url + "/userAvatarURL")!
var request = URLRequest(url: url)
let body = "username=" + user + "&avatarName=" + avatarName
request.httpMethod = "POST"
request.setValue("application/x-www-form-urlencoded; charset=utf-8", forHTTPHeaderField: "Content-Type")
request.httpBody = body.data(using: .utf8)
defaultSession.dataTask(with: request) { data, response, error in
guard let data = data, error == nil, let httpResponse = response as? HTTPURLResponse, 200...299 ~= httpResponse.statusCode else {
print("Error: \(error?.localizedDescription ?? "Unknown error")")
DispatchQueue.main.async { handler(nil) }
return
}
guard let string = String(data: data, encoding: .utf8), let imageURL = URL(string: string) else {
DispatchQueue.main.async { handler(nil) }
return
}
defaultSession.dataTask(with: imageURL) { (data, response, error) in
guard let data = data, error == nil else {
DispatchQueue.main.async { handler(nil) }
return
}
let image = UIImage(data: data)
DispatchQueue.main.async { handler(image) }
}.resume()
}.resume()
}
This is a more subtle point, but you should not use the cell inside the asynchronously called completion handler closure. The cell could have scrolled out of view and you could be updating the cell for a different row of the table. This is likely only to be problematic for really slow network connections, but it's still an issue.
Your asynchronous closure should be determining the index path of the cell and then reloading just that index path with reloadRows(at:with:).
For example:
override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
let cell = tableView.dequeueReusableCell(withIdentifier: "MessageCell", for: indexPath) as! MessageCell
let row = indexPath.row
cell.content.text = messages[row].content
cell.date.text = messages[row].createdDateStrInLocal
cell.messageOwner.text = messages[row].user
if let avatar = cacheImage.object(forKey: messages[row].user as NSString){
cell.profileImageView.image = avatar
} else {
cell.profileImageView.image = nil // make sure to reset this first, in case cell is reused
fetchUserAvatar(avatarName: messages[row].user) { [unowned self] avatar in
guard let avatar = avatar else { return }
self.cacheImage.setObject(avatar, forKey: self.messages[row].user as NSString)
// note, if it's possible rows could have been inserted by the time this async request is done,
// you really should recalculate what the indexPath for this particular message. Below, I'm just
// using the previous indexPath, which is only valid if you _never_ insert rows.
tableView.reloadRows(at: [indexPath], with: .automatic)
}
}
return cell
}
Frankly, there are other subtle issues here (e.g. if your username or avatar name contain reserved characters, your requests will fail; if you scroll quickly on a really slow connection, images for visible cells will get backlogged behind cells that are no longer visible, you risk timeouts, etc.). Rather than spending a lot of time contemplating how to fix these more subtle issues, you might consider using an established UIImageView category that performs asynchronous image requests and supports caching. Typical options include AlamofireImage, KingFisher, SDWebImage, etc.

Related Links

Change textLabel.text without reloading the view?
Table header view not loading iOS
In Firebase, how can I query the most recent 10 child nodes?
My codes do not work for sections in TableView
performSegueWithIdentifier Firing, But Not Displaying
addSubview can not correctly show all the content in the viewController?
If I moved my Xcode project folder to another computer running OS X, Can the app still be built and run there?
How to handle two Segmented Controllers in one View Controller in iOS 9?
How to convert superclass to subclass in iOS
How to set the property for each “subUIBezierPath”
Swift 2.x cannot set MapView Region [closed]
Possible to attach logic to method dynamically?
Passing an IBOutlet variable to another class
Instantiate a view controller on AppDelegate, understanding
Parse JSON and save with Realm
How to pass an id with FBSDKShareOpenGraphObject so when story is selected on Facebook it goes to that particular content in app

Categories

HOME
document
hadoop
jsp
music
outlook
stripe-payments
payment-processing
copy-paste
wso2-das
zend-expressive
smartsheet-c#-sdk-v2
raspberry-pi2
phpbb3
image-gallery
teraterm
oracle11gr2
abaqus
curve-fitting
rational
windows-applications
deadbolt
google-distancematrix-api
aikau
azure-ad-graph-api
line-breaks
easendmail
eonasdan-datetimepicker
google-oauth2
dsl
centos6
toastr
imageshack
deep-copy
data-science
ecmascript-4
vue2
uistoryboardsegue
amazon-elasticsearch
visualsvn-server
jpype
mergesort
karnaugh-map
requirements-management
simplesamlphp
clientscript
federation
smartbanner
sql-mode
orchestrate
ganglia
laravel-routing
glfw
canopen
maven-versions-plugin
catalina
assembly-references
vispy
measurement-protocol
vb4android
balsamiq
cufft
network-traffic
rras
angularjs-filter
jquery-load
google-api-ruby-client
pddl
biojava
push-api
protected
chunk-templates
translucency
keypad
byte-order-mark
alt
twitter-follow
isql
json-web-token
mpj-express
cpp-netlib
before-save
keyboard-navigation
mouseenter
junit-runner
datasift-python
firebird2.1
xhtml-transitional
capitalize
icon-fonts
wpf-4.0
sender
jmdns
mysql-backup
adaboost
expression-encoder-sdk
letter-spacing
ruby-1.9.2
payment-services
duplicate-data
soapheader
emacs-ecb
jquery-pagination
replay
pong
etrade-api
nsindexset
arraycopy
utility
coords
mud
usenet

Resources

Mobile Apps Dev
Database Users
javascript
java
csharp
php
android
MS Developer
developer works
python
ios
c
html
jquery
RDBMS discuss
Cloud Virtualization
Database Dev&Adm
javascript
java
csharp
php
python
android
jquery
ruby
ios
html
Mobile App
Mobile App
Mobile App