IOS download Web image method with GCD

Downloading a network picture with multithreading can cause repeated downloads of wasted traffic.

In order to avoid the above situation, using multi-threaded download image method can be used the following methods.

1. Create a dictionary, if downloaded the picture will be the image address is key, the picture is the value of the save.

2. Create a dictionary, if the child thread has started to download then use the image address as key, and the image address as value to save.

3. If you are downloading, display a placeholder picture

4. If there is a downloaded picture in the dictionary, refresh the picture

Property

@interface Viewcontroller () @property (weak, nonatomic) Iboutlet Uiimageview *imageviewoutlet; @property (Nonatomic, Strong) Nsmutabledictionary *imagedict, @property (nonatomic, strong) Nsmutabledictionary *dispatchdict; @property ( Nonatomic, assign) dispatch_queue_t globalqueue; @property (nonatomic, assign) dispatch_group_t dispatchgroup;@ Property (Nonatomic, Strong) NSData *imagedata; @end

Lazy Loading

-(Nsmutabledictionary *) imagedict {if (!_imagedict) {_imagedict = [nsmutabledictionary dictionary];} return _imagedict;} -(Nsmutabledictionary *) dispatchdict {if (!_dispatchdict) {_dispatchdict = [nsmutabledictionary dictionary];} return _dispatchdict;} -(dispatch_queue_t) Globalqueue {if (!_globalqueue) {_globalqueue = Dispatch_get_global_queue (DISPATCH_QUEUE_ Priority_default, 0);} return _globalqueue;}

Click on the screen to start downloading

-(void) Touchesbegan: (Nsset *) touches withevent: (Uievent *) event {[Self dispatchgroupasync];}

Download picture operation

-(void) Dispatchgroupasync {dispatch_group_t dispatchgroup = Dispatch_group_create (); NSString *imagename = @ "Http://pic29.nipic.com/20130424/11921802_221046126000_2.jpg"; UIImage *image = self.imagedict[imagename];//dictionary without picture if (image = = nil) {NSLog (@ "image is nil"); NSString *dispatchname = self.dispatchdict[imagename];//also not downloaded in if (dispatchname = = nil) {NSLog (@ "Dispatch is nil"); Dispatch_group_async (Dispatchgroup, Self.globalqueue, ^{//add thread dictionary self.dispatchdict[imagename] = imageName; Nsurl *url = [Nsurl urlwithstring:imagename];self.imagedata = [NSData datawithcontentsofurl:url];}); Dispatch_group_notify (Dispatchgroup, Self.globalqueue, ^{nslog (@ "dispatch_group_notify queue finished!"); [self.dispatchdict removeobjectforkey:imagename];self.imagedict[imagename] = self.imagedata; [Self Performselectoronmainthread: @selector (refreshimage) Withobject:nil waituntildone:yes];});} Also in the download else {self.imageViewOutlet.image = [UIImage imagenamed:@ "dufault.jpg"];}} There are already downloaded pictures else {NSLog (@ "refresh picture"); Self.imageviewOutlet.image = [UIImage imagewithdata:self.imagedict[imagename] scale:0.0];}} 

After downloading the picture, go back to the main thread to refresh the picture

-(void) refreshimage {self.imageViewOutlet.image = [UIImage imageWithData:self.imageData scale:0.0];}

Run effect

To test for preventing duplicate download operations, punch multiple screens in a row at the beginning of the download

2015-08-08 22:02:18.055 gcd download Image [2840:163865] image is nil2015-08-08 22:02:18.055 gcd download picture [2840:163865] Dispatch is nil2015-08-08 22:02:18.235 gcd download Image [2840:163865] image is nil2015-08-08 22:02:18.403 gcd download picture [2840:163865] image is nil2015-08-08 22:02:18.560 gcd download Image [2840:163865] image is nil2015-08-08 22:02:18.704 gcd download picture [2840:163865] image is nil2015-08-08 22:02:18.874 gcd download Image [2840:163960] dispatch_group_notify queue finished!2015-08-08 22:02:18.890 gcd download Image [ 2,840:163,865] Refresh picture 2015-08-08 22:02:19.495 gcd download picture [2,840:163,865] Refresh Picture

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Note: If you want to learn to download with nsoperation, please see http://blog.csdn.net/zhangwenhai001/article/details/46723597

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IOS download Web image method with GCD