我想创建一个将图像显示到UICollectionView的应用程序。
问题：我无法将新下载的图像显示到collectionView。
这是我的代码：
-(UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {
BOOL reloaded = NO;
static NSString *cellIdentifier = @"cvCell";
CVCell *cell = (CVCell *)[collectionView dequeueReusableCellWithReuseIdentifier:cellIdentifier forIndexPath:indexPath];
NSMutableArray *data = [self.dataArray objectAtIndex:indexPath.section];
NSString *cellData = [data objectAtIndex:indexPath.row];
dispatch_queue_t queue = dispatch_queue_create("com.justTest.anotherSingleApplication", NULL);
dispatch_async(queue, ^{
//code to be executed in the background
NSString *imageName1 = [[NSString alloc]initWithFormat:@"http://www.abc.com/images/thumb/%@", cellData];
NSString *url_Img1 = imageName1;
UIImage *aImage = [UIImage imageWithData:[NSData dataWithContentsOfURL:[NSURL URLWithString:url_Img1]]];
dispatch_async(dispatch_get_main_queue(), ^{
//code to be executed on the main thread when background task is finished
[cell.cellImage setImage:aImage];
});
});
if (indexPath.row == self.imageArray.count - 1 && !reloaded) {
getOnScrollImages *getImage = [[getOnScrollImages alloc] init]; // class to get image name from server
NSMutableArray *astring = (NSMutableArray *)[getImage getImageNameFromServer:@"list" board:@"111" pin:@"122345"]; // method to get image name from server
[self setNewTestArray:astring]; //adding newly downloaded image name into array
reloaded = YES;
dispatch_async(dispatch_get_main_queue(), ^{
[self.collectionView reloadData];
});
}
return cell;
}
有什么建议吗？
注意：我刚刚开始开发iOS应用程序，这可能是一个非常愚蠢的问题。
答案 0 :(得分：2)
使用异步提取从服务器获取数据并将其显示在collectionView
中- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
YourDataModel *model = self.dataArray[indexPath.row];
YourCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:cellIdentifier forIndexPath:indexPath];
if ([self checkWhetherImageAlreadyExist]) {
[cell.imageView setImage:model.image];
} else {
//show placeholder to avoid nothing in your UI, or your user gets confused
[cell.imageView setImage:placeholderImage];
[self startDownloadImageForIndexPath:indexPath];
}
}
- (void)startDownloadImageForIndexPath:(NSIndexPath *)indexPath
{
//YourImageDownloader is a class to fetch data from server
//imageDownloadsInProgress is a NSMutableDictionary to record the download process, which can avoid repeat download
YourImageDownloader *downloader = [self.imageDownloadsInProgress objectForKey:indexPath];
if (downloader == nil) {
YourDataModel *model = self.dataArray[indexPath.row];
//configure downloader
downloader = [[YourImageDownloader alloc] init];
[downloader setURL:model.url];
[downloader setCompletionHandler:^{
//download the image to local, or you can pass the image to the block
model.image = [UIImage imageWithContentsOfFile:model.localPath];
YourCell *cell = [self.mCollectionView cellForItemAtIndexPath:indexPath];
[cell.imageView setImage:model.image];
//remove downloader from dictionary
[self.imageDownloadsInProgress removeObjectForKey:indexPath];
}];
//add downloader to dictionary
[self.imageDownloadsInProgress setObject:downloader forKey:indexPath];
//start download
[downloader startDownload];
}
}
使用课程下载图像。如果您在一个集合视图中有许多图像，则可以考虑在发生内存警告时将这些图像保存到本地。如果现在很多，只需将图像保留在内存中并将其显示在集合视图中。
后面的代码是将图像保存到本地，并在显示时从本地读取图像数据。
在.h：
#import <Foundation/Foundation.h>
@interface PortraitDownloader : NSObject
@property (nonatomic, copy) NSString *portraitName;
@property (nonatomic, copy) void (^completionHandler)(void);
- (void)startDownload;
- (void)cancelDownload;
@end
in .m
#import "PortraitDownloader.h"
#import <CFNetwork/CFNetwork.h>
#import "NSString+ImagePath.h" // it's a category to get the image local path
@interface PortraitDownloader ()
@property (nonatomic, strong) NSMutableData *activeDownload;
@property (nonatomic, strong) NSURLConnection *portraitConnection;
@end
@implementation PortraitDownloader
- (void)startDownload
{
self.activeDownload = [NSMutableData data];
NSString *urlstr = [NSString serverPortraitPathWithPortrait:self.portraitName];
NSURL *url = [NSURL URLWithString:urlstr];
NSURLRequest *request = [NSURLRequest requestWithURL:url];
self.portraitConnection = [[NSURLConnection alloc] initWithRequest:request delegate:self];
}
- (void)cancelDownload
{
[self.portraitConnection cancel];
self.portraitConnection = nil;
self.activeDownload = nil;
}
#pragma mark - NSURLConnectionDelegate
- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data
{
[self.activeDownload appendData:data];
}
- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error
{
// Clear the activeDownload property to allow later attempts
self.activeDownload = nil;
// Release the connection now that it's finished
self.portraitConnection = nil;
}
- (void)connectionDidFinishLoading:(NSURLConnection *)connection
{
// save to local path
NSString *localSavePath = [NSString localPortraitPathWithPortrait:self.portraitName];
[self.activeDownload writeToFile:localSavePath atomically:YES];
self.activeDownload = nil;
// Release the connection now that it's finished
self.portraitConnection = nil;
// call our delegate and tell it that our icon is ready for display
if (self.completionHandler) {
self.completionHandler();
}
}
@end
如果您想将图像保留在内存中，只需将完成块修改为：
in .h
typedef void (^Completion_handle) (UIImage *image);
@interface PortraitDownloader : NSObject
@property (nonatomic, copy) Completion_handle myCompletionBlock;
和.m
- (void)connectionDidFinishLoading:(NSURLConnection *)connection
{
// get image from data
UIImage *image = [UIImage imageWithData:self.activeDownload];
self.activeDownload = nil;
// Release the connection now that it's finished
self.portraitConnection = nil;
// call our delegate and tell it that our icon is ready for display
if (self.myCompletionBlock) {
self.myCompletionBlock(image);
}
}
并修改方法startDownloadImageForIndexPath，将图像保存到模型中以保留它
答案 1 :(得分：1)
此方法希望立即得到答案：
-(UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
当您的代码响应速度不够快时，该应用通常不会显示任何内容，或者有时会崩溃（取决于您设置的内容）
一种常见的设计模式是将要提供给collectionView的信息存储在类变量中（它不必是属性，但通常是时候）。您始终将SOMETHING存储在该变量中，即使它是旧的或过时的数据。
然后你有了UICollectionViewDataSource协议中定义的方法，直接从类变量中提取他们需要的东西，没有延迟。
其他方法可以获取并检索和吊索更新的数据，一旦完成，您可以在collectionView上调用reloadData:来更新界面。
假设您正在使用的异步调用最终成功检索数据，它们可能对于UICollectionViewDataSource协议方法所期望的速度来说太慢了。
关于如何开始使用的建议是将代码提取到单独的方法，然后将数据分级到一个或多个类变量中，collectionView可以从中可靠地绘制数据。
如果需要，您可以先使用加载到包中的静态数据进行尝试，然后再从Web进行异步提取。
答案 2 :(得分：0)
- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
UICollectionViewCell *cell=[collectionView dequeueReusableCellWithReuseIdentifier:@"cellIdentifier" forIndexPath:indexPath];
UIImageView *imgView=[[UIImageView alloc]initWithImage:[UIImage imageNamed:@"profile_pic.png"]];
NSMutableDictionary *contactData=[NSMutableDictionary new];
contactData = [self.collectionData objectAtIndex:indexPath.row];
imgView.image=[contactData objectForKey:@"image"];
[cell addSubview:imgView];
return cell;
}