iphone - 调用 NSURLConnectionDataDelegate 方法来下载图像只是偶尔有效

Question

我的 Storyboard目前有两个 UIImageView ，其中一个下载我自己的 Facebook 个人资料图片，另一个下载 friend 的个人资料图片。



但是，我的问题是 仅 60% 的时间按预期工作 ，而其他 40% 的时间我自己的个人资料图片出现在我 friend 的图片应该显示在底部的位置，而顶部的框仍然是空的。我不确定这是否是我在下载或完成 View 时调用 NSURLConnectionDataDelegate 方法的结果，还是我调用 Facebook 的请求的性质。

我在 viewDidLoad 中将我的两个请求的精简版粘贴到 Facebook，一个获取我自己的个人资料照片，另一个获取我 friend 的照片:

// ----- Gets my own profile picture, using requestForMe -----
FBRequest *request = [FBRequest requestForMe];
[request startWithCompletionHandler:^(FBRequestConnection *connection, id result, NSError *error) {
    //handle response
    if(!error){
        //Some methods not included for breveity, includes facebookID used below
        NSURL *pictureURL = [NSURL URLWithString:[NSString stringWithFormat:@"https://graph.facebook.com/%@/picture?type=large&return_ssl_resources=1", facebookID]];
        self.imageData = [[NSMutableData alloc] init];
        switcher = 1;
        NSMutableURLRequest *urlRequest = [NSMutableURLRequest requestWithURL:pictureURL cachePolicy:NSURLRequestUseProtocolCachePolicy timeoutInterval:2.0f];
        NSURLConnection *urlConnection = [[NSURLConnection alloc] initWithRequest:urlRequest delegate:self];
        if (!urlConnection){
            NSLog(@"Failed to download picture");
        }
    }
}];
// ----- Gets a profile picture of my friend, using requestForMyFriends -----
FBRequest *requestForFriends = [FBRequest requestForMyFriends]; 
[requestForFriends startWithCompletionHandler:^(FBRequestConnection *connection, id result, NSError *error) {
    if(!error){
        //Other methods not included, including facebookFriendID
        NSURL *friendPictureURL = [NSURL URLWithString:[NSString stringWithFormat:@"https://graph.facebook.com/%@/picture?type=large&return_ssl_resources=1", facebookFriendID]];
        self.supportImageData = [[NSMutableData alloc] init];
        switcher = 2;
        NSMutableURLRequest *urlRequest = [NSMutableURLRequest requestWithURL:friendPictureURL cachePolicy:NSURLRequestUseProtocolCachePolicy timeoutInterval:2.0f];
        NSURLConnection *urlConnection = [[NSURLConnection alloc] initWithRequest:urlRequest delegate:self];
        if (!urlConnection){
            NSLog(@"Failed to download picture");
        }
    }
}];

这两个请求都调用了 NSURLConnectionDataDelegate 方法，我使用 switcher 来决定何时加载哪张图片:

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {
    // As chuncks of the image are received, we build our data file

    if (switcher == 1) [self.imageData appendData:data];
    if (switcher == 2)[self.supportImageData appendData:data];
}

- (void)connectionDidFinishLoading:(NSURLConnection *)connection
{

    //When the entire image is finished downloading
    if (switcher == 1) {
        UIImage *image = [UIImage imageWithData:self.imageData]; //puts the completed picture into the UI
        self.titleImageView.image = image;
        [self.titleImageView setClipsToBounds:YES];
    }

    if (switcher == 2) {
        UIImage *supportImage = [UIImage imageWithData:self.supportImageData];
        self.supportImageView.image = supportImage;
        [self.titleImageView setClipsToBounds:YES];
    }
}

Answer 1

您有两个异步进程，这两个进程都可能导致您的 NSURLConnectionDataDelegate self 中的方法被调用。但如果它们同时发生，它们就会相互叠加(您可能正在使用单个 NSMutableData 变量来引用正在下载的数据)。

要么创建专用类，您可以为两个 NSURLConnection 中的每一个实例化一次。请求(基于 NSOperation 的方法，如 AFNetworking 是理想的)，或使用 sendAsynchronousRequest 反而。但不要使用单个对象作为 delegate两个并发NSURLConnection同时提出要求。

如果你想看一个极简的下载操作，它可能看起来像:

//  NetworkOperation.h

#import <Foundation/Foundation.h>

typedef void(^DownloadCompletion)(NSData *data, NSError *error);

@interface NetworkOperation : NSOperation

- (id)initWithURL:(NSURL *)url completion:(DownloadCompletion)completionBlock;

@property (nonatomic, copy) NSURL *url;
@property (nonatomic, copy) DownloadCompletion downloadCompletionBlock;

@end

和

//  NetworkOperation.m

#import "NetworkOperation.h"

@interface NetworkOperation () <NSURLConnectionDataDelegate>

@property (nonatomic, readwrite, getter = isExecuting) BOOL executing;
@property (nonatomic, readwrite, getter = isFinished)  BOOL finished;
@property (nonatomic, strong) NSMutableData *data;

@end

@implementation NetworkOperation

@synthesize finished  = _finished;
@synthesize executing = _executing;

- (id)initWithURL:(NSURL *)url completion:(DownloadCompletion)downloadCompletionBlock
{
    self = [super init];
    if (self) {
        self.url = url;
        self.downloadCompletionBlock = downloadCompletionBlock;

        _executing = NO;
        _finished = NO;
    }
    return self;
}

#pragma mark - NSOperation related stuff

- (void)start
{
    if ([self isCancelled]) {
        self.finished = YES;
        return;
    }

    self.executing = YES;

    NSURLRequest *request = [NSURLRequest requestWithURL:self.url];
    NSAssert(request, @"%s: requestWithURL failed for URL '%@'", __FUNCTION__, [self.url absoluteString]);
    NSURLConnection *connection = [[NSURLConnection alloc] initWithRequest:request delegate:self startImmediately:NO];
    [connection scheduleInRunLoop:[NSRunLoop mainRunLoop] forMode:NSDefaultRunLoopMode];
    [connection start];
}

- (void)setExecuting:(BOOL)executing
{
    [self willChangeValueForKey:@"isExecuting"];
    _executing = executing;
    [self didChangeValueForKey:@"isExecuting"];
}

- (void)setFinished:(BOOL)finished
{
    [self willChangeValueForKey:@"isFinished"];
    _finished = finished;
    [self didChangeValueForKey:@"isFinished"];
}

- (BOOL)isConcurrent
{
    return YES;
}

#pragma mark NSURLConnectionDataDelegate methods

- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response
{
    self.data = [NSMutableData data];
}

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data
{
    if ([self isCancelled]) {
        [connection cancel];
        self.executing = NO;
        self.finished = YES;
        return;
    }

    [self.data appendData:data];
}

- (void)connectionDidFinishLoading:(NSURLConnection *)connection
{
    if (self.downloadCompletionBlock) {
        [[NSOperationQueue mainQueue] addOperationWithBlock:^{
            self.downloadCompletionBlock(self.data, nil);
            self.downloadCompletionBlock = nil;
        }];
    }

    self.executing = NO;
    self.finished = YES;
}

- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error
{
    if (self.downloadCompletionBlock) {
        [[NSOperationQueue mainQueue] addOperationWithBlock:^{
            self.downloadCompletionBlock(nil, error);
            self.downloadCompletionBlock = nil;
        }];
    }

    self.executing = NO;
    self.finished = YES;
}

@end

然后，当你想使用它时，它可能看起来像:

NSOperationQueue *networkQueue = [[NSOperationQueue alloc] init];
queue.maxConcurrentOperationCount = 4;

// ----- Gets my own profile picture, using requestForMe -----
FBRequest *request = [FBRequest requestForMe];
[request startWithCompletionHandler:^(FBRequestConnection *connection, id result, NSError *error) {
    //handle response
    if(!error) {
        //Some methods not included for breveity, includes facebookID used below
        NSURL *pictureURL = [NSURL URLWithString:[NSString stringWithFormat:@"https://graph.facebook.com/%@/picture?type=large&return_ssl_resources=1", facebookID]];

        [networkQueue addOperation:[[NetworkOperation alloc] requestWithURL:pictureURL completion:^(NSData *data, NSError *error) {
            if (!error) {
                self.meImageView.image = [UIImage imageWithData:data];
            }
        }]];
    }
}];
// ----- Gets a profile picture of my friend, using requestForMyFriends -----
FBRequest *requestForFriends = [FBRequest requestForMyFriends]; 
[requestForFriends startWithCompletionHandler:^(FBRequestConnection *connection, id result, NSError *error) {
    if(!error){
        //Other methods not included, including facebookFriendID
        NSURL *friendPictureURL = [NSURL URLWithString:[NSString stringWithFormat:@"https://graph.facebook.com/%@/picture?type=large&return_ssl_resources=1", facebookFriendID]];

        [networkQueue addOperation:[[NetworkOperation alloc] requestWithURL:friendPictureURL completion:^(NSData *data, NSError *error) {
            if (!error) {
                self.friendImageView.image = [UIImage imageWithData:data];
            }
        }]];
    }
}];