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我正在使用NSURLSessionDataTask从我的类中的 url 检索 JSON JsonClient，这个类有一个委托协议来通知 json 任务何时完成。

json客户端.h

#import <Foundation/Foundation.h>

@protocol JsonDelegate <NSObject>
- (void)jsonFetchComplete;
@end

@interface JsonClient : NSObject

@property (weak,nonatomic) id <JsonDelegate> delegate;

- (void)fetchJsonData;

@end

json客户端.m

#import "JsonClient.h"

@implementation JsonClient

- (void)fetchJsonData {

    [[UIApplication sharedApplication] setNetworkActivityIndicatorVisible:YES];

    NSString *urlString = @"http://api.kivaws.org/v1/loans/search.json?status=fundraising";
    urlString = [urlString stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding];
    //urlString = [urlString stringByAppendingString:@".json"];

    NSURL *url = [NSURL URLWithString:urlString];
    NSURLRequest *request = [NSURLRequest requestWithURL:url cachePolicy:1 timeoutInterval:10];

    NSURLSession *session = [NSURLSession sharedSession];

    NSURLSessionDataTask *task = [session dataTaskWithRequest:request
                                            completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
                                                if (error) {

                                                    NSLog(@"error: %@",error.localizedDescription);

                                                    dispatch_sync(dispatch_get_main_queue(), ^{
                                                        [[UIApplication sharedApplication] setNetworkActivityIndicatorVisible:NO];

                                                        UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"ERROR"
                                                                                                        message:error.localizedDescription
                                                                                                       delegate:self
                                                                                              cancelButtonTitle:@"Cancel"
                                                                                              otherButtonTitles:nil];
                                                        [alert show];
                                                    });

                                                } else {
                                                    NSLog(@"got data");

                                                    [[UIApplication sharedApplication] setNetworkActivityIndicatorVisible:NO];

                                                    NSError *jsonerror = nil;

                                                    NSDictionary *dict = [NSJSONSerialization JSONObjectWithData:data options:kNilOptions error:&jsonerror];

                                                    dispatch_sync(dispatch_get_main_queue(), ^{
                                                        NSLog(@"json is \n %@",dict);
                                                        [self.delegate jsonFetchComplete];
                                                        NSLog(@"done");
                                                    });
                                                }
                                            }];
    [task resume];
}

@end

获取 json 工作正常。但是由于某种原因，委托方法没有在我的第一个视图控制器中被调用。 ViewController呈现ViewController2模态转场。一个按钮在ViewController2点击时获取 json 并返回到第一个ViewController.

视图控制器.m

#import "ViewController.h"
#import "JsonClient.h"

@interface ViewController () <JsonDelegate>
@property (weak,nonatomic) IBOutlet UILabel *label;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    self.label.text = @"Loaded VC";
}

- (void)viewWillAppear:(BOOL)animated {
    NSLog(@"view will appear");

    JsonClient *jc = [[JsonClient alloc] init];
    jc.delegate = self;
}

// delegate method from JsonClient
- (void)jsonFetchComplete {
    NSLog(@"fetch complete");
    self.label.text = @"Completed!";
}

- (IBAction)toHome:(UIStoryboardSegue *)segue {
    // unwind segue
}

@end

视图控制器2.m

#import "ViewController2.h"
#import "JsonClient.h"
#import "ViewController.h"

@interface ViewController2 ()

@end

@implementation ViewController2

- (IBAction)getJsonData:(id)sender {

    JsonClient *jsonClient = [[JsonClient alloc] init];
    [jsonClient fetchJsonData];

    [self dismissViewControllerAnimated:YES completion:nil];
}

@end

为什么ViewController没有调用第一个视图控制器中的委托方法？

4

1 回答 1

1

除非JsonClient是单例，而且您没有告诉我们，您正在创建 的两个实例JsonClient，一个在 的viewWillAppear方法中ViewController，另一个在 的getJsonData方法中ViewController2。

这两个实例分别跟踪它们的委托，而您只设置其中一个。第二个，当你点击按钮时被调用的那个ViewController2，从来没有设置它的委托，所以它默认为nil，并且发送的消息nil什么都不做。

你需要类似的东西：

@implementation ViewController2

- (IBAction)getJsonData:(id)sender {

    JsonClient *jsonClient = [[JsonClient alloc] init];
    jsonClient.delegate = otherViewController;
    [jsonClient fetchJsonData];

    [self dismissViewControllerAnimated:YES completion:nil];
}

@end
于 2013-11-17T22:01:09.497 回答