使用NSURLConnection時圖像空白但使用dataWithContentsOfURL時不使用

Question

當我異步使用NSURLConnection嘗試獲取帶有圖像數據的NSData時，圖像將返回爲空白，但當我同步使用dataWithContentsOfURL時，我沒有問題，圖像數據正確。是否有任何理由爲什麼我的異步方法會失敗？

這工作：

NSData *data = [NSData dataWithContentsOfURL: url]; 
NSLog(@"TEST %@", data); 
UIImage *map = [UIImage imageWithData:data]; 
mapView.image = map; 

這並不：

// 
// MapHttpRequest.m 
// GTWeb 
// 
// Created by Graphic Technologies on 6/21/11. 
// Copyright 2011 __MyCompanyName__. All rights reserved. 
// 

#import "MapHttpRequest.h" 

@implementation MapHttpRequest 
@synthesize receivedData; 
@synthesize dataString; 
@synthesize vc; 

- (void)request:(NSString *)url fromView:(UIViewController *) theVC 
{ 
vc = theVC; 
// Create the request. 
NSURLRequest *theRequest=[NSURLRequest requestWithURL:[NSURL URLWithString:url] 
              cachePolicy:NSURLRequestUseProtocolCachePolicy 
             timeoutInterval:60.0]; 
NSLog(@"URL: %@", url); 
// create the connection with the request 
// and start loading the data 
NSURLConnection *theConnection=[[NSURLConnection alloc] initWithRequest:theRequest delegate:self]; 
if (theConnection) { 
    // Create the NSMutableData to hold the received data. 
    // receivedData is an instance variable declared elsewhere. 
    receivedData = [[NSMutableData data] retain]; 
} else { 
    // Inform the user that the connection failed. 
} 
} 

- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse    *)response 
{ 
// This method is called when the server has determined that it 
// has enough information to create the NSURLResponse. 

// It can be called multiple times, for example in the case of a 
// redirect, so each time we reset the data. 

// receivedData is an instance variable declared elsewhere. 
[receivedData setLength:0]; 
} 

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data 
{ 
// Append the new data to receivedData. 
// receivedData is an instance variable declared elsewhere. 
[receivedData appendData:data]; 
} 

- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error 
{ 
// release the connection, and the data object 
[connection release]; 
// receivedData is declared as a method instance elsewhere 
[receivedData release]; 

// inform the user 
NSLog(@"Connection failed! Error - %@ %@", 
     [error localizedDescription], 
     [[error userInfo] objectForKey:NSURLErrorFailingURLStringErrorKey]); 
} 

- (void)connectionDidFinishLoading:(NSURLConnection *)connection 
{ 
// do something with the data 
// receivedData is declared as a method instance elsewhere 

[vc mapImageConnectionFinished:receivedData]; 

// release the connection, and the data object 
[dataString release]; 
[connection release]; 
[receivedData release]; 
} 

@end 

Answer 1

原來，這是給我一個不好的要求，因爲有一個不可見的回車或空白區域。修剪它：

url = [url stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]]; 

解決了我的問題。

Answer 2

它看起來像你開始尋找在引導上http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/URLLoadingSystem/Tasks/UsingNSURLConnection.html但沒有讀完吧:)

您需要實現將收到有關正在接收的數據信息的方法。

- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response 
- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data 

它的所有描述在我提供的鏈接中。

Answer 3

根據你的代碼，你沒有安排連接來運行：

[connection scheduleInRunLoop:[NSRunLoop currentRunLoop] forMode:NSRunLoopCommonModes]; 
[connection start]; 

你把你委託的回調的痕跡，以確保他們被稱爲？