Cocoa-Touch：performSelectorOnMainThread：怪异的行为+崩溃

问题描述

我有一种情况，我懒加载图像从www。

这是一个项目列表，当一个项目被点击，一个详细视图被推送到导航控制器。

I have a situation where I'm lazy loading images from the www.
It's a list of items, when one item is tapped, a detail view is pushed to a nav controller.

在该详细视图中，该项目有一个图片，首先是默认图片，我想从网址开始加载图片。

In that detail view the item has an image, which first is a default image, and I want to start loading it's image from a URL.

所以我做的是创建一个对象，一旦初始化分离一个新的线程，反过来加载内容，然后通知我的观点，数据加载：

So what I do is create an object which once initialized detaches a new thread which in turn loads the content and afterwards notifies my view that the data is loaded:

// in MyLoader:
- (MyLoader *)initWithUrl:(NSURL *)url requester:(id)requester {
    self.url = url;
    self.requester = requester; // both are nonatomic, retain properties
    [self performSelectorInBackground:@selector(loadIt) withObject:nil];
}

- (void)loadIt {
    NSAutoreleasePool *arp = [[NSAutoreleasePool alloc] init];
    NSData *data = [NSData dataWithContentsOfURL:url];
    [requester performSelectorOnMainThread:@selector(dataReady) withObject:data waitUntilDone:YES;
    [arp release];
}

// in MyRequester:
- (void)somewhere {
    MyLoader *loader = [[[MyLoader] alloc] initWithUrl:someUrl requester:self] autorelease];
    // then I retain loader somewhere, it's more complicated but I have verified that it's properly retained.
}

几个备注：

1. 首先我认为一些变量可能有问题。我在 performSelectorOnMainThread 之前放置一个断点，并确认数据和请求者

1. First I thought there might be a problem with some of the variables. I put a breakpoint right before performSelectorOnMainThread and confirmed that data and requester were both OK.

然后我认为这是由于传递NSData跨线程造成的，所以我改变了 withObject：nil 。它仍然崩溃。

Then I thought it was caused by passing the NSData across the threads, so I changed withObject:nil. It still crashes.

当我进一步调查，崩溃是非常奇怪。我指定 waitUntilDone：YES ，我在请求者的 dataReady 。但是 performSelectorOnMainThread 调用返回（它到达之后的断点），而不到达 dataReady 中的断点。 BTW， - （void）dataReady：（NSData *）的主体现在只包含 int x = 1; （要在其上放置断点）。此外，我尝试设置 waitUntilDone：NO ，它仍然崩溃。

When I further investigated, the crash was very strange. I specified waitUntilDone:YES, I've placed a breakpoint in the requester's dataReady. But the performSelectorOnMainThread call returns (it reaches the breakpoint after it) while not reaching the breakpoint inside dataReady. BTW, - (void)dataReady:(NSData*)'s body for now only contains int x = 1; (to place a breakpoint on). Also, I've tried setting waitUntilDone:NO, it still crashes.

没有人执行（没有达到断点）有什么问题吗？

Does anyone have any idea what's wrong?

这很明显，只是为了清楚，如果我只是注释 [requester performSelectorOnMainThread ... 部分，它不会崩溃。

This is obvious, but just to be clear, if I just comment out the [requester performSelectorOnMainThread... part, it doesn't crash.

此外，这里是一个堆栈跟踪，但它没有任何帮助。

Also, here's a stack trace, but it's not helpful at all.

#0  0x00a71004 in ___TERMINATING_DUE_TO_UNCAUGHT_EXCEPTION___ ()
#1  0x93436e3b in objc_exception_throw ()
#2  0x0028aca6 in __NSThreadPerformPerform ()
#3  0x00a098e1 in CFRunLoopRunSpecific ()
#4  0x00a08c48 in CFRunLoopRunInMode ()
#5  0x0005a78d in GSEventRunModal ()
#6  0x0005a852 in GSEventRun ()
#7  0x0168a003 in UIApplicationMain ()
#8  0x000028d4 in main (argc=1, argv=0xbffff100) at /Users/myName/Document/appName/main.m:14




推荐答案

您拥有：

[requester performSelectorOnMainThread:@selector(dataReady) withObject:data waitUntilDone:YES;

应为：

[requester performSelectorOnMainThread:@selector(dataReady:) withObject:data waitUntilDone:YES;

注意： @selector（dataReady :)（带冒号）
因为你向方法传递一个参数，所以假定数据准备就是这样定义的：

notice: @selector(dataReady:) (with colon)Since you're passing an argument to the method, it's presumed data ready is defined something like:

- (void) dataReady:(NSData *)theData ...

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