iOS启动后台线程 [英] iOS start Background Thread

问题描述

我的iOS设备上有一个小的sqlitedb。当用户按下按钮时，我从sqlite&获取数据。向用户显示。

I have a small sqlitedb in my iOS device. When a user presses a button, I fetch the data from sqlite & show it to user.

这个抓取部分我想在后台线程中执行它（不阻止UI主线程）。我是这样做的 -

This fetching part I want to do it in a background thread (to not block the UI main thread). I do this like so -

[self performSelectorInBackground：@selector（getResultSetFromDB :) withObject：docids];

提取后&一点点处理，我需要更新UI。但是因为（作为一种好的做法）我们不应该从后台线程执行UI更新。我在mainthread上调用选择器，如此 -

After the fetching & a little bit of processing, I need to update the UI. But since (as a good practice) we should not perform UI updation from background threads. I call a selector on mainthread like so -

[self performSelectorOnMainThread：@selector（ showResults）withObject：nil waitUntilDone：NO];

但我的应用程序在第一步崩溃了。即启动后台线程。这不是在iOS中启动后台线程的方法吗？

But my App crashes in the first step. i.e. starting a background thread. Is this not a way to start background threads in iOS?

UPDATE 1： 之后[self performSelectorInBackground .... 我得到这个堆栈跟踪，没有信息是什么 -

UPDATE 1: After [self performSelectorInBackground.... I get this stacktrace, no info what so ever -

更新2：我甚至试过，开始这样的后台线程 -
[NSThread detachNewThreadSelector：@selector（getResultSetFromDB :) toTarget：self withObject：docids]; 但我仍然得到相同的堆栈跟踪。

UPDATE 2: I even tried, starting a background thread like so - [NSThread detachNewThreadSelector:@selector(getResultSetFromDB:) toTarget:self withObject:docids]; but still I get same stacktrace.

这样我就澄清了，当我在主线程上执行此操作时，一切运行顺利...

Just so that I clarify, when I perform this operation on main thread everything runs smooth...

更新3 这是我尝试从后台运行的方法

UPDATE 3 This is the method I am trying to run from background

- (void)getResultSetFromDB:(NSMutableArray *)toProceessDocids
{
    SpotMain *mirror = [[SpotMain alloc] init];
    NSMutableArray *filteredDocids = toProceessDocids;

    if(![gMediaBucket isEqualToString:@""])
        filteredDocids = [mirror FetchDocIdsForMediaBucketWithDocID:filteredDocids mBucket:gMediaBucket numRes:-1];
    if(![gMediaType isEqualToString:@""])
        filteredDocids = [mirror FetchDocIdsForMediaType:filteredDocids mediaType:gMediaType numRes:-1];
    if(![gPlatform isEqualToString:@""])
        filteredDocids = [mirror FetchDocIdsForPlatformID:filteredDocids platformId:@"1" numRes:-1];

    self.resultSet = [mirror FetchObjectFromDocid:filteredDocids];
    [filteredDocids release];
    [mirror release];

    [self performSelectorOnMainThread:@selector(showResults) withObject:nil waitUntilDone:NO];
    return;
}

推荐答案

如果您使用 performSelectorInBackground：withObject：要生成一个新线程，然后执行的选择器负责设置新线程的自动释放池，运行循环和其他配置细节 - 请参阅使用NSObject在Apple的线程编程指南中生成线程。

If you use performSelectorInBackground:withObject: to spawn a new thread, then the performed selector is responsible for setting up the new thread's autorelease pool, run loop and other configuration details – see "Using NSObject to Spawn a Thread" in Apple's Threading Programming Guide.

使用 Grand Central Dispatch ，但是：

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    [self getResultSetFromDB:docids];
});

GCD是一种较新的技术，在内存开销和代码行方面效率更高。

GCD is a newer technology, and is more efficient in terms of memory overhead and lines of code.

已更新，提示 Chris Nolet ，他提出了一个更改，使上述代码更简单，并与Apple最新的GCD代码示例保持一致。

Updated with a hat tip to Chris Nolet, who suggested a change that makes the above code simpler and keeps up with Apple's latest GCD code examples.

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