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[英]dispatch_async(dispatch_get_main_queue() is not working on iPad, but working on iPhone
[英]Why does GCD dispatch_async on main queue causes a deadlock from background queue?
我正在创建一个这样的串行后台队列:
@property (nonatomic, strong) dispatch_queue_t assetCreationQueue;
// in init...
_assetCreationQueue = dispatch_queue_create("com.mycompany.assetCreationQueue", DISPATCH_QUEUE_SERIAL);
然后我在后台枚举ALAsset对象,如下所示:
[group enumerateAssetsUsingBlock:^(ALAsset *asset, NSUInteger index, BOOL *stop){
if (asset){
dispatch_async(weakSelf.assetCreationQueue, ^{
ALAssetRepresentation *assetRepresentation = [asset defaultRepresentation];
NSURL *url = [assetRepresentation url];
if (url) {
AVURLAsset *avAsset = [[AVURLAsset alloc] initWithURL:url options:nil];
// This NSLog fires! avAsset exists.
NSLog(@"AVURLAsset %@", avAsset);
dispatch_async(dispatch_get_main_queue(), ^{
// This NSLog NEVER fires.
// Also tried dispatch_sync.
NSLog(@"add to assets array on main queue");
[weakSelf.assets insertObject:avAsset atIndex:0];
});
}
});
}
}];
assets数组属性定义为:
@property (nonatomic, strong) NSMutableArray *assets;
当我尝试dispatch_sync(dispatch_get_main_queue(), ^{
我只获得一个NSLog(@"AVURLAsset %@", avAsset);
在控制台中它表示dispatch_sync
导致死锁。
但是我怎么能找出原因呢? 我看不到哪里。 assetCreationQueue是后台队列,我必须仅在主队列上对阵列进行操作。
编辑:这是一个更简化的测试,也失败了:
[group enumerateAssetsUsingBlock:^(ALAsset *asset, NSUInteger index, BOOL *stop){
if (asset){
dispatch_async(weakSelf.assetCreationQueue, ^{
if ([NSThread isMainThread]) {
NSLog(@"already main thread"); // gets called often!!
} else {
dispatch_async(dispatch_get_main_queue(), ^{
NSLog(@"dispatch async on main queue"); // never gets called!!
});
}
});
}
}];
所以我不明白的是:为什么我已经在主线程上,即使我调用dispatch_async(weakSelf.assetCreationQueue
。它只会导致邪恶的结论:我创建的队列不是后台队列:
_assetCreationQueue = dispatch_queue_create("com.mycompany.assetCreationQueue", DISPATCH_QUEUE_SERIAL);
为什么?
(不是一个真正的答案,但我想提供足够的信息来推进。)
我尝试过一个非常简单的程序,这根本不会重现。 我接到许多调用“在主队列上调度异步”并且没有调用“已经主线程”(与你期望的相反)。 我在iPhone 5上运行它。完整的代码:
#import "AppDelegate.h"
@import AssetsLibrary;
@interface AppDelegate ()
@property (nonatomic, strong) dispatch_queue_t assetCreationQueue;
@end
@implementation AppDelegate
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
self.assetCreationQueue = dispatch_queue_create("com.mycompany.assetCreationQueue", DISPATCH_QUEUE_SERIAL);
ALAssetsLibrary* library = [[ALAssetsLibrary alloc] init];
AppDelegate __weak *weakSelf = self;
[library enumerateGroupsWithTypes:ALAssetsGroupAll
usingBlock:^(ALAssetsGroup *group, BOOL *stop) {
[group enumerateAssetsUsingBlock:^(ALAsset *asset, NSUInteger index, BOOL *stop){
if (asset){
dispatch_async(weakSelf.assetCreationQueue, ^{
if ([NSThread isMainThread]) {
NSLog(@"already main thread"); // gets called often!!
} else {
dispatch_async(dispatch_get_main_queue(), ^{
NSLog(@"dispatch async on main queue"); // never gets called!!
});
}
});
}
}];
}
failureBlock:nil];
return YES;
}
@end
请注意,在这里使用weakSelf
是不必要和危险的(但如果这是问题,你就会崩溃)。 如果weakSelf
块存储在self
保留的对象的属性中,则只需要weakSelf
。 对于像这样的短寿命块,没有必要或甚至不需要。 你使用它的方式,如果拥有对象在枚举的中间释放(它可以), weakSelf
将变为nil, weakSelf.assetCreationQueue
将为nil
, dispatch_async()
将崩溃。 (但同样,这不太可能是你的问题的原因,因为它应该崩溃。)
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