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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?
我正在創建一個這樣的串行后台隊列:
@property (nonatomic, strong) dispatch_queue_t assetCreationQueue;
// in init...
_assetCreationQueue = dispatch_queue_create("com.mycompany.assetCreationQueue", DISPATCH_QUEUE_SERIAL);
然后我在后台枚舉ALAsset對象,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數組屬性定義為:
@property (nonatomic, strong) NSMutableArray *assets;
當我嘗試dispatch_sync(dispatch_get_main_queue(), ^{
我只獲得一個NSLog(@"AVURLAsset %@", avAsset);
在控制台中它表示dispatch_sync
導致死鎖。
但是我怎么能找出原因呢? 我看不到哪里。 assetCreationQueue是后台隊列,我必須僅在主隊列上對陣列進行操作。
編輯:這是一個更簡化的測試,也失敗了:
[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:為什么我已經在主線程上,即使我調用dispatch_async(weakSelf.assetCreationQueue
。它只會導致邪惡的結論:我創建的隊列不是后台隊列:
_assetCreationQueue = dispatch_queue_create("com.mycompany.assetCreationQueue", DISPATCH_QUEUE_SERIAL);
為什么?
(不是一個真正的答案,但我想提供足夠的信息來推進。)
我嘗試過一個非常簡單的程序,這根本不會重現。 我接到許多調用“在主隊列上調度異步”並且沒有調用“已經主線程”(與你期望的相反)。 我在iPhone 5上運行它。完整的代碼:
#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
請注意,在這里使用weakSelf
是不必要和危險的(但如果這是問題,你就會崩潰)。 如果weakSelf
塊存儲在self
保留的對象的屬性中,則只需要weakSelf
。 對於像這樣的短壽命塊,沒有必要或甚至不需要。 你使用它的方式,如果擁有對象在枚舉的中間釋放(它可以), weakSelf
將變為nil, weakSelf.assetCreationQueue
將為nil
, dispatch_async()
將崩潰。 (但同樣,這不太可能是你的問題的原因,因為它應該崩潰。)
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