dispatch_async() 与 throwables swift 2 Xcode 7
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dispatch_async() with throwables swift 2 Xcode 7
提问by justin shores
Trying to use dispatch_async in which i need a throwable call, but Swift's new error handling, and method calls are confusing me, if anyone could show me how to do this correctly, or point me in the right direction, I would greatly appreciate it.
尝试使用 dispatch_async 其中我需要一个 throwable 调用，但 Swift 的新错误处理和方法调用让我感到困惑，如果有人能告诉我如何正确地做到这一点，或者指出我正确的方向，我将不胜感激。
Code:
代码：
func focusAndExposeAtPoint(point: CGPoint) {
dispatch_async(sessionQueue) {
var device: AVCaptureDevice = self.videoDeviceInput.device
do {
try device.lockForConfiguration()
if device.focusPointOfInterestSupported && device.isFocusModeSupported(AVCaptureFocusMode.AutoFocus) {
device.focusPointOfInterest = point
device.focusMode = AVCaptureFocusMode.AutoFocus
}
if device.exposurePointOfInterestSupported && device.isExposureModeSupported(AVCaptureExposureMode.AutoExpose) {
device.exposurePointOfInterest = point
device.exposureMode = AVCaptureExposureMode.AutoExpose
}
device.unlockForConfiguration()
} catch let error as NSError {
print(error)
}
}
}
Warning:
警告：
: Invalid conversion from throwing function of type '() throws -> _' to non-throwing function type '@convention(block) () -> Void'
: 从类型 '() throws -> _' 到非抛出函数类型 '@convention(block) () -> Void' 的抛出函数的无效转换
回答by matt
FINAL EDIT: This bug is fixed in Swift 2.0 final (Xcode 7 final).
最终编辑：此错误已在 Swift 2.0 最终版（Xcode 7 最终版）中修复。
Change
改变
} catch let error as NSError {
to
到
} catch {
The effect is exactly the same — you can still print(error)— but the code will compile and you'll be on your way.
效果完全一样——你仍然可以print(error)——但代码会编译，你会继续前进。
EDITHere's why I think (as I said in a comment) that what you've found is a bug. This compiles just fine:
编辑这就是为什么我认为（正如我在评论中所说）您发现的是一个错误。这编译得很好：
func test() {
do {
throw NSError(domain: "howdy", code: 1, userInfo:nil)
} catch let error as NSError {
print(error)
}
}
The compiler does not complain, and in particular does not force you to write func test() throws— thus proving that the compiler thinks this catchis exhaustive.
编译器不会抱怨，特别是不会强迫您编写func test() throws- 从而证明编译器认为这catch是详尽无遗的。
But this does not compile:
但这不能编译：
func test() {
dispatch_async(dispatch_get_main_queue()) {
do {
throw NSError(domain: "howdy", code: 1, userInfo:nil)
} catch let error as NSError {
print(error)
}
}
}
But it's the exact same do/catchblocks! So why doesn't it compile here? I think it's because the compiler is somehow confused by the surrounding GCD block (hence all the stuff in the error message about the @convention(block)function).
但它是完全相同的do/catch块！那么为什么不在这里编译呢？我认为这是因为编译器在某种程度上被周围的 GCD 块混淆了（因此错误消息中有关该@convention(block)函数的所有内容）。
So, what I'm saying is, either they should both compile or they should both fail to compile. The fact that one does and the other doesn't is, I think, a bug in the compiler, and I have submitted a bug report on precisely that basis.
所以，我要说的是，要么它们都应该编译，要么它们都不能编译。我认为，一个有而另一个没有的事实是编译器中的一个错误，我正是在此基础上提交了一份错误报告。
EDIT 2: Here's another pair that illustrates the bug (this comes from @fqdn's comment). This does notcompile:
编辑 2：这是另一对说明错误的对（这来自@fqdn 的评论）。这并不会编译：
func test() {
dispatch_async(dispatch_get_main_queue()) {
do {
throw NSError(domain: "howdy", code: 1, userInfo:nil)
} catch is NSError {
}
}
}
But this doescompile even though it is exactly the same thing:
但这确实可以编译，即使它完全相同：
func test() {
func inner() {
do {
throw NSError(domain: "howdy", code: 1, userInfo:nil)
} catch is NSError {
}
}
dispatch_async(dispatch_get_main_queue(), inner)
}
That inconsistency is the bug.
这种不一致就是错误。
