新建一个playground，将代码全部替换为下面演示代码
import UIKit
enum ero : Error{
case essEro
case essEro2
}
func alwaysThrows() throws {
throw ero.essEro
}
func someFunction(callback: ()throws->Void, callback2: ()throws->Void) rethrows{
do{
try callback2()
try callback()
// try alwaysThrows() // Invalid, alwaysThrows() isn't a throwing parameter
}catch{
// print("2")
throw ero.essEro
}
}
do{
try someFunction(callback: {
print("\(ero.essEro)+\(#line)")
throw ero.essEro
}, callback2: {
print("\(ero.essEro2)+\(#line)")
throw ero.essEro2
})
}catch(ero.essEro) {
print("\(ero.essEro)+\(#line)")
}
共同点：A rethrowing function or method can contain a throw statement only inside a catch clause.trows function is also too.
以上代码可以正常运行。打印信息如下
essEro2+45
essEro+53
但是以下try callback()块中的语句并没有执行，因为trycallback2()块中的代码print("\(ero.essEro2)+\(#line)")
throw ero.essEro2中有throw关键词，具有与return相当的功能，trycallback2()后的代码并不会执行。将rehtrows改为throws效果一样，所以是共同点。
如果想调用另一个throws函数，可以在catch语句中调用：This lets you call the throwing function inside a do-catch block and handle errors in the catch clause by throwing a different error. 即将改成如下代码：
func someFunction(callback: ()throws->Void, callback2: ()throws->Void) rethrows{
do{
try callback2()
// try alwaysThrows() // Invalid, alwaysThrows() isn't a throwing parameter
}catch{
// print("2")
try callback()
throw ero.essEro
}
}
打印信息变为：
essEro2+45
essEro+39
essEro+53
区别一： the catch clause must handle only errors thrown by one of the rethrowing function’s throwing parameters.
若将以下语句注释打开，编译器会提示：A function declared 'rethrows' may only throw if its parameter does，因为函数alwaysThrows（）不是参数，rethrows限制了do语句中throws的代码只能是参数中带throws的块
//try alwaysThrows() // Invalid, alwaysThrows() isn't a throwing parameter
如果将
func someFunction(callback: ()throws->Void, callback2: ()throws->Void) rethrows
改为
func someFunction(callback: ()throws->Void, callback2: ()throws->Void) throws
就不会报错了，但还是不会执行alwaysThrows() 函数，因为其在trycallback2()块代码之后，原因即共同点.
区别二：A throwing method can’t override a rethrowing method, and a throwing method can’t satisfy a protocol requirement for a rethrowing method. That said, a rethrowing method can override a throwing method, and a rethrowing method can satisfy a protocol requirement for a throwing method.
我的理解是类似于rethrowing是throwing的子类，throwing方法不能override rethrowing方法，但是rethrowing方法可以override throwing方法，协议中throwing方法不可以满足rethrowing方法的需求，反之则可以，猜测rethrowing方法的限制比throwing方法的更多
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