Learn Swift from scratch Learning Note (day)--do-try-catch error handling mode

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Swift 1.x error-handling mode, for example: in order to save time in programming, give Error parameter to pass a Nil , or when the method call is complete, do not judge Error whether to Nil , no error handling is performed.

Let contents =nsstring (Contentsoffile:filepath, encoding:nsutf8stringencoding, Error:nil)//error parameters pass a nil or Var    Err:nserror? Let contents =nsstring (Contentsoffile:filepath, encoding:nsutf8stringencoding, error: &err)

bad programming habits, because objective-c and the Swift 1.x There is no enforcement mechanism, so the program crashes once a real error occurs.

the same example of reading a string from a file, if you use the Swift2 The error handling mode code is as follows:

Import Foundation do {//To do some operations let str = try NSString (Contentsoffile:filepath, ENC oding:nsutf8stringencoding)//What to try to do} catch let err as nserror{//if failed enter catch code block Err.Description}

Do-try-catch This error mode is associated with Java the exception handling mechanism is very similar, the intention is to try ( Try ) To do one thing, if the failure is captured ( Catch ) processing.  

Catch error

Full- Do-try-catch the syntax for error handling mode is as follows:

Do {Try statement successfully processed statement group} catch match error {error handling statement group}

in the Try The statement can produce an error, of course, it may not produce an error, if an error occurs, Catch will handle the error. A catch code block can have multiple, and the error is determined by which catch code block is handled by the error after the catch . The number of error types determines How many catch can be. Let's introduce the error type first.  

Type of error

in the Swift the type of error must follow ErrorType protocol, followed by the matching of the error type, it should be designed as an enumeration type, which is ideal for associating a set of related values.

If we write access to the database table program and implement operations such as inserting, deleting, modifying, and querying the table data, we will need an error type similar to the following code:

Enum Daoerror:errortype {case NoData case Primarykeynull}

NoData indicates no data condition, Primarykeynull represents the primary key of the table ( Primary Key ) is an empty case.

Then we can catch the error with the following code.

Do {//try accesses a data table function or method} catch Daoerror.nodata {print ("no data. ")} catchdaoerror.primarykeynull {print (" PRIMARY key is empty. ")}

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Learn Swift from scratch Learning Note (day)--do-try-catch error handling mode