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

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The error handling mode of the Swift 1.x has many drawbacks, for example: in order to save time in programming, passing a nil to the error parameter, or not judging whether the error is nil after the method call is complete, and not handling error.

Let contents = nsstring (contentsoffile:filepath,        êencoding:nsutf8stringencoding, Error:nil)// the error parameter passes a nil

Or

var err:nserror? = NSString (contentsoffile:filepath,        &err)    

Bad programming habits, because Objective-c and Swift 1.x do not have a mandatory processing mechanism, so once a real error occurs, the program will crash.

The same example of reading a string from a file, if using the Swift 2 error handling mode code is as follows:

Import do {        // to do some action      try  nsstring ( Contentsoffile:filepath,                             êencoding:nsutf8stringencoding)    // what to try to do   Catch// if failed, enter catch code block     Err.Description}

Do-try-catch This error pattern is very similar to the exception handling mechanism in Java, which is intended to be an attempt to do one thing and catch (catch) processing if it fails.

Catch error

The syntax for the complete Do-try-catch error handling pattern is as follows:

 Do {    trycatch  match error {    error-handling statement group}

An error can be generated in a try statement, and of course it may not produce an error, and if an error occurs, the catch handles 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 swift, the error type must conform to the ErrorType protocol, followed by the match of the error type, which 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 that there is no data condition, and Primarykeynull indicates that the table's primary key (Primary key) is empty.

Then we can catch the error with the following code.

 Do {     //try to access the data table function or method  catch  daoerror.nodata {    print (" No data. "catch  daoerror.primarykeynull {    print (" The primary key is empty.) ")}

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