Foreword

In the project development, I often check the iOS closure how to write, because its syntax is too weird, the two languages ​​are written differently, often confusing, simply record the commonly used writing method.

Closure definition

A closure is a block of code that can contain free (unbound to a specific object) variable; these variables are not defined within this code block or in any global context, but are defined in the context in which the code block is defined (local variables) . The term "closure" comes from a combination of two: the block of code to execute (since free variables are included in the code block, these free variables and the objects they reference are not released) and provide bindings for free variables. Computing environment (scope).

Declare a closure

typedef NSString *(^blockCallback)(NSString *parameter1, NSString *parameter2);

//callback closure for evaluation after success
    typealias callbackFunc = (_ orderId:String ) -> Void
    var evaluationBlockCallback:callbackFunc?

Define a closure

>First way Use statement
@property (nonatomic,strong) blockCallbackType blockCallbackName;

_blockCallbackName = ^(NSString *parameter1,NSString *parameter2){
        return parameter1;
    };
 >The second way does not use the statement
@property (nonatomic,strong) NSString *(^blockCallbackName2)(NSString *parameter1, NSString *parameter2);

_blockCallbackName2 = ^(NSString *parameter1,NSString *parameter2){
        return parameter1;
    };

let blockCallbackName:String = {(parameter1:String,parameter2:String) ->String in
            return parameter1
        }("parameter1","parameter2")

Function definition closure

-(void)functionUseBlock:(NSString* (^)(NSString *parameter1, NSString *parameter2))callBackOne callBackTwo:(blockCallbackType)callBackTwo{
    if (callBackOne) {
        callBackOne(@"parameter1",@"parameter2");
    }
    if (callBackTwo) {
        callBackTwo(@"parameter1",@"parameter2");
    }
}

func functionUseBlock(callBack:(String,String) ->String) -> Void  {
        print(callBack("parameter1","parameter2"))
    }

Call function closure (block)

[self functionUseBlock:^NSString *(NSString *parameter1, NSString *parameter2) {
        return parameter1;
    } callBackTwo:^NSString *(NSString *parameter1, NSString *parameter2) {
        return parameter1;
    }];

self.functionUseBlock = { (parameter1, parameter2) -> String in
            return parameter2
        }

end

The Swift version has a lot of abbreviations based on parameters and return values, so I won't write them here.