Objective-C study note _ Block

Objective-C study note _ Block
I. Block syntax

Block: Block syntax. It is essentially an anonymous function (a function without a name). Block variables are used to store functions. can Block variables be directly called? Function. There is no Block in Standard C, and the C language will be extended later, plus? Anonymous function. C ++, JS, Swift, etc ?, There are similar syntaxes called closures. The Block syntax is similar to the function pointer.

Block Type: int (^) (int) Block Variable: myBlock Block Value: ^ int (int num) {return 7 * num;} That is: ^ return value type (parameter list) {function body} the return value type can be omitted.

/ * Block type: int (^) (int, int)
          * Block variable: block1
          * Block value: ^ int (int x, int y) {return x + y;};
            ^ Return value type (parameter list) {function body} (return value type can be omitted)
          * /

         / * 1. Block without return value, without parameters * /
         void (^ block3) () = ^ ()
         {
             NSLog (@Hello, World!);
         };
         block3 (); / * Block call * /

         / * 2. No return value, with parameters * /
         void (^ block4) (int, int) = ^ (int x, int y)
         {
             NSLog (@% d, x + y);
         };
         block4 (3, 45);

         / * 3. With return value, no parameters * /
         int (^ block5) () = ^ ()
         {
             return 100;
         };
         NSLog (@% d, block5 ());

         / * 4. With return value, with parameters * /
         int (^ block6) (int, int) = ^ (int x, int y)
         {
             return x> y? x: y;
         };
         NSLog (@% d, block6 (3, 5));

Ii. Block?

int (^ block1) (int x, int y) = ^ (int x, int y) {
   return x + y;
};
int a = block1 (32, 34); // The use of block is similar to the use of function pointers
NSLog (@ “% d”, a); // print result: 66

Block? Typedef

Typedef int (^ BlockType) (int x, int y)

Original Type: int (^) (int x, int y)

New Type: BlockType

/* Block typedef */
typedef int(^blockType)(int, int);

blockType block1 = ^(int x, int y) {
            return x + y;
        };
NSLog(@%d, block1(3, 5));

Block writing

int (^block1)(int x, int y) = ^(int x, int y) {
  return x + y;
};
BlockType block1 = ^(int x, int y) {
  return x + y;
};

The above two implementations are equivalent.

Blcok and local variables and global variables

/ * Global variables defined outside the main function * /
         int n = 100;

         / * Knowledge point 4 Block and local variables Global variables * /
         / * Local variables * /
         int a = 100;

         int (^ block) () = ^ () {
             return a;
         };
         NSLog (@% d, block ());

         int (^ block2) () = ^ () {
// a = 200;
             return a;
         };
         / * Summary: By default, blocks can access but cannot change local variables * /

         __block int b = 200;
         int (^ block3) () = ^ () {
             b = 300;
             return b;
         };
         NSLog (@% d, block3 ());
         / * Summary: Local variables decorated with __block, the value can be changed in Block * /


         / * Block and global variables * /

         int (^ block4) () = ^ () {
             n = 200;
             return n;
         };
         NSLog (@n =% d, block4 ());

         static int num = 1;
         int (^ block5) () = ^ () {
             num = 8;
             return num;
         };
         NSLog (@num =% d, block5 ());