Introduction to block in iOS

I. Overview

Block is a C-level syntax and runtime feature. Block is similar to the C function, but the block is more flexible than the C function, which is reflected in the stack memory, heap memory reference, and we can even pass a block as a parameter to other functions or block.

Second, warm up

Let's look at a relatively simple block example:

1. int multiplier = 7;
2. Int (^myblock) (int) = ^ (int num) {
3. return num * multiplier;
4. };

In this example, Myblock is a block variable that takes a parameter of type int and returns a value of type int. Does it look like a C function?

Come on, let's have a typedef.

1. typedef VOID (^boolblock) (BOOL); A block that accepts only one bool parameter and no return value
2. typedef INT (^intblock) (void); A block with no parameters that returns an int
3. typedef boolblock (^hugeblock) (Intblock); //See, this hugeblock parameter and return value are block

Third, more detailed examples

Note: The typedef above are also valid ~

Take the initiative to call:

1. -(void) SomeMethod
2. {
3. Boolblock Ablock = ^ (BOOL bValue) {
4. NSLog (@"Bool block!");
5. };
6. Ablock ();
7. }

Returned as a parameter:

1. typedef VOID (^boolblock) (BOOL);
2. -(Boolblock) foo ()
3. {
4. Boolblock Ablock = ^ (BOOL bValue) {
5. NSLog (@"Bool block!");
6. };
7. return [[ablock copy] autorelease]; //must copy, copy it to the heap, more detailed principles, will be explained in the following chapters
8. }

A member of the class:

1. @interface Obj1:nsobject
2. @property (nonatomic, copy) Boolblock block; //Reason ditto, students
3. @end
5. OBJ1 *obj1 = ...
6. Obj1.block = ^ (BOOL bValue) {
7. NSLog (@"Bool block!");
8. };

Parameters for other functions:

1. -(void) foo (Boolblock block)
2. {
3. if (block) {
4. Block ();
5. }
6. }

Even other block parameters:

1. Boolblock bblock = ^ (BOOL BV) {if (BV) {/*do some thing*/}};
2. Hugeblock Hblock = ^ (boolblock bb) {BB ();};
4. Hbolck (bblock);

Ah, global Variables! ：

1. static int (^maxintblock) (int, int) = ^ (int A, int b) {return a>b?a:b;};
2. int main ()
3. {
4. printf ("%d\n", Maxintblock (2,10));
5. return 0;
6. }

Well, you know how the block might be used.

Four, special markings, __block

If you want to modify the stack variables declared outside of block within a block, be sure to add __block tags to the variable:

1. int main ()
2. {
3. __block int i = 1024x768;
4. Boolblock bblock = ^ (BOOL BV) {
5. if (BV) {
6. i++; //If there is no __block tag, it cannot be compiled.
7. }
8. };
9. }

Article Source: http://www.cnblogs.com/shouce/p/5522180.html

Introduction to block in iOS