什么是block

块是添加在 c，object-c 和 c++ 语言中的语言级别的形式，它允许你编写一个独特的代码段，这个代码段能够在作为值方法和函数中传递。
块是 object-c 的对象，这意味着他们能够被添加到像NASArray或是NSDictionary的集合中.

没有参数和返回值的块

定义一个块

^{
     NSLog(@"This is a block");
}

声明块变量

void (^simpleBlock)(void);

块变量赋值

void (^simpleBlock)(void) = ^{
    NSLog(@"This is a block");
};

块变量赋值的简约形式

simpleBlock = ^{
    NSLog(@"This is a block");
};

调用块命令

simpleBlock();

有参数和返回值的块

定义一个块

//1.

^ (double firstValue, double secondValue) {
    return firstValue * secondValue;
}

//2.
^ double (double firstValue, double secondValue) {
    return firstValue * secondValue;
}

声明块变量

double (^multiplyTwoValues)(double, double);

调用块命令

double (^multiplyTwoValues)(double, double) =
                          ^(double firstValue, double secondValue) {
                              return firstValue * secondValue;
                          };

double result = multiplyTwoValues(2,4);

NSLog(@"The result is %f", result);

块的值域

如果你在一个方法中声明了块，他可以捕捉到任何在方法域中可以访问到的值

- (void)testMethod {
int anInteger = 42;

void (^testBlock)(void) = ^{
    NSLog(@"Integer is: %i", anInteger);
};

testBlock();
}

一旦捕获后，即使在捕获后和调用期间改变值，不受影响.块不能够改变初始变量的值，或者是被捕获的值

int anInteger = 42;

void (^testBlock)(void) = ^{
    NSLog(@"Integer is: %i", anInteger);
};

anInteger = 84;

testBlock();  

//Integer is: 42

__block变量（块变量的改变）

变量和块共享存储空间。变量改变，块内的也改变

__block int anInteger = 42;

void (^testBlock)(void) = ^{
NSLog(@"Integer is: %i", anInteger);
};

anInteger = 84;
testBlock(); 

//Integer is：84

变量可以在块内被改变

__block int anInteger = 42;

void (^testBlock)(void) = ^{
    NSLog(@"Integer is: %i", anInteger);
    anInteger = 100;
};

testBlock();
NSLog(@"Value of original variable is now: %i", anInteger); 

//Integer is: 42
//Value of original variable is now: 100

块作为方法或函数的参数

- (void)beginTaskWithCallbackBlock:(void (^)(void))callbackBlock;


- (void)beginTaskWithCallbackBlock:(void (^)(void))callbackBlock {
...
callbackBlock();
}

- (void)doSomethingWithBlock:(void (^)(double, double))block {
...
block(21.0, 2.0);
}

块应该总是一个方法的最后一个参数

//声明
- (void)beginTaskWithName:(NSString *)name completion:(void(^)(void))callback;

//调用
[self beginTaskWithName:@"MyTask" completion:^{
    NSLog(@"The task is complete");
}];

块也用于回调 — 定义了当一个任务结束时将被执行的代码

- (IBAction)fetchRemoteInformation:(id)sender {
[self showProgressIndicator];

XYZWebTask *task = ...

[task beginTaskWithCallbackBlock:^{
    [self hideProgressIndicator];
}];
}

定义块的数据类型

typedef void (^XYZSimpleBlock)(void);


XYZSimpleBlock anotherBlock = ^{
    ...
};


- (void)beginFetchWithCallbackBlock:(XYZSimpleBlock)callbackBlock {

...
callbackBlock();
}

该 complexBlock 变量是指一个块需要另一块作为一个参数（ABLOCK）并返回另一个块

void (^(^complexBlock)(void (^)(void)))(void) = ^ (void (^aBlock)(void)) {
...
return ^{
    ...
};
};

重写代码使用类型定义使其可读性更高，将块定义成数据类型

XYZSimpleBlock (^betterBlock)(XYZSimpleBlock) = ^ (XYZSimpleBlock aBlock) {
...
return ^{
    ...
};
};

定义块为属性

@interface XYZObject : NSObject
@property (copy) void (^blockProperty)(void);  //通常使用copy,why?
@end


self.blockProperty = ^{
    ...
};
self.blockProperty();


使用块属性声明的类型定义

typedef void (^XYZSimpleBlock)(void);

@interface XYZObject : NSObject
@property (copy) XYZSimpleBlock blockProperty;
@end

Avoid Strong Reference Cycles when Capturing self

Blocks maintain strong references to any captured objects, including self, which means that it’s easy to end up with a strong reference cycle。 block对任何捕获的对象是强引用。

@interface XYZBlockKeeper : NSObject
@property (copy) void (^block)(void);
@end

@implementation XYZBlockKeeper
- (void)configureBlock {
self.block = ^{
    [self doSomething];    // capturing a strong reference to self
                           // creates a strong reference cycle
};
}
...
@end

正确做法如下

- (void)configureBlock {
XYZBlockKeeper * __weak weakSelf = self;
self.block = ^{
    [weakSelf doSomething];   // capture the weak reference
                              // to avoid the reference cycle
}
}

By capturing the weak pointer to self, the block won’t maintain a strong relationship back to the XYZBlockKeeper object. If that object is deallocated before the block is called, the weakSelf pointer will simply be set to nil.

Blocks