《Programming with Objective-C》第八章 Working with Blocks

標籤：

Blocks are Objective-C objects, which means they can be added to collections like NSArray or NSDictionary.

 

Block文法——無參數版本定義（Block的值）

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

 

聲明

void (^simpleBlock)(void);

類似int i;

 

賦值

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

類似i = 2;

 

聲明的時候定義

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

類似int i = 2;

 

調用

simpleBlock();

 

Block文法——帶參數版本定義

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

or

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

 

聲明

double (^multiplyTwoValues)(double, double);

 

賦值

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

 

聲明的時候定義

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

 

調用

double result = multiplyTwoValues(2,3);

 

__block修飾符

int anInteger = 42;void (^testBlock)(void) = ^{    //此時只是Block定義，並沒有執行裡面的函數    NSLog(@"Integer is: %i", anInteger);};anInteger = 84;testBlock();    //Block調用 輸出42

 

Value is captured when the block is defined.

Block定義的時候，將值複製一份給自己，所以該值已經不受外界影響。

 

__block int anInteger = 42;void (^testBlock)(void) = ^{    //此時只是Block定義，並沒有執行裡面的函數    NSLog(@"Integer is: %i", anInteger);    };anInteger = 84;testBlock();    //Block調用 輸出84

 

Because anInteger is declared as a __block variable, its storage is shared with the block declaration.

此時Block裡面的值與外面的值共用同一份記憶體

 

Block與self的恩怨情仇

It’s important to take care when capturing self because it’s easy to create a strong reference cycle when capturing self.

 

因為變數預設是__strong修飾（詳見這裡），所以要時刻注意在block裡面對self的引用（只要出現了self關鍵字就算引用了，因為block會自動capture）

假如self裡面定義了一個block，那麼self有一個指向block的strong指標（比如該block是self的一個strong成員變數）；假如block裡面使用了self，則block也預設拷貝了一個指向self的strong指標，此時形成strong reference cycle.

解決方案：在Block前面建立一個__weak類型的指向self的指標，並在block裡面使用該指標。

例子

__weak typeof(self) weakSelf = self;    //學習下這種寫法哦 typeof(self)self.simpleBlock = ^{    [weakSelf f];};...self.simpleBlock();

但是，假如Block裡面又有一個Block，怎麼辦？最好是強引用weakSelf，此時strongSelf強引用的是weakSelf而不是self，所以不會形成strong reference cycle

__weak typeof(self) weakSelf = self;    //學習下這種寫法哦 typeof(self)self.simpleBlock = ^{    [weakSelf f];    __strong typeof(weakSelf) strongSelf = weakSelf;    self.simpleBlock2 = ^{        [strongSelf f];    };    self.simpleBlock2();};...self.simpleBlock();

圖解

一個函數最好只有一個Block參數，且最好是在最後一個

A Block Should Always Be the Last Argument to a Method.

It’s best practice to use only one block argument to a method.

 

使用typedef定義一個block

typedef int (^Sum)(int, int);Sum mySum = ^(int a, int b){    return a+b;}

or

typedef void (^XYZSimpleBlock)(void);@property (copy) XYZSimpleBlock blockProperty;

 

使用copy修飾block的property

@property (nonatomic, copy) Sum mySum;@property (nonatomic, copy) void (^blockProperty)(void);

非ARC下就必須寫copy，because a block needs to be copied to keep track of its captured state outside of the original scope
在ARC下寫不寫copy都無所謂，so it‘s a best practice to set copy property for block whether it‘s ARC or not.

 

《Programming with Objective-C》第八章 Working with Blocks