用OBJC編程 7 - Working with Blocks
Block 是語(yǔ)言級(jí)別的特性。它是一個(gè)OBJC的對(duì)象，可以被加入容器如NSArray或NSDictionary。它可以捕獲所處作用域的數(shù)值，非常類(lèi)似其它語(yǔ)言的closure或者lambda。語(yǔ)法^{
NSLog(@"This is a block");
}
可以像函數(shù)指針那樣聲明一個(gè)變量來(lái)持有這個(gè)blockvoid (^simpleBlock)(void);
simpleBlock = ^{
NSLog(@"This is a block");
}; // 注意這里有一個(gè)分號(hào)
也可以這樣寫(xiě)void (^simpleBlock)(void) = ^{
NSLog(@"This is a block");
};
調(diào)用這個(gè)blocksimpleBlock();
帶上參數(shù)和返回值^double (double firstValue, double secondValue){
return firstValue*secondValue;
}
// ,,,,,,,,,,,
double (^multiplyTwoValues)(double, double) =
^(double firstValue, double secondValue) { // 返回值類(lèi)型可以省略
return firstValue*secondValue;
};
double result = multiplyTwoValues(2,4);
NSLog(@"The result is %f", result);
捕獲Enclosing Scope內(nèi)的值，一旦捕獲，這個(gè)值就不會(huì)變化，即便后續(xù)改變這個(gè)值-(void)testMethod{
int anInteger = 42;
void (^testBlock)(void) = ^{
NSLog(@"Integer is: %i", anInteger);
}
anInteger = 84;
testBlock(); // 仍然輸出42
}
使用__block__block int anInteger = 42;
void (^testBlock)(void) = ^{
NSLog(@"Integer is: %i", anInteger);
};
anInteger = 84:
testBlock(); // output 84;
// ,,,,,,,,,,,,,
_block int anInteger = 42;
void (^testBlock)(void) = ^{
NSLog(@"Integer is: %i", anInteger); // output 42
anInteger = 100;
};
testBlock();
NSLog(@"Value of original variable is now: %i", anInteger); // output 100;
通過(guò)參數(shù)傳遞Block，例如實(shí)現(xiàn)一個(gè)回調(diào)-(IBAction)fetchRemoveInformation:(id)sender{
[self showProgressIndicator];
XYZWebTask *task = //,,,
[task beginTaskWithCallbackBlock:^{
[self hideProgressIndicator];
}];
}
// beginTaskWithCallbackBlock 的定義是這樣的
-(void)beginTaskWithCallbackBlock:(void)(^)(void))callbackBlock{
//,,,
callbackBlock();
}
最佳實(shí)踐是將block作為最后一個(gè)參數(shù)，這樣便于閱讀。也可以使用typedef簡(jiǎn)化語(yǔ)法typedef void (^XYZSimpleBlock)(void);
//,,,,,,,,,,,,,,,
XYZSimpleBlock anotherBlock = ^{ /*,,,*/ };
//,,,,,,,,,,,,
-(void)beginFetchWithCallbackBlock:(XYZSimpleBlock)callbackBlock{
//,,,,
callbackBlock();
}
可以將block作為屬性@interface XYZObject : NSObject
@property (copy) void (^blockProperty)(void); // 必須使用copy
@end
//,,,,,,,,,,,,,,
self.blockProperty = ^{ /* ,,, */ };
self.blockProperty();
避免強(qiáng)引用循環(huán)在block里捕獲self，諸如在一個(gè)callback block里，會(huì)引入內(nèi)存管理問(wèn)題。block會(huì)會(huì)維護(hù)一個(gè)捕獲對(duì)象的強(qiáng)引用，包括self@interface XYZBlockKeeper : NSObject
@property (copy) void (^block)(void);
@end
////////////////////////
@implementation XYZBlockKeeper
-(void)configureBlock{
self.block = ^{
[self doSomething]; // 捕獲了一個(gè)self的強(qiáng)引用
// 建立了一個(gè)強(qiáng)引用循環(huán)
}
}
@end
上述代碼會(huì)產(chǎn)生一個(gè)編譯警告，為了避免這種情況，最佳實(shí)踐是捕獲一個(gè)self的弱引用-(void)configureBlock{
XYZBlockKeeper * __weak weakSelf = self;
self.block = ^ {
[weakSelf doSomething];
};
}
block可以簡(jiǎn)化枚舉（略）block可以簡(jiǎn)化并發(fā)任務(wù)（略）