在Objective-C塊中編輯參數

Question

我想在塊中編輯UIImage指針，但不允許。

-(void)downloadImage:(NSURL *)url ofPointer:(UIImage *)imagePointer 
{ 
    __weak typeof(self) weakSelf = self; 
    [SDWebImageManager.sharedManager downloadWithURL:url 
              options:0 
              progress:^(NSUInteger receivedSize, long long expectedSize) {} 
              completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished) { 
               imagePointer = image; 
               [weakSelf setNeedsDisplay]; 
              }]; 

}

我試圖通過爭論與__block但它不允許太多。

-(void)downloadImage:(NSURL *)url ofPointer:(__block UIImage *)imagePointer

有什麼辦法來編輯作爲參數傳遞的指針？

Answer 1

您需要使用指向指針的指針（UIImage **），因爲圖像是不可變的對象，所以即使您可以在塊內部更改它，也不會產生所需的效果。

你應該使用委託或回調塊將下載的圖像從塊中傳遞給將要使用它的實例。

Answer 2

您可以做到這一點，通過傳遞指針的指針要設置：

#import <Foundation/Foundation.h> 

@interface Dinsdale : NSObject 
- (void)setThisPointer:(NSString *__autoreleasing *)s; 
@end 

@implementation Dinsdale 

- (void)setThisPointer:(NSString *__autoreleasing *)s 
{ 
    void (^b)(void) = ^{ 
     *s = @"Lemon curry?"; 
    }; 

    b(); 
} 

@end 

int main(int argc, const char * argv[]) 
{ 

    @autoreleasepool { 

     Dinsdale * d = [Dinsdale new]; 
     NSString * s = @"Semprimi!"; 
     [d setThisPointer:&s]; 
     NSLog(@"%@", s); // Prints "Lemon curry?" 

    } 
    return 0; 
} 

，但它會更好只使用一個setter方法：

-(void)downloadImageFromURL:(NSURL *)url 
{ 
    __weak typeof(self) weakSelf = self; 
    [SDWebImageManager.sharedManager downloadWithURL:url 
              options:0 
              progress:^(NSUInteger receivedSize, long long expectedSize) {} 
              completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished) { 
               [weakSelf setWhateverImage:image] 
               [weakSelf setNeedsDisplay]; 
              }]; 
}