block 块实现传值

使用块（block）传值比较简单，并且用块写出来的代码更为整洁。

代码模拟：

声明文件：

#import <Foundation/Foundation.h>

typedef void (^GBCompletionHandler)(NSData* data, NSError *error);

@interface GBUpload : NSObject

/**
 *  upload data to some where
 *
 *  @param data             which to be upload
 *  @param completionHandle do some logic when upload finish.
 */
- (void) upload:(NSData*) data completionHandler:(GBCompletionHandler) completionHandle;

@end

实现文件：

#import "GBUpload.h"

@implementation GBUpload

- (void) upload:(NSData*) data completionHandler:(GBCompletionHandler) completionHandle {
    NSLog(@"uploading......");

    sleep(3);

    NSLog(@"upload success");

    if (completionHandle) {
        completionHandle(data, nil);
    }
}

@end

函数调用：

GBUpload *upload = [[GBUpload alloc] init];

    [upload upload:nil completionHandler:^(NSData *data, NSError *error) {
        if (error) {
            NSLog(@"upload failed");
        } else {
            NSLog(@"upload finish, update UI");
        }
    }];

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