iOS/Objective-C相当于Android的AsyncTask

Question

我熟悉在Android中使用AsyncTask：创建一个子类，在子类的实例上调用execute，在UI线程或主线程上调用onPostExecute。iOS中的等价物是什么？

Answer 1

如果您的目标早于iOS版本（大中央调度iOS4），您可以使用NSObject performSelector方法

>

在后台线程上执行performSelectorInbackground： with Object：

并在主线程上执行performSelectorOnMainThread:withObject:waitUntilDone:

这是一个例子：

[self performSelectorInBackground:@selector(executeInBackground) withObject:nil];


-(void) executeInBackground
{
    NSLog(@"executeInBackground");

    [self performSelectorOnMainThread:@selector(executeOnMainThread) withObject:nil waitUntilDone:NO];
}

-(void) executeOnMainThread
{
    NSLog(@"executeOnMainThread");
}

Answer 2

iOS中没有类，但您可以使用队列模拟它。您可以调用：

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    //Your code to execute in background...
});

对于异步任务和异步代码调用下一个队列在视图中执行某些操作...：

dispatch_async(dispatch_get_main_queue(), ^{
    //Your code to execute on UIthread (main thread)
});

然后，使用这两个队列，您可以创建一个asyncTask类，将该类添加到您的项目中以实现它们：

//
//  AsyncTask.h
//
//  Created by Mansour Boutarbouch Mhaimeur on 25/10/13.
//

#import <Foundation/Foundation.h>

@interface AsyncTask : NSObject

- (void) executeParameters: (NSArray *) params;
- (void) preExecute;
- (NSInteger) doInBackground: (NSArray *) parameters;
- (void) postExecute: (NSInteger) result;
@end

//
//  AsyncTask.m
//
//  Created by Mansour Boutarbouch Mhaimeur on 25/10/13.
//

#import "AsyncTask.h"

@implementation AsyncTask

- (void) executeParameters: (NSArray *) params{
    [self preExecute];
    __block NSInteger result;
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        result = [self doInBackground:params];
        dispatch_async(dispatch_get_main_queue(), ^{
            [self postExecute:result];
        });
    });
}

- (void) preExecute{
    //Method to override
    //Run on main thread (UIThread)
}

- (NSInteger) doInBackground: (NSArray *) parameters{
    //Method to override
    //Run on async thread (Background)
    return 0;
}

- (void) postExecute: (NSInteger) result{
    //Method to override
    //Run on main thread (UIThread)
}

@end

这是我在项目中使用的一个示例：

#import "AsyncTask.h"
#import "Chat.h"

@interface SendChatTask : AsyncTask{
    NSArray * chatsNotSent;
}

@end

#import "SendChatTask.h"

@implementation SendChatTask

- (void) preExecute{
    //Method to override
}

- (NSInteger) doInBackground: (NSArray *) parameters{
    //Method to override
    NSString *sendChatsURL = [NSString stringWithFormat:@"%@%@%@",HOST, NAMESPACE,URL_SEND_CHAT];
    chatsNotSent = [parameters objectAtIndex:0];

    NSString *response;
    NSMutableDictionary *params = [[NSMutableDictionary alloc] init];
    //...
    NSError *error;
    NSData *jsonData = [NSJSONSerialization dataWithJSONObject:[ChatJSONParser wrapChatArray: chatsNotSent] options:0 error:&error];
    NSString *JSONString = [[NSString alloc] initWithBytes:[jsonData bytes] length:[jsonData length] encoding:NSUTF8StringEncoding];

    [params setObject:JSONString forKey:@"chats"];

    response = [HTTPClient executePOST:sendChatsURL parameters:params];

    if([respuesta isEqualToString:@"true"]){
        return 1;
    }else{
        return -1;
    }
}

- (void) postExecute: (NSInteger) result{
    //Method to override
    if (result == 1) {
        for (Chat *chat in chatsNotSent) {
            chat.state = STATE_NOT_SENT;
            [chat save];
            AppDelegate *appDelegate = (AppDelegate *)[[UIApplication sharedApplication] delegate];
            [appDelegate refreshChat];
        }
    } else {

    }
}

@end

以及以下呼叫：

[[[SendChatTask alloc] init] executeParameters:[NSArray arrayWithObjects: chatsNotSent, nil]];

您可以添加Publishgres（）更新方法和相应的...我暂时不使用它，因为我在后台服务中调用我的异步任务。

我希望这有帮助。

Answer 3

Grand Central Dispatch（GCD）提供了一种在后台执行任务的机制，尽管其工作方式在结构上与AsyncTask不同。要异步执行某些操作，只需创建一个队列（如线程），然后将一个块传递给要在后台执行的dispatch\u async（）。我发现它比AsyncTask更简洁，因为不涉及子类；它或多或少是即插即用的，只要你有代码想在后台执行。例如：

dispatch_queue_t queue = dispatch_queue_create("com.yourdomain.yourappname", NULL);
dispatch_async(queue, ^{
    //code to be executed in the background
});

如果要在后台执行任务，并在后台任务完成时更新UI（或在另一个线程上执行某些操作），只需嵌套调度调用：

dispatch_queue_t queue = dispatch_queue_create("com.yourdomain.yourappname", NULL);
dispatch_async(queue, ^{
    //code to be executed in the background
    dispatch_async(dispatch_get_main_queue(), ^{
        //code to be executed on the main thread when background task is finished
    });
});

创建队列时，您还可以使用dispatch\u get\u global\u queue（）函数获取具有特定优先级的全局调度队列（例如dispatch\u queue\u priority\u HIGH）。这些队列是普遍可访问的，当您想要将多个任务分配给同一个线程/队列时，它们非常有用。请注意，内存完全由iOS管理。

内存管理和调度队列有时会出现一些混淆，因为它们有自己的功能。但是，请放心，ARC将它们视为Objective-C对象，因此您无需担心调用这些函数。参考rob mayoff关于GCD和ARC的伟大回答，您可以看到文档描述了GCD队列与Objective-C对象的等价性：

* By default, libSystem objects such as GCD and XPC objects are declared as
* Objective-C types when building with an Objective-C compiler. This allows
* them to participate in ARC, in RR management by the Blocks runtime and in
* leaks checking by the static analyzer, and enables them to be added to Cocoa
* collections.
*
* NOTE: this requires explicit cancellation of dispatch sources and xpc
*       connections whose handler blocks capture the source/connection object,
*       resp. ensuring that such captures do not form retain cycles (e.g. by
*       declaring the source as __weak).
*
* To opt-out of this default behavior, add -DOS_OBJECT_USE_OBJC=0 to your
* compiler flags.
*
* This mode requires a platform with the modern Objective-C runtime, the
* Objective-C GC compiler option to be disabled, and at least a Mac OS X 10.8
* or iOS 6.0 deployment target.

我要补充的是，如果一个任务在多个异步活动完成之前无法继续，GCD有一个分组接口支持同步多个异步块。Jörn Eyrich和ɲeuroburɳ在这里对这个主题进行了慷慨的解释。如果您需要此功能，我强烈建议您花几分钟时间仔细阅读他们的两个答案，并了解它们之间的差异。

如果你有这样的倾向，留档有大量关于这个主题的信息。