Swift 3

Question

我熟悉在Android中使用AsyncTask：创建子类，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

回答原始问题：

Grand Central Dispatch（GCD）提供了一种在后台执行任务的机制，尽管它的工作方式与AsyncTask结构不同。要异步执行某些操作，您只需创建一个队列（如线程），然后将块传递给dispatch_async()以在后台执行。我发现它比AsyncTask更整洁，因为没有涉及子类化;它或多或少是即插即用的，无论你有什么代码，你想在后台执行。一个例子：

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

其他要点：

1）回调

如果您想在后台执行任务并在后台任务完成时更新UI（或在其他线程上执行某些操作），您可以简单地嵌套调度调用：

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
    });
});

2）全局队列

创建队列时，您还可以使用dispatch_get_global_queue()函数获取具有特定优先级的全局调度队列（例如DISPATCH_QUEUE_PRIORITY_HIGH）。这些队列是普遍可访问的，在您希望将多个任务分配给同一个线程/队列时非常有用。请注意，内存是由iOS完全管理的。

3）记忆

有时候对内存管理和调度队列存在一些混淆，因为它们有自己的dispatch_retain / dispatch_release函数。但是，请放心，它们被ARC视为Objective-C对象，因此您无需担心调用这些函数。引用有关GCD和ARC的rob mayoff's great answer，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.

4）多个任务/块

我将补充说，如果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。 JörnEyrich和ɲeuroburɳ对这一主题提供了慷慨的解释here。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答案，并了解它们之间的差异。

如果您愿意，documentation有关于该主题的大量信息。

Answer 2

iOS中没有类，但您可以使用队列进行模拟。你可以打电话：

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内部调用下一个队列来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，将此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例子：

#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

以下电话：

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

您可以添加publishProgress()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了异步任务。

我希望它有用。

Answer 3

如果您定位早期的iOS版本（比用于Grand Central Dispatch的iOS 4），您可以使用NSObject performSelector方法

在后台执行帖子performSelectorInBackground:withObject:
在MainThread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 4

Swift 3

在Android中，当我想在后台线程上运行任务，然后在完成后更新UI时，我使用了AsyncTask（example）。现在当我制作我的应用程序的iOS版本时，我使用Grand Central Dispatch（GCD）来做同样的事情。以下是Swift的完成方式：

DispatchQueue.global(qos: .background).async {

    // code to be run on a background task

    DispatchQueue.main.async {

        // code to be run on the main thread after the background task is finished
    }
}

注释

Grand Central Dispatch Tutorial for Swift

Answer 5

这是带有PusblishProgress的C＃Xamarin.iOS版本：

internal abstract class AsyncTask : NSObject
{
    protected abstract nint DoInBackground(NSArray parameters);

    protected abstract void PostExecute(nint result);

    public void ExecuteParameters(NSArray @params)
    {
        this.PreExecute();

        DispatchQueue.GetGlobalQueue(DispatchQueuePriority.Default).DispatchAsync(() =>
        {
            //We're on a Background thread
            var result = this.DoInBackground(@params);
            DispatchQueue.MainQueue.DispatchAsync(() => {
                // We're on the main thread
                this.PostExecute(result);
            });
        });

    }

    protected abstract void PreExecute();

    protected void PublishProgress(NSArray parameters)
    {
        InvokeOnMainThread(() => {
            // We're on the main thread
            this.OnProgressUpdate(parameters);
        });
    }

    protected abstract void OnProgressUpdate(NSArray parameters);
}

实施：

internal class MyAsyncTask : AsyncTask
{
    protected override void OnProgressUpdate(NSArray parameters)
    {
        // This runs on the UI Thread
    }

    protected override nint DoInBackground(NSArray parameters)
    {
        // Do some background work
        // ....
        var progress = NSArray.FromObjects(1, "Done step 1");
        PublishProgress(progress);

        return 0;
     }

     protected override void PostExecute(nint result)
     {
         // This runs on the UI Thread

     }

     protected override void PreExecute()
     {
        // This runs on the UI Thread

     }
}

iOS / Objective-C相当于Android的AsyncTask

5 个答案:

回答原始问题：

其他要点：

1）回调

2）全局队列

3）记忆

4）多个任务/块

Swift 3

注释