IOS学习之路七(使用 Operation 异步运行任务)

在 application delegate 头文件(.h)中声明一个 operation 队列和两个 invocation operations: 

#import <UIKit/UIKit.h>
@interface Running_Tasks_Asynchronously_with_OperationsAppDelegate
: UIResponder <UIApplicationDelegate> @property (nonatomic, strong) UIWindow *window;
@property (nonatomic, strong) NSOperationQueue *operationQueue; @property (nonatomic, strong) NSInvocationOperation *firstOperation; @property (nonatomic, strong) NSInvocationOperation *secondOperation; @end

application delegate 的实现文件(.m 文件)如下: 

#import "Running_Tasks_Asynchronously_with_OperationsAppDelegate.h" @implementation Running_Tasks_Asynchronously_with_OperationsAppDelegate @synthesize window = _window;
@synthesize firstOperation;
@synthesize secondOperation;
@synthesize operationQueue;
- (void) firstOperationEntry:(id)paramObject{
NSLog(@"%s", __FUNCTION__);
NSLog(@"Parameter Object = %@", paramObject); NSLog(@"Main Thread = %@", [NSThread mainThread]); NSLog(@"Current Thread = %@", [NSThread currentThread]);
}
- (void) secondOperationEntry:(id)paramObject{
NSLog(@"%s", __FUNCTION__);
NSLog(@"Parameter Object = %@", paramObject); NSLog(@"Main Thread = %@", [NSThread mainThread]); NSLog(@"Current Thread = %@", [NSThread currentThread]);
}
- (BOOL) application:(UIApplication *)application
didFinishLaunchingWithOptions:(NSDictionary *)launchOptions{
NSNumber *firstNumber = [NSNumber numberWithInteger:111]; NSNumber *secondNumber = [NSNumber numberWithInteger:222];
self.firstOperation =[[NSInvocationOperation alloc] initWithTarget:self
selector:@selector(firstOperationEntry:)
object:firstNumber]; self.secondOperation = [[NSInvocationOperation alloc]
initWithTarget:self selector:@selector(secondOperationEntry:) object:secondNumber];
self.operationQueue = [[NSOperationQueue alloc] init]; /* Add the operations to the queue */

[self.operationQueue addOperation:self.firstOperation]; [self.operationQueue addOperation:self.secondOperation];
NSLog(@"Main thread is here");
self.window = [[UIWindow alloc] initWithFrame: [[UIScreen mainScreen] bounds]];
self.window.backgroundColor = [UIColor whiteColor]; [self.window makeKeyAndVisible];
return YES;
} @end

在实现的代码里面都发生了什么呢:
有两个方法:firstOperationEntry:和 secondOperationEntry:,每个方法都接收一个对象 

作为参数,并且在控制台窗口打印出当前线程、主线程和参数。这两个入口函数的

invocation operation 将被添加到一个 operation 队列中。
我们初始化两个 NSInvocationOperation 类型对象,并给每个 operation 设置目标

selector 入口点,如之前所述。
然后我们初始化一个 NSOperationQueue 类型对象。(当然也可以在入口方法前面创

建)队列对象将负责管理 operation 对象的并发。
 我们调用 NSOperationQueue 的实例方法 addOperation:把每个 invocation operation 添加

到 operation 队列中。在这里,operation 队列可能会也可能不会立即通过 nvocationoperation 的 start 方法启动 invocation operation。但是,需要牢记重要的一点:添加operations 至 operation 队列后,你不能手动启动 operations,必须交由 operation 队列负责。

现在,我们运行一次示例代码,在控制台窗口可以看到如下结果:

[Running_Tasks_Asynchronously_with_OperationsAppDelegate firstOperationEntry:]Main thread is here
Parameter Object = 111
[Running_Tasks_Asynchronously_with_OperationsAppDelegate secondOperationEntry:]Main Thread = <NSThread: 0x6810260>{name = (null), num = 1}

Parameter Object = 222
Current Thread = <NSThread: 0x6805c20>{name = (null), num = 3}Main Thread = <NSThread: 0x6810260>{name = (null), num = 1}Current Thread = <NSThread: 0x6b2d1d0>{name = (null), num = 4} 

如果我们子类化了一个 NSOperation 类,并且把这个子类的实例对象添加到了 operation队列,我们需要做稍微的改动。记住以下几点:

 由于当把 NSOperation 的子类对象添加到一个 operation 队列中,该对象会异步运行。由此,你必须 overrideNSOperation 的实例方法 isConcurrent,在该方法中返回 YES。 在 start 方法里面执行 main 任务之前,需要定期的调用 isCancelled 方法来检测该函数的返回值,以确定是退出 start 方法还是开始运行 operation。在这里,当 operation 添加到队列中后,operation 的 start 方法将会被 operation 队列调用,start 方法中,调用isCanelled 方法确定 operation 是否被取消。如果 operation 被取消了,只需要从 start 

方法中简单的返回即可。如果没被取消,会在 start 方法中调用 main 方法。

 在 main task 实现部分中 override main 函数,main 函数将被 operation 执行。在这个函数里面确保分配和初始化 autorelease pool,并且在返回之前释放这个 pool。

 重载 operation 的 isFinished 和 isExecuting 方法,这两个函数返回对应的 BOOL 值,代表 operation 是执行完毕还是在执行中。

下面是我们的 operation 声明(.h 文件): 

#import <Foundation/Foundation.h>
@interface SimpleOperation : NSOperation
/* Designated Initializer */
- (id) initWithObject:(NSObject *)paramObject; @end
The implementation of the operation is as follows:
#import "SimpleOperation.h" @implementation SimpleOperation NSObject *givenObject;
BOOL finished;
BOOL executing;
- (id) init {
NSNumber *dummyObject = [NSNumber numberWithInteger:123]; return([self initWithObject:dummyObject]);
}
- (id) initWithObject:(NSObject *)paramObject{
self = [super init];
if (self != nil){
/* Keep these values for the main method */
givenObject = paramObject;
}
return(self);
}
- (void) start {
/* If we are cancelled before starting, then
we have to return immediately and generate the
required KVO notifications */
if ([self isCancelled]){
/* If this operation *is* cancelled */
/* KVO compliance */
[self willChangeValueForKey:@"isFinished"];
finished = YES;
[self didChangeValueForKey:@"isFinished"];
return;

} else {
/* If this operation is *not* cancelled */
/* KVO compliance */
[self willChangeValueForKey:@"isExecuting"]; executing = YES;
/* Call the main method from inside the start method */ [self main];
[self didChangeValueForKey:@"isExecuting"];
}
}
- (void) main {
@try {
@autoreleasepool {
/* Keep a local variable here that must get set to YES whenever we are done with the task */

BOOL taskIsFinished = NO;
/* Create a while loop here that only exists
if the taskIsFinished variable is set to YES or
the operation has been cancelled */
while (taskIsFinished == NO &&
[self isCancelled] == NO){
/* Perform the task here */
NSLog(@"%s", __FUNCTION__);
NSLog(@"Parameter Object = %@", givenObject); NSLog(@"Main Thread = %@", [NSThread mainThread]); NSLog(@"Current Thread = %@", [NSThread currentThread]); /* Very important. This way we can get out of the
loop and we are still complying with the cancellation
rules of operations */
taskIsFinished = YES;
}
/* KVO compliance. Generate the
required KVO notifications */
[self willChangeValueForKey:@"isFinished"]; [self willChangeValueForKey:@"isExecuting"]; finished = YES;
executing = NO;
[self didChangeValueForKey:@"isFinished"]; [self didChangeValueForKey:@"isExecuting"]; }
}
@catch (NSException * e) { NSLog(@"Exception %@", e);
}
}
- (BOOL) isConcurrent{
return YES;
}
- (BOOL) isFinished{
/* Simply return the value */
return finished;
}
- (BOOL) isExecuting{
/* Simply return the value */
return executing;
}
@end

现在可以在其他任何类中使用上面定义的这个 operation 类了,比如在 applicationdelegate 中。下面是 application delegate 的声明,使用了新的 operation 类,并将其添加到了新的 operation 队列中: 

#import <UIKit/UIKit.h>
@class SimpleOperation;
@interface Running_Tasks_Asynchronously_with_OperationsAppDelegate : UIResponder <UIApplicationDelegate>
@property (nonatomic, strong) UIWindow *window;
@property (nonatomic, strong) NSOperationQueue *operationQueue; @property (nonatomic, strong) SimpleOperation *firstOperation; @property (nonatomic, strong) SimpleOperation *secondOperation;
@end

application delegate 的实现部分如下: 

#import "Running_Tasks_Asynchronously_with_OperationsAppDelegate.h" #import "SimpleOperation.h"
@implementation Running_Tasks_Asynchronously_with_OperationsAppDelegate @synthesize window = _window;
@synthesize firstOperation;
@synthesize secondOperation;
@synthesize operationQueue;
- (BOOL) application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions{ NSNumber *firstNumber = [NSNumber numberWithInteger:111]; NSNumber *secondNumber = [NSNumber numberWithInteger:222]; self.firstOperation = [[SimpleOperation alloc]
initWithObject:firstNumber];
self.secondOperation = [[SimpleOperation alloc] initWithObject:secondNumber];
self.operationQueue = [[NSOperationQueue alloc] init]; /* Add the operations to the queue */ [self.operationQueue addOperation:self.firstOperation]; [self.operationQueue addOperation:self.secondOperation]; NSLog(@"Main thread is here");
self.window = [[UIWindow alloc] initWithFrame: [[UIScreen mainScreen] bounds]]; self.window.backgroundColor = [UIColor whiteColor]; [self.window makeKeyAndVisible];
return YES;
}
@end

打印到控制台窗口的结果与之前使用并发 invocation operation 类似: 

Main thread is here
-[SimpleOperation main]
-[SimpleOperation main]
Parameter Object = 222
Parameter Object = 222
Main Thread = <NSThread: 0x6810260>{name = (null), num = 1}Main Thread = <NSThread: 0x6810260>{name = (null), num = 1}Current Thread = <NSThread: 0x6a10b90>{name = (null), num = 3}Current Thread = <NSThread: 0x6a13f50>{name = (null), num = 4}