高性能iOS应用开发笔记05

#import "Page3VC.h"
#import "DataUpdateOp.h"

@interface Page3VC ()

@property (nonatomic, strong) DataUpdateOp *updateOp;
@property (nonatomic, assign) BOOL refreshing;
@property (weak, nonatomic) IBOutlet UIImageView *imageView;

@end

- (IBAction)onClickBtn:(id)sender
{
    if(self.refreshing == NO)
    {
        self.refreshing = YES;
        
        if(self.updateOp == nil)
        {
            self.updateOp = [[DataUpdateOp alloc] init];
        }
        
        [self.updateOp startWithDelegate:self withSelector:@selector(onDataAvailable:)];
    }
}

- (void)onDataAvailable:(id)sender
{
    self.refreshing = NO;
    
    self.updateOp = nil;
    
    UIImage *image = (UIImage *)sender;
    
    self.imageView.image = image;
}

@implementation DataUpdateOp

- (void)startWithDelegate:(id)delegate withSelector:(SEL)selector
{
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSURL *imgURL = [[NSURL alloc] initWithString:@"http://macdown.uranusjr.com/static/images/logo-160.png"];
        NSData *data = [[NSData alloc] initWithContentsOfURL:imgURL];
        UIImage *image = [[UIImage alloc] initWithData:data];
        
        dispatch_async(dispatch_get_main_queue(), ^{
            if(delegate && [delegate respondsToSelector:selector])
            {
                [delegate performSelector:selector withObject:image];
            }
        });
    });
}

@end

[self.updateOp startWithDelegate:self withSelector:@selector(onDataAvailable:)]; 这里把self传入且VC对updateOp有引用这里会产生循环引用

===
更新

- (IBAction)onClickBtn:(id)sender
{
    if(self.refreshing == NO)
    {
        self.refreshing = YES;
        
        DataUpdateOp *updateOp = [[DataUpdateOp alloc] init];
        [updateOp startWithDelegate:self withSelector:@selector(onDataAvailable:)];
    }
}

- (void)onDataAvailable:(id)sender
{
    self.refreshing = NO;
    
    UIImage *image = (UIImage *)sender;
    
    self.imageView.image = image;
}

采用局部变量 updateOp 会持有VC 但是VC不会持有updateOp 解决了循环引用问题但是 updateOp 无法被其他地方使用

一种case 网络请求在排队中 APP在使用中切换页面 VC理想情况下应该立即回收但是VC被网络操作持有所以不会立即被回收虽然没有循环引用但是会造成内存使用峰值

解决方法
委托对操作是强引用
操作对委托是弱引用
操作引入cancel方法

改进如下

- (IBAction)onClickCloseVC:(id)sender
{
    [self dismissViewControllerAnimated:NO completion:nil];
}

- (IBAction)onClickBtn:(id)sender
{
    if(self.refreshing == NO)
    {
        self.refreshing = YES;
        
        if(self.updateOp == nil)
        {
            self.updateOp = [[DataUpdateOp alloc] init];
        }
        
        self.updateOp.delegate = self;
        
        [self.updateOp start];
    }
}

- (void)onDataAvailable:(id)sender
{
    self.refreshing = NO;
    
    self.updateOp = nil;
    
    UIImage *image = (UIImage *)sender;
    
    self.imageView.image = image;
}

- (void)dealloc
{
    NSLog(@"VC dealloc");
    
    if(self.updateOp != nil)
    {
        [self.updateOp cancel];
    }
}

#import "DataUpdateOp.h"

@interface DataUpdateOp ()

@property (nonatomic, strong) NSURLSessionDownloadTask *downloadTask;

@end

@implementation DataUpdateOp

- (void)start
{
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSURL *imgURL = [[NSURL alloc] initWithString:@"http://macdown.uranusjr.com/static/images/logo-160.png"];
        
        NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:imgURL];
        
        NSURLSession *session = [NSURLSession sharedSession];
        
        self.downloadTask = [session dataTaskWithRequest:request completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
            UIImage *image = [[UIImage alloc] initWithData:data];
            dispatch_async(dispatch_get_main_queue(), ^{
                id<DataUpdateOpDelegate> delegate = self.delegate;
                
                if(!delegate)
                {
                    
                }
                else
                {
                    [delegate onDataAvailable:image];
                }
            });
        }];
        
        [self.downloadTask resume];
    });
}

- (void)cancel
{
    NSLog(@"request cancel");
    
    [self.downloadTask cancel];
    
    self.delegate = nil;
}

@end

补充（http://www.cocoachina.com/bbs/read.php?tid=1726514#）
id<DataUpdateOpDelegate> delegate = self.delegate;
self.delegate 本身是 weak
处理过程中需要保证一直有效
=右边赋值强持有一下
保证在 block 执行期间 delegate 一直有效

高性能iOS应用开发 笔记05

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高性能iOS应用开发笔记05