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