由於下載和保存圖像而導致的內存壓力

Question

幸運的是，我知道我的內存壓力問題是從哪里來的，並且我嘗試了許多技術，例如將一個塊包裝在@autorelease塊中並將對象設置為nil，但仍然沒有成功。

很抱歉在這里轉儲太多代碼，我試圖將其縮減為基本內容。 這是用於下載和保存圖像的代碼：

NSMuttableArray *photosDownOps = [NSMuttableArray array];
NSURL *URL = [...];
NSURLRequest *request = [...];
AFHTTPRequestOperation *op = [[AFHTTPRequestOperation alloc] initWithRequest:request];
op.responseSerializer = [AFImageResponseSerializer serializer];

[op setCompletionBlockWithSuccess:^(AFHTTPRequestOperation *operation, id responseObject) {    
    dispatch_queue_t amBgSyncQueue = dispatch_queue_create("writetoFileThread", NULL);
    dispatch_async(amBgSyncQueue, ^{
        [self savePhotoToFile:(UIImage *)responseObject usingFileName:photo.id];
    });    
} failure:^(AFHTTPRequestOperation *operation, NSError *error) {
    if ([error code] !=  NSURLErrorCancelled)
        NSLog(@"Error occured downloading photos: %@", error);
}];
[photosDownOps addObject:op];

NSArray *photosDownloadOperations = [AFURLConnectionOperation batchOfRequestOperations:photosDownloadOperatons 
                                                                         progressBlock:^(NSUInteger nof, NSUInteger tno) {        
} completionBlock:^(NSArray *operations) {
    NSLog(@"all photo downloads completed");
}];

[self.photosDownloadQueue addOperations:photosDownloadOperations waitUntilFinished:NO];

+ (void) savePhotoToFile:(UIImage *)imageToSave usingFileName:(NSNumber *)photoID{
    @autoreleasepool {
        NSData * binaryImageData = UIImageJPEGRepresentation(imageToSave, 0.6);
        NSString *filePath = [Utilities fullPathForPhoto:photoID];
        [binaryImageData writeToFile:filePath atomically:YES];
        binaryImageData = nil;
        imageToSave = nil;
    }
}

盡管這種情況僅發生在經過我測試的iPhone 4s設備上，但在iPhone 5機型上卻沒有發生。

Answer 1

我設法通過擴展NSOperation並在接收到將數據寫到文件中的數據后立即在主塊內解決此問題：

- (void)main{
    @autoreleasepool {
        //...
        NSData *imageData = [[NSData alloc] initWithContentsOfURL:imageUrl];        
        if (imageData) {
            NSError *error = nil;
            [imageData writeToFile:imageSavePath options:NSDataWritingAtomic error:&error];
        }
        //...
    }
}

然后，此NSOperation對象添加了我已經擁有的NSOperationQueue。

Answer 2

嘗試創建自己的類以使用NSUrlConnection下載圖像，然后在委托方法中將該數據附加到文件中，請參見以下代碼

-(void)connection:(NSURLConnection*)connection didReceiveData:(NSData*)data {

NSFileHandle *fileHandle = [NSFileHandle fileHandleForWritingAtPath:aPath]; 
[fileHandle seekToEndOfFile]; 
[fileHandle writeData:data]; 
[fileHandle closeFile];

}

這將幫助您進行內存管理，因為不需要緩存所有下載的數據。