调度队列和异步RNCryptor

Question

这是使用RNCryptor在iOS上异步解密大文件的后续行动。

我用本文中描述的方法异步解密了一个大的下载文件(60 by )，Calman在他的回答中对此进行了更正。

基本上是这样的：

int blockSize = 32 * 1024;
NSInputStream *cryptedStream = [NSInputStream inputStreamWithFileAtPath:...];
NSOutputStream *decryptedStream = [NSOutputStream output...];

[cryptedStream open];
[decryptedStream open];

RNDecryptor *decryptor = [[RNDecryptor alloc] initWithPassword:@"blah" handler:^(RNCryptor *cryptor, NSData *data) {
    NSLog("Decryptor recevied %d bytes", data.length);
    [decryptedStream write:data.bytes maxLength:data.length];
    if (cryptor.isFinished) {
        [decryptedStream close];
        // call my delegate that I'm finished with decrypting
    }
}];

while (cryptedStream.hasBytesAvailable) {
    uint8_t buf[blockSize];
    NSUInteger bytesRead = [cryptedStream read:buf maxLength:blockSize];
    NSData *data = [NSData dataWithBytes:buf length:bytesRead];

    [decryptor addData:data];
    NSLog("Sent %d bytes to decryptor", bytesRead);
}

[cryptedStream close];
[decryptor finish];

然而，我仍然面临一个问题:在解密之前，整个数据被加载到内存中。我可以看到一堆“发送X字节到解密器”，在此之后，控制台中同样的“解密器回收X字节”，当我希望看到“发送、接收、发送、接收”时。

对于小文件(2Mb)或模拟器上的大文件(60 my )来说，这是可以的；但是在一个真实的iPad1上，它会因为内存限制而崩溃，所以显然我不能在我的生产应用程序中保留这个过程。

我觉得我需要使用dispatch_async将数据发送到解密器，而不是盲目地在while循环中发送数据，但是我完全迷路了。我试过：

• 在while之前创建自己的队列，并使用dispatch_async(myQueue, ^{ [decryptor addData:data]; });
• 相同，但是在while循环中分配整个代码
• 相同，但是分配整个while循环
• 使用RNCryptor-provided responseQueue而不是我自己的队列

在这4种变体中，没有什么起作用的。

我还没有完全理解调度队列，我觉得问题就在这里。如果有人能澄清这件事，我会很高兴的。

Answer 1

如果一次只处理一个块，那么只在第一个块调用回您时处理一个块。您不需要一个信号量来完成这个任务，您只需要在回调中执行下一个读取。您可能需要@autoreleasepool块在readStreamBlock中，但我认为您不需要它。

当我有时间时，我可能会将它直接包装到RNCryptor中。我为此打开了Issue#47。我愿意撤回请求。

// Make sure that this number is larger than the header + 1 block.
// 33+16 bytes = 49 bytes. So it shouldn't be a problem.
int blockSize = 32 * 1024;

NSInputStream *cryptedStream = [NSInputStream inputStreamWithFileAtPath:@"C++ Spec.pdf"];
NSOutputStream *decryptedStream = [NSOutputStream outputStreamToFileAtPath:@"/tmp/C++.crypt" append:NO];

[cryptedStream open];
[decryptedStream open];

// We don't need to keep making new NSData objects. We can just use one repeatedly.
__block NSMutableData *data = [NSMutableData dataWithLength:blockSize];
__block RNEncryptor *decryptor = nil;

dispatch_block_t readStreamBlock = ^{
  [data setLength:blockSize];
  NSInteger bytesRead = [cryptedStream read:[data mutableBytes] maxLength:blockSize];
  if (bytesRead < 0) {
    // Throw an error
  }
  else if (bytesRead == 0) {
    [decryptor finish];
  }
  else {
    [data setLength:bytesRead];
    [decryptor addData:data];
    NSLog(@"Sent %ld bytes to decryptor", (unsigned long)bytesRead);
  }
};

decryptor = [[RNEncryptor alloc] initWithSettings:kRNCryptorAES256Settings
                                         password:@"blah"
                                          handler:^(RNCryptor *cryptor, NSData *data) {
                                            NSLog(@"Decryptor recevied %ld bytes", (unsigned long)data.length);
                                            [decryptedStream write:data.bytes maxLength:data.length];
                                            if (cryptor.isFinished) {
                                              [decryptedStream close];
                                              // call my delegate that I'm finished with decrypting
                                            }
                                            else {
                                              // Might want to put this in a dispatch_async(), but I don't think you need it.
                                              readStreamBlock();
                                            }
                                          }];

// Read the first block to kick things off    
readStreamBlock();

Answer 2

西里尔

由于内存限制，应用程序崩溃的原因是RNCryptor缓冲区超出了设备的能力。

基本上，您读取文件内容的速度远远快于RNCryptor所能处理的速度。因为它不能足够快地解密，所以它会缓冲传入的流，直到它能够处理它。

我还没有时间深入研究RNCryptor代码，并弄清楚它是如何使用GCD来管理所有内容的，但是您可以使用信号量来强制读取等待上一个块被解密。

下面的代码可以成功地解密iPad 1上的225 an文件，而不会崩溃。

它有一些问题，我不太满意，但它应该给你一个不错的起点。

有些事情要注意：

• 我将while循环的内部封装在一个@autoreleasepool块中，以强制释放数据。没有它，在while循环结束之前，发行版才会发生。(马特·加洛韦有一篇很棒的文章在这里解释：从ARC的引擎盖下面看一看
• 对dispatch_semaphore_wait的调用会阻止执行，直到接收到dispatch_semaphore_signal为止。这意味着如果发送太多UI更新和应用程序冻结的可能性(因此检查bytesRead > 0)。

就我个人而言，我认为必须有一个更好的解决方案，但我还没有时间来研究更多。

我希望这能帮到你。

- (IBAction)decryptWithSemaphore:(id)sender {
    dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);

    __block int total = 0;
    int blockSize = 32 * 1024;

    NSArray *docPaths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *input = [[docPaths objectAtIndex:0] stringByAppendingPathComponent:@"zhuge.rncryptor"];
    NSString *output = [[docPaths objectAtIndex:0] stringByAppendingPathComponent:@"zhuge.decrypted.pdf"];

    NSInputStream *cryptedStream = [NSInputStream inputStreamWithFileAtPath:input];
    __block NSOutputStream *decryptedStream = [NSOutputStream outputStreamToFileAtPath:output append:NO];
    __block NSError *decryptionError = nil;

    [cryptedStream open];
    [decryptedStream open];

    RNDecryptor *decryptor = [[RNDecryptor alloc] initWithPassword:@"12345678901234567890123456789012" handler:^(RNCryptor *cryptor, NSData *data) {
        @autoreleasepool {
            NSLog(@"Decryptor recevied %d bytes", data.length);
            [decryptedStream write:data.bytes maxLength:data.length];
            dispatch_semaphore_signal(semaphore);

            data = nil;
            if (cryptor.isFinished) {
                [decryptedStream close];
                decryptionError = cryptor.error;
                // call my delegate that I'm finished with decrypting
            }
        }
    }];

    while (cryptedStream.hasBytesAvailable) {
        @autoreleasepool {
            uint8_t buf[blockSize];
            NSUInteger bytesRead = [cryptedStream read:buf maxLength:blockSize];
            if (bytesRead > 0) {
                NSData *data = [NSData dataWithBytes:buf length:bytesRead];

                total = total + bytesRead;
                [decryptor addData:data];
                NSLog(@"New bytes to decryptor: %d Total: %d", bytesRead, total);

                dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
            }
        }
    }

    [cryptedStream close];
    [decryptor finish];

    dispatch_release(semaphore);

}

Answer 3

在过去的两天试图让我的MBProgress hud用Calman的代码更新它的进度之后，我想出了以下内容。使用的内存仍然很低，用户界面也会更新。

- (IBAction)decryptWithSemaphore:(id)sender {

__block int total = 0;
int blockSize = 32 * 1024;

NSArray *docPaths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *input = [[docPaths objectAtIndex:0] stringByAppendingPathComponent:@"zhuge.rncryptor"];
NSString *output = [[docPaths objectAtIndex:0] stringByAppendingPathComponent:@"zhuge.decrypted.pdf"];

NSInputStream *cryptedStream = [NSInputStream inputStreamWithFileAtPath:input];
__block NSOutputStream *decryptedStream = [NSOutputStream outputStreamToFileAtPath:output append:NO];
__block NSError *decryptionError = nil;
__block RNDecryptor *encryptor=nil;
NSDictionary *attributes = [[NSFileManager defaultManager] attributesOfItemAtPath:input error:NULL];
__block long long fileSize = [attributes fileSize];
[cryptedStream open];
[decryptedStream open];
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0);
dispatch_async(queue, ^{
    dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);

encryptor = [[RNDecryptor alloc] initWithPassword:@"12345678901234567890123456789012" handler:^(RNCryptor *cryptor, NSData *data) {
    @autoreleasepool {
        NSLog(@"Decryptor recevied %d bytes", data.length);
        [decryptedStream write:data.bytes maxLength:data.length];
        dispatch_semaphore_signal(semaphore);

        data = nil;
        if (cryptor.isFinished) {
            [decryptedStream close];
            decryptionError = cryptor.error;
            [cryptedStream close];
            [encryptor finish];
            // call my delegate that I'm finished with decrypting
        }
    }
}];

while (cryptedStream.hasBytesAvailable) {
    @autoreleasepool {
        uint8_t buf[blockSize];
        NSUInteger bytesRead = [cryptedStream read:buf maxLength:blockSize];
        if (bytesRead > 0) {
            NSData *data = [NSData dataWithBytes:buf length:bytesRead];

            total = total + bytesRead;
            [encryptor addData:data];
            NSLog(@"New bytes to decryptor: %d Total: %d", bytesRead, total);

            dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
            dispatch_async(dispatch_get_main_queue(), ^{
                HUD.progress = (float)total/fileSize;
            });

                           }
                           }
}
    });

}