调度队列和异步 RNCryptor

Question

这是Asynchronously decrypt a large file with RNCryptor on iOS的后续行动

我已经设法使用本文中描述的方法异步解密一个下载的大型文件 (60Mb)，Calman 在他的回答中进行了更正。

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];

但是，我仍然面临一个问题：整个数据在解密之前已加载到内存中。我可以看到一堆“向解密器发送 X 字节”，然后在控制台中看到同一束“解密器收到 X 字节”，当我想看到“发送，接收，发送，接收，.. 。”。

这对于小型 (2Mb) 文件或模拟器上的大型 (60Mb) 文件都适用；但在真正的 iPad1 上，由于内存限制，它会崩溃，所以显然我不能为我的生产应用程序保留这个过程。

我觉得我需要使用dispatch_async 将数据发送到解密器，而不是在while 循环中盲目发送，但是我完全迷路了。我试过了：

• 在while 之前创建我自己的队列，并使用dispatch_async(myQueue, ^{ [decryptor addData:data]; });
• 相同，但在 while 循环内调度整个代码
• 相同，但调度整个 while 循环
• 使用RNCryptor-provided responseQueue 而不是我自己的队列

这 4 种变体都不起作用。

我还没有完全理解调度队列；我觉得问题出在这里。如果有人能对此有所了解，我会很高兴。

Answer 1

如果您只想一次处理一个块，则仅在第一个块回调您时处理一个块。您不需要信号量来执行此操作，您只需要在回调中执行下一次读取。您可能需要在 readStreamBlock 内添加一个 @autoreleasepool 块，但我认为您不需要它。

当我有时间时，我可能会将其直接包装到 RNCryptor 中。我为此打开了Issue#47。我愿意接受拉取请求。

// 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，

您的应用由于内存限制而崩溃的原因是 RNCryptor 缓冲区的增长超出了设备的能力。

基本上，您读取文件内容的速度比 RNCryptor 处理它的速度要快得多。由于它不能足够快地解密，它会缓冲传入的流，直到它可以处理它。

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到前一个块被解密。

下面的代码可以在 iPad 1 上成功解密一个 225MB 的文件而不会崩溃。

它有一些我不太满意的问题，但它应该给你一个不错的起点。

注意事项：

• 我将 while 循环的内部封装在 @autoreleasepool 块中以强制释放数据。没有它，直到 while 循环结束才会释放。 （Matt Galloway 有一篇很棒的帖子在这里解释它：A Look under ARC's hood
• 调用 dispatch_semaphore_wait 会阻塞执行，直到收到 dispatch_semaphore_signal。这意味着没有 UI 更新，并且如果您发送太多，应用程序可能会冻结（因此检查 bytesRead > 0）。

我个人觉得这一定有更好的解决方案，但我还没有时间去研究更多。

我希望这会有所帮助。

- (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 2 天试图让我的 MBProgress hud 使用 Calman 的代码更新其进度后，我想出了以下内容。 使用的内存仍然很低，并且 UI 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;
            });

                           }
                           }
}
    });

}