带有NSData的UIImage initWithData返回nil

UIImage with NSData initWithData returns nil

本文关键字：返回 nil initWithData UIImage NSData 带有更新时间：2023-10-16

我有一个新问题!因为真正的问题不是在c++转换，而是我需要将返回的字符串数据字节转换为CGImageRef。如果有人知道怎么做，请去那个链接回答这个问题。

谢谢。

OK。我没有用protobuf的东西来混淆这个问题，而是简化了我的测试方法来模拟对protobuf的调用。

该测试方法完成以下两部分。第1部分获取一个UIImage并将其转换为std::string。

take a UIImage
获取NSData
将数据转换为unsigned char *
将无符号char *填充到std::string

该字符串是我们将从protobuf调用接收到的字符串。第2部分从字符串中获取数据并将其转换回NSData格式以填充UIImage。以下是执行该操作的步骤:

将std::字符串转换为char数组

将char数组转换为const char *

将char *放入NSData

返回NSData

- (NSData *)testProcessedImage:(UIImage *)processedImage
{
    // UIImage to unsigned char *
    CGImageRef imageRef = processedImage.CGImage;
    NSData *data = (NSData *) CFBridgingRelease(CGDataProviderCopyData(CGImageGetDataProvider(imageRef)));
    unsigned char *pixels = (unsigned char *)[data bytes];
    unsigned long size = [data length];
    // ***************************************************************************
    // This is where we would call transmit and receive the bytes in a std::string
    // ***************************************************************************
    // unsigned char * to string
    std::string byteString(pixels, pixels + size);

    // string to char array to const char *
    char myArray[byteString.size()+1];//as 1 char space for null is also required
    strcpy(myArray, byteString.c_str());
    const char *bytes = (const char *)myArray;
    // put byte array back into NSData format
    NSUInteger usize = byteString.length();
    data = [NSData dataWithBytes:(const void *)bytes length:sizeof(unsigned char)*usize];
    NSLog(@"examine data");
    return data;
}

是返回数据时的代码:

    NSData *data = [self.messageCommand testProcessedImage:processedImage];
    // But when I try to alloc init a UIImage with the data, the image is nil
    UIImage *image = [[UIImage alloc] initWithData:data];
    NSLog(@"examine image");

一切似乎都按计划进行，直到我尝试用数据创建UIImage。用那个数据Alloc初始化UIImage返回nil。

好的，所以问题很可能与Objective-C, C和c++数据结构之间的重复转换有关。总的来说，您需要确保将字符串初始化为字节数组，而不是文本C字符串，并且您希望返回没有空终止符的原始字节。我认为这样可以正确保存数据:

- (void)onDataReceived:(NSNotification *)note {
    if ([note.name isEqualToString:@"DataReceived"]) {
        NSDictionary *userData = note.userInfo;
        NSData *imageData = [userData objectForKey:@"ImageData"];
 // Note the two-argument string constructor -- this is necessary for non-textual data!
        std::string byteString = std::string(static_cast<const char*>([imageData bytes]), imageData.length);

 // We get the image back as a std::string
        std::string imageStr = [self.message parseMessage:byteString ofSize:byteString.size()];
        NSLog(@"examine imageStr");
 // We get the data from the std::string
        char *imageCStr = new char[imageStr.size()];
        imageStr.copy(imageCStr, imageStr.size());
        NSData *data = [NSData dataWithBytes:imageCStr length:imageStr.size()];
        delete[] imageCStr;
 // But when I try to alloc init a UIImage with the data, the image is nil
        UIImage *image = [[UIImage alloc] initWithData:data];
        NSLog(@"examine image");
    }
}

我试着回答。为了消除错误，我需要做一些小的修改。此外，我还更改了一些变量名，以尽量减少混淆。这仍然会为UIImage返回nil。

- (void)onObjectReceived:(NSNotification *)note {
    if ([note.name isEqualToString:@"ObjectReceived"]) {
        NSDictionary *userData = note.userInfo;
        NSData *objectData = [userData objectForKey:@"ObjectData"];
        // Added this because bytes is used below.  Or did you mean something else?
        const char *bytes = (const char *)[objectData bytes];
        // Note the two-argument string constructor -- this is necessary for non-textual data!
        std::string byteString = std::string(static_cast<const char*>(bytes), objectData.length);
        // This is an out parameter in the parseMessage method.
        long unsigned int *msgSize = (long unsigned *)malloc(sizeof(long unsigned int));
        // We get the image back as a std::string
        std::string imageStr = [self.message parseMessage:byteString outMsgSize:msgSize];
        NSLog(@"examine imageStr");
        // We get the data from the std::string
        char *imageCStr = new char[imageStr.size()];
        imageStr.copy(imageCStr, imageStr.size());
        NSData *data = [NSData dataWithBytes:imageCStr length:imageStr.size()];
        delete[] imageCStr;
        // But when I try to alloc init a UIImage with the data, the image is nil
        UIImage *image = [[UIImage alloc] initWithData:data];
        NSLog(@"examine image");
    }
}

我试着删除中间的一切，它工作。下面是代码:

- (NSData *)testProcessedImage:(UIImage *)processedImage
{
    // UIImage to unsigned char *
    CGImageRef imageRef = processedImage.CGImage;
    NSData *data1 = (NSData *) CFBridgingRelease(CGDataProviderCopyData(CGImageGetDataProvider(imageRef)));
    UIImage *image = [UIImage imageWithCGImage:imageRef];

这告诉我NSData dataWithBytes不会工作，因为我使用的是CGImageRef。我的图像是来自相机的原始数据(不是PNG或JPEG)。

我在网上找到了这个答案，这很有帮助。询者甚至发表了这样的评论，"看起来很简单包装(数据)在CFData对象，然后CGDataProviderCreateWithCFData。"

我在SO上找到了另一个答案，也很有帮助。它展示了如何使用字符串。

创建CFDataRef。有很多有用的信息在那里，但仍然没有找到我需要的。我要问另一个问题，当我得到答案时，把它引用到这里。

谢谢。