我正在使用 Dropbox Core API，但在寻找获取图像文件尺寸的方法时遇到了困难。我在设备上检索缩略图，但我需要知道图像的宽度和高度才能处理对它们的修改。

而且我绝对不想在手机上下载整个文件来检查其尺寸。你想过什么技巧来得到它们吗？我在元数据中唯一拥有的是文件大小，这对我来说毫无用处。

非常感谢。

最佳答案

我想出了我的答案。我使用 UIImage 类别而不是通过 URL 下载部分文件。一旦它获得足够的数据来定义大小，它就会停止下载。

我做了一些测试，它下载了大约 30 kB 来获取图片的尺寸，无论文件大小是 300 kB 还是 10 MB，这都非常快。

它可以用于任何图像文件，而不仅仅是 Dropbox API。

这是类别的标题:

#import <UIKit/UIKit.h>

typedef void (^UIImageSizeRequestCompleted) (NSURL* imgURL, CGSize size);

@interface UIImage (RemoteSize)

+ (void) requestSizeFor: (NSURL*) imgURL completion: (UIImageSizeRequestCompleted) completion;

@end

这是源文件:

#import "UIImage+RemoteSize.h"
#import <objc/runtime.h>`

#import <objc/runtime.h>

static char *kSizeRequestDataKey = "NSURL.sizeRequestData";
static char *kSizeRequestTypeKey = "NSURL.sizeRequestType";
static char *kSizeRequestCompletionKey = "NSURL.sizeRequestCompletion";

typedef uint32_t dword;

@interface NSURL (RemoteSize)

@property (nonatomic, strong) NSMutableData* sizeRequestData;
@property (nonatomic, strong) NSString* sizeRequestType;
@property (nonatomic, copy) UIImageSizeRequestCompleted sizeRequestCompletion;

@end

@implementation NSURL (RemoteSize)

- (void) setSizeRequestCompletion: (UIImageSizeRequestCompleted) block {
objc_setAssociatedObject(self, &kSizeRequestCompletionKey, block, OBJC_ASSOCIATION_COPY);
}

- (UIImageSizeRequestCompleted) sizeRequestCompletion {
return objc_getAssociatedObject(self, &kSizeRequestCompletionKey);
}

- (void) setSizeRequestData:(NSMutableData *)sizeRequestData {
objc_setAssociatedObject(self, &kSizeRequestDataKey, sizeRequestData, OBJC_ASSOCIATION_RETAIN);
}

- (NSMutableData*) sizeRequestData {
return objc_getAssociatedObject(self, &kSizeRequestDataKey);
}

- (void) setSizeRequestType:(NSString *)sizeRequestType {
objc_setAssociatedObject(self, &kSizeRequestTypeKey, sizeRequestType, OBJC_ASSOCIATION_RETAIN);
}

- (NSString*) sizeRequestType {
return objc_getAssociatedObject(self, &kSizeRequestTypeKey);
}

#pragma mark - NSURLConnectionDelegate

- (void) connection: (NSURLConnection*) connection didReceiveResponse:(NSURLResponse *)response {
[self.sizeRequestData setLength: 0];    //Redirected => reset data
}

- (void) connection: (NSURLConnection*) connection didReceiveData:(NSData *)data {
NSMutableData* receivedData = self.sizeRequestData;

if( !receivedData ) {
    receivedData = [NSMutableData data];
    self.sizeRequestData = receivedData;
}

[receivedData appendData: data];

//Parse metadata
const unsigned char* cString = [receivedData bytes];
const NSInteger length = [receivedData length];
const char pngSignature[8] = {137, 80, 78, 71, 13, 10, 26, 10};
const char bmpSignature[2] = {66, 77};
const char gifSignature[2] = {71, 73};
const char jpgSignature[2] = {255, 216};

if(!self.sizeRequestType ) {
    if( memcmp(pngSignature, cString, 8) == 0 ) {
        self.sizeRequestType = @"PNG";
    }
    else if( memcmp(bmpSignature, cString, 2) == 0 ) {
        self.sizeRequestType = @"BMP";
    }
    else if( memcmp(jpgSignature, cString, 2) == 0 ) {
        self.sizeRequestType = @"JPG";
    }
    else if( memcmp(gifSignature, cString, 2) == 0 ) {
        self.sizeRequestType = @"GIF";
    }
}

if( [self.sizeRequestType isEqualToString: @"PNG"] ) {
    char type[5];
    int offset = 8;

    dword chunkSize = 0;
    int chunkSizeSize = sizeof(chunkSize);

    if( offset+chunkSizeSize > length )
        return;

    memcpy(&chunkSize, cString+offset, chunkSizeSize);
    chunkSize = OSSwapInt32(chunkSize);
    offset += chunkSizeSize;

    if( offset + chunkSize > length )
        return;

    memcpy(&type, cString+offset, 4); type[4]='\0';
    offset += 4;

    if( strcmp(type, "IHDR") == 0 ) {   //Should always be first
        dword width = 0, height = 0;
        memcpy(&width, cString+offset, 4);
        offset += 4;
        width = OSSwapInt32(width);

        memcpy(&height, cString+offset, 4);
        offset += 4;
        height = OSSwapInt32(height);

        if( self.sizeRequestCompletion ) {
            self.sizeRequestCompletion(self, CGSizeMake(width, height));
        }

        self.sizeRequestCompletion = nil;

        [connection cancel];
    }
}
else if( [self.sizeRequestType isEqualToString: @"BMP"] ) {
    int offset = 18;
    dword width = 0, height = 0;
    memcpy(&width, cString+offset, 4);
    offset += 4;

    memcpy(&height, cString+offset, 4);
    offset += 4;

    if( self.sizeRequestCompletion ) {
        self.sizeRequestCompletion(self, CGSizeMake(width, height));
    }

    self.sizeRequestCompletion = nil;

    [connection cancel];
}
else if( [self.sizeRequestType isEqualToString: @"JPG"] ) {
    int offset = 4;
    dword block_length = cString[offset]*256 + cString[offset+1];

    while (offset<length) {
        offset += block_length;

        if( offset >= length )
            break;
        if( cString[offset] != 0xFF )
            break;
        if( cString[offset+1] == 0xC0 ||
           cString[offset+1] == 0xC1 ||
           cString[offset+1] == 0xC2 ||
           cString[offset+1] == 0xC3 ||
           cString[offset+1] == 0xC5 ||
           cString[offset+1] == 0xC6 ||
           cString[offset+1] == 0xC7 ||
           cString[offset+1] == 0xC9 ||
           cString[offset+1] == 0xCA ||
           cString[offset+1] == 0xCB ||
           cString[offset+1] == 0xCD ||
           cString[offset+1] == 0xCE ||
           cString[offset+1] == 0xCF ) {

            dword width = 0, height = 0;

            height = cString[offset+5]*256 + cString[offset+6];
            width = cString[offset+7]*256 + cString[offset+8];

            if( self.sizeRequestCompletion ) {
                self.sizeRequestCompletion(self, CGSizeMake(width, height));
            }

            self.sizeRequestCompletion = nil;

            [connection cancel];

        }
        else {
            offset += 2;
            block_length = cString[offset]*256 + cString[offset+1];
        }

    }
}
else if( [self.sizeRequestType isEqualToString: @"GIF"] ) {
    int offset = 6;
    dword width = 0, height = 0;
    memcpy(&width, cString+offset, 2);
    offset += 2;

    memcpy(&height, cString+offset, 2);
    offset += 2;

    if( self.sizeRequestCompletion ) {
        self.sizeRequestCompletion(self, CGSizeMake(width, height));
    }

    self.sizeRequestCompletion = nil;

    [connection cancel];
}
}

- (void) connection: (NSURLConnection*) connection didFailWithError:(NSError *)error {
if( self.sizeRequestCompletion )
    self.sizeRequestCompletion(self, CGSizeZero);
}

- (NSCachedURLResponse *)connection:(NSURLConnection *)connection willCacheResponse:(NSCachedURLResponse *)cachedResponse {
return cachedResponse;
}

- (void) connectionDidFinishLoading: (NSURLConnection *)connection {
// Basically, we failed to obtain the image size using metadata and the
// entire image was downloaded...

if(!self.sizeRequestData.length) {
    self.sizeRequestData = nil;
}
else {
    //Try parse to UIImage
    UIImage* image = [UIImage imageWithData: self.sizeRequestData];

    if( self.sizeRequestCompletion && image) {
        self.sizeRequestCompletion(self, [image size]);
        return;
    }
}

self.sizeRequestCompletion(self, CGSizeZero);
}

@end

@implementation UIImage (RemoteSize)

+ (void) requestSizeFor: (NSURL*) imgURL completion: (UIImageSizeRequestCompleted) completion {

if( [imgURL isFileURL] ) {
    //Load from file stream
}
else {
    imgURL.sizeRequestCompletion = completion;

    NSURLRequest* request = [NSURLRequest requestWithURL: imgURL];
    NSURLConnection* conn = [NSURLConnection connectionWithRequest: request delegate: imgURL];
    [conn scheduleInRunLoop: [NSRunLoop mainRunLoop] forMode: NSDefaultRunLoopMode];
    [conn start];
}
}

@end

非常感谢这篇文章对我有很大帮助: Remote image size without downloading

希望对你也有帮助。

关于ios - 通过文件URL远程获取图片文件尺寸，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/23609063/