具體分享看微信開放平臺文檔
壓縮圖片 參考 iOS 網絡圖片轉為UIImage 和微信分享時縮略圖片的壓縮

 
// 生成圖片
UIGraphicsBeginImageContextWithOptions(self.quoteImgView.frame.size, NO, 0.0);
    [self.view.layer renderInContext:UIGraphicsGetCurrentContext()];
    UIImage *shareImg=UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    // 下面就是組裝要分享到微信的內容
#pragma mark - ==============微信、朋友圈分享圖片===========
- (void)weixinShareWithTarget:(int)nTarget withImagePath:(NSString *)imagePath {
    
    if ([WXApi isWXAppInstalled]) {
        UIImage *image = [UIImage imageWithContentsOfFile:imagePath];
        NSData *thumbImgData = [WeiXinPayReq compressImage:image toByte:25 * 1024];
        WXMediaMessage *message = [WXMediaMessage message];
        message.thumbData = thumbImgData;
        WXImageObject *imageObject = [WXImageObject object];
        imageObject.imageData = UIImagePNGRepresentation(image);
        message.mediaObject = imageObject;
        SendMessageToWXReq *req = [[SendMessageToWXReq alloc]init];
        req.bText = NO;
        req.message = message;
        if (nTarget == 0) {
            req.scene = WXSceneSession;
        } else {
            req.scene = WXSceneTimeline;
        }
        [WXApi sendReq:req];
    } else {
        mAlert = [[UIAlertView alloc]initWithTitle:@"您尚未安裝微信" message:@"我們已將分享內容復制，請自行分享" delegate:self cancelButtonTitle:@"好的" otherButtonTitles: nil];
        [mAlert show];
        UIPasteboard *pasteboard = [UIPasteboard generalPasteboard];
        pasteboard.string = [NSString stringWithFormat:@"%@很好玩的，去下載和我一起玩吧!",APP_DisplayName];
    }
}

/**壓縮圖片到指定字節長度*/
+ (NSData *)compressImage:(UIImage *)image toByte:(NSUInteger)maxLength {
    // Compress by quality
    CGFloat compression = 1;
    NSData *data = UIImageJPEGRepresentation(image, compression);
    if (data.length < maxLength) return data;
    CGFloat max = 1;
    CGFloat min = 0;
    for (int i = 0; i < 6; ++i) {
        compression = (max + min) / 2;
        data = UIImageJPEGRepresentation(image, compression);
        if (data.length < maxLength * 0.9) {
            min = compression;
        } else if (data.length > maxLength) {
            max = compression;
        } else {
            break;
        }
    }
    UIImage *resultImage = [UIImage imageWithData:data];
    if (data.length < maxLength) return data;
    
    // Compress by size
    NSUInteger lastDataLength = 0;
    while (data.length > maxLength && data.length != lastDataLength) {
        lastDataLength = data.length;
        CGFloat ratio = (CGFloat)maxLength / data.length;
        CGSize size = CGSizeMake((NSUInteger)(resultImage.size.width * sqrtf(ratio)),
                                 (NSUInteger)(resultImage.size.height * sqrtf(ratio))); // Use NSUInteger to prevent white blank
        UIGraphicsBeginImageContext(size);
        [resultImage drawInRect:CGRectMake(0, 0, size.width, size.height)];
        resultImage = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();
        data = UIImageJPEGRepresentation(resultImage, compression);
    }
    return data;
}