使用 AVFoundation 显示来自 AVVideoCapture 会话的异步静止图像输出答案

【问题标题】：DIsplaying Asynchronous stillImageOutput from AVVideoCapture Session using AVFoundation使用 AVFoundation 显示来自 AVVideoCapture 会话的异步静止图像输出
【发布时间】：2019-02-08 23:35:03
【问题描述】：

我正在以预览模式捕捉视频，并希望显示相机捕捉到的静止图像。

我目前将图像和捕获输出保存到接口中定义的 ivars 为：

UIImage *snapshot
AVCaptureStillImageOutput* stillImageOutput;

视频显示正常。但是，当我尝试捕获并显示静止图像时，什么都没有出现，事实上，调试器显示 stillImageOutput 和图像为零。我认为这可能是异步捕获的时间问题，我需要使用完成处理程序，但我对完成处理程序很弱。

在不占用 UI 的情况下立即显示静止图像的正确方法是什么：

捕获静止的代码：

- (void)takeSnapshot {
 AVCaptureConnection *videoConnection = nil;
 for (AVCaptureConnection *connection in stillImageOutput.connections) {
 for (AVCaptureInputPort *port in [connection inputPorts]) {
 if ([[port mediaType] isEqual:AVMediaTypeVideo]) {
 videoConnection = connection;
 break;
 }
 }
 if (videoConnection) {
 break;
 }
 }
 [stillImageOutput captureStillImageAsynchronouslyFromConnection:videoConnection
 completionHandler:^(CMSampleBufferRef imageDataSampleBuffer, NSError *error) {
 if (imageDataSampleBuffer != NULL) {
 NSData *imageData = [AVCaptureStillImageOutput jpegStillImageNSDataRepresentation:imageDataSampleBuffer];
 snapshot = [UIImage imageWithData:imageData];
 }
 }];
 }

显示静止的代码。请注意缺少完成处理程序，这可能是个问题，但是，我不确定如何编写...

 [self takeSnapshot];
 self.imageView.image = snapshot;

【问题讨论】：

标签： ios objective-c avfoundation avcapture

【解决方案1】：

我会更改 takeSnapshot 方法以接收完成块，然后在其他异步方法的完成块中调用该完成块： captureStillImageAsynchronouslyFromConnection:completionHandler

这是一个方法示例，该方法获取一个完成块，然后在内部调用的方法的完成块中回调它：

// this correlates to your takeSnapshot method
// you want to add a completion portion to this method
- (void)doSomethingAsynchronouslyWithCompletion:(void (^)(NSData *completionData))completion {
    // call your other async method
    [self anotherAsyncMethodWithItsOwnCompletion:^(NSData *completionDataFromSecondMethod) {
        if (completionDataFromSecondMethod.length > 0) {
            // this is where you would receive the CMSampleBufferRef from the completion handler of captureStillImageAsynchronouslyFromConnection:completionHandler
            // and convert it over to to data
            // make sure the completion block isn't nil if it's nullable
            if (completion) {
                // you would want to pass back the NSData imageData in the completion block here
                completion(completionDataFromSecondMethod);
            }
        }
    }];
}

// this method would simulate the captureStillImageAsynchronouslyFromConnection:completionHandler: method
- (void)anotherAsyncMethodWithItsOwnCompletion:(void (^)(NSData * completionDataFromSecondMethod))anotherCompletion {
    // this is just to simulate some time waiting for the asnyc task to complete
    // never call sleep in your own code
    sleep(3);
    if (anotherCompletion) {
        // this simulates the fake CFSampleBufferRef passed back by the captureStillImage...
        NSData *fakeCompletionData = [@"FakeCompletionString" dataUsingEncoding:NSUTF8StringEncoding];
        anotherCompletion(fakeCompletionData);
    }
}

还有一个你怎么称呼它的例子：

    [self doSomethingAsynchronouslyWithCompletion:^(NSData *completionData) {
        if (completionData.length > 0) {
            // come back on the main queue to modify any UI Elements
            [[NSOperationQueue mainQueue] addOperationWithBlock:^{
                // this is where you want want to set your self.imageView.image
                // self.imageView.image = [UIImage imageWithData:{{dataFromCompletion}}]
                NSLog(@"The completionString result = %@", [[NSString alloc] initWithData:completionData encoding:NSUTF8StringEncoding]);
            }];
        }
    }];

此链接可能有助于您开始使用块语法：http://goshdarnblocksyntax.com

【讨论】：