ios - 使用 AVFoundation 显示来自 AVVideoCapture session 的异步 stillImageOutput

Question

我正在预览模式下拍摄视频，并希望显示相机拍摄的静止图像。

我目前将图像和捕获输出保存到接口(interface)中定义的 ivars 中:

UIImage *snapshot
AVCaptureStillImageOutput* stillImageOutput;

视频显示正常。但是，当我 try catch 和显示静止图像时，什么也没有出现，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，但我不擅长完成处理程序。

在不占用 UI 的情况下捕获静止图像后立即显示静止图像的正确方法是什么:

捕获静止图像的代码:

- (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];
 }
 }];
 }

静态显示的代码。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;

Answer 1

我将更改 takeSnapshot 方法以接受完成 block ，然后在其他异步方法的完成 block 中调用该完成 block :captureStillImageAsynchronouslyFromConnection:completionHandler

下面是一个方法的示例，该方法采用完成 block ，然后在内部调用的方法的完成 block 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);
    }
}

以及如何调用它的示例:

    [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 block 语法:http://goshdarnblocksyntax.com