Question

我正在以预览模式捕获视频，并且想显示相机捕获的静止图像。

我当前将图像保存并捕获输出到界面中定义的ivars中：

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，什么也没有出现，实际上，调试器显示stillImageOutput和图像为nil。我认为这可能是异步捕获的时间问题，需要使用完成处理程序，但是我对完成处理程序不满意。

捕获静止图像后立即显示静止图像而不占用UI的正确方法是什么：

要捕获的代码：

- (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];
 }
 }];
 }

仍然显示的代码。请注意，可能没有问题的完成处理程序，但是，我不确定该如何写...

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

Answer 1

我将更改takeSnapshot方法以获取一个完成块，然后在另一个异步方法的完成块中调用该完成块： captureStillImageAsynchronouslyFromConnection:completionHandler

这是一个方法的示例，该方法先获取一个完成块，然后在内部调用的方法的完成块中对其进行回调：

// 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);
    }
}

以及如何命名的示例：

    [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入门使用块语法：http://goshdarnblocksyntax.com

使用AVFoundation从AVVideoCapture会话中显示异步stillImageOutput

1 个答案: