ios - 是否可以使用 Quartz 2D 在另一个线程上制作 UIImage ？

Question

我想移动一些需要几秒钟才能在另一个线程上生成 UIImage 的代码，但是我在使用时遇到上下文错误

    UIGraphicsBeginImageContextWithOptions(size,false,0);

在调用调度生成图像之前，我尝试执行的每个操作都显示“无效上下文 0x0”。这是可能吗？

Answer 1

What's New in iOS: iOS 4.0是这样说的:

• Drawing to a graphics context in UIKit is now thread-safe. Specifically:
  • The routines used to access and manipulate the graphics context can now correctly handle contexts residing on different threads.
  • String and image drawing is now thread-safe.
  • Using color and font objects in multiple threads is now safe to do.

听起来你试过这样的事情:

UIGraphicsBeginImageContextWithOptions(size,false,0);
dispatch_async(someQueue, ^{
    [UIColor.whiteColor setFill];
    UIRectFill(0, 0, 20, 20);
    dispatch_async(dispatch_get_main_queue(), ^{
        UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();
        self.imageView.image = image;
    });
};

这行不通，因为每个线程都有自己的图形上下文堆栈(从 iOS 4.0 开始)。你需要这样做:

dispatch_async(someQueue, ^{
    UIGraphicsBeginImageContextWithOptions(size,false,0);
    [UIColor.whiteColor setFill];
    UIRectFill(0, 0, 20, 20);
    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    dispatch_async(dispatch_get_main_queue(), ^{
        self.imageView.image = image;
    });
};

更新

documentation UIGraphicsBeginImageContextWithOptions 和其他 UIKit 图形函数现在说

In iOS 4 and later, you may call this function from any thread of your app.

documentation对于 UIColor 说

Color objects are immutable and so it is safe to use them from multiple threads in your app.

documentation对于 UIFont 说

Font objects are immutable and so it is safe to use them from multiple threads in your app.

但是，documentation对于 UIKit NSString-drawing additions says

The methods described in this class extension must be used from your app’s main thread.

因此，您一定不在后台线程中尝试类似[@"hello"drawAtPoint:CGPointZero withAttributes:attrs]的操作。