如何在Swift里用UnsafeMutablePointer

下午在适配iPadUI的时候,用到了UIPopoverPresentationController,然后在转屏的时候需要调用UIPopoverPresentationControllerDelegate来返回一个适配后的view和CGRect,这里先看下在OC里的写法:
- (void)popoverPresentationController: (nonnull UIPopoverPresentationController *) popoverPresentationController willRepositionPopoverToRect:(inout nonnull CGRect *)rect inView:(inout UIView *__autoreleasing __nonnull * __nonnull)view {
*view = self.someView;
*rect = self.someView.bounds;
}

在OC里面你可以很方便的修改这里的参数值来返回正确的指针地址,但是在swift里面方法是这样子的:

func popoverPresentationController( popoverPresentationController:UIPopoverPresentationController, willRepositionPopoverToRect rect: UnsafeMutablePointer, inView view: AutoreleasingUnsafeMutablePointer) {
     // What to do here?
}

UnsafeMutablePointer和AutoreleasingUnsafeMutablePointer是什么鬼?这俩玩意要怎么返回值给他们?
先说结论
   在Swift里面rect.memory就是Objective-C里面的*rect

原因:

这两个参数并不是swift本身的写法混乱,而是为了让swift和Cocoa和UIKit下的Objective-C和C的frameworks 相互转换兼容,查阅apple的SDK你可以发现UnsafeMutablePointer就是一个结构体:

struct UnsafeMutablePointer

你可以把它看成一个Memory的指针,那现在再来看protocol里面的两个参数:

  • UnsafeMutablePointer是一个CGRect的指针
  • AutoreleasingUnsafeMutablePointer是一个可选View的指针

所以这里你可以通过它的memory属性来访问它的引用存储值:

/// Access the underlying raw memory, getting and setting 
values.public var memory: Memory { get nonmutating set }

例如:

// Objective-C assuming 
CGRect *rect;
CGRect oldRect = *rect;
*rect = newRect;

// Swift assuming 
rect: UnsafeMutablePointer
oldRect = rect.memory
rect.memory = newRect

简单来说:在Swift里面rect.memory就是Objective-C里面的*rect
所以最后我们可以在Swift里面这么写:

func popoverPresentationController( popoverPresentationController:UIPopoverPresentationController, willRepositionPopoverToRect rect: UnsafeMutablePointer, inView view: AutoreleasingUnsafeMutablePointer) { 
    view.memory = someView 
    rect.memory = someView.bounds
}

参考:
Swift Standard Library Reference - UnsafeMutablePointer Structure Reference
How to Dereference an Unsafe Mutable Pointer in Swift

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