如何在Swift里用UnsafeMutablePointer

Posted 2020-08-14 brave-sailor

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<CGRect>, inView view: AutoreleasingUnsafeMutablePointer<UIView?>) {
     // 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<Memory>

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<CGRect>是一个CGRect的指针
• AutoreleasingUnsafeMutablePointer<UIView?>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<CGRect>
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<CGRect>, inView view: AutoreleasingUnsafeMutablePointer<UIView?>) { 
    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