当我们想找到一个视图的子视图相对于当前控制器中的坐标位置时，用坐标转换就可以很方便的找到了，如下图所示：
如图所示：从下到上依次是 self.view --> redView -->yellowView
redView的坐标：{{20, 20}, {335, 627}}，加在控制器的view上
yellowView的坐标：{{20, 20}, {295, 587}}，加在redView上
CGSize size = self.view.frame.size;
UIView *redView = [[UIView alloc] initWithFrame:CGRectMake(20, 20, size.width - 40, size.height - 40)];
redView.backgroundColor = [UIColor redColor];
[self.view addSubview:redView];
UIImageView *yellowView = [[UIImageView alloc] initWithFrame:CGRectMake(20, 20, redView.frame.size.width - 40, redView.frame.size.height - 40)];
yellowView.backgroundColor = [UIColor yellowColor];
[redView addSubview:yellowView];
NSLog(@"%@ --- %@", NSStringFromCGRect(redView.frame), NSStringFromCGRect(yellowView.frame));
// 输出{{20, 20}, {335, 627}} --- {{20, 20}, {295, 587}}
//把 yellowView.frame 从 redView 的坐标系转换到 self.view 的坐标系
// 如果传入的是 yellowView.frame，则要用 yellowView 的父视图来调用，toView：传入的参数是最终要转换到的那个视图。
// CGRect rect = [redView convertRect:yellowView.frame toView:self.view]; // 这个和下面一句一样
// 如果传入的是 yellowView.bounds，则要用 yellowView 本身来调用，toView：传入的参数是最终要转换到的那个视图。
CGRect rect = [yellowView convertRect:yellowView.bounds toView:self.view]; // 这个和上面一句一样
CGRect frame = [self.view convertRect:yellowView.frame fromView:redView];
NSLog(@"%@, %@", NSStringFromCGRect(rect), NSStringFromCGRect(frame));
// 输出{{40, 40}, {295, 587}}, {{40, 40}, {295, 587}}
//把 yellowView.frame.origin 从 redView 的坐标系转换到 self.view 的坐标系
CGPoint point = [redView convertPoint:yellowView.frame.origin toView:self.view];
CGPoint point1 = [self.view convertPoint:yellowView.frame.origin fromView:redView];
NSLog(@"%@, %@", NSStringFromCGPoint(point), NSStringFromCGPoint(point1));
// 输出{40, 40}, {40, 40}
