临时读者注意：尽管有标题，但这个问题与UIScrollView属性bounces（滚动相关）或bouncesZoom.
我正在使用UIScrollView将缩放添加到自定义视图。自定义视图使用子图层来绘制其内容。每个子层都是一个CALayer实例，使用 . 添加到视图的主层[CALayer addSublayer:]。子层使用 CoreGraphics 来渲染它们的内容。
每次缩放完成后，自定义视图需要以新的缩放比例重新绘制其内容，以使内容再次显得清晰锐利。我目前正在尝试使用此 SO question中显示的工作方法，即在每次缩放操作后将滚动视图的zoomScale属性重置为 1.0，然后调整minimumZoomScale和maximumZoomScale属性，以便用户不能放大/缩小比原来更多故意的。
内容重绘已经正常工作（！），但我缺少的是平滑的 GUI 更新，以便缩放的内容被重绘到位而不会出现移动。使用我当前的解决方案（代码示例在这个问题的底部），我观察到一种“反弹”效果：缩放操作结束后，缩放的内容会短暂移动到不同的位置，然后立即移动回原来的位置地点。
我不完全确定“反弹”效果的原因是什么：要么有两个 GUI 更新周期（一个用于重置zoomScale到 1.0，另一个用于重置setNeedsDisplay），或者正在发生某种动画，使这两个更改都可见，一个在另一个之后。
我的问题是：如何防止上述“反弹”效应？
更新：以下是一个最小但完整的代码示例，您可以简单地复制和粘贴来观察我正在谈论的效果。
- 使用“空应用程序”模板创建一个新的 Xcode 项目。
- 将下面的代码分别添加到
AppDelegate.h和。AppDelegate.m - 在项目的链接构建阶段，添加对
QuartzCore.framework.
进入的东西AppDelegate.h：
#import <UIKit/UIKit.h>
@class LayerView;
@interface AppDelegate : UIResponder <UIApplicationDelegate, UIScrollViewDelegate>
@property (nonatomic, retain) UIWindow* window;
@property (nonatomic, retain) LayerView* layerView;
@end
进入的东西AppDelegate.m：
#import "AppDelegate.h"
#import <QuartzCore/QuartzCore.h>
@class LayerDelegate;
@interface LayerView : UIView
@property (nonatomic, retain) LayerDelegate* layerDelegate;
@end
@interface LayerDelegate : NSObject
@property(nonatomic, retain) CALayer* layer;
@property (nonatomic, assign) CGFloat zoomScale;
@end
static CGFloat kMinimumZoomScale = 1.0;
static CGFloat kMaximumZoomScale = 5.0;
@implementation AppDelegate
- (void) dealloc
{
self.window = nil;
self.layerView = nil;
[super dealloc];
}
- (BOOL) application:(UIApplication*)application didFinishLaunchingWithOptions:(NSDictionary*)launchOptions
{
[UIApplication sharedApplication].statusBarHidden = YES;
self.window = [[[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]] autorelease];
self.window.backgroundColor = [UIColor whiteColor];
UIScrollView* scrollView = [[[UIScrollView alloc] initWithFrame:self.window.bounds] autorelease];
[self.window addSubview:scrollView];
scrollView.contentSize = scrollView.bounds.size;
scrollView.delegate = self;
scrollView.minimumZoomScale = kMinimumZoomScale;
scrollView.maximumZoomScale = kMaximumZoomScale;
scrollView.zoomScale = 1.0f;
scrollView.bouncesZoom = NO;
self.layerView = [[[LayerView alloc] initWithFrame:scrollView.bounds] autorelease];
[scrollView addSubview:self.layerView];
[self.window makeKeyAndVisible];
return YES;
}
- (UIView*) viewForZoomingInScrollView:(UIScrollView*)scrollView
{
return self.layerView;
}
- (void) scrollViewDidEndZooming:(UIScrollView*)scrollView withView:(UIView*)view atScale:(float)scale
{
CGPoint contentOffset = scrollView.contentOffset;
CGSize contentSize = scrollView.contentSize;
scrollView.maximumZoomScale = scrollView.maximumZoomScale / scale;
scrollView.minimumZoomScale = scrollView.minimumZoomScale / scale;
// Big change here: This resets the scroll view's contentSize and
// contentOffset, and also the LayerView's frame, bounds and transform
// properties
scrollView.zoomScale = 1.0f;
CGFloat newZoomScale = self.layerView.layerDelegate.zoomScale * scale;
self.layerView.layerDelegate.zoomScale = newZoomScale;
self.layerView.frame = CGRectMake(0, 0, contentSize.width, contentSize.height);
scrollView.contentSize = contentSize;
[scrollView setContentOffset:contentOffset animated:NO];
[self.layerView setNeedsDisplay];
}
@end
@implementation LayerView
- (id) initWithFrame:(CGRect)frame
{
self = [super initWithFrame:frame];
if (self)
{
self.layerDelegate = [[[LayerDelegate alloc] init] autorelease];
[self.layer addSublayer:self.layerDelegate.layer];
// super's initWithFrame already invoked setNeedsDisplay, but we need to
// repeat because at that time our layerDelegate property was still empty
[self setNeedsDisplay];
}
return self;
}
- (void) dealloc
{
self.layerDelegate = nil;
[super dealloc];
}
- (void) setNeedsDisplay
{
[super setNeedsDisplay];
// Zooming changes the view's frame, but not the frame of the layer
self.layerDelegate.layer.frame = self.bounds;
[self.layerDelegate.layer setNeedsDisplay];
}
@end
@implementation LayerDelegate
- (id) init
{
self = [super init];
if (self)
{
self.layer = [CALayer layer];
self.layer.delegate = self;
self.zoomScale = 1.0f;
}
return self;
}
- (void) dealloc
{
self.layer = nil;
[super dealloc];
}
- (void) drawLayer:(CALayer*)layer inContext:(CGContextRef)context
{
CGRect layerRect = self.layer.bounds;
CGFloat radius = 25 * self.zoomScale;
CGFloat centerDistanceFromEdge = 5 * self.zoomScale + radius;
CGPoint topLeftCenter = CGPointMake(CGRectGetMinX(layerRect) + centerDistanceFromEdge,
CGRectGetMinY(layerRect) + centerDistanceFromEdge);
[self drawCircleWithCenter:topLeftCenter radius:radius fillColor:[UIColor redColor] inContext:context];
CGPoint layerCenter = CGPointMake(CGRectGetMidX(layerRect), CGRectGetMidY(layerRect));
[self drawCircleWithCenter:layerCenter radius:radius fillColor:[UIColor greenColor] inContext:context];
CGPoint bottomRightCenter = CGPointMake(CGRectGetMaxX(layerRect) - centerDistanceFromEdge,
CGRectGetMaxY(layerRect) - centerDistanceFromEdge);
[self drawCircleWithCenter:bottomRightCenter radius:radius fillColor:[UIColor blueColor] inContext:context];
}
- (void) drawCircleWithCenter:(CGPoint)center
radius:(CGFloat)radius
fillColor:(UIColor*)color
inContext:(CGContextRef)context
{
const int startRadius = [self radians:0];
const int endRadius = [self radians:360];
const int clockwise = 0;
CGContextAddArc(context, center.x, center.y, radius,
startRadius, endRadius, clockwise);
CGContextSetFillColorWithColor(context, color.CGColor);
CGContextFillPath(context);
}
- (double) radians:(double)degrees
{
return degrees * M_PI / 180;
}
@end