在 ARC 中保留局部变量的对象引用

Question

我在 ARC 下创建了一个类，其中包含一些接受块的方法。问题是应用程序不断崩溃，我认为崩溃的原因是对象被 ARC 释放。我的问题是，我该如何解决这个问题，即如何保留对象的引用，以便在处理块之前不会释放对象。

这是 .h 类

#if NS_BLOCKS_AVAILABLE
typedef void (^KelaMagicalControlCompletionBlock)();
#endif

@interface KelaMagicalControl : NSObject

+(KelaMagicalControl *)controlWithTitle:(NSString *)title message:(NSString *)message;
-(id)initWithTitle:(NSString *)title message:(NSString *)message;

-(void)showWithTouchCompletionBlock:(KelaMagicalControlCompletionBlock)completionBlock;

@end

这是 .m 类

#import "KelaMagicalControl.h"

@interface KelaMagicalControl()

@property (nonatomic, strong) NSString * title;
@property (nonatomic, strong) NSString * message;

@property (copy) KelaMagicalControlCompletionBlock completionBlock;

@end

@implementation KelaMagicalControl

-(void)dealloc
{
   NSLog(@"deallocated");
}

+ (KelaMagicalControl *)toastWithTitle:(NSString *)title message:(NSString *)message
{
   KelaMagicalControl * magicalControl = [[KelaMagicalControl alloc] initWithTitle:title   message:message];
   return magicalControl;
}
-(id)initWithTitle:(NSString *)title message:(NSString *)message
{
    if(self = [super init])
    {
        _title = title;
        _message = message;
    }
    return self;
}

-(void)showWithTouchCompletionBlock:(void (^)())completionBlock
{

    UIWindow * mainWindow = [[UIApplication sharedApplication]keyWindow];
    UIView * viewTemp = [[UIView alloc] initWithFrame:CGRectMake(10, 10, 300, 100)];
    [viewTemp setTag:10001];
    [viewTemp setBackgroundColor:[UIColor redColor]];
    [mainWindow addSubview:viewTemp];

    UITapGestureRecognizer * tapGestureRecognizer = [[UITapGestureRecognizer alloc] initWithTarget:self action:@selector(mainViewTapped)];
    [viewTemp addGestureRecognizer:tapGestureRecognizer];

    self.completionBlock = completionBlock;

}

-(void)mainViewTapped
{
    if(self.completionBlock)
    {
        self.completionBlock();
        self.completionBlock = nil;
    }
}

从控制器类，我向自定义类的方法发送消息，如下所示：

-(IBAction)showMagicalControl:(id)sender
{
    NSString * title = @"Title";
    NSString * message = @"This is a very long message";


    KelaMagicalControl * magicalControl = [[KelaMagicalControl alloc] initWithTitle:title message:message];
    [magicalControl showWithTouchCompletionBlock:^{
        NSLog(@"control tapped");
    }];
}

所以它显示控件很好，但是当我点击它时，它没有执行块，而是崩溃并出现错误“obj_msgsend”。它甚至没有达到 showMagicalControl 方法。我认为，当我使用 ARC 时，它会自动释放，我可以看到 dealloc 被立即调用（在执行块之前）。如果我创建 magicRecord 的属性并使用它，它不会崩溃，但根据我的要求，我不想使用全局 iVar 或属性来调用这个块代码。

有什么建议吗？

Answer 1

问题是您的 KelaMagicalControl 在 showMagicalControl: 方法结束时被释放，它没有被保留在任何地方。只有您在 showWithTouchCompletionBlock: 中创建的 UIView 被保留，因为您将它添加到了超级视图，在本例中为窗口。这就是正确显示弹出窗口的原因。但是一个目标总是 unsafe_unretained，所以当你点击那个视图时，gestureRecognizer 会尝试调用你已经释放的 KelaMagicalControl，所以你会崩溃。

您可以通过将 KelaMagicalControl 设为 UIView 的子类来轻松解决此问题。我快速输入了您必须进行的更改：

.h 文件

#import <UIKit/UIKit.h>

#if NS_BLOCKS_AVAILABLE
typedef void (^KelaMagicalControlCompletionBlock)();
#endif

@interface KelaMagicalControl : UIView
{
    NSString* _title;
    NSString* _message;
}

-(id)initWithTitle:(NSString *)title message:(NSString *)message;
-(void)showWithTouchCompletionBlock:(KelaMagicalControlCompletionBlock)completionBlock;

@end

.m 文件

#import "KelaMagicalControl.h"

@interface KelaMagicalControl()

@property (nonatomic, strong) NSString * title;
@property (nonatomic, strong) NSString * message;

@property (copy) KelaMagicalControlCompletionBlock completionBlock;

@end

@implementation KelaMagicalControl

-(void)dealloc
{
    NSLog(@"deallocated");
}

+ (KelaMagicalControl *)toastWithTitle:(NSString *)title message:(NSString *)message
{
    KelaMagicalControl * magicalControl = [[KelaMagicalControl alloc] initWithTitle:title   message:message];
    return magicalControl;
}

-(id)initWithTitle:(NSString *)title message:(NSString *)message
{
    self = [super initWithFrame:CGRectMake(10, 10, 300, 300)];
    if (self)
    {
        _title = title;
        _message = message;
    }
    return self;
}

-(void)showWithTouchCompletionBlock:(void (^)())completionBlock
{
    UIWindow * mainWindow = [[UIApplication sharedApplication]keyWindow];
    [self setTag:10001];
    [self setBackgroundColor:[UIColor redColor]];
    [mainWindow addSubview:self];

    UITapGestureRecognizer * tapGestureRecognizer = [[UITapGestureRecognizer alloc] initWithTarget:self action:@selector(mainViewTapped)];
    [self addGestureRecognizer:tapGestureRecognizer];

    self.completionBlock = completionBlock;
}

-(void)mainViewTapped
{
    if(self.completionBlock)
    {
        self.completionBlock();
        self.completionBlock = nil;
    }
}
@end

由于您的 KelaMagicalControl 现在是您正在显示的 UIView，它会自动保留，因为它有一个超级视图。当您点击视图时，完成块现在将按照您的意愿执行。确保在完成块的末尾将其从其父视图中删除，否则它将永远不会被释放。

Answer 2

一种解决方案是为UIGestureRecognizer 使用block API（互联网上有很多版本），然后在block 中调用[self mainViewTapped]。这将保留您的 KelaMagicalControl，并确保只要手势识别器调用它，KelaMagicalControl 就可用。

Answer 3

你是对的，magicControl 被释放是因为它结束了他的作用域。我尚未测试以下内容，但它应该可以工作。

KelaMagicalControl * magicalControl = [[KelaMagicalControl alloc] initWithTitle:title message:message];
    [magicalControl showWithTouchCompletionBlock:^{
        KelaMagicalControl *retainedVar = magicalControl;
        NSLog(@"control tapped");
    }];

在块内声明强引用将保留magicControl。