Objective-C中NSView子类实例未初始化报错问题求解

问题背景 #

尝试将简单Swift应用转换为Objective-C版本，以此学习NSLayoutAnchor在Objective-C中的使用方式。原Swift应用逻辑如下：

• 创建NSView子类，通过NSColor分类生成随机颜色，设置为子类视图layer的背景色
• 在AppDelegate的application:didFinishLaunchingWithOptions方法中创建两个该子类的实例
• 生成约束数组，将两个视图添加到父视图后激活约束即可正常运行

异常表现 #

转换后的Objective-C版本中，两个自定义NSView子类实例值始终为nil，单步调试可确认实例未完成初始化，应用启动时控制台输出如下崩溃信息：

-[__NSPlaceholderArray initWithObjects:count:]: attempt to insert nil object from objects[0]

初步判断为NSView子类初始化方式存在问题，经多轮排查未定位错误点。

相关代码 #

ColorView.m实现（RandomColor.h为实现randomColor方法的分类头文件）

#import "ColorView.h"
#import "RandomColor.h"

@implementation ColorView

- (id)initWithFrame:(NSRect)frameRect {
    self = [super initWithFrame:frameRect];
    
    if (self) {        
        self.wantsLayer = YES;
        self.layer = [[CALayer alloc] init];
        
        self.layer.backgroundColor = [[[NSColor alloc] randomColor] CGColor];
    }
    
    NSLog(@"Color View Initialized");
    return self;
}

@end

NSColor分类实现（经单独测试该部分逻辑无异常）

@implementation NSColor (RandomColor)
- (NSColor *)randomColor {
    CGFloat r = drand48();
    CGFloat g = drand48();
    CGFloat b = drand48();
    CGFloat alpha = 1.0;
    
    return [NSColor colorWithRed:r green:g blue:b alpha:alpha];
}

@end

AppDelegate相关代码

AppDelegate.h中声明了两个ColorView类型的属性，分别命名为leftView和rightView，AppDelegate.m实现代码如下：

@synthesize leftView;
@synthesize rightView;

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification {
    leftView.translatesAutoresizingMaskIntoConstraints = NO;
    rightView.translatesAutoresizingMaskIntoConstraints = NO;
    
    NSView *view = [self.window contentView];
    [self.window setBackgroundColor:[NSColor systemMintColor]];
    
    if (view) {
        [view addSubview:leftView];
        [view addSubview:rightView];
        
        [NSLayoutConstraint activateConstraints:@[
            [leftView.topAnchor constraintEqualToAnchor:[view topAnchor]],
            [leftView.leftAnchor constraintEqualToAnchor:[view leftAnchor]],
            [leftView.rightAnchor constraintEqualToAnchor:[view centerXAnchor]],
            [leftView.widthAnchor constraintGreaterThanOrEqualToConstant:150],
            [leftView.heightAnchor constraintEqualToConstant:100],
            [rightView.centerXAnchor constraintEqualToAnchor:[view centerXAnchor] constant:50],
            [rightView.centerYAnchor constraintEqualToAnchor:[view centerYAnchor]],
            [rightView.widthAnchor constraintEqualToConstant:100],
            [rightView.heightAnchor constraintEqualToConstant:100]
        ]];
    }
}

已查阅NSView子类化相关官方文档，但文档内容多数无法直接编译，未提供有效参考。

错误原因 #

崩溃的直接原因是**leftView和rightView两个属性从未执行初始化**：Objective-C中声明的对象属性默认值为nil，直接对nil对象调用方法、传入NSLayoutConstraint的字面量数组时，就会触发数组不能存储nil对象的崩溃。

代码中另外存在两处逻辑问题：

• randomColor方法定义为实例方法，但功能上属于工具类方法，应当声明为类方法，当前调用时需要先alloc NSColor实例，属于冗余且不符合设计逻辑的写法
• AppKit中NSView开启layer-backed模式（设置wantsLayer = YES）后，系统会自动生成适配的backing layer，无需手动创建CALayer实例赋值给self.layer，手动创建赋值反而可能引发显示异常。

修复方案 #

1. 修正NSColor分类方法定义 #

将randomColor改为类方法，去掉冗余的实例初始化逻辑：

// RandomColor.h中声明改为类方法
@interface NSColor (RandomColor)
+ (NSColor *)randomColor;
@end

// RandomColor.m实现对应修改
@implementation NSColor (RandomColor)
+ (NSColor *)randomColor {
    CGFloat r = drand48();
    CGFloat g = drand48();
    CGFloat b = drand48();
    return [NSColor colorWithRed:r green:g blue:b alpha:1.0];
}
@end

2. 修正ColorView初始化逻辑 #

删除手动创建CALayer的代码，直接调用类方法获取随机色：

@implementation ColorView

- (instancetype)initWithFrame:(NSRect)frameRect {
    self = [super initWithFrame:frameRect];
    if (self) {        
        self.wantsLayer = YES;
        self.layer.backgroundColor = [[NSColor randomColor] CGColor];
    }
    NSLog(@"Color View Initialized");
    return self;
}

@end

3. 在AppDelegate中初始化两个视图实例 #

在操作视图属性、添加约束前，先完成两个ColorView实例的初始化：

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification {
    // 初始化两个自定义视图，初始frame传NSZeroRect即可，后续会被自动布局约束覆盖
    self.leftView = [[ColorView alloc] initWithFrame:NSZeroRect];
    self.rightView = [[ColorView alloc] initWithFrame:NSZeroRect];

    leftView.translatesAutoresizingMaskIntoConstraints = NO;
    rightView.translatesAutoresizingMaskIntoConstraints = NO;
    
    NSView *contentView = [self.window contentView];
    [self.window setBackgroundColor:[NSColor systemMintColor]];
    
    if (contentView) {
        [contentView addSubview:leftView];
        [contentView addSubview:rightView];
        
        [NSLayoutConstraint activateConstraints:@[
            [leftView.topAnchor constraintEqualToAnchor:contentView.topAnchor],
            [leftView.leftAnchor constraintEqualToAnchor:contentView.leftAnchor],
            [leftView.rightAnchor constraintEqualToAnchor:contentView.centerXAnchor],
            [leftView.widthAnchor constraintGreaterThanOrEqualToConstant:150],
            [leftView.heightAnchor constraintEqualToConstant:100],
            [rightView.centerXAnchor constraintEqualToAnchor:contentView.centerXAnchor constant:50],
            [rightView.centerYAnchor constraintEqualToAnchor:contentView.centerYAnchor],
            [rightView.widthAnchor constraintEqualToConstant:100],
            [rightView.heightAnchor constraintEqualToConstant:100]
        ]];
    }
}

修改完成后重新运行，两个视图会按照约束规则正常显示，不会再出现nil插入数组的崩溃。

内容的提问来源于stack exchange，提问作者NotationMaster