在一个App中，弹窗一直是一个使用频率较高的提示类控件。苹果对用户体验方面的重视程度有多高，在弹窗的处理上就能体现出这一点来。不知你是否留意过新安装的App上的弹窗显示顺序？通常是这样的，如果先出现的是通知权限弹窗然后是定位权限弹窗，前一个弹窗会暂时隐藏，等用户关掉后一个弹窗后才再次弹出来。显然苹果认为后面的弹窗更重要，所以应当优先被用户处理。苹果对弹窗的弹出顺序进行了LIFO（last in, first out）后进先出的管理。
本文后面会讲解如何使用Runtime和多线程实现LIFO和FIFO （first in, last out）先进先出。先看一下最终效果，这是一个自定义转场动画（TransitionAnimation）的UIViewController。
最终效果
UIAlertView支持自动后进先出管理。但是，它从iOS8开始就已经被废弃了，再次使用UIAlertView，代码中会出现黄色警告提示。以下是UIAlertView被废弃的说明，苹果让我们使用UIAlertController去替代前者。
UIAlert View is deprecated in iOS 8. (Note that UIAlert View Delegate is also deprecated.) To create and manage alerts in iOS 8 and later, instead use UIAlert Controller with a preferred Style of UIAlert Controller Style Alert.
UIAlertController是继承于UIViewController并自定义了转场动画的控制器，和其它modal控制器一样调用presentViewController:animated:completion:方法弹出。但是，每个控制器只能拥有一个presentedController，也就是每次只能present一个别的控制器，强行present新的会出现以下提示：
Warning: Attempt to present <UIAlertController: 0x7fdb635045d0> on <ViewController: 0x7fdb660090f0> which is already presenting <UIAlertController: 0x7fdb635084a0>
很多情况下，异步请求结束后，我们需要根据服务器的返回信息进行弹窗提示。却无法保证当时的所在控制器是否已经present了别的UIAlertController，新的弹窗是弹不出来的。所以使用UIAlertController会给我们带来很大的困扰。
还有一种是把自定义的UIView盖到UIWindow的方式进行弹窗。首先这种方式不支持弹出顺序管理，其次同时弹出多个就是多次执行addSubview方法，很多半透明背景遮罩和view叠加在一起，显示效果不言而喻。更重要的是，很多系统控件也使用了window作为父控件，比如键盘的window是UITextEffectsWindow，频繁使用window可能会出现很多意想不到的问题。
总结一下上述3种弹窗方式：
| 弹窗方式 | 存在的问题 |
|---|---|
| UIAlertView | iOS8开始已经被废弃 |
| UIAlertController | 每次只能弹一个 |
| -[UIWindow addSubview:] | 同时弹出多个的显示效果较差 |
现在开始讲解如何用FIFO和LIFO管理modal控制器
先讲相对简单的FIFO
FIFO流程图
效果如下
ps: gif图中，弹窗2的点击事件中弹出弹窗4，观察在FIFO和LIFO中的区别
自定义UIViewController
UIAlertController
首先，新建一个UIViewController的分类，设计成分类的目的是做到百分百解耦，供控制器调用，并支持对所有继承于UIViewController的控制器进行FIFO和LIFO的modal管理。
分类的头文件只有一个方法，用来替代系统的presentViewController:animated:completion:方法。该方法接收一个UIViewController参数，一个present完成的回调和dismiss完成的回调。
// 枚举，要用哪种方式管理modal控制器
typedef NS_OPTIONS (NSUInteger, JCPresentType) {
JCPresentTypeLIFO = 0, // last in, first out
JCPresentTypeFIFO // first in, last out
};
// 新的present方法
- (void)jc_presentViewController:(UIViewController *)controller presentType:(JCPresentType)presentType presentCompletion:(void (^)(void))presentCompletion dismissCompletion:(void (^)(void))dismissCompletion;
.m文件中，方法的实现是这样的
// 判断JCPresentType枚举类型，跳转到具体方法
- (void)jc_presentViewController:(UIViewController *)controller presentType:(JCPresentType)presentType presentCompletion:(void (^)(void))presentCompletion dismissCompletion:(void (^)(void))dismissCompletion {
if (presentType == JCPresentTypeLIFO) {
[self lifoPresentViewController:controller presentCompletion:presentCompletion dismissCompletion:dismissCompletion];
} else {
[self fifoPresentViewController:controller presentCompletion:presentCompletion dismissCompletion:dismissCompletion];
}
}
// 核心方法
- (void)fifoPresentViewController:(UIViewController *)controller presentCompletion:(void (^)(void))presentCompletion dismissCompletion:(void (^)(void))dismissCompletion {
dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
NSBlockOperation *operation = [NSBlockOperation blockOperationWithBlock:^{
[controller setDeallocCompletion:^{
if (dismissCompletion) {
dismissCompletion();
}
// got to next operation
dispatch_semaphore_signal(semaphore);
}];
dispatch_async(dispatch_get_main_queue(), ^{
[self presentViewController:controller animated:YES completion:presentCompletion];
});
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
}];
// put in queue
if ([self getOperationQueue].operations.lastObject) {
[operation addDependency:[self getOperationQueue].operations.lastObject];
}
[[self getOperationQueue] addOperation:operation];
}
// NSOperationQueue单例，用于添加operation
- (NSOperationQueue *)getOperationQueue {
static NSOperationQueue *operationQueue = nil;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
operationQueue = [NSOperationQueue new];
});
return operationQueue;
}
// 使用关联对象存取deallocCompletion这个block
- (void)setDeallocCompletion:(void (^)(void))completion {
objc_setAssociatedObject(self, @selector(getDeallocCompletion), completion, OBJC_ASSOCIATION_COPY_NONATOMIC);
}
- (void (^)(void))getDeallocCompletion {
return objc_getAssociatedObject(self, _cmd);
}
// hook控制器的viewDidDisappear方法，在这个时候调deallocCompletion这个block
+ (void)load {
SEL oldSel = @selector(viewDidDisappear:);
SEL newSel = @selector(jc_viewDidDisappear:);
Method oldMethod = class_getInstanceMethod([self class], oldSel);
Method newMethod = class_getInstanceMethod([self class], newSel);
BOOL didAddMethod = class_addMethod(self, oldSel, method_getImplementation(newMethod), method_getTypeEncoding(newMethod));
if (didAddMethod) {
class_replaceMethod(self, newSel, method_getImplementation(oldMethod), method_getTypeEncoding(oldMethod));
} else {
method_exchangeImplementations(oldMethod, newMethod);
}
}
- (void)jc_viewDidDisappear:(BOOL)animated {
[self jc_viewDidDisappear:animated];
if ([self getDeallocCompletion] && ![self isTemporarilyDismissed]) {
[self getDeallocCompletion]();
}
}
其中dispatch_semaphore_t是多线程中的信号量，dispatch_semaphore_signal函数使信号量加一，dispatch_semaphore_wait使信号量减一，并且在信号量小于0的时候暂停当前线程。
presentViewController是一个耗时操作，我把这个操作放在NSBlockOperation中，用dispatch_semaphore_t暂停线程直到present完成。并设置每个NSBlockOperation的前后依赖，最后加到NSOperationQueue中，组成一个串行的先进先出队列。
下面开始讲LIFO
LIFO流程图
效果如下
自定义UIViewController
UIAlertController
// 核心方法
- (void)lifoPresentViewController:(UIViewController *)controller presentCompletion:(void (^)(void))presentCompletion dismissCompletion:(void (^)(void))dismissCompletion {
dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
// put in stack
NSMutableArray *stackControllers = [self getStackControllers];
if (![stackControllers containsObject:controller]) {
[stackControllers addObject:controller];
}
NSBlockOperation *operation = [NSBlockOperation blockOperationWithBlock:^{
__weak typeof(controller) weakController = controller;
[controller setPresentCompletion:presentCompletion];
[controller setDismissCompletion:dismissCompletion];
[controller setDeallocCompletion:^{
if (dismissCompletion) {
dismissCompletion();
}
// fetch new next controller if exists, because button action after dismiss completion
[weakController setDismissing:YES];
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(CGFLOAT_MIN * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
[weakController setDismissing:NO];
// if the dismiss controller is the last one
if (stackControllers.lastObject == controller) {
[stackControllers removeObject:weakController];
// is there any previous controllers
if (stackControllers.count > 0) {
UIViewController *preController = [stackControllers lastObject];
[self lifoPresentViewController:preController presentCompletion:[preController getPresentCompletion] dismissCompletion:[preController getDismissCompletion]];
}
} else {
NSUInteger index = [stackControllers indexOfObject:weakController];
[stackControllers removeObject:weakController];
// is there any next controllers
NSArray *nextControllers = [stackControllers objectsAtIndexes:[NSIndexSet indexSetWithIndexesInRange:NSMakeRange(index, stackControllers.count - index)]];
for (UIViewController *nextController in nextControllers) {
[self lifoPresentViewController:nextController presentCompletion:[nextController getPresentCompletion] dismissCompletion:[nextController getDismissCompletion]];
}
}
});
}];
// if the previous controller is dismissing, wait it's completion
if (stackControllers.count > 1) {
for (UIViewController *preController in stackControllers) {
if ([preController isDismissing]) {
return ;
}
}
}
// present a new controller before dismissing the presented controller if exists
dispatch_async(dispatch_get_main_queue(), ^{
if (self.presentedViewController) {
[self.presentedViewController temporarilyDismissViewControllerAnimated:YES completion:^{
[self presentViewController:controller animated:YES completion:^{
dispatch_semaphore_signal(semaphore);
}];
}];
} else {
[self presentViewController:controller animated:YES completion:^{
dispatch_semaphore_signal(semaphore);
if (presentCompletion) {
presentCompletion();
}
}];
}
});
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
}];
// put in queue
if ([self getOperationQueue].operations.lastObject) {
[operation addDependency:[self getOperationQueue].operations.lastObject];
}
[[self getOperationQueue] addOperation:operation];
}
// 使用关联对象存取dismissCompletion
- (void)setDismissCompletion:(void (^)(void))completion {
objc_setAssociatedObject(self, @selector(getDismissCompletion), completion, OBJC_ASSOCIATION_COPY_NONATOMIC);
}
- (void (^)(void))getDismissCompletion {
return objc_getAssociatedObject(self, _cmd);
}
// 使用关联对象存取presentCompletion
- (void)setPresentCompletion:(void (^)(void))completion {
objc_setAssociatedObject(self, @selector(getPresentCompletion), completion, OBJC_ASSOCIATION_COPY_NONATOMIC);
}
- (void (^)(void))getPresentCompletion {
return objc_getAssociatedObject(self, _cmd);
}
// 使用关联对象存取temporarilyDismissed，用于判断是临时隐藏还是用户关闭控制器
- (void)setTemporarilyDismissed:(BOOL)temporarilyDismissed {
objc_setAssociatedObject(self, @selector(isTemporarilyDismissed), @(temporarilyDismissed), OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}
- (BOOL)isTemporarilyDismissed {
NSNumber *num = objc_getAssociatedObject(self, _cmd);
return [num boolValue];
}
// 使用关联对象存取dismissing
- (void)setDismissing:(BOOL)dismissing {
objc_setAssociatedObject(self, @selector(isDismissing), @(dismissing), OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}
- (BOOL)isDismissing {
NSNumber *num = objc_getAssociatedObject(self, _cmd);
return [num boolValue];
}
// 数组栈，用于缓存所有传进来的控制器
- (NSMutableArray *)getStackControllers {
static NSMutableArray *stackControllers = nil;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
stackControllers = [NSMutableArray array];
});
return stackControllers;
}
// 临时dismiss方法
- (void)temporarilyDismissViewControllerAnimated: (BOOL)flag completion: (void (^ __nullable)(void))completion {
[self setTemporarilyDismissed:YES];
[self dismissViewControllerAnimated:flag completion:^{
[self setTemporarilyDismissed:NO];
if (completion) {
completion();
}
}];
}
和FIFO方法的区别是，LIFO方法中，每次进来会先判断前面是否已经有控制器了，如果有就先临时dismiss。并且，控制器在被用户关闭的时候，优先判断栈后面有没有还没有弹出来的控制器，然后才判断栈前面有没有控制器。
这样，一个具有FIFO和LIFO驱动的present分类方法就完成了，敢紧试试吧。
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