Runtime学习与使用（一）：为UITextField添加类目实现被键盘遮住后视图上移,点击空白回收键盘

OC中类目无法直接添加属性，可以通过runtime实现在类目中添加属性。

在学习的过程中，试着为UITextField添加了一个类目，实现了当TextField被键盘遮住时视图上移的功能，顺便也添加了点击空白回收键盘功能。
效果预览
使用时不需要一句代码就可以实现上述功能

[github链接](https://github.com/a1419430265/CHTTextFieldHealper)

.h文件

 //
 //  UITextField+CHTPositionChange.h
 //  CHTTextFieldHealper
 //
 //  Created by risenb_mac on 16/8/17.
 //  Copyright © 2016年 risenb_mac. All rights reserved.
 //

 #import <UIKit/UIKit.h>

 @interface UITextField (CHTHealper)

 /**
  *  是否支持视图上移
  */
 @property (nonatomic, assign) BOOL canMove;
 /**
  *  点击回收键盘、移动的视图，默认是当前控制器的view
  */
 @property (nonatomic, strong) UIView *moveView;
 /**
  *  textfield底部距离键盘顶部的距离
  */
 @property (nonatomic, assign) CGFloat heightToKeyboard;

 @property (nonatomic, assign, readonly) CGFloat keyboardY;
 @property (nonatomic, assign, readonly) CGFloat keyboardHeight;
 @property (nonatomic, assign, readonly) CGFloat initialY;
 @property (nonatomic, assign, readonly) CGFloat totalHeight;
 @property (nonatomic, strong, readonly) UITapGestureRecognizer *tapGesture;
 @property (nonatomic, assign, readonly) BOOL hasContentOffset;

 @end

在.h文件中声明属性之后需要在.m中重写setter，getter方法
首先定义全局key用作关联唯一标识符

 static char canMoveKey;
 static char moveViewKey;

@implementation UITextField (CHTHealper)
@dynamic canMove;
@dynamic moveView;

具体实现

 - (void)setCanMove:(BOOL)canMove {
 // 参数意义：关联对象 ，关联标识符，关联属性值，关联策略
     objc_setAssociatedObject(self, &canMoveKey, @(canMove), OBJC_ASSOCIATION_RETAIN_NONATOMIC);
 }

 - (BOOL)canMove {
 // 关联属性值为对象类型，需要转换
     return [objc_getAssociatedObject(self, &canMoveKey) boolValue];
 }

想要实现键盘遮住TextField后视图上移，首先应确定TextField是否被键盘遮住，需要知道TextField在整个屏幕中的位置

// 此方法可以获得TextField左上角在当前window中的坐标
[self convertPoint:self.bounds.origin toView:[UIApplication sharedApplication].keyWindow]

还需要知道键盘高度，这点需要接受系统通知，但是什么时候接受通知、注销通知？
我的思路是在TextField成为第一响应者的时候，为TextField添加通知，但是如果直接重写becomeFirstResponder方法会覆盖掉UITextField本身的方法，造成的最明显的后果就是没有光标了……为了避免这个问题，我用了runtime另外一个强大的功能，方法交换
为了保证方法交换只进行一次，使用dispatch_once
为了保证方法交换尽早执行，写在了load方法中

 + (void)load {
     static dispatch_once_t onceToken;
     dispatch_once(&onceToken, ^{
         SEL systemSel = @selector(initWithFrame:);
         SEL mySel = @selector(setupInitWithFrame:);
         [self exchangeSystemSel:systemSel bySel:mySel];

         SEL systemSel2 = @selector(becomeFirstResponder);
         SEL mySel2 = @selector(newBecomeFirstResponder);
         [self exchangeSystemSel:systemSel2 bySel:mySel2];

         SEL systemSel3 = @selector(resignFirstResponder);
         SEL mySel3 = @selector(newResignFirstResponder);
         [self exchangeSystemSel:systemSel3 bySel:mySel3];

         SEL systemSel4 = @selector(initWithCoder:);
         SEL mySel4 = @selector(setupInitWithCoder:);
         [self exchangeSystemSel:systemSel4 bySel:mySel4];
     });
     [super load];
 }

具体交换步骤

 // 交换方法
 + (void)exchangeSystemSel:(SEL)systemSel bySel:(SEL)mySel {
     Method systemMethod = class_getInstanceMethod([self class], systemSel);
     Method myMethod = class_getInstanceMethod([self class], mySel);
     //首先动态添加方法，实现是被交换的方法，返回值表示添加成功还是失败
     BOOL isAdd = class_addMethod(self, systemSel, method_getImplementation(myMethod), method_getTypeEncoding(myMethod));
     if (isAdd) {
         //如果成功，说明类中不存在这个方法的实现
         //将被交换方法的实现替换到这个并不存在的实现
         class_replaceMethod(self, mySel, method_getImplementation(systemMethod), method_getTypeEncoding(systemMethod));
     }else{
         //否则，交换两个方法的实现
         method_exchangeImplementations(systemMethod, myMethod);
     }
 }

在上面我交换了四组方法，两组init方法，是为了保证无论是代码创建的还是xib拖得TextField都进行初始化

 - (instancetype)setupInitWithCoder:(NSCoder *)aDecoder {
     [self setup];
     return [self setupInitWithCoder:aDecoder];
 }

 - (instancetype)setupInitWithFrame:(CGRect)frame {
     [self setup];
     return [self setupInitWithFrame:frame];
 }

 - (void)setup {
     self.heightToKeyboard = ;
     self.canMove = YES;
     self.keyboardY = ;
     self.totalHeight = ;
     self.tapGesture = [[UITapGestureRecognizer alloc] initWithTarget:self action:@selector(tapAction)];
 }

在TextField成为第一响应者时，为self添加通知接收，为moveView添加点击事件（实现点击空白回收键盘），注销第一响应者时，注销通知，移除点击事件

 - (BOOL)newBecomeFirstResponder {
 // 如果没有设置moveView 默认为当前控制器的view
     if (self.moveView == nil) {
         self.moveView = [self viewController].view;
     }
 // 保证moveView只有一个本TextField的点击事件
     if (![self.moveView.gestureRecognizers containsObject:self.tapGesture]) {
         [self.moveView addGestureRecognizer:self.tapGesture];
     }
 // 当重复点击当前TextField时（重复成为第一响应者）或设置为不可移动 不再添加通知
     if ([self isFirstResponder] || !self.canMove) {
         return [self newBecomeFirstResponder];
     }
     [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(showAction:) name:UIKeyboardWillShowNotification object:nil];
     [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(hideAction:) name:UIKeyboardWillHideNotification object:nil];
     return [self newBecomeFirstResponder];
 }

 - (BOOL)newResignFirstResponder {
 // 确保当前moveView有当前点击事件，移除
     if ([self.moveView.gestureRecognizers containsObject:self.tapGesture]) {
         [self.moveView removeGestureRecognizer:self.tapGesture];
     }
     if (!self.canMove) {
         return [self newResignFirstResponder];
     }
     BOOL result = [self newResignFirstResponder];
     [[NSNotificationCenter defaultCenter] removeObserver:self name:UIKeyboardWillShowNotification object:nil];
     [[NSNotificationCenter defaultCenter] removeObserver:self name:UIKeyboardWillHideNotification object:nil];
 // 当另外一个TextField成为第一响应者，当前TextField注销第一响应者时不会回收键盘，手动调用moveView改变方法
     [self hideKeyBoard:];
     return result;
 }
 //获取当前TextField所在controller
 - (UIViewController *)viewController {
     UIView *next = self;
     while () {
         UIResponder *nextResponder = [next nextResponder];
         if ([nextResponder isKindOfClass:[UIViewController class]]) {
             return (UIViewController *)nextResponder;
         }
         next = next.superview;
     }
     return nil;
 }

接收到弹出键盘后调用的方法

 - (void)showAction:(NSNotification *)sender {
     if (!self.canMove) {
         return;
     }
 // 获取键盘高度以及键盘的Y坐标
     self.keyboardY = [sender.userInfo[UIKeyboardFrameEndUserInfoKey] CGRectValue].origin.y;
     self.keyboardHeight = [sender.userInfo[UIKeyboardFrameEndUserInfoKey] CGRectValue].size.height;
     [self keyboardDidShow];
 }

 - (void)hideAction:(NSNotification *)sender {
     if (!self.canMove || self.keyboardY == ) {
         return;
     }
     [self hideKeyBoard:0.25];
 }

 - (void)keyboardDidShow {
     if (self.keyboardHeight == ) {
         return;
     }
 // 获取TextField在window中的Y坐标
     CGFloat fieldYInWindow = [self convertPoint:self.bounds.origin toView:[UIApplication sharedApplication].keyWindow].y;
 // 确定是否需要视图上移，以及移动的距离
     CGFloat height = (fieldYInWindow + self.heightToKeyboard + self.frame.size.height) - self.keyboardY;
     CGFloat moveHeight = height >  ? height : ;

     [UIView animateWithDuration:0.25 animations:^{
 // 判断是否是scrollView并进行相应移动
         if (self.hasContentOffset) {
             UIScrollView *scrollView = (UIScrollView *)self.moveView;
             scrollView.contentOffset = CGPointMake(scrollView.contentOffset.x, scrollView.contentOffset.y + moveHeight);
         } else {
             CGRect rect = self.moveView.frame;
             self.initialY = rect.origin.y;
             rect.origin.y -= moveHeight;
             self.moveView.frame = rect;
         }
 // 记录当前TextField使得moveView移动的距离
         self.totalHeight += moveHeight;
     }];
 }

 - (void)hideKeyBoard:(CGFloat)duration {
     [UIView animateWithDuration:duration animations:^{
         if (self.hasContentOffset) {
             UIScrollView *scrollView = (UIScrollView *)self.moveView;
             scrollView.contentOffset = CGPointMake(scrollView.contentOffset.x, scrollView.contentOffset.y - self.totalHeight);
         } else {
             CGRect rect = self.moveView.frame;
             rect.origin.y += self.totalHeight;
             self.moveView.frame = rect;
         }
 // moveView回复状态后将移动距离置0
         self.totalHeight = ;
     }];
 }

点击事件当前controllerview endediting

- (void)tapAction {
    [[self viewController].view endEditing:YES];
}