Runtime學習與使用（一）：為UITextField添加類目實現被鍵盤遮住後視圖上移

OC中類目無法直接添加屬性，可以通過runtime實現在類目中添加屬性。

在學習的過程中，試著為UITextField添加了一個類目，實現了當TextField被鍵盤遮住時視圖上移的功能，順便也添加了點擊空白回收鍵盤功能。

效果預覽
使用時不需要一句代碼就可以實現上述功能
github鏈接

.h文件

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

#import 

@interface UITextField (CHTHealper)

/**
* 是否支持視圖上移
*/
@property (nonatomic, assign) BOOL canMove;
/**
* 點擊回收鍵盤、移動的視圖，默認是當前控制器的view
*/
@property (nonatomic, strong) UIView *moveView;
/**
* textfield底部距離鍵盤頂部的距離
*/
@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文件中聲明屬性之後需要在.m中重寫setter，getter方法
首先定義全局key用作關聯唯一標識符

static char canMoveKey;
static char moveViewKey;

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

具體實現

- (void)setCanMove:(BOOL)canMove {
// 參數意義：關聯對象 ，關聯標識符，關聯屬性值，關聯策略
  objc_setAssociatedObject(self, &canMoveKey, @(canMove), OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

- (BOOL)canMove {
// 關聯屬性值為對象類型，需要轉換
  return [objc_getAssociatedObject(self, &canMoveKey) boolValue];
}

想要實現鍵盤遮住TextField後視圖上移，首先應確定TextField是否被鍵盤遮住，需要知道TextField在整個屏幕中的位置

// 此方法可以獲得TextField左上角在當前window中的坐標
[self convertPoint:self.bounds.origin toView:[UIApplication sharedApplication].keyWindow]

還需要知道鍵盤高度，這點需要接受系統通知，但是什麼時候接受通知、注銷通知？
我的思路是在TextField成為第一響應者的時候，為TextField添加通知，但是如果直接重寫becomeFirstResponder方法會覆蓋掉UITextField本身的方法，造成的最明顯的後果就是沒有光標了……為了避免這個問題，我用了runtime另外一個強大的功能，方法交換
為了保證方法交換只進行一次，使用dispatch_once
為了保證方法交換盡早執行，寫在了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];
}

具體交換步驟

// 交換方法
+ (void)exchangeSystemSel:(SEL)systemSel bySel:(SEL)mySel {
  Method systemMethod = class_getInstanceMethod([self class], systemSel);
  Method myMethod = class_getInstanceMethod([self class], mySel);
  //首先動態添加方法，實現是被交換的方法，返回值表示添加成功還是失敗
  BOOL isAdd = class_addMethod(self, systemSel, method_getImplementation(myMethod), method_getTypeEncoding(myMethod));
  if (isAdd) {
    //如果成功，說明類中不存在這個方法的實現
    //將被交換方法的實現替換到這個並不存在的實現
    class_replaceMethod(self, mySel, method_getImplementation(systemMethod), method_getTypeEncoding(systemMethod));
  }else{
    //否則，交換兩個方法的實現
    method_exchangeImplementations(systemMethod, myMethod);
  }
}

在上面我交換了四組方法，兩組init方法，是為了保證無論是代碼創建的還是xib拖得TextField都進行初始化

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

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

- (void)setup {
  self.heightToKeyboard = 10;
  self.canMove = YES;
  self.keyboardY = 0;
  self.totalHeight = 0;
  self.tapGesture = [[UITapGestureRecognizer alloc] initWithTarget:self action:@selector(tapAction)];
}

在TextField成為第一響應者時，為self添加通知接收，為moveView添加點擊事件（實現點擊空白回收鍵盤），注銷第一響應者時，注銷通知，移除點擊事件

- (BOOL)newBecomeFirstResponder {
// 如果沒有設置moveView 默認為當前控制器的view
  if (self.moveView == nil) {
    self.moveView = [self viewController].view;
  }
// 保證moveView只有一個本TextField的點擊事件
  if (![self.moveView.gestureRecognizers containsObject:self.tapGesture]) {
    [self.moveView addGestureRecognizer:self.tapGesture];
  }
// 當重復點擊當前TextField時（重復成為第一響應者）或設置為不可移動 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 {
// 確保當前moveView有當前點擊事件，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];
// 當另外一個TextField成為第一響應者，當前TextField注銷第一響應者時不會回收鍵盤，手動調用moveView改變方法
  [self hideKeyBoard:0];
  return result;
}
//獲取當前TextField所在controller
- (UIViewController *)viewController {
  UIView *next = self;
  while (1) {
    UIResponder *nextResponder = [next nextResponder];
    if ([nextResponder isKindOfClass:[UIViewController class]]) {
      return (UIViewController *)nextResponder;
    }
    next = next.superview;
  }
  return nil;
}

接收到彈出鍵盤後調用的方法

- (void)showAction:(NSNotification *)sender {
  if (!self.canMove) {
    return;
  }
// 獲取鍵盤高度以及鍵盤的Y坐標
  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 == 0) {
    return;
  }
  [self hideKeyBoard:0.25];
}

- (void)keyboardDidShow {
  if (self.keyboardHeight == 0) {
    return;
  }
// 獲取TextField在window中的Y坐標
  CGFloat fieldYInWindow = [self convertPoint:self.bounds.origin toView:[UIApplication sharedApplication].keyWindow].y;
// 確定是否需要視圖上移，以及移動的距離
  CGFloat height = (fieldYInWindow + self.heightToKeyboard + self.frame.size.height) - self.keyboardY;
  CGFloat moveHeight = height > 0 ? height : 0;
 
  [UIView animateWithDuration:0.25 animations:^{
// 判斷是否是scrollView並進行相應移動
    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;
    }
// 記錄當前TextField使得moveView移動的距離
    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回復狀態後將移動距離置0
    self.totalHeight = 0;
  }];
}

點擊事件當前controllerview endediting

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