ViewGroup存在的目的就是爲了對其子View進行管理,爲其子View添加顯示、響應的規則。因此,自定義ViewGroup通常需要重寫onMeasure()方法來對子View進行測量,重寫onLayout()方法來確定子View的位置,重寫onTouchEvent()方法增加響應事件。
本例將實現一個類似Android原生控件ScrollView的自定義ViewGroup,且在滑動的過程中,增加一個粘性的效果,效果圖如下:
代碼如下:
package com.example.huangfei.myapplication;
import android.content.Context;
import android.util.AttributeSet;
import android.util.DisplayMetrics;
import android.view.MotionEvent;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Scroller;
/**
* Created by huangfeihong on 2016/5/15.
* 具有粘性效果的ScrollView
* 先讓自定義ViewGroup實現類似ScrollView的功能,再添加粘性效果
*/
public class MyScrollView extends ViewGroup {
private int mScreenHeight;
private Scroller mScroller;
private int mLastY;
private int mStart;
private int mEnd;
public MyScrollView(Context context) {
super(context);
initView(context);
}
public MyScrollView(Context context, AttributeSet attrs) {
super(context, attrs);
initView(context);
}
public MyScrollView(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
initView(context);
}
private void initView(Context context) {
DisplayMetrics metrics = context.getResources().getDisplayMetrics();
mScreenHeight = metrics.heightPixels; //讓每個子View都顯示完整的一屏
mScroller = new Scroller(context);
}
/**
* 先對其子View進行測量
* @param widthMeasureSpec
* @param heightMeasureSpec
*/
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
int count = getChildCount();
for (int i = 0; i < count; i++) {
View childView = getChildAt(i);
measureChild(childView, widthMeasureSpec, heightMeasureSpec);
}
}
/**
* 對其子View進行放置位置的設定
* @param changed
* @param l
* @param t
* @param r
* @param b
*/
@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
int childCount = getChildCount();
// 設置ViewGroup的高度
MarginLayoutParams mlp = (MarginLayoutParams) getLayoutParams();
mlp.height = childCount * mScreenHeight;
setLayoutParams(mlp);
//讓每個子View依次下排
for (int i = 0; i < childCount; i++) {
View childView = getChildAt(i);
if(childView.getVisibility() != View.GONE){
childView.layout(l, i * mScreenHeight, r, (i + 1) * mScreenHeight);
}
}
}
/**
* 在ViewGroup中添加滑動事件,可以使用scrollBy()方法來輔助滑動
* @param event
* @return
*/
@Override
public boolean onTouchEvent(MotionEvent event) {
int y = (int) event.getY();
switch (event.getAction()){
case MotionEvent.ACTION_DOWN:
mLastY = y;
//記錄觸摸起點
mStart = getScrollY();
break;
case MotionEvent.ACTION_MOVE:
if(!mScroller.isFinished()){
mScroller.abortAnimation();
}
int dy = mLastY - y;
if(getScrollY() < 0 && dy < 0)
dy = 0;
if(getScrollY() > getHeight() - mScreenHeight && dy > 0)
dy = 0;
scrollBy(0, dy);
mLastY = y;
break;
case MotionEvent.ACTION_UP:
//記錄觸摸終點
//mEnd = getScrollY();
//int dScrollY = mEnd - mStart;
int dScrollY = checkAlignment();
//實現粘性效果,滑動距離大於子View的1/3,則使用Scroller類來平滑到下一個子View,否則就會回滾
//到原來的位置
if(dScrollY > 0){//向上滑動
if(dScrollY < mScreenHeight / 3){
mScroller.startScroll(0, getScrollY(), 0, -dScrollY);
}else{
mScroller.startScroll(0, getScrollY(), 0, mScreenHeight - dScrollY);
}
}else{//向下滑動
if(-dScrollY < mScreenHeight / 3){
mScroller.startScroll(0, getScrollY(), 0, -dScrollY);
}else{
mScroller.startScroll(0, getScrollY(), 0, - mScreenHeight - dScrollY);
}
}
break;
}
postInvalidate();
return true;
}
/**
* 獲取手指滑動的距離
* @return
*/
private int checkAlignment() {
int mEnd = getScrollY();
boolean isUp = ((mEnd - mStart) > 0) ? true : false;
int lastPrev = mEnd % mScreenHeight;
int lastNext = mScreenHeight - lastPrev;
if (isUp) {
//向上的
return lastPrev;
} else {
return -lastNext;
}
}
@Override
public void computeScroll() {
super.computeScroll();
if(mScroller.computeScrollOffset()){
scrollTo(0, mScroller.getCurrY());
postInvalidate();
}
}
}