前言
ViewPager是我们常用的控件之一,此篇文章我们用Scroller等知识实现一个简单的ViewPager。
效果:(源码在文章结尾)
涉及知识点onMeasure和onLayout
此点若不了解可以参考郭霖前辈的文章:
Android视图绘制流程完全解析,带你一步步深入了解View(二)
事件分发机制
此点可以参考笔者文章:
完全理解android事件分发机制
scrollTo:以View的初始位置为起点进行移动
scrollBy:以View的当前位置为起点进行移动
TouchSlop
系统可以识别出的被认为是滑动的最小距离。如果大于这个距离则是滑动。
getScrollX()获取到的值是屏幕的最左侧在整个空间中所占位置的X值。
打个比方:View是一条6米的绳子,而屏幕只能看到2~4米的绳子。那么getScrollX()的值就为2。如果屏幕看到的是3~5米的绳子,那么getScrollX()的值就为3。
Scroller的使用主要为3步:
1、初始化Scroller
2、重写computeScroll()方法
computeScroll()是在View的draw的时候调用的,而invalidate会导致View重绘,所以在重写computeScroll()之后,我们要使用invalidate()来间接调用它。
过程为:invalidate()->draw()->computeScroll()
3、使用startScroll()开启滑动
代码:
MainActivity:
package com.example.double2.scrollertest;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;
import android.widget.Toast;
public class MainActivity extends AppCompatActivity implements View.OnClickListener {
private Button btnOne;
private Button btnTwo;
private Button btnThree;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
btnOne=(Button)findViewById(R.id.btn_one);
btnTwo=(Button)findViewById(R.id.btn_two);
btnThree=(Button)findViewById(R.id.btn_three);
btnOne.setOnClickListener(this);
btnTwo.setOnClickListener(this);
btnThree.setOnClickListener(this);
}
@Override
public void onClick(View v) {
switch (v.getId()){
case R.id.btn_one:
Toast.makeText(this, "One", Toast.LENGTH_SHORT).show();
break;
case R.id.btn_two:
Toast.makeText(this, "Two", Toast.LENGTH_SHORT).show();
break;
case R.id.btn_three:
Toast.makeText(this, "Three", Toast.LENGTH_SHORT).show();
break;
}
}
}
ScrollerLayout:
package com.example.double2.scrollertest;
import android.content.Context;
import android.support.v4.view.ViewConfigurationCompat;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.View;
import android.view.ViewConfiguration;
import android.view.ViewGroup;
import android.widget.Scroller;
/**
* 项目名称:ScrollerTest
* 创建人:Double2号
* 创建时间:2017.2.23 12:02
* 修改备注:
*/
public class ScrollerLayout extends ViewGroup {
private Scroller mScroller;
private int mTouchSlop;
private float downX;
private float moveX;
private float lastMoveX;
private int leftBorder;
private int rightBorder;
public ScrollerLayout(Context context, AttributeSet attrs) {
super(context, attrs);
mScroller = new Scroller(context);
ViewConfiguration cOnfiguration= ViewConfiguration.get(context);
mTouchSlop = ViewConfigurationCompat.getScaledPagingTouchSlop(configuration);
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
int childCount = getChildCount();
for (int i = 0; i View childView = getChildAt(i);
measureChild(childView, widthMeasureSpec, heightMeasureSpec);
}
}
@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
if (changed) {
int childCount = getChildCount();
for (int i = 0; i View childView = getChildAt(i);
childView.layout(i * childView.getMeasuredWidth(), 0, (i + 1) * childView.getMeasuredWidth(), childView.getMeasuredHeight());
}
leftBorder = getChildAt(0).getLeft();
rightBorder = getChildAt(getChildCount() - 1).getRight();
}
}
@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
switch (ev.getAction()) {
case MotionEvent.ACTION_DOWN:
downX = ev.getRawX();
lastMoveX = downX;
break;
case MotionEvent.ACTION_MOVE:
moveX = ev.getRawX();
float diff = Math.abs(moveX - downX);
lastMoveX = moveX;
if (diff > mTouchSlop) {
return true;
}
break;
}
return super.onInterceptTouchEvent(ev);
}
@Override
public boolean onTouchEvent(MotionEvent event) {
switch (event.getAction()) {
case MotionEvent.ACTION_DOWN:
return true;
case MotionEvent.ACTION_MOVE:
moveX = event.getRawX();
int scrolledX = (int) (lastMoveX - moveX);
if (getScrollX() + scrolledX scrollTo(leftBorder, 0);
return true;
} else if (getScrollX() + getWidth() + scrolledX > rightBorder) {
scrollTo(rightBorder - getWidth(), 0);
return true;
}
scrollBy(scrolledX, 0);
lastMoveX = moveX;
break;
case MotionEvent.ACTION_UP:
int targetIndex = (getScrollX() + getWidth() / 2) / getWidth();
int dx = targetIndex * getWidth() - getScrollX();
mScroller.startScroll(getScrollX(), 0, dx, 0);
invalidate();
break;
}
return super.onTouchEvent(event);
}
@Override
public void computeScroll() {
if (mScroller.computeScrollOffset()) {
scrollTo(mScroller.getCurrX(), mScroller.getCurrY());
invalidate();
}
}
}
activity_main:
<com.example.double2.scrollertest.ScrollerLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
>
<Button
android:id="@+id/btn_one"
android:layout_width="match_parent"
android:layout_height="100dp"
android:text="One"/>
<Button
android:id="@+id/btn_two"
android:layout_width="match_parent"
android:layout_height="100dp"
android:text="Two"/>
<Button
android:id="@+id/btn_three"
android:layout_width="match_parent"
android:layout_height="100dp"
android:text="Three"/>
com.example.double2.scrollertest.ScrollerLayout>
源码地址:
http://download.csdn.net/detail/double2hao/9762018