自定义View有多种形式，可以继承自View，也可以继承自ViewGroup，还可以直接继承Andorid系统现有的View组件，比如TextView、ImageView、LinearLayout等，且每种方式都有它的使用场景。
继承View
这种方式需要重写onDraw方法，主要用于实现一些不规则的布局效果，通过xml布局不容易实现的情况下使用该方式，采用这种方式需要我们自己支持wrap_content并且处理padding等。继承ViewGroup
该方式主要用于实现自定义的布局，把几个View重新组合在一起，形成一个整体的View。这种方式比较复杂，需要合适地处理ViewGroup的测量和布局这两个过程，并同时处理子元素的测量和布局过程。继承系统现有的View
这种方式适用于扩展现有View的功能，比较常见，开发者不需要自己处理wrap_content并且处理padding等。继承系统现有的ViewGroup
和方法2类似，但方法2更接近View的底层实现。
自定义View的注意事项
1、直接继承View或者ViewGroup的控件，如果不在onMeasure方法中对wrao_content做特出处理，那么当外界在布局中使用wrap_content时就无法达到预期的效果，必须要让这种方式自定义的View支持wrap_content。
2、另外，还需要处理padding，如果不处理padding，那么padding属性在这个自定义View中是不起作用的，如果是继承自ViewGroup，还需要在onMeasure和onLayout方法中考虑padding和子元素的margin对布局造成的影响，否则自定义View的padding和子元素的margin会不起作用。
3、当自定义View中需要停止线程或者动画时，可以在onDetachedFromWindow方法中执行停止动画或线程的操作。因为当包含此View的Activity退出或者当前View被remove时，View的onDetachedFromWindow方法会被调用，和此方法对应的是onAttachedToWindow，当包含此View的Activity启动时，View的onAttachedToWindow方法会被调用。且当View变得不可见时我们也需要停止线程和动画，否则可能会造成内存泄漏。
4、当自定义View带有滑动嵌套情形时，要处理好滑动冲突，否则会影响View的效果。
5、在自定义View的内部最好不要使用Handler，因为它内部已经提供了post系统的方法，可完全代替Handler的作用。
针对第一二点，需要处理支持wrap_content的情况时，我们可重写onMeasure方法。
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
int widthMode = MeasureSpec.getMode(widthMeasureSpec);
int heightMode = MeasureSpec.getMode(heightMeasureSpec);
int widthSize = MeasureSpec.getSize(widthMeasureSpec);
int heightSize = MeasureSpec.getSize(heightMeasureSpec);
if (widthMode == MeasureSpec.AT_MOST && heightMode == MeasureSpec.AT_MOST) {
setMeasuredDimension(mWidth, mHeight);
} else if (widthMode == MeasureSpec.AT_MOST) {
setMeasuredDimension(mWidth, heightMeasureSpec);
} else {
setMeasuredDimension(widthMeasureSpec, mHeight);
}
}
当然我们需要先定义mWidth和mHeight这两个宽高默认值，这个就随便定义了。接下来处理padding的情况，也就是计算时我们要考虑四个padding值，比如画圆心时，需要这样写：
@Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
int paddingLeft = getPaddingLeft();
int paddingRight = getPaddingRight();
int paddingTop = getPaddingTop();
int paddingBottom = getPaddingBottom();
int width = getWidth() - paddingLeft - paddingRight;
int height = getHeight() - paddingBottom - paddingTop;
// 大圆半径
int radius = Math.min(width, height) / 2;
canvas.drawCircle(paddingLeft + width / 2, paddingTop + height / 2, radius, borderPaint);
// 小圆半径
int smallRadius = radius - (int) borderWidth;
canvas.drawCircle(paddingLeft + width / 2, paddingTop + height / 2, smallRadius, mPaint);
}
开发者还可以根据自己的需求为自定义View添加自定义属性，一般在xml中属性以android开头的都是系统自带的属性，添加自定义属性有如下几个步骤：
首先，在values文件夹下创建自定义属性的xml，一般是attrs.xml，然后在这个xml文件中定义自定义属性集合名，集合中就可以自定义多个属性了。比如：
<?xml version="1.0" encoding="utf-8"?>
<resources>
<declare-styleable name="CircleView">
<attr name="circle_color" format="color" />
<attr name="border_color" format="color" />
<attr name="border_width" format="dimension" />
</declare-styleable>
</resources>
在自定义View的初始化方法中需要获取到这几个自定义的属性:
public class CircleView extends View {
private Paint mPaint, borderPaint;
private int mWidth, mHeight;
private int circleColor, borderColor;
private float borderWidth;
public CircleView(Context context) {
this(context, null);
}
public CircleView(Context context, @Nullable AttributeSet attrs) {
this(context, attrs, 0);
}
public CircleView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
init(context, attrs);
}
}
private void init(Context context, AttributeSet attrs) {
TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.CircleView);
circleColor = a.getColor(R.styleable.CircleView_circle_color, Color.GREEN);
borderColor = a.getColor(R.styleable.CircleView_border_color, Color.WHITE);
borderWidth = a.getDimension(R.styleable.CircleView_border_width, 0);
a.recycle();
mPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
mPaint.setColor(circleColor);
borderPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
borderPaint.setColor(borderColor);
mWidth = mHeight = 200;
}
最后在xml中使用自定义View并添加自定义属性：
<com.shenhuniurou.viewdemo.CircleView
android:id="@+id/circleView"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_margin="20dp"
android:background="@color/colorAccent"
android:padding="10dp"
app:border_color="@android:color/holo_blue_light"
app:border_width="5dp"
app:circle_color="@android:color/holo_orange_light" />
效果如图所示：
我们知道在xml中使用自定义属性时，前缀是app:，而且必须在布局文件开头添加schemas声明：xmlns:app="http://schemas.android.com/apk/res-auto"，当然这个前缀app可以自定义，但必须保证这个前缀和布局中自定义属性的前缀保持一致。比如你的声明是xmlns:shenhuniurou="http://schemas.android.com/apk/res-auto"，那么自定义属性就得这么写：shenhuniurou:border_width="5dp"。
以上是我们基于View来继承的自定义View的实现，下面我们来试试继承ViewGroup的自定义View。
package com.shenhuniurou.viewdemo;
import android.content.Context;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.VelocityTracker;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Scroller;
/**
* Created by Daniel on 2017/8/5.
*/
public class CustomViewPager extends ViewGroup {
private Scroller mScroller;
private VelocityTracker mVelocityTracker;
private int mChildWidth;// 子View的宽度
private int mChildIndex;// 子View的位置索引
private int mChildrenSize;// 子View个数
private int mLastXIntercept = 0;
private int mLastYIntercept = 0;
private int mLastX = 0;
private int mLastY = 0;
public CustomViewPager(Context context) {
this(context, null);
}
public CustomViewPager(Context context, AttributeSet attrs) {
this(context, attrs, 0);
}
public CustomViewPager(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
init();
}
private void init() {
mScroller = new Scroller(getContext());
mVelocityTracker = VelocityTracker.obtain();
}
@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
boolean intercepted = false;
int x = (int) ev.getX();
int y = (int) ev.getY();
switch (ev.getAction()) {
case MotionEvent.ACTION_DOWN:
intercepted = false;
if (!mScroller.isFinished()) {
mScroller.abortAnimation();
intercepted = true;
}
break;
case MotionEvent.ACTION_MOVE:
int deltaX = x - mLastXIntercept;
int deltaY = y - mLastYIntercept;
if (Math.abs(deltaX) > Math.abs(deltaY)) {
// 表示水平滑动，父容器需要拦截
intercepted = true;
} else {
intercepted = false;
}
break;
case MotionEvent.ACTION_UP:
intercepted = false;
break;
}
mLastX = x;
mLastY = y;
mLastXIntercept = x;
mLastYIntercept = y;
return intercepted;
}
@Override
public boolean onTouchEvent(MotionEvent event) {
mVelocityTracker.addMovement(event);
int x = (int) event.getX();
int y = (int) event.getY();
switch (event.getAction()) {
case MotionEvent.ACTION_DOWN:
if (!mScroller.isFinished()) {
mScroller.abortAnimation();
}
break;
case MotionEvent.ACTION_MOVE:
int deltaX = x - mLastX;
int deltaY = y - mLastY;
scrollBy(-deltaX, 0);
break;
case MotionEvent.ACTION_UP:
int scrollX = getScrollX();
mVelocityTracker.computeCurrentVelocity(1000);
// 计算水平速度
float xVelocity = mVelocityTracker.getXVelocity();
// 这里了是模拟ViewPager快速滑动时，即使只滑动了一小段距离，也可以滑到下一页去
if (Math.abs(xVelocity) >= 50) {
mChildIndex = xVelocity > 0 ? mChildIndex - 1 : mChildIndex + 1;
} else {
mChildIndex = (scrollX + mChildWidth / 2) /mChildWidth;
}
mChildIndex = Math.max(0, Math.min(mChildIndex, mChildrenSize - 1));
int dx = mChildIndex * mChildWidth - scrollX;
mScroller.startScroll(getScrollX(), 0, dx, 0, 500);
invalidate();
mVelocityTracker.clear();
break;
}
mLastX = x;
mLastY = y;
return true;
}
@Override
public void computeScroll() {
if (mScroller.computeScrollOffset()) {
scrollTo(mScroller.getCurrX(), mScroller.getCurrY());
postInvalidate();
}
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
int measuredWidth = 0;
int measuredHeight = 0;
final int childCount = getChildCount();
measureChildren(widthMeasureSpec, heightMeasureSpec);
int widthSpaceSize = MeasureSpec.getSize(widthMeasureSpec);
int widthSpecMode = MeasureSpec.getMode(widthMeasureSpec);
int heightSpaceSize = MeasureSpec.getSize(heightMeasureSpec);
int heightSpecMode = MeasureSpec.getMode(heightMeasureSpec);
if (childCount == 0) {
setMeasuredDimension(0, 0);
} else if (widthSpecMode == MeasureSpec.AT_MOST && heightSpecMode == MeasureSpec.AT_MOST) {
final View childView = getChildAt(0);
measuredWidth = childView.getMeasuredWidth() * childCount;
measuredHeight = childView.getMeasuredHeight();
setMeasuredDimension(measuredWidth, measuredHeight);
} else if (heightSpecMode == MeasureSpec.AT_MOST) {
final View childView = getChildAt(0);
measuredHeight = childView.getMeasuredHeight();
setMeasuredDimension(widthSpaceSize, measuredHeight);
} else if (widthSpecMode == MeasureSpec.AT_MOST) {
final View childView = getChildAt(0);
measuredWidth = childView.getMeasuredWidth() * childCount;
setMeasuredDimension(measuredWidth, heightSpaceSize);
}
}
@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
int childLeft = 0;
final int childCount = getChildCount();
mChildrenSize = childCount;
for (int i = 0; i < childCount; i++) {
final View childView = getChildAt(i);
if (childView.getVisibility() != View.GONE) {
final int childWidth = childView.getMeasuredWidth();
mChildWidth = childWidth;
childView.layout(childLeft, 0, childLeft + childWidth, childView.getMeasuredHeight());
childLeft += childWidth;
}
}
}
@Override
protected void onDetachedFromWindow() {
mVelocityTracker.recycle();
super.onDetachedFromWindow();
}
}
package com.shenhuniurou.viewdemo;
import android.content.Context;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.widget.ListView;
/**
* Created by Daniel on 2017/8/5.
*/
public class CustomListView extends ListView {
private CustomViewPager customViewPager;
// 分别记录上次滑动的坐标
private int mLastX = 0;
private int mLastY = 0;
public CustomListView(Context context) {
this(context, null);
}
public CustomListView(Context context, AttributeSet attrs) {
this(context, attrs, 0);
}
public CustomListView(Context context, AttributeSet attrs, int defStyle) {
super(context, attrs, defStyle);
}
public void setCustomViewPager(CustomViewPager customViewPager) {
this.customViewPager = customViewPager;
}
@Override
public boolean dispatchTouchEvent(MotionEvent event) {
int x = (int) event.getX();
int y = (int) event.getY();
switch (event.getAction()) {
case MotionEvent.ACTION_DOWN: {
customViewPager.requestDisallowInterceptTouchEvent(true);
break;
}
case MotionEvent.ACTION_MOVE: {
int deltaX = x - mLastX;
int deltaY = y - mLastY;
if (Math.abs(deltaX) > Math.abs(deltaY)) {
customViewPager.requestDisallowInterceptTouchEvent(false);
}
break;
}
case MotionEvent.ACTION_UP: {
break;
}
default:
break;
}
mLastX = x;
mLastY = y;
return super.dispatchTouchEvent(event);
}
}
<com.shenhuniurou.viewdemo.CustomViewPager
android:id="@+id/container"
android:layout_width="match_parent"
android:layout_height="match_parent">
</com.shenhuniurou.viewdemo.CustomViewPager>
private void initView() {
LayoutInflater inflater = getLayoutInflater();
mListContainer = (CustomViewPager) findViewById(R.id.container);
int screenWidth = MyUtils.getScreenMetrics(this).widthPixels;
// 往ViewGroup中添加ListView，这里是把含有ListView的一整个布局加进去
for (int i = 0; i < 5; i++) {
ViewGroup layout = (ViewGroup) inflater.inflate(R.layout.content_layout, mListContainer, false);
layout.getLayoutParams().width = screenWidth;
TextView textView = (TextView) layout.findViewById(R.id.title);
textView.setText("第 " + (i + 1) + "页");
layout.setBackgroundColor(Color.rgb(255 / (i + 10), 255 / (i + 10), 10));
addListView(layout);
mListContainer.addView(layout);
}
}
private void addListView(ViewGroup layout) {
CustomListView listView = (CustomListView) layout.findViewById(R.id.list);
List<String> datas = new ArrayList<>();
for (int i = 0; i < 50; i++) {
datas.add("item " + i);
}
ArrayAdapter<String> adapter = new ArrayAdapter<>(this, R.layout.content_list_item, R.id.name, datas);
listView.setCustomViewPager(mListContainer);
listView.setAdapter(adapter);
listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
Toast.makeText(MainActivity.this, "click item", Toast.LENGTH_SHORT).show();
}
});
}
主要关注的就是onMeasure方法中对于宽高是wrap_content的处理和onLayout方法中对子元素的布局，CustomListView是为了解决横向滑动冲突而自定义的ListView，最后的效果图：
当然这都是一些最基本的自定义View，想要达到随心所欲的自定义View的境界，推荐看这两个系列：
