自定義View有多種形式膜蛔，可以繼承自View，也可以繼承自ViewGroup脖阵，還可以直接繼承Andorid系統(tǒng)現(xiàn)有的View組件皂股，比如TextView、ImageView命黔、LinearLayout等呜呐，且每種方式都有它的使用場景。
繼承View
這種方式需要重寫onDraw方法悍募，主要用于實(shí)現(xiàn)一些不規(guī)則的布局效果蘑辑，通過xml布局不容易實(shí)現(xiàn)的情況下使用該方式，采用這種方式需要我們自己支持wrap_content并且處理padding等坠宴。繼承ViewGroup
該方式主要用于實(shí)現(xiàn)自定義的布局洋魂，把幾個(gè)View重新組合在一起，形成一個(gè)整體的View喜鼓。這種方式比較復(fù)雜副砍，需要合適地處理ViewGroup的測量和布局這兩個(gè)過程，并同時(shí)處理子元素的測量和布局過程庄岖。繼承系統(tǒng)現(xiàn)有的View
這種方式適用于擴(kuò)展現(xiàn)有View的功能址晕，比較常見，開發(fā)者不需要自己處理wrap_content并且處理padding等顿锰。繼承系統(tǒng)現(xiàn)有的ViewGroup
和方法2類似谨垃，但方法2更接近View的底層實(shí)現(xiàn)。
自定義View的注意事項(xiàng)
1硼控、直接繼承View或者ViewGroup的控件刘陶，如果不在onMeasure方法中對wrao_content做特出處理，那么當(dāng)外界在布局中使用wrap_content時(shí)就無法達(dá)到預(yù)期的效果牢撼，必須要讓這種方式自定義的View支持wrap_content匙隔。
2、另外熏版，還需要處理padding纷责，如果不處理padding，那么padding屬性在這個(gè)自定義View中是不起作用的撼短，如果是繼承自ViewGroup再膳，還需要在onMeasure和onLayout方法中考慮padding和子元素的margin對布局造成的影響，否則自定義View的padding和子元素的margin會不起作用曲横。
3喂柒、當(dāng)自定義View中需要停止線程或者動畫時(shí)不瓶，可以在onDetachedFromWindow方法中執(zhí)行停止動畫或線程的操作。因?yàn)楫?dāng)包含此View的Activity退出或者當(dāng)前View被remove時(shí)灾杰，View的onDetachedFromWindow方法會被調(diào)用蚊丐，和此方法對應(yīng)的是onAttachedToWindow，當(dāng)包含此View的Activity啟動時(shí)艳吠，View的onAttachedToWindow方法會被調(diào)用麦备。且當(dāng)View變得不可見時(shí)我們也需要停止線程和動畫，否則可能會造成內(nèi)存泄漏昭娩。
4凛篙、當(dāng)自定義View帶有滑動嵌套情形時(shí)，要處理好滑動沖突题禀，否則會影響View的效果鞋诗。
5膀捷、在自定義View的內(nèi)部最好不要使用Handler迈嘹，因?yàn)樗鼉?nèi)部已經(jīng)提供了post系統(tǒng)的方法，可完全代替Handler的作用全庸。
針對第一二點(diǎn)秀仲，需要處理支持wrap_content的情況時(shí)，我們可重寫onMeasure方法壶笼。
@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);
}
}
當(dāng)然我們需要先定義mWidth和mHeight這兩個(gè)寬高默認(rèn)值神僵，這個(gè)就隨便定義了。接下來處理padding的情況覆劈，也就是計(jì)算時(shí)我們要考慮四個(gè)padding值保礼，比如畫圓心時(shí)，需要這樣寫：
@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;
// 大圓半徑
int radius = Math.min(width, height) / 2;
canvas.drawCircle(paddingLeft + width / 2, paddingTop + height / 2, radius, borderPaint);
// 小圓半徑
int smallRadius = radius - (int) borderWidth;
canvas.drawCircle(paddingLeft + width / 2, paddingTop + height / 2, smallRadius, mPaint);
}
開發(fā)者還可以根據(jù)自己的需求為自定義View添加自定義屬性责语，一般在xml中屬性以android開頭的都是系統(tǒng)自帶的屬性炮障，添加自定義屬性有如下幾個(gè)步驟：
首先，在values文件夾下創(chuàng)建自定義屬性的xml坤候，一般是attrs.xml胁赢，然后在這個(gè)xml文件中定義自定義屬性集合名，集合中就可以自定義多個(gè)屬性了白筹。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>
在自定義View的初始化方法中需要獲取到這幾個(gè)自定義的屬性:
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中使用自定義View并添加自定義屬性：
<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" />
效果如圖所示：
我們知道在xml中使用自定義屬性時(shí)智末，前綴是app:，而且必須在布局文件開頭添加schemas聲明：xmlns:app="http://schemas.android.com/apk/res-auto"徒河，當(dāng)然這個(gè)前綴app可以自定義系馆，但必須保證這個(gè)前綴和布局中自定義屬性的前綴保持一致。比如你的聲明是xmlns:shenhuniurou="http://schemas.android.com/apk/res-auto"顽照，那么自定義屬性就得這么寫：shenhuniurou:border_width="5dp"它呀。
以上是我們基于View來繼承的自定義View的實(shí)現(xiàn)，下面我們來試試?yán)^承ViewGroup的自定義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的寬度
private int mChildIndex;// 子View的位置索引
private int mChildrenSize;// 子View個(gè)數(shù)
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)) {
// 表示水平滑動纵穿，父容器需要攔截
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);
// 計(jì)算水平速度
float xVelocity = mVelocityTracker.getXVelocity();
// 這里了是模擬ViewPager快速滑動時(shí)下隧，即使只滑動了一小段距離，也可以滑到下一頁去
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;
// 分別記錄上次滑動的坐標(biāo)
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谓媒，這里是把含有ListView的一整個(gè)布局加進(jìn)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) + "頁");
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();
}
});
}
主要關(guān)注的就是onMeasure方法中對于寬高是wrap_content的處理和onLayout方法中對子元素的布局淆院，CustomListView是為了解決橫向滑動沖突而自定義的ListView，最后的效果圖：
當(dāng)然這都是一些最基本的自定義View句惯，想要達(dá)到隨心所欲的自定義View的境界土辩，推薦看這兩個(gè)系列：
