View的工作原理
2018-09-26 本文已影响0人
要学的东西太多了
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ViewRoot对应于ViewRootImpl类，是连接Windowmanager和DecorView的纽带，View的三大流程（measure-测量View的宽高，通过getMeasureWidth和getMeasureHeight方法能拿到宽高，一般来说等于最终宽高、layout-确定View的位置即四个顶点的坐标和View的最终宽高、draw-决定View在屏幕上的显示）均由ViewRoot完成。
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MeasureSpec用来测量View的宽高，它是一个32位int值，高两位表示测量模式（specMode），低30表示这个模式下的规格大小（specSize），由父容器和LayoutParams共同决定。规则如下图：
View创建Measure值的规则.png
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specMode分三类：无限制（UNSPECIFIED），要多大给多大；精确大小（EXACTLY），对应`LayoutParams的match_parent和具体数值；最大值（ATMOST），不超过这个范围，对应LayoutParams的wrap_parent。
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直接继承View的自定义控件，要在OnMeasure方法中对wrap_content做特殊处理，如下所示：
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
int defautWidth=20,defaultHeight=20;//wrap_content下设置的默认值
int widthMode = MeasureSpec.getMode(widthMeasureSpec);
int widthSize = MeasureSpec.getSize(widthMeasureSpec);
int heightMode = MeasureSpec.getMode(heightMeasureSpec);
int heightSize = MeasureSpec.getSize(heightMeasureSpec);
if(widthMode == MeasureSpec.AT_MOST && heightMode == MeasureSpec.AT_MOST){
setMeasuredDimension(defautWidth,defaultHeight);
}else if(widthMode == MeasureSpec.AT_MOST){
setMeasuredDimension(defautWidth,heightSize);
}else if(heightMode == MeasureSpec.AT_MOST){
setMeasuredDimension(widthSize,defaultHeight);
}else{
super.onMeasure(widthMeasureSpec,heightMeasureSpec);
}
}
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ViewGroup是一个抽象类，自定义 Viewgroup要重写OnMeasure方法，同时提供了一个measureChildren的方法，在里面去循环遍历调用子元素的measure方法。
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系统可能会多次measure才能确定宽高，要拿到View的宽高最好在onlayout方法里取。
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在Activity的创建过程中去拿View的宽高都是0，因为Activity创建过程和View的测量过程不是同步的，要拿到宽高可以用下面几种方案：
(1)onWindowFoucsChanged,不过要注意，这个方法会被频繁调用，典型代码如下：
@Override
public void onWindowFocusChanged(boolean hasFocus) {
super.onWindowFocusChanged(hasFocus);
if(hasFocus){
int width = view.getMeasuredWidth();
int heigt = view.getMeasuredHeight();
}
}
（2）view.post(runnable)。典型代码如下：
@Override
protected void onStart() {
super.onStart();
view.post(new Runnable(){
@Override
public void run() {
int width = view.getMeasuredWidth();
int heigt = view.getMeasuredHeight();
}
});
}
（3）ViewTreeObserver。典型代码如下：
@Override
protected void onStart() {
super.onStart();
ViewTreeObserver viewTreeObserver = view.getViewTreeObserver();
viewTreeObserver.addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
@Override
public void onGlobalLayout() {
view.getViewTreeObserver().removeOnGlobalLayoutListener(this);
int width = view.getMeasuredWidth();
int heigt = view.getMeasuredHeight();
}
});
}
（4）view.measure(int widthMeasureSpec , int heightMeasureSpec)。根据view的LayoutParams来处理，具体情况如下：
1.match_parent，直接放弃，无法拿到父容器的剩余空间。
2.具体数值，如下：
int widthMeasureSpec = MeasureSpec.makeMeasureSpec(100,MeasureSpec.EXACTLY);
int heightMeasureSpec = MeasureSpec.makeMeasureSpec(100,MeasureSpec.EXACTLY);
view.measure(widthMeasureSpec,heightMeasureSpec);
3.wrap_content，如下：
int widthMeasureSpec = MeasureSpec.makeMeasureSpec((1<<30)-1, MeasureSpec.AT_MOST);
int heightMeasureSpec = MeasureSpec.makeMeasureSpec((1<<30)-1, MeasureSpec.AT_MOST);
view.measure(widthMeasureSpec,heightMeasureSpec);
- 这几种方案里面，post是最常用的，GlobalLayoutListener的方式在布局发生变化的时候都会调用，在里面尽量不要生成大量对象，且一定记得remove。此外，post还可以切换到主线程。post为什么能拿到宽高并切换线程呢，具体看下它的实现：
public boolean post(Runnable action) {
final AttachInfo attachInfo = mAttachInfo;
if (attachInfo != null) {
return attachInfo.mHandler.post(action);
}
getRunQueue().post(action);
return true;
}
下面分情况分析：
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如果attachInfo不等于空，就直接调用它的handler发送消息，不等于空就放入HandlerActionQueue的mActions里面等待调用。HandlerActionQueue有一个executeActions方法，会把队列里面的消息全部发送出去。executeActions是在什么时候调用呢，view的dispatchAttachedToWindow，这个方法是在ViewRootImpl的performTraversals调用。在performTraversals里面，executeActions之后才会调用performMeasure、performLayout和performDraw，那么为什么post能拿到宽高呢，因为executeActions只是把消息放到handler的消息队列里面，performTraversals本来就是一个消息的处理，执行完才会去执行下一个消息，所以post的消息被处理的时候，已经测量好了。
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那么attachInfo 是在什么时候初始化的呢？答案也在dispatchAttachedToWindow里面，dispatchAttachedToWindow又是在哪里调用的呢？viewParent的实现类ViewRootImpl的performTraversals里面调用，并把mAttachInfo传进去，这个mAttachInfo是在ViewRootImpl的构造方法里面初始化的，它持有的mHandler是一个常量，由于ViewRootImpl是在主线程里面初始化的，所以handler处理消息也是在主线程，那么post就能做到线程切换。
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其实viewTreeObserver.addOnGlobalLayoutListener也在performTraversals有调用，都在performMeasure和performLayout之后，所以也能拿到宽高。如下：
mAttachInfo.mTreeObserver.dispatchOnGlobalLayout();
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自定义View主要分为四种方式：继承View重写onDraw方法；继承ViewGroup派生特殊的Layout；继承特定的View（如TextView）；继承特定的ViewGroup（如LinearLayout）。
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自定义View的注意事项：
（1）让View支持wrap_content。如果不在onMeasure中对wrap_content做处理，那么wrap_content是不起作用的，相当于match_parent。因为view默认的onMeasure方法如下：
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
getDefaultSize方法就是根据mode返回不同的size值，可以看出AT_MOST和EXACTLY都是返回的父容器可提供的最大值，UNSPECIFIED一般用于系统测量，这里返回的是传入的getSuggestedMinimumWidth，如下：
public static int getDefaultSize(int size, int measureSpec) {
int result = size;
int specMode = MeasureSpec.getMode(measureSpec);
int specSize = MeasureSpec.getSize(measureSpec);
switch (specMode) {
case MeasureSpec.UNSPECIFIED:
result = size;
break;
case MeasureSpec.AT_MOST:
case MeasureSpec.EXACTLY:
result = specSize;
break;
}
return result;
}
getSuggestedMinimumWidth里面返回的判断逻辑如下：
protected int getSuggestedMinimumWidth() {
return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());
}
mMinWidth /mMinHeight ，即xml文件中的minWidth和minHeight，mBackground.getMinimumWidth()表示背景图片的实际大小，例如shape这一类背景是没有实际大小的，bitmap则有。
（2）如果有必要，支持padding。继承View的控件不在draw中处理padding的话，padding不起作用；继承ViewGroup的控件在onMeasure和onLayout中要考虑padding和子元素的margin带来的影响。
（3）尽量不要在View中使用handler，因为View本身提供了post系列方法。
（4）View中有线程和动画要及时停止，在onDetachedFromWindow方法中处理，否则可能内存泄漏。
（5）合理处理滑动冲突。
示例：
1.继承View：
public class CircleView extends View{
private int defalultColor = Color.parseColor("#000000");
private int circleColor = defalultColor;
private float defalultRidus = 5.0f;
private float ridus = defalultRidus;
private Paint paint;
private Context context;
public CircleView(Context context) {
this(context,null);
}
public CircleView(Context context, @Nullable AttributeSet attrs) {
super(context, attrs);
this.context=context;
init(attrs);
}
private void init(AttributeSet attrs){
paint = new Paint(Paint.ANTI_ALIAS_FLAG);
if(attrs!=null){
TypedArray typedArray = context.getResources().obtainAttributes(attrs,R.styleable.CircleView);
circleColor = typedArray.getColor(R.styleable.CircleView_circleColor, defalultColor);
ridus = typedArray.getFloat(R.styleable.CircleView_ridus,defalultRidus);
typedArray.recycle();
}
paint.setColor(circleColor);
ridus = DisPlayUtils.dp2px(context,ridus);
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
int defautWidth=200,defaultHeight=200;//wrap_content下设置的默认值
int widthMode = MeasureSpec.getMode(widthMeasureSpec);
int widthSize = MeasureSpec.getSize(widthMeasureSpec);
int heightMode = MeasureSpec.getMode(heightMeasureSpec);
int heightSize = MeasureSpec.getSize(heightMeasureSpec);
if(widthMode == MeasureSpec.AT_MOST && heightMode == MeasureSpec.AT_MOST){
setMeasuredDimension(defautWidth,defaultHeight);
}else if(widthMode == MeasureSpec.AT_MOST){
setMeasuredDimension(defautWidth,heightSize);
}else if(heightMode == MeasureSpec.AT_MOST){
setMeasuredDimension(widthSize,defaultHeight);
}else{
super.onMeasure(widthMeasureSpec,heightMeasureSpec);
}
}
@Override
protected void onDraw(Canvas canvas) {
int width = getWidth()-getPaddingLeft()-getPaddingRight();
int height = getHeight()-getPaddingBottom()-getPaddingTop();
canvas.drawCircle(getPaddingLeft()+width/2,getPaddingTop()+height/2,ridus,paint);
}
}
2.继承ViewGroup：
public class HorizontalScrollView extends ViewGroup{
private Context context;
private Scroller scroller;
private VelocityTracker velocityTracker;//记录滑动速度
private int lastX=0,lastY=0;//记录上次滑动的坐标
private int lastInterceptX=0,lastInterceptY=0;//记录上次intercept中的坐标
private int mChildIndex=0;
private int mChildWidth=0;
private int mChildSize = 0;
public HorizontalScrollView(Context context) {
this(context,null);
}
public HorizontalScrollView(Context context, AttributeSet attrs) {
this(context, attrs,0);
}
public HorizontalScrollView(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
this.context=context;
init();
}
private void init(){
scroller = new Scroller(context);
velocityTracker = VelocityTracker.obtain();
}
@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
boolean isIntercept =false;
int x = (int) ev.getX();
int y = (int) ev.getY();
switch (ev.getAction()){
case MotionEvent.ACTION_DOWN:
isIntercept = false;
if(!scroller.isFinished()){
scroller.abortAnimation();
isIntercept=true;
}
break;
case MotionEvent.ACTION_MOVE:
int delX = x-lastInterceptX;
int delY = y-lastInterceptY;
if(Math.abs(delX)>Math.abs(delY)){//横向移动的距离大于纵向的就拦截自己处理
isIntercept = true;
}else{
isIntercept = false;
}
break;
case MotionEvent.ACTION_UP:
isIntercept=false;
break;
}
lastX = x;
lastY = y;
lastInterceptX = x;
lastInterceptY = y;
return isIntercept;
}
@Override
public boolean onTouchEvent(MotionEvent event) {
velocityTracker.addMovement(event);
int x = (int) event.getX();
int y = (int) event.getY();
switch (event.getAction()){
case MotionEvent.ACTION_DOWN:
if(!scroller.isFinished()){
scroller.abortAnimation();
}
break;
case MotionEvent.ACTION_MOVE:
int delX = x-lastX;
scrollBy(-delX,0);
break;
case MotionEvent.ACTION_UP:
int scrollX = getScrollX();
velocityTracker.computeCurrentVelocity(1000);
float xVelocity = velocityTracker.getXVelocity();
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,mChildSize-1));
int dx = getTotalViewWidth() - scrollX;
smoothScrollBy(dx,0);
velocityTracker.clear();
break;
}
lastX = x;
lastY = y;
return true;
}
private int getTotalViewWidth(){
int total = 0;
for(int i=0;i<mChildIndex;i++){
View view = getChildAt(i);
MarginLayoutParams lp = (MarginLayoutParams) view.getLayoutParams();
total+=view.getMeasuredWidth()+lp.leftMargin+lp.rightMargin;
}
total += getPaddingLeft() + getPaddingRight();
return total;
}
private void smoothScrollBy(int dx,int dy){
scroller.startScroll(getScrollX(),0,dx,0,500);
invalidate();
}
@Override
public void computeScroll() {
if(scroller.computeScrollOffset()){
scrollTo(scroller.getCurrX(),scroller.getCurrY());
postInvalidate();
}
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec,heightMeasureSpec);
int measureWidth = 0 , measureHeight = 0;
int childCount = getChildCount();
measureChildren(widthMeasureSpec,heightMeasureSpec);
int widthSpedMode = MeasureSpec.getMode(widthMeasureSpec);
int widthSpecSize = MeasureSpec.getSize(widthMeasureSpec);
int heightSpedMode = MeasureSpec.getMode(heightMeasureSpec);
int heightSpecSize = MeasureSpec.getSize(heightMeasureSpec);
int paddingLeft = getPaddingLeft();
int paddingRight = getPaddingRight();
int paddingTop = getPaddingTop();
int paddingBottom = getPaddingBottom();
if(childCount==0){
if(widthSpedMode == MeasureSpec.AT_MOST && heightSpedMode == MeasureSpec.AT_MOST){
setMeasuredDimension(paddingLeft+paddingRight,paddingTop+paddingBottom);
}else if(widthSpedMode == MeasureSpec.AT_MOST){
setMeasuredDimension(paddingLeft+paddingRight,heightSpecSize );
}else if(heightSpedMode == MeasureSpec.AT_MOST){
setMeasuredDimension(widthSpecSize ,paddingTop+paddingBottom);
}else{
setMeasuredDimension(widthSpecSize ,heightSpecSize );
}
}else{
if(widthSpedMode == MeasureSpec.AT_MOST){
for(int i=0;i<childCount;i++) {
View view = getChildAt(i);
MarginLayoutParams lp = (MarginLayoutParams) view.getLayoutParams();
measureWidth+=view.getMeasuredWidth()+lp.leftMargin+lp.rightMargin;
}
measureWidth+=paddingLeft+paddingRight;
}else{
measureWidth = widthSpecSize;
}
if(heightSpedMode == MeasureSpec.AT_MOST){
for(int i=0;i<childCount;i++) {
View view = getChildAt(i);
MarginLayoutParams lp = (MarginLayoutParams) view.getLayoutParams();
measureHeight = Math.max(measureHeight,view.getMeasuredHeight()+lp.topMargin+lp.bottomMargin);
}
measureHeight+=paddingBottom+paddingTop;
}else{
measureHeight = heightSpecSize;
}
setMeasuredDimension(measureWidth,measureHeight);
}
}
@Override
protected void onLayout(boolean isChanged, int left, int top, int right, int bottom) {
int mLeft = getPaddingLeft();
int childCount = getChildCount();
mChildSize = childCount;
for(int n=0;n<childCount;n++){
View childView = getChildAt(n);
mChildWidth = childView.getMeasuredWidth();
MarginLayoutParams lp =(MarginLayoutParams) childView.getLayoutParams();
if(childView.getVisibility() != GONE){
int childWidth = childView.getMeasuredWidth();
int mTop = getPaddingTop()+lp.topMargin;
mLeft+=lp.leftMargin;
childView.layout(mLeft,mTop,mLeft+childWidth,mTop+childView.getMeasuredHeight());
mLeft+=lp.rightMargin+childWidth;
}
}
}
@Override
public LayoutParams generateLayoutParams(AttributeSet attrs) {//要支持margin必须重写这个方法
return new MarginLayoutParams(context,attrs);
}
@Override
protected void onDetachedFromWindow() {//释放资源，关闭线程等
velocityTracker.recycle();
super.onDetachedFromWindow();
}
}
