从DecorView开始分析
DecorView是窗口大小和自身LayoutParams两者共同确定自己的MeasureSpec. 此处是通过getRootMeasureSpec来获取返回值. private static int getRootMeasureSpec(int windowSize, int rootDimension) { int measureSpec; switch (rootDimension) { case ViewGroup.LayoutParams.MATCH_PARENT: // Window can't resize. Force root view to be windowSize. measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY); break; case ViewGroup.LayoutParams.WRAP_CONTENT: // Window can resize. Set max size for root view. measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST); break; default: // Window wants to be an exact size. Force root view to be that size. measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY); break; } return measureSpec; } 调用performMeasure方法执行measure函数. private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) { if (mView == null) { return; } Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure"); try { mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);//在view中measure方法为final,不可更改 } finally { Trace.traceEnd(Trace.TRACE_TAG_VIEW); } } measure方法中会调用onMeasure方法所以DecorView(继承于FrameLayout)中重写了onMeasure. 同时FrameLayout(继承于viewGroup, viewGroup继承于view)自己也重写了onMeasure方法. 之后DecorView将处理结果(自身宽高的精确值)通过super.onMeasure传递到FrameLayout中处理.FrameLayout中的onMeasure方法会对子view进行遍历同时调用measureChildWithMargins. measureChildWithMargins又会调用view.measure, measure又会调用onMeasure. 直到最后只剩下view(不是viewGroup)时, 在onMeasure中调用setMeasuredDimension设置单独view的宽高.当每个字view宽高都计算完时, 加上子view各自的margin, 父view的padding后, viewGroup就能得出自身的大小.在第5条中的measureChildWithMargins, 调用了getChildMeasureSpec方法. 这个方法是普通view利用父MeasureSpec和自身lp来创建自己MeasureSpec的规则. public static int getChildMeasureSpec(int spec, int padding, int childDimension) { int specMode = MeasureSpec.getMode(spec); int specSize = MeasureSpec.getSize(spec); int size = Math.max(0, specSize - padding);//父容器对padding做了处理 int resultSize = 0; int resultMode = 0; switch (specMode) { // Parent has imposed an exact size on us case MeasureSpec.EXACTLY: if (childDimension >= 0) { resultSize = childDimension; resultMode = MeasureSpec.EXACTLY; } else if (childDimension == LayoutParams.MATCH_PARENT) { // Child wants to be our size. So be it. resultSize = size; resultMode = MeasureSpec.EXACTLY; } else if (childDimension == LayoutParams.WRAP_CONTENT) { // Child wants to determine its own size. It can't be // bigger than us. resultSize = size; resultMode = MeasureSpec.AT_MOST; } break; // Parent has imposed a maximum size on us case MeasureSpec.AT_MOST: if (childDimension >= 0) { // Child wants a specific size... so be it resultSize = childDimension; resultMode = MeasureSpec.EXACTLY; } else if (childDimension == LayoutParams.MATCH_PARENT) { // Child wants to be our size, but our size is not fixed. // Constrain child to not be bigger than us. resultSize = size; resultMode = MeasureSpec.AT_MOST; } else if (childDimension == LayoutParams.WRAP_CONTENT) { // Child wants to determine its own size. It can't be // bigger than us. resultSize = size; resultMode = MeasureSpec.AT_MOST; } break; // Parent asked to see how big we want to be case MeasureSpec.UNSPECIFIED: if (childDimension >= 0) { // Child wants a specific size... let him have it resultSize = childDimension; resultMode = MeasureSpec.EXACTLY; } else if (childDimension == LayoutParams.MATCH_PARENT) { // Child wants to be our size... find out how big it should // be resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size; resultMode = MeasureSpec.UNSPECIFIED; } else if (childDimension == LayoutParams.WRAP_CONTENT) { // Child wants to determine its own size.... find out how // big it should be resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size; resultMode = MeasureSpec.UNSPECIFIED; } break; } //noinspection ResourceType return MeasureSpec.makeMeasureSpec(resultSize, resultMode); } 以上总结的表格 根据规则可知, 当自定义view的属性为wrap时, SpecSize都是parentSize(即父view的剩余空间大小).所以此时在自定义view中, 要重写onMeasure方法, 在SpecMode模式为AT_MOST时设定一个默认的内部宽高.手机上看不全下面代码HorizontalScrollViewEx
import android.content.Context; import android.util.AttributeSet; import android.util.Log; import android.view.MotionEvent; import android.view.VelocityTracker; import android.view.View; import android.view.ViewGroup; import android.widget.Scroller; public class HorizontalScrollViewEx extends ViewGroup { private static final String TAG = "HorizontalScrollViewEx"; private int mChildrenSize; private int mChildWidth; private int mChildIndex; // 分别记录上次滑动的坐标 private int mLastX = 0; private int mLastY = 0; // 分别记录上次滑动的坐标(onInterceptTouchEvent) private int mLastXIntercept = 0; private int mLastYIntercept = 0; private Scroller mScroller; private VelocityTracker mVelocityTracker; public HorizontalScrollViewEx(Context context) { super(context); init(); } public HorizontalScrollViewEx(Context context, AttributeSet attrs) { super(context, attrs); init(); } public HorizontalScrollViewEx(Context context, AttributeSet attrs, int defStyle) { super(context, attrs, defStyle); init(); } private void init() { if (mScroller == null) { mScroller = new Scroller(getContext()); mVelocityTracker = VelocityTracker.obtain(); } } @Override public boolean onInterceptTouchEvent(MotionEvent event) { boolean intercepted = false; int x = (int) event.getX(); int y = (int) event.getY(); switch (event.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)) { intercepted = true; } else { intercepted = false; } break; } case MotionEvent.ACTION_UP: { intercepted = false; break; } default: break; } Log.d(TAG, "intercepted=" + intercepted); 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); float xVelocity = mVelocityTracker.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, mChildrenSize - 1)); int dx = mChildIndex * mChildWidth - scrollX; smoothScrollBy(dx, 0); mVelocityTracker.clear(); break; } default: break; } mLastX = x; mLastY = y; return true; } @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, childView.getMeasuredHeight()); } 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; } } } private void smoothScrollBy(int dx, int dy) { mScroller.startScroll(getScrollX(), 0, dx, 0, 500); invalidate(); } @Override public void computeScroll() { if (mScroller.computeScrollOffset()) { scrollTo(mScroller.getCurrX(), mScroller.getCurrY()); postInvalidate(); } } @Override protected void onDetachedFromWindow() { mVelocityTracker.recycle(); super.onDetachedFromWindow(); } }