View与ViewGroup layout 过程
View与ViewGroup layout 过程
这个过程相比measure要简单一点,我们先从View 的layout()看起,ViewGroup的layout主要还是判断一些条件之后调用View 的 layout()
View.java
public void layout(int l, int t, int r, int b) { if (DBG_SYSTRACE_LAYOUT) { Trace.traceBegin(Trace.TRACE_TAG_VIEW, "layout : " + getClass().getSimpleName()); } // !=0 表示在measure 的时候设置这个标志,也就是measure的时候根据measure cache 测量,而不是直接调用onMeasure(),所以这里要重新onMeasure一次 if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) { /// M: Monitor onLayout time if longer than 3s print log. if (DBG_LAYOUT || DBG_MEASURE_LAYOUT) { Xlog.d(VIEW_LOG_TAG, "view onMeasure start (measure cache), this =" + this + ", widthMeasureSpec = " + MeasureSpec.toString(mOldWidthMeasureSpec) + ", heightMeasureSpec = " + MeasureSpec.toString(mOldHeightMeasureSpec)); } long logTime = System.currentTimeMillis(); onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec); long nowTime = System.currentTimeMillis(); if (nowTime - logTime > DBG_TIMEOUT_VALUE) { Xlog.d(VIEW_LOG_TAG, "[ANR Warning]onMeasure time too long, this =" + this + "time =" + (nowTime - logTime) + " ms"); } if (DBG_LAYOUT || DBG_MEASURE_LAYOUT) { Xlog.d(VIEW_LOG_TAG, "view onMeasure end (measure cache), this =" + this + ", mMeasuredWidth = " + mMeasuredWidth + ", mMeasuredHeight = " + mMeasuredHeight + ", time =" + (nowTime - logTime) + " ms"); } mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;//将标志位恢复 } int oldL = mLeft; int oldT = mTop; int oldB = mBottom; int oldR = mRight; //视觉/光学边界布局,一般是false boolean changed = isLayoutModeOptical(mParent) ? setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b); if (DBG_LAYOUT || DBG_MEASURE_LAYOUT) { Xlog.d(VIEW_LOG_TAG, "view layout start, this = " + this + ", mLeft = " + mLeft + ", mTop = " + mTop + ", mRight = " + mRight + ", mBottom = " + mBottom + ", changed = " + changed); } //PFLAG_LAYOUT_REQUIRED这个标志也是在measure 的时候设置的. if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) { /// M: Monitor onLayout time if longer than 3s print log. long logTime = System.currentTimeMillis(); onLayout(changed, l, t, r, b);//这个方法view和ViewGroup都没有实现,具体是有子类去实现 long nowTime = System.currentTimeMillis(); if (nowTime - logTime > DBG_TIMEOUT_VALUE) { Xlog.d(VIEW_LOG_TAG, "[ANR Warning]onLayout time too long, this =" + this + "time =" + (nowTime - logTime) + " ms"); } if (DBG_LAYOUT || DBG_MEASURE_LAYOUT) { Xlog.d(VIEW_LOG_TAG, "view layout end, this =" + this + ", mLeft = " + mLeft + ", mTop = " + mTop + ", mRight = " + mRight + ", mBottom = " + mBottom + ", time =" + (nowTime - logTime) + " ms"); } mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;//将标志位恢复 ListenerInfo li = mListenerInfo; if (li != null && li.mOnLayoutChangeListeners != null) { ArrayList<OnLayoutChangeListener> listenersCopy = (ArrayList<OnLayoutChangeListener>)li.mOnLayoutChangeListeners.clone(); int numListeners = listenersCopy.size(); for (int i = 0; i < numListeners; ++i) { listenersCopy.get(i).onLayoutChange(this, l, t, r, b, oldL, oldT, oldR, oldB);//接口回调 } } } else { if (DBG_LAYOUT || DBG_MEASURE_LAYOUT) { Xlog.d(VIEW_LOG_TAG, "view layout end 2 (use previous layout), this = " + this + ", mLeft = " + mLeft + ", mTop = " + mTop + ", mRight = " + mRight + ", mBottom = " + mBottom); } } mPrivateFlags &= ~PFLAG_FORCE_LAYOUT; mPrivateFlags3 |= PFLAG3_IS_LAID_OUT; if (DBG_SYSTRACE_LAYOUT) { Trace.traceEnd(Trace.TRACE_TAG_VIEW); }}
从上面的代码可以看到,layout 流程先会根据之前在measure流程中是否设置了 PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT 变量来判断是否需要重新measure一遍,然后调用 setFrame 来确定自己的4个点的位置,4个点分别是left,top.right,bottom.有了这4个点,后续才能准确的绘制.
接着根据在measure里面设置的 PFLAG_LAYOUT_REQUIRED 变量来执行 onLayout 流程,其实这个方法View和ViewGroup都没有去实现,真正的实现是在各个子类里面的,后面会分析.再这之后,如果这个视图设置过 OnLayoutChangeListener 接口,还会回调这些接口的 onLayoutChange() 方法.
这里暂时不分析 setFrame()流程,后续在draw()里面再看.
上面已经提到 View和ViewGroup没有自己去实现onLayout(),是需要子类是去实现.这里还是以LinearLayout为例分析一下实现过程.
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { if (mOrientation == VERTICAL) { measureVertical(widthMeasureSpec, heightMeasureSpec); } else { measureHorizontal(widthMeasureSpec, heightMeasureSpec); }}
这里我们只分析垂直布局
void layoutVertical(int left, int top, int right, int bottom) { final int paddingLeft = mPaddingLeft; int childTop; int childLeft; // Where right end of child should go final int width = right - left; int childRight = width - mPaddingRight; // Space available for child int childSpace = width - paddingLeft - mPaddingRight; final int count = getVirtualChildCount(); final int majorGravity = mGravity & Gravity.VERTICAL_GRAVITY_MASK; final int minorGravity = mGravity & Gravity.RELATIVE_HORIZONTAL_GRAVITY_MASK; switch (majorGravity) { case Gravity.BOTTOM: // mTotalLength contains the padding already childTop = mPaddingTop + bottom - top - mTotalLength; break; // mTotalLength contains the padding already case Gravity.CENTER_VERTICAL: childTop = mPaddingTop + (bottom - top - mTotalLength) / 2; break; case Gravity.TOP: default: childTop = mPaddingTop; break; } for (int i = 0; i < count; i++) { final View child = getVirtualChildAt(i); if (child == null) { childTop += measureNullChild(i); } else if (child.getVisibility() != GONE) { final int childWidth = child.getMeasuredWidth(); final int childHeight = child.getMeasuredHeight(); final LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams) child.getLayoutParams(); int gravity = lp.gravity; if (gravity < 0) { gravity = minorGravity; } final int layoutDirection = getLayoutDirection(); final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection); switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) { case Gravity.CENTER_HORIZONTAL: childLeft = paddingLeft + ((childSpace - childWidth) / 2) + lp.leftMargin - lp.rightMargin; break; case Gravity.RIGHT: childLeft = childRight - childWidth - lp.rightMargin; break; case Gravity.LEFT: default: childLeft = paddingLeft + lp.leftMargin; break; } if (hasDividerBeforeChildAt(i)) { childTop += mDividerHeight; } childTop += lp.topMargin; setChildFrame(child, childLeft, childTop + getLocationOffset(child), childWidth, childHeight); childTop += childHeight + lp.bottomMargin + getNextLocationOffset(child); i += getChildrenSkipCount(child, i); } }}
private void setChildFrame(View child, int left, int top, int width, int height) { child.layout(left, top, left + width, top + height);}
从上面的代码可以看到,是先根据 Gravity 属性来确定第一个child 的top 位置,然后循环来获取childTop和childLeft,同时获取宽高来调用 setChildFrame ().而 setChildFrame ()只是调用child 的layout流程.就这样递归调用各个"child"的layout流程.