View繪製的三部曲，測量，佈局，繪畫現在我們分析佈局部分測量部分在上篇文章中已經分析過了。不瞭解的可以去我的博客里找一下View的佈局和測量一樣，都是從ViewRootImpl中發起，ViewRootImpl先通過measure來初始化整個的view樹之後會調用onLayout方法來佈局,View ...
View繪製的三部曲，測量，佈局，繪畫
現在我們分析佈局部分
測量部分在上篇文章中已經分析過了。不瞭解的可以去我的博客里找一下
View的佈局和測量一樣，都是從ViewRootImpl中發起，ViewRootImpl先通過measure來初始化整個的view樹
之後會調用onLayout方法來佈局,ViewRootImpl是通過performLayout函數來發起重繪的
比較重要的部分我會寫註釋，註意看註釋就行
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth, int desiredWindowHeight) { mLayoutRequested = false; mScrollMayChange = true; mInLayout = true; final View host = mView; if (DEBUG_ORIENTATION || DEBUG_LAYOUT) { Log.v(TAG, "Laying out " + host + " to (" + host.getMeasuredWidth() + ", " + host.getMeasuredHeight() + ")"); } Trace.traceBegin(Trace.TRACE_TAG_VIEW, "layout"); try { //通過調用DecorView的layout函數，來發起整個view視圖的重繪 host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight()); mInLayout = false; int numViewsRequestingLayout = mLayoutRequesters.size(); if (numViewsRequestingLayout > 0) { // requestLayout() was called during layout. // If no layout-request flags are set on the requesting views, there is no problem. // If some requests are still pending, then we need to clear those flags and do // a full request/measure/layout pass to handle this situation. ArrayList<View> validLayoutRequesters = getValidLayoutRequesters(mLayoutRequesters, false); if (validLayoutRequesters != null) { // Set this flag to indicate that any further requests are happening during // the second pass, which may result in posting those requests to the next // frame instead mHandlingLayoutInLayoutRequest = true; // Process fresh layout requests, then measure and layout int numValidRequests = validLayoutRequesters.size(); for (int i = 0; i < numValidRequests; ++i) { final View view = validLayoutRequesters.get(i); Log.w("View", "requestLayout() improperly called by " + view + " during layout: running second layout pass"); view.requestLayout(); } measureHierarchy(host, lp, mView.getContext().getResources(), desiredWindowWidth, desiredWindowHeight); mInLayout = true; host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight()); mHandlingLayoutInLayoutRequest = false; // Check the valid requests again, this time without checking/clearing the // layout flags, since requests happening during the second pass get noop'd validLayoutRequesters = getValidLayoutRequesters(mLayoutRequesters, true); if (validLayoutRequesters != null) { final ArrayList<View> finalRequesters = validLayoutRequesters; // Post second-pass requests to the next frame getRunQueue().post(new Runnable() { @Override public void run() { int numValidRequests = finalRequesters.size(); for (int i = 0; i < numValidRequests; ++i) { final View view = finalRequesters.get(i); Log.w("View", "requestLayout() improperly called by " + view + " during second layout pass: posting in next frame"); view.requestLayout(); } } }); } } } } finally { Trace.traceEnd(Trace.TRACE_TAG_VIEW); } mInLayout = false; }
這個函數主要功能就是調用view的layout方法，接下來要分析的就是layout函數了。這個函數在View中，是觸發onLayout函數的方法
@SuppressWarnings({"unchecked"})
public void layout(int l, int t, int r, int b) {
//先判斷一下是否需要重新測量
if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) {
onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec);
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}
int oldL = mLeft;
int oldT = mTop;
int oldB = mBottom;
int oldR = mRight;
//判斷是否使用 optical bound 佈局,並且繪製Frame出來
boolean changed = isLayoutModeOptical(mParent) ?
setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);
//如果需要重新layout的話，就開始調用DecorView的onLayout方法，我們簡單看一下
if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
onLayout(changed, l, t, r, b);
mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;
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);
}
}
}
mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;
mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;
}
這個函數的工作就是分發整個的佈局流程，先是DecorView，在FrameLayout ....直到整個view tree佈局完畢
@Override protected void onLayout(boolean changed, int left, int top, int right, int bottom) { super.onLayout(changed, left, top, right, bottom); //獲取界面的邊框如果有偏移，就需要偏移一下view視窗 getOutsets(mOutsets); if (mOutsets.left > 0) { offsetLeftAndRight(-mOutsets.left); } if (mOutsets.top > 0) { offsetTopAndBottom(-mOutsets.top); } }
這個onLayout是在DecorView中，他調用了super，也就是FrameLayout下邊的onLayout方法，我們在繼續看FrameLayout
/** * {@inheritDoc} */ @Override protected void onLayout(boolean changed, int left, int top, int right, int bottom) { layoutChildren(left, top, right, bottom, false /* no force left gravity */); }
這個函數頁很簡單，直接調用了layoutChildren方法去佈局各種子view
void layoutChildren(int left, int top, int right, int bottom, boolean forceLeftGravity) { final int count = getChildCount(); final int parentLeft = getPaddingLeftWithForeground(); final int parentRight = right - left - getPaddingRightWithForeground(); final int parentTop = getPaddingTopWithForeground(); final int parentBottom = bottom - top - getPaddingBottomWithForeground(); //開始佈局，目前這個是FrameLayout，特性就是預設左上角，且會z軸覆蓋 for (int i = 0; i < count; i++) { final View child = getChildAt(i); if (child.getVisibility() != GONE) { final LayoutParams lp = (LayoutParams) child.getLayoutParams(); final int width = child.getMeasuredWidth(); final int height = child.getMeasuredHeight(); int childLeft; int childTop; //處理對齊方式 int gravity = lp.gravity; if (gravity == -1) { gravity = DEFAULT_CHILD_GRAVITY; } final int layoutDirection = getLayoutDirection(); final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection); final int verticalGravity = gravity & Gravity.VERTICAL_GRAVITY_MASK; switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) { case Gravity.CENTER_HORIZONTAL: childLeft = parentLeft + (parentRight - parentLeft - width) / 2 + lp.leftMargin - lp.rightMargin; break; case Gravity.RIGHT: if (!forceLeftGravity) { childLeft = parentRight - width - lp.rightMargin; break; } case Gravity.LEFT: default: childLeft = parentLeft + lp.leftMargin; } switch (verticalGravity) { case Gravity.TOP: childTop = parentTop + lp.topMargin; break; case Gravity.CENTER_VERTICAL: childTop = parentTop + (parentBottom - parentTop - height) / 2 + lp.topMargin - lp.bottomMargin; break; case Gravity.BOTTOM: childTop = parentBottom - height - lp.bottomMargin; break; default: childTop = parentTop + lp.topMargin; } //佈局子view，以此類推，會佈局完整個view樹 child.layout(childLeft, childTop, childLeft + width, childTop + height); } } }
上面方法運行完後，整個的佈局過程就結束了。view這塊的設計非常棒，採用了組合模式去設計，在上邊迴圈中去調用layout方法，layout在去觸發子view的onLayout來按照各自的規則去佈局，直到整個view樹迴圈完畢
