[Android FrameWork 6.0源碼學習] View的重繪過程之Layout

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裡找一下

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注釋， 注意看注釋就行

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 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 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 listenersCopy = (ArrayList)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); //獲取介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， 特性就是默認左上角， 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