Activity的resume
上次在Activity的啟動流程中，我們知道了Activity的生命周期都是通過Handler發(fā)送消息來執(zhí)行的匀奏，而View的繪制就是在onResume之后：
// H();
case RESUME_ACTIVITY:
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityResume");
SomeArgs args = (SomeArgs) msg.obj;
handleResumeActivity((IBinder) args.arg1, true, args.argi1 != 0, true,
args.argi3, "RESUME_ACTIVITY");
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
break;
final void handleResumeActivity(IBinder token, boolean clearHide, boolean isForward, boolean reallyResume, int seq, String reason) {
// 調(diào)用onResume方法
r = performResumeActivity(token, clearHide, reason);
// 將decorView添加到屏幕中
View decor = r.window.getDecorView();
decor.setVisibility(View.INVISIBLE);
ViewManager wm = a.getWindowManager();
WindowManager.LayoutParams l = r.window.getAttributes();
a.mDecor = decor;
wm.addView(decor, l);
}
// WindowManagerImpl.java
@Override
public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
applyDefaultToken(params);
mGlobal.addView(view, params, mContext.getDisplay(), mParentWindow);
}
-
handleResumeActivity方法首先調(diào)用performResumeActivity也就是我們Activity中的onResume方法煌张。 - 在通過獲取到decorView早歇，我們的布局其實已經(jīng)加載到decorView中了焰檩，在通過wm的addView方法將decorView傳遞出去憔涉。
- wm其實就是
WindowManagerImpl，而在WindowManagerImpl中又調(diào)用了WindowManagerGlobal的addView方法析苫。
// WindowManagerGlobal.java
public void addView(View view, ViewGroup.LayoutParams params,
Display display, Window parentWindow) {
ViewRootImpl root;
root = new ViewRootImpl(view.getContext(), display);
view.setLayoutParams(wparams);
mViews.add(view);
mRoots.add(root);
mParams.add(wparams);
root.setView(view, wparams, panelParentView);
}
// ViewRootImpl.java
public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
requestLayout();
}
@Override
public void requestLayout() {
if (!mHandlingLayoutInLayoutRequest) {
checkThread();
mLayoutRequested = true;
scheduleTraversals();
}
}
void scheduleTraversals() {
mChoreographer.postCallback(
Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
}
final TraversalRunnable mTraversalRunnable = new TraversalRunnable();
final class TraversalRunnable implements Runnable {
@Override
public void run() {
doTraversal();
}
}
在addView方法中首先構(gòu)建了ViewRootImpl這個非常關(guān)鍵的類兜叨，這個類就是操作View的繪制流程的類，在通過setView方法設置View衩侥。
而在setView方法中我們又看到了非常熟悉的一個方法requestLayout国旷，我們在自定義ViewGroup的時候，如果要更新布局就會調(diào)用這個方法刷新界面茫死，其實就是調(diào)用了View的重新繪制流程跪但。
經(jīng)過一系列的調(diào)用之后，最終調(diào)用了doTraversal方法峦萎。
measure
void doTraversal() {
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
}
private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");
try {
mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
}
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
onMeasure(widthMeasureSpec, heightMeasureSpec);
}
在doTraversal中調(diào)用了performMeasure屡久，而performMeasure又調(diào)用了View的onMeasure方法，這個才開始真正的測量爱榔。
我們以LinearLayout的VERTICAL來分析：
void measureVertical(int widthMeasureSpec, int heightMeasureSpec) {
for (int i = 0; i < count; ++i) {
final View child = getVirtualChildAt(i);
measureChildBeforeLayout(child, i, widthMeasureSpec, 0,
heightMeasureSpec, usedHeight);
}
if (useLargestChild &&
(heightMode == MeasureSpec.AT_MOST || heightMode == MeasureSpec.UNSPECIFIED)) {
mTotalLength = 0;
for (int i = 0; i < count; ++i) {
final View child = getVirtualChildAt(i);
if (child == null) {
mTotalLength += measureNullChild(i);
continue;
}
if (child.getVisibility() == GONE) {
i += getChildrenSkipCount(child, i);
continue;
}
final LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams)
child.getLayoutParams();
// Account for negative margins
final int totalLength = mTotalLength;
mTotalLength = Math.max(totalLength, totalLength + largestChildHeight +
lp.topMargin + lp.bottomMargin + getNextLocationOffset(child));
}
}
// Add in our padding
mTotalLength += mPaddingTop + mPaddingBottom;
int heightSize = mTotalLength;
int heightSizeAndState = resolveSizeAndState(heightSize, heightMeasureSpec, 0);
setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
heightSizeAndState);
}
void measureChildBeforeLayout(View child, int childIndex,
int widthMeasureSpec, int totalWidth, int heightMeasureSpec,
int totalHeight) {
measureChildWithMargins(child, widthMeasureSpec, totalWidth,
heightMeasureSpec, totalHeight);
}
protected void measureChildWithMargins(View child,
int parentWidthMeasureSpec, int widthUsed,
int parentHeightMeasureSpec, int heightUsed) {
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin
+ widthUsed, lp.width);
final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin
+ heightUsed, lp.height);
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
首先遍歷所有的子View被环，然后通過子View的measure方法來繼續(xù)測量，如果子View是ViewGroup則繼續(xù)遍歷详幽，如果是View則直接可以測量子View的寬高筛欢。而在View的measure中調(diào)用了onMeasure方法，這個方法就是我們平時寫的自定義View的時候覆蓋的方法唇聘，來指定我們自定義View的寬高版姑。
通過深入優(yōu)先遍歷的算法，從最里層的View開始測量雳灾，最終測量到最外層的ViewGroup漠酿。
當測量到最外層的時候，會再次遍歷自己的子View將子View的寬高計算谎亩。
最終調(diào)用setMeasuredDimension將寬高設置好炒嘲。
layout
void doTraversal() {
performLayout(lp, mWidth, mHeight);
}
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
int desiredWindowHeight) {
host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
}
public void layout(int l, int t, int r, int b) {
onLayout(changed, l, t, r, b);
}
當performMeasure方法執(zhí)行完之后，doTraversal又調(diào)用了performLayout來進行確定子View的位置匈庭。而performLayout則繼續(xù)調(diào)用View的layout方法夫凸，在這里我們又見到了熟悉的onLayout。
同樣以LinearLayout的VERTICAL來分析
void layoutVertical(int left, int top, int right, int bottom) {
for (int i = 0; i < count; i++) {
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);
}
layoutVertical會遍歷所有的子View并調(diào)用setChildFrame來為子View指定對應的位置阱持。
這個時候layout和measure的計算方式又有所區(qū)別夭拌，measure是通過深度優(yōu)先遍歷的方式，首先獲取子View的寬高，而layout指定View的位置的時候鸽扁，是從外向內(nèi)蒜绽，依次遍歷指定View的位置。
draw
void doTraversal() {
performDraw();
}
private void performDraw() {
try {
// 調(diào)用繪制函數(shù)
draw(fullRedrawNeeded);
} finally {
mIsDrawing = false;
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
}
private void draw(boolean fullRedrawNeeded) {
// 獲取surface
Surface surface = mSurface;
if (!surface.isValid()) {
return;
}
// 繪制需要更新
if (!dirty.isEmpty() || mIsAnimating || accessibilityFocusDirty) {
// 使用硬件加速
if (mAttachInfo.mHardwareRenderer != null && mAttachInfo.mHardwareRenderer.isEnabled()) {
// 使用硬件渲染繪制
mAttachInfo.mHardwareRenderer.draw(mView, mAttachInfo, this);
} else {
// 使用cpu繪制
if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset, scalingRequired, dirty)) {
return;
}
}
}
if (animating) {
mFullRedrawNeeded = true;
scheduleTraversals();
}
}
當performMeasure方法執(zhí)行完之后桶现，doTraversal又調(diào)用了performDraw開始繪制視圖躲雅。
在draw函數(shù)中獲取需要繪制的區(qū)域，再判斷是否使用硬件加速骡和。
通常情況下都是使用cpu繪制相赁，也就是直接調(diào)用drawSoftware：
private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
boolean scalingRequired, Rect dirty) {
// Draw with software renderer.
final Canvas canvas;
try {
final int left = dirty.left;
final int top = dirty.top;
final int right = dirty.right;
final int bottom = dirty.bottom;
// 獲取canvas對象，用于framework層繪制
canvas = mSurface.lockCanvas(dirty);
} catch (Surface.OutOfResourcesException e) {
return false;
}
try {
// 開始繪制慰于，從decorView開始繪制
mView.draw(canvas);
} finally {
// 釋放canvas鎖钮科，并通知SurfaceFlinger更新
// private static native void nHwuiDraw(long renderer); 調(diào)用native方法
surface.unlockCanvasAndPost(canvas);
}
return true;
}
從上面的函數(shù)可以看出繪制的過程：
- 判斷使用gpu還是cpu渲染
- 獲取繪制的surface對象
- 通過surface獲取并鎖住canvas對象
- 從DecorView開始發(fā)起整個view樹的繪制
- 解鎖canvas對象，并通知surfaceFlinger對象更新視圖婆赠，調(diào)用native方法绵脯。
我們來看View的draw函數(shù)：
public void draw(Canvas canvas) {
drawBackground(canvas);
if (!verticalEdges && !horizontalEdges) {
// Step 3, draw the content
if (!dirtyOpaque) onDraw(canvas);
// Step 4, draw the children
dispatchDraw(canvas);
// Overlay is part of the content and draws beneath Foreground
if (mOverlay != null && !mOverlay.isEmpty()) {
mOverlay.getOverlayView().dispatchDraw(canvas);
}
// Step 6, draw decorations (foreground, scrollbars)
onDrawForeground(canvas);
// we're done...
return;
}
}
View的draw方法會調(diào)用dispatchDraw(canvas)來傳遞繪制過程，dispatchDraw會遍歷調(diào)用所有子元素的draw方法页藻，這樣draw就會一層一層的傳遞下去桨嫁，知道最后一個View繪制完成。
這個時候View的整個繪制流程就完成了份帐。
