View繪制基本流程
Android系統(tǒng)View的繪制流程：依次執(zhí)行View類里面的如下三個方法：
measure(int ,int) :測量View的大小
layout(int ,int ,int ,int) ：設(shè)置子View的位置
draw(Canvas) ：繪制View內(nèi)容到Canvas畫布上
整個View樹的繪圖流程是在ViewRoot.Java類的performTraversals()函數(shù)展開的，該函數(shù)做的執(zhí)行過程可簡單概況為根據(jù)之前設(shè)置的狀態(tài)用押，判斷是否需要重新計算視圖大小(measure)、是否重新需要安置視圖的位置(layout)、以及是否需要重繪 (draw)
mesarue()測量過程
主要作用：為整個View樹計算實際的大小，即設(shè)置實際的高(mMeasuredHeight)和寬(mMeasureWidth)描融，每個View的控件的實際寬高都是由父視圖和本身視圖決定的吕喘。
具體的調(diào)用如下：
ViewRootImpl 的performTraversals方法中，調(diào)用measureHierarchy厅翔，然后調(diào)用performMeasure
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);
}
}
ViewRoot根對象地屬性mView(其類型一般為ViewGroup類型)調(diào)用measure()方法去計算View樹的大小，回調(diào)View/ViewGroup對象的onMeasure()方法搀突，該方法實現(xiàn)的功能如下：
1刀闷、設(shè)置本View視圖的最終大小，該功能的實現(xiàn)通過調(diào)用setMeasuredDimension()方法去設(shè)置實際的高(mMeasuredHeight)和寬(mMeasureWidth)
2仰迁、如果該View對象是個ViewGroup類型甸昏，需要重寫onMeasure()方法，對其子視圖進行遍歷的measure()過程徐许。
對每個子視圖的measure()過程施蜜，是通過調(diào)用父類ViewGroup.java類里的measureChildWithMargins()方法去實現(xiàn)，該方法內(nèi)部只是簡單地調(diào)用了View對象的measure()方法雌隅。
整個measure調(diào)用流程就是個樹形的遞歸過程measure()方法兩個參數(shù)都是父View傳遞過來的翻默，也就是代表了父view的規(guī)格。他由兩部分組成恰起，高2位表示MODE修械，定義在MeasureSpec類（View的內(nèi)部類）中，有三種類型检盼，MeasureSpec.EXACTLY表示確定大小肯污， MeasureSpec.AT_MOST表示最大大小， MeasureSpec.UNSPECIFIED不確定吨枉。低30位表示size蹦渣，也就是父View的大小。對于系統(tǒng)Window類的DecorVIew對象Mode一般都為MeasureSpec.EXACTLY 貌亭，而size分別對應(yīng)屏幕寬高柬唯。對于子View來說大小是由父View和子View共同決定的。
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
boolean optical = isLayoutModeOptical(this);
if (optical != isLayoutModeOptical(mParent)) {
Insets insets = getOpticalInsets();
int opticalWidth = insets.left + insets.right;
int opticalHeight = insets.top + insets.bottom;
measuredWidth += optical ? opticalWidth : -opticalWidth;
measuredHeight += optical ? opticalHeight : -opticalHeight;
}
mMeasuredWidth = measuredWidth;
mMeasuredHeight = measuredHeight;
mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;
}
layout布局過程
主要作用 ：為將整個根據(jù)子視圖的大小以及布局參數(shù)將View樹放到合適的位置上圃庭。
具體的調(diào)用如下：
ViewRootImpl 的performTraversals方法中权逗，調(diào)用performLayout
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 {
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;
}
draw()繪圖過程
ViewRootImpl 的performTraversals方法中美尸，調(diào)用了mView的draw方法
mView.draw()開始繪制，draw()方法實現(xiàn)的功能如下：
1 斟薇、繪制該View的背景
2 师坎、為顯示漸變框做一些準備操作
3、調(diào)用onDraw()方法繪制視圖本身 (每個View都需要重載該方法堪滨，ViewGroup不需要實現(xiàn)該方法)
4胯陋、調(diào)用dispatchDraw ()方法繪制子視圖(如果該View類型不為ViewGroup，即不包含子視圖袱箱，不需要重載該方法)
值得說明的是遏乔，ViewGroup類已經(jīng)為我們重寫了dispatchDraw ()的功能實現(xiàn)，應(yīng)用程序一般不需要重寫該方法发笔，但可以重載父類函數(shù)實現(xiàn)具體的功能盟萨。
dispatchDraw()方法內(nèi)部會遍歷每個子視圖，調(diào)用drawChild()去重新回調(diào)每個子視圖的draw()方法了讨。
5捻激、繪制滾動條
刷新視圖
Android中實現(xiàn)view的更新有兩個方法，一個是invalidate前计，另一個是postInvalidate胞谭，其中前者是在UI線程自身中使用，而后者在非UI線程中使用男杈。
requestLayout()方法 ：會導致調(diào)用measure()過程 和 layout()過程 丈屹。
說明：只是對View樹重新布局layout過程包括measure()和layout()過程，不會調(diào)用draw()過程伶棒，但不會重新繪制
任何視圖包括該調(diào)用者本身旺垒。
一般引起invalidate()操作的函數(shù)如下：
1、直接調(diào)用invalidate()方法肤无，請求重新draw()先蒋，但只會繪制調(diào)用者本身。
2舅锄、setSelection()方法 ：請求重新draw()鞭达，但只會繪制調(diào)用者本身司忱。
3皇忿、setVisibility()方法 ： 當View可視狀態(tài)在INVISIBLE轉(zhuǎn)換VISIBLE時，會間接調(diào)用invalidate()方法坦仍，繼而繪制該View鳍烁。
4 、setEnabled()方法 ： 請求重新draw()繁扎，但不會重新繪制任何視圖包括該調(diào)用者本身幔荒。
