Android中Activity是做爲應用程序的載體存在,表明着一個完整的用戶界面,提供了一個窗口來繪製各類視圖,當Activity啓動時,咱們會經過setContentView方法來設置一個內容視圖,這個內容視圖就是用戶看到的界面。
PhoneWindow是Android系統中最基本的窗口系統,每一個Activity會建立一個。PhoneWindow是Activity和View系統交互的接口。一個PhoneWindow對應一個DecorView跟一個ViewRootImpl,DecorView是ViewTree裏面的頂層佈局,是繼承於FrameLayout,是Activity中全部View的祖先。ViewRootImpl創建DecorView和Window之間的聯繫。
下面介紹一些相關的概念:canvas
當一個應用啓動時,會啓動一個主Activity,Android系統會根據Activity的佈局來對它進行繪製。繪製會從根視圖ViewRootImpl的performTraversals()方法開始,從上到下遍歷整個視圖樹,每一個View控制負責繪製本身,而ViewGroup還須要負責通知本身的子View進行繪製操做。View的繪製流程主要是指measure、layout、draw這三大流程,即測量、佈局和繪製,其中measure肯定View的測量寬高,layout根據測量的寬高肯定View在其父View中的四個頂點的位置,而draw則將View繪製到屏幕上。經過ViewGroup的遞歸遍歷,一個View樹就展示在屏幕上了。
performTraversals()方法在類ViewRootImpl內,其核心代碼以下:ide
int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width); int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height); ... // 測量 performMeasure(childWidthMeasureSpec, childHeightMeasureSpec); ... // 佈局 performLayout(lp, mWidth, mHeight); ... // 繪製 performDraw();
爲了更好地理解View的測量過程,咱們須要理解MeasureSpec,它是View的一個內部類,它表示對View的測量規格。
在Google官方文檔中是這麼定義MeasureSpec的:佈局
A MeasureSpec encapsulates the layout requirements passed from parent to child. Each MeasureSpec represents a requirement for either the width or the height. A MeasureSpec is comprised of a size and a mode.ui
MeasureSpec是一個32位二進制的整數,由SpecMode和SpecSize兩部分組成。其中,高2位爲SpecMode(測量模式),低30位爲SpecSize(測量大小)。
SpecMode的取值可爲如下三種:this
ViewGroup在它的measureChild方法中傳遞給子View。ViewGroup經過遍歷自身全部的子View,並逐個調用子View的measure方法實現測量操做。ViewGroup在遍歷完子View後,須要根據子元素的測量結果來決定本身最終的測量大小,並調用setMeasuredDimension方法保存測量寬高值。spa
// 遍歷測量 ViewGroup 中全部的 View protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec) { final int size = mChildrenCount; final View[] children = mChildren; for (int i = 0; i < size; ++i) { final View child = children[i]; if ((child.mViewFlags & VISIBILITY_MASK) != GONE) { measureChild(child, widthMeasureSpec, heightMeasureSpec); } } } //測量某個指定的View protected void measureChild(View child, int parentWidthMeasureSpec, int parentHeightMeasureSpec) { final LayoutParams lp = child.getLayoutParams(); final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec, mPaddingLeft + mPaddingRight, lp.width); final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec, mPaddingTop + mPaddingBottom, lp.height); child.measure(childWidthMeasureSpec, childHeightMeasureSpec); }
View的MeasureSpec是由父容器的MeasureSpec和本身的LayoutParams決定的,可是對於DecorView來講有點不一樣,由於它沒有父類。在ViewRootImpl中的measureHierarchy方法中有以下一段代碼展現了DecorView的MeasureSpec的建立過程,其中desiredWindowWidth和desireWindowHeight是屏幕的尺寸大小。rest
private boolean measureHierarchy(final View host, final WindowManager.LayoutParams lp, final Resources res, final int desiredWindowWidth, final int desiredWindowHeight) { ... childWidthMeasureSpec = getRootMeasureSpec(desiredWindowWidth, lp.width); childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height); performMeasure(childWidthMeasureSpec, childHeightMeasureSpec); ... } private static int getRootMeasureSpec(int windowSize, int rootDimension) { int measureSpec; switch (rootDimension) { case ViewGroup.LayoutParams.MATCH_PARENT: // Window can't resize. Force root view to be windowSize. measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY); break; case ViewGroup.LayoutParams.WRAP_CONTENT: // Window can resize. Set max size for root view. measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST); break; default: // Window wants to be an exact size. Force root view to be that size. measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY); break; } return measureSpec; }
DecorView是FrameLyaout的子類,屬於ViewGroup,對於ViewGroup來講,除了完成本身的measure過程外,還會遍歷去調用全部子元素的measure方法,各個子元素再遞歸去執行這個過程。和View不一樣的是,ViewGroup是一個抽象類,他沒有重寫View的onMeasure方法,這裏很好理解,由於每一個具體的ViewGroup實現類的功能是不一樣的,如何測量應該讓它本身決定,好比LinearLayout和RelativeLayout。
所以在具體的ViewGroup中須要遍歷去測量子View,這裏咱們看看ViewGroup中提供的測量子View的measureChildWithMargins方法:code
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); }
上述方法會對子元素進行measure,在調用子元素的measure方法以前會先經過getChildMeasureSpec方法來獲得子元素的MeasureSpec。從代碼上看,子元素的MeasureSpec的建立與父容器的MeasureSpec和自己的LayoutParams有關,此外和View的margin和父類的padding有關,如今看看getChildMeasureSpec的具體實現:orm
public static int getChildMeasureSpec(int spec, int padding, int childDimension) { int specMode = MeasureSpec.getMode(spec); int specSize = MeasureSpec.getSize(spec); int size = Math.max(0, specSize - padding); int resultSize = 0; int resultMode = 0; switch (specMode) { // Parent has imposed an exact size on us case MeasureSpec.EXACTLY: if (childDimension >= 0) { resultSize = childDimension; resultMode = MeasureSpec.EXACTLY; } else if (childDimension == LayoutParams.MATCH_PARENT) { // Child wants to be our size. So be it. resultSize = size; resultMode = MeasureSpec.EXACTLY; } else if (childDimension == LayoutParams.WRAP_CONTENT) { // Child wants to determine its own size. It can't be // bigger than us. resultSize = size; resultMode = MeasureSpec.AT_MOST; } break; // Parent has imposed a maximum size on us case MeasureSpec.AT_MOST: if (childDimension >= 0) { // Child wants a specific size... so be it resultSize = childDimension; resultMode = MeasureSpec.EXACTLY; } else if (childDimension == LayoutParams.MATCH_PARENT) { // Child wants to be our size, but our size is not fixed. // Constrain child to not be bigger than us. resultSize = size; resultMode = MeasureSpec.AT_MOST; } else if (childDimension == LayoutParams.WRAP_CONTENT) { // Child wants to determine its own size. It can't be // bigger than us. resultSize = size; resultMode = MeasureSpec.AT_MOST; } break; // Parent asked to see how big we want to be case MeasureSpec.UNSPECIFIED: if (childDimension >= 0) { // Child wants a specific size... let him have it resultSize = childDimension; resultMode = MeasureSpec.EXACTLY; } else if (childDimension == LayoutParams.MATCH_PARENT) { // Child wants to be our size... find out how big it should // be resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size; resultMode = MeasureSpec.UNSPECIFIED; } else if (childDimension == LayoutParams.WRAP_CONTENT) { // Child wants to determine its own size.... find out how // big it should be resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size; resultMode = MeasureSpec.UNSPECIFIED; } break; } //noinspection ResourceType return MeasureSpec.makeMeasureSpec(resultSize, resultMode); }
當父View的mode是EXACTLY的時候:說明父View的size是肯定的。
子View的寬或高是具體數值:子view的size已經固定了,子View的size就是固定這個數值,mode=EXACTLY。
子View的寬或高是MATCH_PARENT:子View的size=父View的size,mode=EXACTLY。
子View的寬或高是WRAP_CONTENT:子View是包裹布局,說明子View的size還不肯定,因此子View的size最大不能超過父View的size,mode=AT_MOST。對象
當父View的mode是AT_MOST的時候:說明父View的size是不肯定的。
子View的寬或高是具體數值:子view的size已經固定了,子View的size就是固定這個數值,mode=EXACTLY。
子View的寬或高是MATCH_PARENT:父View的size是不肯定的,子View是填充佈局狀況,也不能肯定size,因此子View的size不能超過父View的size,mode=AT_MOST。
子View的寬或高是WRAP_CONTENT:子View是包裹布局,size不能超過父View的size,mode=AT_MOST。
當父View的mode是UNSPECIFIED的時候:說明父View不指定測量模式,父View沒有限制子視圖的大小,子View能夠是想要的任何尺寸。
子View的寬或高是具體數值:子view的size已經固定了,子View的size就是固定這個數值,mode=EXACTLY。
子View的寬或高是MATCH_PARENT:子視圖能夠是想要的任何尺寸,mode=UNSPECIFIED。
子View的寬或高是WRAP_CONTENT:子視圖能夠是想要的任何尺寸,mode=UNSPECIFIED。
須要注意一點就是,此時的MeasureSpec並非View真正的大小,只有setMeasuredDimension以後才能真正肯定View的大小。
關於具體ViewGroup的onMeasure過程這裏不作分析,因爲每種佈局的測量方式不同,不可能逐個分析,但在它們的onMeasure裏面的步驟是有必定規律的:
1.根據各自的測量規則遍歷Children元素,調用getChildMeasureSpec方法獲得Child的measureSpec;
2.調用Child的measure方法;
3.調用setMeasuredDimension肯定最終的大小。
View的measure過程由其measure方法來完成,measure方法是一個final類型的方法,這意味着子類不能重寫此方法,在View的measure方法裏面會去調用onMeasure方法,咱們能夠經過複寫onMeasure()方法去測量設置View的大小。以下:
public final void measure(int widthMeasureSpec, int heightMeasureSpec) { ... onMeasure(widthMeasureSpec, heightMeasureSpec); .... } //若是須要自定義測量,子類需重寫這個方法 protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec), getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec)); } //將測量好的寬跟高進行存儲 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; } setMeasuredDimensionRaw(measuredWidth, measuredHeight); } //若是View沒有重寫onMeasure方法,默認會直接調用getDefaultSize public static int getDefaultSize(int size, int measureSpec) { int result = size; //獲取父View傳遞過來的模式 int specMode = MeasureSpec.getMode(measureSpec); //獲取父View傳遞過來的大小 int specSize = MeasureSpec.getSize(measureSpec); switch (specMode) { case MeasureSpec.UNSPECIFIED: //View的大小父View未定,設置爲建議最小值 result = size; break; case MeasureSpec.AT_MOST: case MeasureSpec.EXACTLY: result = specSize; break; } return result; }
從上述代碼能夠得出,View的寬/高由specSize決定,直接繼承View的自定義控件須要重寫onMeasure方法並設置wrap_content時的自身大小,不然在佈局中使用wrap_content就至關於使用match_parent。
上述就是View的measure大體過程,在measure完成以後,經過getMeasuredWidth/Height方法就能夠得到測量後的寬高,這個寬高通常狀況下就等於View的最終寬高,由於View的layout佈局的時候就是根據measureWidth/Height來設置寬高的,除非在layout中修改了measure值。
measure()方法中咱們已經測量出View的大小,根據這些大小,咱們接下來就要肯定View在父View的佈局位置。Layout的做用是ViewGroup用來肯定子元素的位置,當ViewGroup的位置被肯定後,它在onLayout中會遍歷全部的子元素並調用其layout方法。簡單的來講就是,layout方法肯定View自己的位置,而onLayout方法則會肯定全部子元素的位置。
View的layout方法代碼:
public void layout(int l, int t, int r, int b) { ... onLayout(changed, l, t, r, b); ... } //空方法,子類若是是ViewGroup類型,則重寫這個方法,實現ViewGroup中全部View控件佈局 protected void onLayout(boolean changed, int left, int top, int right, int bottom) { }
能夠看到這是一個空實現,和onMeasure方法相似,onLayout的實現和具體的佈局有關,具體ViewGroup的子類須要重寫onLayout方法,並根據具體佈局規則遍歷調用Children的layout方法。
經過上面的分析,能夠獲得兩個結論:
View經過layout方法來確認本身在父容器中的位置。
ViewGroup經過onLayout方法來肯定View在容器中的位置。
FrameLayout的onLayout方法代碼:
@Override protected void onLayout(boolean changed, int left, int top, int right, int bottom) { layoutChildren(left, top, right, bottom, false); } 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(); 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; 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; } child.layout(childLeft, childTop, childLeft + width, childTop + height); } } }
一、獲取父View的內邊距padding的值。
二、遍歷子View,處理子View的layout_gravity屬性、根據View測量後的寬和高、父View的padding值、來肯定子View的佈局參數。
三、調用child.layout方法,對子View進行佈局。
通過前面的測量和佈局以後,接下來就是繪製了,也就是真正把View繪製在屏幕可見視圖上。Draw操做用來將控件繪製出來,繪製的流程從performDraw()方法開始。performDraw()方法在類ViewRootImpl內,其核心代碼以下:
private void performDraw() { ... boolean canUseAsync = draw(fullRedrawNeeded); ... } private boolean draw(boolean fullRedrawNeeded) { ... if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset, scalingRequired, dirty, surfaceInsets)) { return false; } ... } private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff, boolean scalingRequired, Rect dirty, Rect surfaceInsets) { ... mView.draw(canvas); ... }
最終調用到View的draw方法繪製每一個具體的View,繪製基本上能夠分爲七個步驟。
public void draw(Canvas canvas) { ... // Step 1, draw the background, if needed int saveCount; if (!dirtyOpaque) { drawBackground(canvas); } ... // Step 2, If necessary, save the canvas' layers to prepare for fading saveCount = canvas.getSaveCount(); canvas.saveUnclippedLayer(left, top, right, top + length); ... // Step 3, draw the content if (!dirtyOpaque) onDraw(canvas); ... // Step 4, draw the children dispatchDraw(canvas); ... // Step 5, If necessary, draw the fading edges and restore layers canvas.drawRect(left, top, right, top + length, p); ... canvas.restoreToCount(saveCount); ... // Step 6, draw decorations (foreground, scrollbars) onDrawForeground(canvas); ... // Step 7, draw the default focus highlight drawDefaultFocusHighlight(canvas); ... }
第一步:drawBackground(canvas):做用就是繪製View的背景。
第二步:saveUnclippedLayer:保存畫布的圖層。
第三步:onDraw(canvas):繪製View的內容。View的內容是根據本身需求本身繪製的,因此方法是一個空方法,View的繼承類本身複寫實現繪製內容。
第四步:dispatchDraw(canvas):遍歷子View進行繪製內容。在View裏面是一個空實現,ViewGroup裏面纔會有實現。View的繪製過程的傳遞經過dispatchDraw來實現的,dispatchDraw會遍歷調用全部子元素的draw方法,如此draw事件就一層層地傳遞了下去。在自定義ViewGroup通常不用複寫這個方法,由於它在裏面的實現幫咱們實現了子View的繪製過程,基本知足需求。
第五步:drawRect:繪製邊緣和恢復畫布的圖層。
第六步:onDrawForeground(canvas):對前景色跟滾動條進行繪製。
第七步:drawDefaultFocusHighlight(canvas):繪製默認焦點高亮。
若是是自定義ViewGroup的話,須要重寫onMeasure方法,在onMeasure方法裏面遍歷測量子元素,同理onLayout方法也是同樣,最後實現onDraw方法繪製本身; 若是自定義View的話,則須要從寫onMeasure方法,處理wrap_content的狀況,不須要處理onLayout,最後實現onDraw方法繪製本身。