abstract：前言Android自定義View的詳細(xì)步驟是我們每一個(gè)Android開發(fā)人員都必須掌握的技能，因?yàn)樵陂_發(fā)中總會(huì)遇到自定義View的需求。為了提高自己的技術(shù)水平，自己就系統(tǒng)的去研究了一下，在這里寫下一點(diǎn)心得，有不足之處希望大家及時(shí)指出。流程在Android中對(duì)于布局的請(qǐng)求繪制是在Android framework層開始處理的。繪制是從根節(jié)點(diǎn)開始，對(duì)布局樹進(jìn)行measure與draw。在RootVie
前言
Android自定義View的詳細(xì)步驟是我們每一個(gè)Android開發(fā)人員都必須掌握的技能，因?yàn)樵陂_發(fā)中總會(huì)遇到自定義View的需求。為了提高自己的技術(shù)水平，自己就系統(tǒng)的去研究了一下，在這里寫下一點(diǎn)心得，有不足之處希望大家及時(shí)指出。
流程
在Android中對(duì)于布局的請(qǐng)求繪制是在Android framework層開始處理的。繪制是從根節(jié)點(diǎn)開始，對(duì)布局樹進(jìn)行measure與draw。在RootViewImpl中的performTraversals展開。它所做的就是對(duì)需要的視圖進(jìn)行measure（測(cè)量視圖大?。?、layout（確定視圖的位置）與draw（繪制視圖）。下面的圖能很好的展現(xiàn)視圖的繪制流程：
當(dāng)用戶調(diào)用requestLayout時(shí)，只會(huì)觸發(fā)measure與layout，但系統(tǒng)開始調(diào)用時(shí)還會(huì)觸發(fā)draw
下面來詳細(xì)介紹這幾個(gè)流程。
measure
measure是View中的final型方法不可以進(jìn)行重寫。它是對(duì)視圖的大小進(jìn)行測(cè)量計(jì)算，但它會(huì)回調(diào)onMeasure方法，所以我們?cè)谧远xView的時(shí)候可以重寫onMeasure方法來對(duì)View進(jìn)行我們所需要的測(cè)量。它有兩個(gè)參數(shù)widthMeasureSpec與heightMeasureSpec。其實(shí)這兩個(gè)參數(shù)都包含兩部分，分別為size與mode。size為測(cè)量的大小而mode為視圖布局的模式
我們可以通過以下代碼分別獲?。?/p>
int widthSize = MeasureSpec.getSize(widthMeasureSpec); int heightSize = MeasureSpec.getSize(heightMeasureSpec); int widthMode = MeasureSpec.getMode(widthMeasureSpec); int heightMode = MeasureSpec.getMode(heightMeasureSpec);
獲取到的mode種類分為以下三種：
setMeasuredDimension
通過以上邏輯獲取視圖的寬高，最后要調(diào)用setMeasuredDimension方法將測(cè)量好的寬高進(jìn)行傳遞出去。其實(shí)最終是調(diào)用setMeasuredDimensionRaw方法對(duì)傳過來的值進(jìn)行屬性賦值。調(diào)用super.onMeasure()的調(diào)用邏輯也是一樣的。
下面以自定義一個(gè)驗(yàn)證碼的View為例，它的onMeasure方法如下：
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
int widthSize = MeasureSpec.getSize(widthMeasureSpec);
int heightSize = MeasureSpec.getSize(heightMeasureSpec);
int widthMode = MeasureSpec.getMode(widthMeasureSpec);
int heightMode = MeasureSpec.getMode(heightMeasureSpec);
if (widthMode == MeasureSpec.EXACTLY) {
//直接獲取精確的寬度
width = widthSize;
} else if (widthMode == MeasureSpec.AT_MOST) {
//計(jì)算出寬度(文本的寬度+padding的大小)
width = bounds.width() + getPaddingLeft() + getPaddingRight();
}
if (heightMode == MeasureSpec.EXACTLY) {
//直接獲取精確的高度
height = heightSize;
} else if (heightMode == MeasureSpec.AT_MOST) {
//計(jì)算出高度(文本的高度+padding的大小)
height = bounds.height() + getPaddingBottom() + getPaddingTop();
}
//設(shè)置獲取的寬高
setMeasuredDimension(width, height);
}可以對(duì)自定義View的layout_width與layout_height進(jìn)行設(shè)置不同的屬性，達(dá)到不同的mode類型，就可以看到不同的效果
measureChildren
如果你是對(duì)繼承ViewGroup的自定義View那么在進(jìn)行測(cè)量自身的大小時(shí)還要測(cè)量子視圖的大小。一般通過measureChildren(int widthMeasureSpec, int heightMeasureSpec)方法來測(cè)量子視圖的大小。
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);
}
}
}通過上面的源碼會(huì)發(fā)現(xiàn)，它其實(shí)是遍歷每一個(gè)子視圖，如果該子視圖不是隱藏的就調(diào)用measureChild方法，那么來看下measureChild源碼：
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);
}會(huì)發(fā)現(xiàn)它首先調(diào)用了getChildMeasureSpec方法來分別獲取寬高，最后再調(diào)用的就是View的measure方法，而通過前面的分析我們已經(jīng)知道它做的就是對(duì)視圖大小的計(jì)算。而對(duì)于measure中的參數(shù)是通過getChildMeasureSpec獲取，再來看下其源碼：
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);
}是不是容易理解了點(diǎn)呢。它做的就是前面所說的根據(jù)mode的類型，獲取相應(yīng)的size。根據(jù)父視圖的mode類型與子視圖的LayoutParams類型來決定子視圖所屬的mode,最后再將獲取的size與mode通過MeasureSpec.makeMeasureSpec方法整合返回。最后傳遞到measure中，這就是前面所說的widthMeasureSpec與heightMeasureSpec中包含的兩部分的值。整個(gè)過程為measureChildren->measureChild->getChildMeasureSpec->measure->onMeasure->setMeasuredDimension,所以通過measureChildren就可以對(duì)子視圖進(jìn)行測(cè)量計(jì)算。
layout
layout也是一樣的內(nèi)部會(huì)回調(diào)onLayout方法，該方法是用來確定子視圖的繪制位置，但這個(gè)方法在ViewGroup中是個(gè)抽象方法，所以如果要自定義的View是繼承ViewGroup的話就必須實(shí)現(xiàn)該方法。但如果是繼承View的話就不需要了，View中有一個(gè)空實(shí)現(xiàn)。而對(duì)子視圖位置的設(shè)置是通過View的layout方法通過傳遞計(jì)算出來的left、top、right與bottom值，而這些值一般都要借助View的寬高來計(jì)算,視圖的寬高則可以通過getMeasureWidth與getMeasureHeight方法獲取，這兩個(gè)方法獲取的值就是上面onMeasure中setMeasuredDimension傳遞的值，即子視圖測(cè)量的寬高。
getWidth、getHeight與getMeasureWidth、getMeasureHeight是不同的，前者是在onLayout之后才能獲取到的值，分別為left-right與top-bottom；而后者是在onMeasure之后才能獲取到的值。只不過這兩種獲取的值一般都是相同的，所以要注意調(diào)用的時(shí)機(jī)。
下面以定義一個(gè)把子視圖放置于父視圖的四個(gè)角的View為例：
@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
int count = getChildCount();
MarginLayoutParams params;
int cl;
int ct;
int cr;
int cb;
for (int i = 0; i < count; i++) {
View child = getChildAt(i);
params = (MarginLayoutParams) child.getLayoutParams();
if (i == 0) {
//左上角
cl = params.leftMargin;
ct = params.topMargin;
} else if (i == 1) {
//右上角
cl = getMeasuredWidth() - params.rightMargin - child.getMeasuredWidth();
ct = params.topMargin;
} else if (i == 2) {
//左下角
cl = params.leftMargin;
ct = getMeasuredHeight() - params.bottomMargin - child.getMeasuredHeight()
- params.topMargin;
} else {
//右下角
cl = getMeasuredWidth() - params.rightMargin - child.getMeasuredWidth();
ct = getMeasuredHeight() - params.bottomMargin - child.getMeasuredHeight()
- params.topMargin;
}
cr = cl + child.getMeasuredWidth();
cb = ct + child.getMeasuredHeight();
//確定子視圖在父視圖中放置的位置
child.layout(cl, ct, cr, cb);
}
}至于onMeasure的實(shí)現(xiàn)源碼我后面會(huì)給鏈接，如果要看效果圖的話，我后面也會(huì)貼出來，前面的那個(gè)驗(yàn)證碼的也是一樣
draw
draw是由dispatchDraw發(fā)動(dòng)的，dispatchDraw是ViewGroup中的方法，在View是空實(shí)現(xiàn)。自定義View時(shí)不需要去管理該方法。而draw方法只在View中存在，ViewGoup做的只是在dispatchDraw中調(diào)用drawChild方法，而drawChild中調(diào)用的就是View的draw方法。那么我們來看下draw的源碼：
public void draw(Canvas canvas) {
final int privateFlags = mPrivateFlags;
final boolean dirtyOpaque = (privateFlags & PFLAG_DIRTY_MASK) == PFLAG_DIRTY_OPAQUE &&
(mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);
mPrivateFlags = (privateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_DRAWN;
/*
* Draw traversal performs several drawing steps which must be executed
* in the appropriate order:
*
* 1. Draw the background
* 2. If necessary, save the canvas' layers to prepare for fading
* 3. Draw view's content
* 4. Draw children
* 5. If necessary, draw the fading edges and restore layers
* 6. Draw decorations (scrollbars for instance)
*/
// Step 1, draw the background, if needed
int saveCount;
if (!dirtyOpaque) {
drawBackground(canvas);
}
// skip step 2 & 5 if possible (common case)
final int viewFlags = mViewFlags;
boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;
boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;
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;
}
//省略2&5的情況
....
}源碼已經(jīng)非常清晰了draw總共分為6步；
繪制背景
如果需要的話，保存layers
繪制自身文本
繪制子視圖
如果需要的話，繪制fading edges
繪制scrollbars
其中 第2步與第5步不是必須的。在第3步調(diào)用了onDraw方法來繪制自身的內(nèi)容，在View中是空實(shí)現(xiàn)，這就是我們?yōu)槭裁丛谧远xView時(shí)必須要重寫該方法。而第4步調(diào)用了dispatchDraw對(duì)子視圖進(jìn)行繪制。還是以驗(yàn)證碼為例：
@Override
protected void onDraw(Canvas canvas) {
//繪制背景
mPaint.setColor(getResources().getColor(R.color.autoCodeBg));
canvas.drawRect(0, 0, getMeasuredWidth(), getMeasuredHeight(), mPaint);
mPaint.getTextBounds(autoText, 0, autoText.length(), bounds);
//繪制文本
for (int i = 0; i < autoText.length(); i++) {
mPaint.setColor(getResources().getColor(colorRes[random.nextInt(6)]));
canvas.drawText(autoText, i, i + 1, getWidth() / 2 - bounds.width() / 2 + i * bounds.width() / autoNum
, bounds.height() + random.nextInt(getHeight() - bounds.height())
, mPaint);
}
//繪制干擾點(diǎn)
for (int j = 0; j < 250; j++) {
canvas.drawPoint(random.nextInt(getWidth()), random.nextInt(getHeight()), pointPaint);
}
//繪制干擾線
for (int k = 0; k < 20; k++) {
int startX = random.nextInt(getWidth());
int startY = random.nextInt(getHeight());
int stopX = startX + random.nextInt(getWidth() - startX);
int stopY = startY + random.nextInt(getHeight() - startY);
linePaint.setColor(getResources().getColor(colorRes[random.nextInt(6)]));
canvas.drawLine(startX, startY, stopX, stopY, linePaint);
}
}其實(shí)很簡(jiǎn)單，就是一些繪制的業(yè)務(wù)邏輯。好了基本就到這里了，下面上傳一張示例的效果圖，與源碼鏈接
示例圖
對(duì)了還有自定義屬性，這里簡(jiǎn)單說一下。自定義View時(shí)一般都要自定義屬性，所以都會(huì)在res/values/attr.xml中定義attr與declare-styleable,最后在自定義View中通過TypedArray獲取。