view顯示到屏幕上,大概來講,需要走如下的流程:從調用方法的角度來看,即需要走三個方法:measure(),layout(),draw(),這三個方法是view的方法,走完這三個方法view就會顯示到屏幕上
而且這三個方法是final修飾的,系統不允許我們重寫這些方法,要求我們遵循這個框架流程,但是又不能不讓我們自定義自己的測量,佈局和繪畫過程,所以給出了onMeasure(),onLayout(),onDraw()可重寫的方法,它們分別對應於measure(),layout(),draw()方法,分別在measure(),layout(),draw()內部調用,(這裏用到了模板方法模式)我們可以通過重寫on開頭的這些方法達到自定義的效果。
下面,看一下view的onMeasure()方法的默認實現,讀代碼發現默認實現wrap_content和match_parent獲取的測量值是一樣的,即都是通過int specSize = MeasureSpec.getSize(measureSpec);獲取的,那麼這裏的measureSpec(即onMeasure(int widthMeasureSpec, int heightMeasureSpec)方法傳過來的的兩個參數,widthMeasureSpec 和 heightMeasureSpec從哪裏來的呢?發現,是通過如下流程傳過來的,從父容器開始,在父容器中的measure()方法中調用了measureChildren()方法,在measureChildren()方法中遍歷了所有的子view,調用了measureChild()方法,在measureChild()方法中getChildMeasureSpec()方法獲取了子View的大小和測量模式,然後再通過調用子view的measure()傳給了子view的onMeasure()方法。簡述如下
父measure() --> 父measureChildren()--> 父measureChild() -- > 父 getChildMeasureSpec()-- > 子measure() --> 子onMeasure()
上面的方法的調用在具體的控件中可能並不是準確的(具體到某個方法),但是整個流程是正確的,當把某個自定義view放到某個系統已經實現好的現有的父容器中時(比如Linear Layout,FrameLayout),由於父容器已經重寫了ViewGroup的onMeasure()方法,加上了測量子view的邏輯(ViewGroup默認是沒有對子View的操作的,只是提供了一些操作的方法,如measureChildren(),measureChild(),measureChildWithMargins()等),所以會調用getChildMeasureSpec()方法獲取子view的大小和測量模式。
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
//獲取測量後的大小(getDefaultSize)
//並進行設置(setMeasuredDimension)
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
public static int getDefaultSize(int size, int measureSpec) {
int result = size;
int specMode = MeasureSpec.getMode(measureSpec);
int specSize = MeasureSpec.getSize(measureSpec);
switch (specMode) {
case MeasureSpec.UNSPECIFIED:
result = size;
break;
case MeasureSpec.AT_MOST://對應wrap_content
case MeasureSpec.EXACTLY://對應match_parent和確定的高寬值
result = specSize;
break;
}
return result;
}
另:protected int getSuggestedMinimumWidth() {
return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());
}
protected void measureChild(View child, int parentWidthMeasureSpec,
int parentHeightMeasureSpec) {
final LayoutParams lp = child.getLayoutParams();
//getChildMeasureSpec獲取子View的測量規格
final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
mPaddingLeft + mPaddingRight, lp.width);
final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
mPaddingTop + mPaddingBottom, lp.height);
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
//注:在LinearLayout中測量子view的方法調用的應該是下面這個方法(待驗證)
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);
}
經過上面的分析結合 measureChild()方法發現getDefaultSize()方法參數中的 measureSpec而是通過getChildMeasureSpec()方法獲取的,下面看一下getChildMeasureSpec()方法,可以看出,父容器是EXACTLY和AT_MOST時(UNSPECIFIED一般是系統使用,暫不考慮,無論子view是match_parent還是wrap_content,其獲得的size(即方法中你的resultSize)都是父容器的size減去父容器自身的padding : int size = Math.max(0, specSize - padding);
public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
//父容器的測量模式
int specMode = MeasureSpec.getMode(spec);
//父容器的尺寸
int specSize = MeasureSpec.getSize(spec);
int size = Math.max(0, specSize - padding);
int resultSize = 0;
int resultMode = 0;
//對不同的父容器測量模式分別進行處理
switch (specMode) {
// Parent has imposed an exact size on us
//父容器是EXACTLY模式
case MeasureSpec.EXACTLY:
//子View 尺寸 >= 0,即指定了確切的寬高值。
if (childDimension >= 0) {
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
//子View是MATCH_PARENT
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size. So be it.
resultSize = size;
resultMode = MeasureSpec.EXACTLY;
//子View是WRAP_CONTENT
} 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
//父容器是AT_MOST模式
case MeasureSpec.AT_MOST:
//子View 尺寸 >= 0,即指定了確切的寬高值。
if (childDimension >= 0) {
// Child wants a specific size... so be it
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
//子View是MATCH_PARENT
} 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;
//子View是WRAP_CONTENT
} 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
//父容器是UNSPECIFIED模式
case MeasureSpec.UNSPECIFIED:
//子View 尺寸 >= 0,即指定了確切的寬高值。
if (childDimension >= 0) {
// Child wants a specific size... let him have it
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
//子View是MATCH_PARENT
} 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;
//子View是WRAP_CONTENT
} 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);
}
解決辦法:如下,即設置一個默認的寬高
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
// 獲取寬的大小
int widthSize = MeasureSpec.getSize(widthMeasureSpec);
// 獲取高的大小
int heightSize = MeasureSpec.getSize(heightMeasureSpec);
//默認寬高大小
int defaultWidth = 300;
int defaultHeight = 300;
if (getLayoutParams().width == ViewGroup.LayoutParams.WRAP_CONTENT) {
widthSize = defaultWidth;
}
if (getLayoutParams().height == ViewGroup.LayoutParams.WRAP_CONTENT) {
heightSize = defaultHeight;
}
//重新設置大小
setMeasuredDimension(widthSize, heightSize);
}