Customize the layout of view Foundation

When the previous section is completed measure After the operation , Next layout The process is actually relatively simple , The purpose is that the parent view follows
The size and layout parameters of the subview , Place the subview in the appropriate position . The core of layout parameters is to deal with gravity Parameters .

layout Process design ideas

Initial call layout() Function is from ViewRoot Class performTraversals(),host It's a View object ,View Class layout() The type of function is final, Its subclasses cannot overload the function , In order to make sure View In the system layout The process remains the same .

stay layout() Function , First call setFrame() Function to set the position specified in the parameter for the current view , And then back onLayou() function .ViewGroup Class is overloaded onLayout() function , And set its function type to a abstract type , therefore , be-all ViewGroup Instance must implement onLayout().View The system expects programmers to be able to onLayout() Function to modify the child views contained in the view layout operation , When the view is ViewGroup when , Programmers usually work in onLayout() Call child view in function chart child Of layout() Method , So as to complete the position allocation of the sub view . Of course , If the subview is also a ViewGroup example , The corresponding onLayout() function .

View class in layout() The prototype of the function is ：

public void layout(int l, int t, int r, int b) 

Parameter specifies the left position of the child view in the parent view 、 On 、 Right 、 The next position . Of this function
The internal process is as follows .

1. call setFrame(l, t, r, b) Save the position parameters .

These parameters will be saved to View Internal variables （mLeft、mTop、 mRight、mBottom), Before saving variables , We will first compare whether these parameters are the same as the original , If the same , Then do nothing , If it is different, it will be reassigned , And before the assignment , Will give mPrivateFlags Add DRAWN identification , At the same time call invalidate tell View The original position occupied by the system needs to be redrawn . Note that first call invalidate(), Then assigned , Because what needs to be redrawn is the old area .

2. Callback onLayout().

View As defined in onLayout() Function does nothing by default ,View The system provides onLayout() The function is used to enable the parent view containing child views to be displayed in the system onLayout() Function to allocate the position of the child view , Because of that , If it is the parent view , It has to be overloaded onLayout(), That's why ,ViewGroup Class caiba onLayout() The overload is modified to a abstract type .

3. eliminate mPrivateFlags Medium LAYOUT_REQUIRED identification , because layout Your operation has been completed .

LinearLayout in onLayout() Internal process

LinearLayout There are two ways to arrange the subviews in , One is horizontal , The other is vertical , This section only describes the vertical mode
Workflow under .
onLayout() Letter Count in The first First root According to the mOrientation change The amount sentence break yes water flat also yes Hang straight , Such as fruit yes Hang straight , Call layoutVertical() To begin layout operation .

1. Get the available width of the subview .

Here is the vertical layout , Why use the available width , Not the available height ？

because , Even in the vertical direction , The subview itself is also centered horizontally , To center, you need to know what width is available , So as to calculate the position of the left edge of the sub view , Such as chart Shown , The altitude information will be obtained in the following steps .

2. According to... In the parent view gravity attribute , Determine the starting position of the subview .
By default ,gravity yes TOP, application The program can be set to BOTTOM or person CENTER_VERTICAL, The corresponding starting positions of the three are shown in the figure .

3. The vertical position of the subview has been determined , And so it began for() Loop through assigning locations to each child view .

(1) Take out the... Of the subview LayoutParams, And get the gravity Value . Pay attention to this gravity Values are not in subviews
android:gravity Value , It is android:layou_gravity Value , See the next section for an explanation of this .

( 2 ) root According to the gravity The value of determines the starting position in the horizontal direction , Divided into three , branch other yes LEFT、 CENTER HORIZONTAL And RIGHT, The corresponding starting positions of the three are shown in the figure .

( 3 ) transfer use setChildFrame(), This function is actually called internally child.layout() Set the layout location for the subview .

thus ,LinearLayout This completes the process of assigning layout positions to the included sub views .

TextView in gravity And layout The relationship between

As mentioned in the previous section , In the subview gravity The value of is not android:gravity Value , and yes android:layout__gravity, This is a
What's going on ？
This question actually contains an important concept , That's it LayoutParams How is it assigned . Applications can be found in layout.xml Use in documents such as < LinearLayout >、< TextView > The tag defines an interface . Whether it's ViewGroup still View, These tags define just one view , And cannot define LayoutParams, because LayoutParams As its name says , It is a layout attribute , When a view is not laid out, where does the layout attribute come from ？

and stay too Go to Of open Hair the Examination in , Big home A few Almost all send use too android:layout_height、android:layout_width、 android:gravity etc. XM L The syntax is a LinearLayout or person TextView Add the corresponding layout properties , And these values seem to work properly , On the surface , It is the corresponding view LayoutParams value , How does this explain ？

in addition , stay LayoutInflater There is one in the class inflate(int resId,View root) function , in the majority of cases , You call this
Function time , The second parameter root All is empty . What is the meaning of this parameter ？ Why should it be empty in most cases ？

To answer the above questions , It is necessary to figure out how the internal system is from a XM L Create a corresponding file View Tree object ,
In the following paragraph XML Code as an example to illustrate the process :

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical">
        <TextView
            android:layout_width="200dp"
            android:layout_height="60dp"
            android:background="@color/white"
            android:text="am i center_vertical"
            android:gravity="center_horizontal"/>
        <ListView
            android:layout_width="300dp"
            android:layout_height="100dp"
            android:background="@color/black"/>
    </LinearLayout>

This code contains a LinearLayout, It also contains two sub views , It's a TextView And one ListView.

from XML The corresponding... Is constructed in the file View The tree is in Layoutlnflater in Of inflate() Function , If this code is used as setContentView() Parameters of , Will be treated as Activity The interface of , that LinearLayout As defined in layout_width And height It all makes sense . But if this code is just an application call inflate() Function to create a View object , that Well layout_width And height There's no point .

inflate() The function will first determine the parameters root Is it empty , If it is empty , So XML The root view in the file constructs a temporary
Of View object , Here is LinearLayout object , At this point the LinearLayout Declarative layout_width And height Is of no use . Next, continue to read XML In file TextView, Read TextView after , It is necessary to TextView Add to LinearLayout in . Because to add , So there has to be a LayoutParams object , therefore ,inflate() In the function TextView As defined in layout_width And height Is the parameter , return transfer TextView Parent view generateLayoutParams() function .
TextView The parent view of is LinearLayout, And in the LinearLayout Of generateLayoutParams(attrs) Function , Parameters attrs It is from TextView All attribute values declared in , except layout_width and height Xibu , Also contains background、text And all declared attribute values , but generateLayoutParams(atts) But I am only interested in four of them :

public LayoutParams(Context c, AttributeSet attrs) {
            super(c, attrs);
            TypedArray a =
                    c.obtainStyledAttributes(attrs, com.android.internal.R.styleable.LinearLayout_Layout);

            weight = a.getFloat(com.android.internal.R.styleable.LinearLayout_Layout_layout_weight, 0);
            gravity = a.getInt(com.android.internal.R.styleable.LinearLayout_Layout_layout_gravity, -1);

            a.recycle();
        }

In the above code ,super(c,attrs) Will read layout_height and layout_width Properties of , As shown in the following code . Then continue to read weight Properties and gravity attribute , The names of these two attributes are layout_weight and layout_ gravity.
generateLayoutParams(atts) Finally, according to TextView The above four attributes in the construct a LinearLayout.ayoutParams object , And with this object TextView Add to LinearLayout in .

Let's look back at the questions raised above ,< TextView > Included in the label android:layout_width、height Etc. is not defined LayoutParams object , It just provides the structure LayoutParams The value required for the parameter , And the real structure LayoutParams Object is its parent view , What will these parameters ultimately produce LayoutParams It depends on TextView In the parent view of generateLayoutParams(attrs) The concrete realization of .

That's why we used... In the previous section layout_gravity instead of gravity Why , Because in LinearLayout
Of generateLayoutParams() Function , It's a android:layout_gravity The attribute value is assigned to LayoutParams.gravity, instead of android:gravity. in fact ,android:gravity Properties will be used as TextView Internal parameters ,TextView Inside onDraw() The function will be based on android:gravity The value of determines where the text is displayed .

And when you call setContentView(int resld) when , The system also calls inflate() Method from the specified XML In the document View Trees , And call time parameters root The setting is an internal one FrameLayout object .root The meaning of the parameter is that it will raise for generateLayoutParams() function , This function produces LayoutParams Object will be used as XML The views contained in the file are added to root Layout properties when .

If you're in one XM L Only one... Is defined in the file TextView label , And then call inflate(xml, null) From this XML
In file infalte a TextView object , Then the TextView Included in the tag android:layout_width、height It won't make any sense . When you Want to hold infalte Out Come on Of TextView Object is added to a ViewGroup when , You must also construct a LayoutParams object .