Custom view incomplete to be continued

Customize View

Purple wish _ The world is perfect , A teenager committed to internship in the factory during the sophomore summer vacation , First written in 2022/2/13

What is customization View

Customize View And customization ViewGroup The difference between ？

• Customize View Generally, you need to realize your own onMeasure() and onDraw() Method .

• Customize ViewGroup Need to achieve onMeasure() and onLayout() Method .

Customize View Steps for

Three steps ：

1. Layout onLayout(){}
2. draw onDraw(){}
3. Touch event onTouchEvent(){}

Layout

Layout , About to confirm View The size and location of , If not set, then sub View Will be in the upper left corner of the parent control . Then according to the size of the child control , Determine the size you need .

First of all, you need the child controls to measure themselves , That is, call the... Of the child control onMeasure() Method , Let child controls measure themselves . Before the layout The measurement operation must be carried out first , otherwise get When the width and height of the child control are obtained, there will be multiple times of obtaining Inconsistent values The situation of . Then call the sub control onMeasure() Method time , Including rewriting your own onMeasure() Method time , You will encounter two parameters widthMeasureSpec: Int and heightMeasureSpec: Int, Why are there these two parameters ？ How to set the values of these two parameters ？ How to use the values of these two parameters ？ I believe these problems are difficult for beginners and must be solved .

widthMeasureSpec and heightMeasureSpec What is it?

Let's see what these two words mean first ：

width measure spec： Width measurement specification

height measure spec： Height measurement specification

Through the name, we can easily understand , Is the father View Tell me View Recommended width of / Height value .

So father View How to set the recommended width with only one parameter / Height value to child View Clearly described ？ This is wonderful , Really amazed Google The ingenuity of the engineer . Don't underestimate this variable , He also stored specMode and specSize Two messages . How to do it ？ We know that integer in computer is 4 Binary representation of bytes , That is, by 32 individual 0 perhaps 1. In general ,Android The pixel value of the width or height of the device of the system does not exceed 2³⁰,Google Engineers put the high-ranking first 31 and 30 Bits specifically store other intention information .

Although there are only two , There are three kinds of information stored in it , It happens to be doomed dp value 、wrap_content and match_parent, Cooperate with parent view Limitations on oneself and self pairs View The limitation of , When you put it together, you have 9 In this case , this 9 This situation covers all the current situations —— Father View yes match_parent, Son View yes wrap_content; Father view yes wrap_content, Son view yes match_parent And so on . So the rest 30 Bit stores information about the specific value size . This way of storing information in high places, which is not used at all, is a new idea .

Let's see Google How does the official figure out Spec The result of

public static int makeMeasureSpec(@IntRange(from = 0, to = (1 << MeasureSpec.MODE_SHIFT) - 1) int size,
                                          @MeasureSpecMode int mode) {
    
    if (sUseBrokenMakeMeasureSpec) {
    
        return size + mode;
    } else {
    
        return (size & ~MODE_MASK) | (mode & MODE_MASK);
    }
}

sUseBrokenMakeMeasureSpec by true When , yes API17 And the calculation method of previous versions , It's going to be size and mode Add up , The sequence of the passed in parameters is inconsistent, and it can still be executed normally , But any overflow of the two values will affect MeasureSpec Result , this Caused ConstraintsLayout Of bug,Google The official replaced it with a more rigorous bit calculation method to solve this problem . So now sUseBrokenMakeMeasureSpec The values for are as follows ：

private static final int MODE_SHIFT = 30;
private static final int MODE_MASK  = 0x3 << MODE_SHIFT;	//  The first 31、30 Position as 1, The rest are 0
// Use the old (broken) way of building MeasureSpecs.
private static boolean sUseBrokenMakeMeasureSpec = false;

The default is false, Then it will be (size & ~MODE_MASK) | (mode & MODE_MASK) To calculate .

widthMeasureSpec and heightMeasureSpec What value should the parameter be set to

The system has given a very good conversion to WidthMeasureSprc or heightMeasureSpec Methods

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 them 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);
}

in other words , Here you only need to pass three parameters int spec, int padding, int childDimension, How to pass parameters ？

• spec It can be directly transferred into its own widthMeasureSpec and heightMeasureSpec, Used to get subclasses widthMeasureSpec and heightMeasureSpec
• padding You need to set the corresponding padding Pass in
• childDimension Transitions View Of layoutParams The width or height of

Here is my general way of writing ：

val childLayoutParams = childView.layoutParams

val chileWidthMeasureSpec = getChildMeasureSpec(
    widthMeasureSpec, paddingLeft + paddingRight, childLayoutParams.width
)
val chileHeightMeasureSpec = getChildMeasureSpec(
    heightMeasureSpec, paddingTop + paddingBottom, childLayoutParams.height
)
    //  Finally, according to business needs , Calculate your own width and height settings 

Some small problems

onMeasure() Methods may be used by parents View Call several times , Therefore, you need to reset and initialize the used object attributes . Here we can see an example ：

// sdk30 -FrameLayout.java Source code 
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    mMatchParentChildren.clear();
    /**
     *  Omitted code 
     */
    for (int i = 0; i < count; i++) {       
        if (mMeasureAllChildren || child.getVisibility() != GONE) {
			measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
        }
    }
    if (count > 1) {
        for (int i = 0; i < count; i++) {
            child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
        }
    }
}

Here we can see that subclasses are called twice onMeasure Method , and FrameLayout In itself onMeasure I also emptied my own ArrayList.

draw

Update after learning

Touch event

Update after learning

Purple wish _ The world is perfect , A teenager committed to internship in the factory during the sophomore summer vacation , First written in 2022/2/13