android - 自定义 View 的 onMeasure : how to get width based on height

Question

我的问题之前的版本太罗嗦了。人们无法理解它，所以下面是一个完整的重写。如果您对旧版本感兴趣，请参阅 edit history。

RelativeLayout 的父级将 MeasureSpec s 发送到其 subview 的 onMeasure 方法，以查看 child 想要多大。这发生在几个 channel 中。

我的自定义 View

我有一个 custom view 。随着 View 内容的增加， View 的高度也会增加。当 View 达到父级允许的最大高度时，任何其他内容的 View 宽度都会增加(只要为宽度选择了 wrap_content)。因此，自定义 View 的宽度直接取决于父级所说的最大高度。

(不和谐的)父子对话

onMeasure 传递 1

RelativeLayout 父级告诉我的观点，“你可以是任何宽度 up to 900 和任何高度 up to 600 。你说什么？”

我的观点是，“嗯，在那个高度，我可以容纳宽度为 100 的所有东西。所以我将采用 100 的宽度和 600 的高度。”

onMeasure 第 2 遍

RelativeLayout 父级告诉我的观点，“你上次告诉我你想要一个 100 的宽度，所以让我们将其设置为 exact 宽度。现在，基于这个宽度，你想要什么样的高度？任何 up to 500 没问题。”

“嘿!”我的看法答复。 “如果你只给我一个 500 的最大高度，那么 100 太窄了。我需要一个 200 的宽度来达到那个高度。但是很好，随你的便。我不会违反规则_(还...)。我将采用 100 的宽度和 500 的高度。"

最终结果

RelativeLayout 父级为 View 分配最终大小，宽度为 100，高度为 500。这对于 View 来说当然太窄了，并且部分内容被剪裁了。

“叹息，”我的看法是。 “为什么我的 parent 不让我变宽？空间很大。也许 Stack Overflow 上的人可以给我一些建议。”

Answer 1

更新:修改代码以修复一些问题。

首先，让我说您提出了一个很好的问题并且很好地提出了问题(两次!)这是我的解决方案:

onMeasure 似乎发生了很多事情，从表面上看，这并没有多大意义。既然如此，我们就让 onMeasure 照常运行，最后通过 onLayout 判断 View 的边界将 mStickyWidth 设置为我们将接受的新最小宽度。在 onPreDraw 中，使用 ViewTreeObserver.OnPreDrawListener，我们将强制另一个布局 (requestLayout)。来自 documentation (强调):

boolean onPreDraw ()

Callback method to be invoked when the view tree is about to be drawn. At this point, all views in the tree have been measured and given a frame. Clients can use this to adjust their scroll bounds or even to request a new layout before drawing occurs.

onLayout 中设置的新的最小宽度现在将由 onMeasure 强制执行，它现在可以更智能地了解可能的情况。

我已经用您的示例代码对此进行了测试，它似乎工作正常。它将需要更多的测试。可能还有其他方法可以做到这一点，但这是该方法的要点。

CustomView.java

import android.content.Context;
import android.util.AttributeSet;
import android.util.Log;
import android.view.View;
import android.view.ViewTreeObserver;

public class CustomView extends View
        implements ViewTreeObserver.OnPreDrawListener {
    private int mStickyWidth = STICKY_WIDTH_UNDEFINED;

    public CustomView(Context context) {
        super(context);
    }

    public CustomView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        logMeasureSpecs(widthMeasureSpec, heightMeasureSpec);
        int desiredHeight = 10000; // some value that is too high for the screen
        int desiredWidth;

        int widthMode = MeasureSpec.getMode(widthMeasureSpec);
        int widthSize = MeasureSpec.getSize(widthMeasureSpec);
        int heightMode = MeasureSpec.getMode(heightMeasureSpec);
        int heightSize = MeasureSpec.getSize(heightMeasureSpec);

        int width;
        int height;

        // Height
        if (heightMode == MeasureSpec.EXACTLY) {
            height = heightSize;
        } else if (heightMode == MeasureSpec.AT_MOST) {
            height = Math.min(desiredHeight, heightSize);
        } else {
            height = desiredHeight;
        }

        // Width
        if (mStickyWidth != STICKY_WIDTH_UNDEFINED) {
            // This is the second time through layout and we are trying renogitiate a greater
            // width (mStickyWidth) without breaking the contract with the View.
            desiredWidth = mStickyWidth;
        } else if (height > BREAK_HEIGHT) { // a number between onMeasure's two final height requirements
            desiredWidth = ARBITRARY_WIDTH_LESSER; // arbitrary number
        } else {
            desiredWidth = ARBITRARY_WIDTH_GREATER; // arbitrary number
        }

        if (widthMode == MeasureSpec.EXACTLY) {
            width = widthSize;
        } else if (widthMode == MeasureSpec.AT_MOST) {
            width = Math.min(desiredWidth, widthSize);
        } else {
            width = desiredWidth;
        }

        Log.d(TAG, "setMeasuredDimension(" + width + ", " + height + ")");
        setMeasuredDimension(width, height);
    }

    @Override
    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
        int w = right - left;
        int h = bottom - top;

        super.onLayout(changed, left, top, right, bottom);
        // Here we need to determine if the width has been unnecessarily constrained.
        // We will try for a re-fit only once. If the sticky width is defined, we have
        // already tried to re-fit once, so we are not going to have another go at it since it
        // will (probably) have the same result.
        if (h <= BREAK_HEIGHT && (w < ARBITRARY_WIDTH_GREATER)
                && (mStickyWidth == STICKY_WIDTH_UNDEFINED)) {
            mStickyWidth = ARBITRARY_WIDTH_GREATER;
            getViewTreeObserver().addOnPreDrawListener(this);
        } else {
            mStickyWidth = STICKY_WIDTH_UNDEFINED;
        }
        Log.d(TAG, ">>>>onLayout: w=" + w + " h=" + h + " mStickyWidth=" + mStickyWidth);
    }

    @Override
    public boolean onPreDraw() {
        getViewTreeObserver().removeOnPreDrawListener(this);
        if (mStickyWidth == STICKY_WIDTH_UNDEFINED) { // Happy with the selected width.
            return true;
        }

        Log.d(TAG, ">>>>onPreDraw() requesting new layout");
        requestLayout();
        return false;
    }

    protected void logMeasureSpecs(int widthMeasureSpec, int heightMeasureSpec) {
        int widthMode = MeasureSpec.getMode(widthMeasureSpec);
        int widthSize = MeasureSpec.getSize(widthMeasureSpec);
        int heightMode = MeasureSpec.getMode(heightMeasureSpec);
        int heightSize = MeasureSpec.getSize(heightMeasureSpec);
        String measureSpecHeight;
        String measureSpecWidth;

        if (heightMode == MeasureSpec.EXACTLY) {
            measureSpecHeight = "EXACTLY";
        } else if (heightMode == MeasureSpec.AT_MOST) {
            measureSpecHeight = "AT_MOST";
        } else {
            measureSpecHeight = "UNSPECIFIED";
        }

        if (widthMode == MeasureSpec.EXACTLY) {
            measureSpecWidth = "EXACTLY";
        } else if (widthMode == MeasureSpec.AT_MOST) {
            measureSpecWidth = "AT_MOST";
        } else {
            measureSpecWidth = "UNSPECIFIED";
        }

        Log.d(TAG, "Width: " + measureSpecWidth + ", " + widthSize + " Height: "
                + measureSpecHeight + ", " + heightSize);
    }

    private static final String TAG = "CustomView";
    private static final int STICKY_WIDTH_UNDEFINED = -1;
    private static final int BREAK_HEIGHT = 1950;
    private static final int ARBITRARY_WIDTH_LESSER = 200;
    private static final int ARBITRARY_WIDTH_GREATER = 800;
}