Android P应用显示宽高比maxAspectRatio使用及原理

android:maxAspectRatio属性使用

default 情况 maxAspectRatio = 1.86f;
在AndroidManiferst.xml中使用

<activity android:name=".MainActivity"
            android:maxAspectRatio="1.98">

注意：此属性生效的前提是

android:resizeableActivity="false"
//如果为true会根据手机屏幕自动的调整显示大小。

maxAspectRatio原理

在PMS解析AndroidManifest.xml的时候对maxAspectRatio处理如下：

// 解析activity时候如设置了maxAspectRatio就将设置好的传输进去。
 private Activity parseActivity(Package owner, Resources res,
            XmlResourceParser parser, int flags, String[] outError, CachedComponentArgs cachedArgs,
            boolean receiver, boolean hardwareAccelerated)
            throws XmlPullParserException, IOException {
	if (sa.hasValue(R.styleable.AndroidManifestActivity_maxAspectRatio)
                    && sa.getType(R.styleable.AndroidManifestActivity_maxAspectRatio)
                    == TypedValue.TYPE_FLOAT) {
                a.setMaxAspectRatio(sa.getFloat(R.styleable.AndroidManifestActivity_maxAspectRatio,
                        0 /*default*/));
            }
            ....
//接着看一下setMaxAspectRatio，先判断是否可以resize,也就是前面的android:resizeableActivity="false"。
private void setMaxAspectRatio(float maxAspectRatio) {
            if (info.resizeMode == RESIZE_MODE_RESIZEABLE
                    || info.resizeMode == RESIZE_MODE_RESIZEABLE_VIA_SDK_VERSION) {
                // Resizeable activities can be put in any aspect ratio.
                return;
            }

            if (maxAspectRatio < 1.0f && maxAspectRatio != 0) {
                // Ignore any value lesser than 1.0.
                return;
            }

            info.maxAspectRatio = maxAspectRatio;
            mHasMaxAspectRatio = true;
        }

//如果没有该属性 再设置默认的。
    private static final float DEFAULT_PRE_O_MAX_ASPECT_RATIO = 1.86f;
    ....
    /**
     * Sets every the max aspect ratio of every child activity that doesn't already have an aspect
     * ratio set.
     */
    private void setMaxAspectRatio(Package owner) {
        // Default to (1.86) 16.7:9 aspect ratio for pre-O apps and unset for O and greater.
        // NOTE: 16.7:9 was the max aspect ratio Android devices can support pre-O per the CDD.
        float maxAspectRatio = owner.applicationInfo.targetSdkVersion < O
                ? DEFAULT_PRE_O_MAX_ASPECT_RATIO : 0;
                //android o 之前的用默认1.86，之后的就根据手机；屏幕。

        if (owner.applicationInfo.maxAspectRatio != 0) {
            // Use the application max aspect ration as default if set.
            maxAspectRatio = owner.applicationInfo.maxAspectRatio;
        } else if (owner.mAppMetaData != null
                && owner.mAppMetaData.containsKey(METADATA_MAX_ASPECT_RATIO)) {
            maxAspectRatio = owner.mAppMetaData.getFloat(METADATA_MAX_ASPECT_RATIO, maxAspectRatio);
        }

        for (Activity activity : owner.activities) {
            // If the max aspect ratio for the activity has already been set, skip.
            if (activity.hasMaxAspectRatio()) {
                continue;
                //如果activity已经有这个属性了就不在重新设置，如果没有在设置一个默认的。
            }

            // By default we prefer to use a values defined on the activity directly than values
            // defined on the application. We do not check the styled attributes on the activity
            // as it would have already been set when we processed the activity. We wait to process
            // the meta data here since this method is called at the end of processing the
            // application and all meta data is guaranteed.
            final float activityAspectRatio = activity.metaData != null
                    ? activity.metaData.getFloat(METADATA_MAX_ASPECT_RATIO, maxAspectRatio)
                    : maxAspectRatio;

            activity.setMaxAspectRatio(activityAspectRatio);
        }
    }

这个属性的使用：

// frameworks/base/services/core/java/com/android/server/am/ActivityRecord.java
/**
     * Computes the bounds to fit the Activity within the bounds of the {@link Configuration}.
     */
    // TODO(b/36505427): Consider moving this method and similar ones to ConfigurationContainer.
    private void computeBounds(Rect outBounds) {
        outBounds.setEmpty();
        final float maxAspectRatio = info.maxAspectRatio;
        final ActivityStack stack = getStack();
        if (task == null || stack == null || !task.mFullscreen || maxAspectRatio == 0
                || isInVrUiMode(getConfiguration())) {
            // We don't set override configuration if that activity task isn't fullscreen. I.e. the
            // activity is in multi-window mode. Or, there isn't a max aspect ratio specified for
            // the activity. This is indicated by an empty {@link outBounds}. We also don't set it
            // if we are in VR mode.
            return;
            //如果是0直接就pass了没有后面计算过程了。
        }
        ...