Question

我正在学习Java和XML，并且正在尝试将TextView设置为其父RelativeLayout的中心。我的应用程序仅在我在setContentView(homeScreen)

之前注释掉最后3行时加载

这是我的XML：

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity">

</RelativeLayout>

这是我的Java：

package com.example.android.testerapp1;

import android.graphics.Color;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.Gravity;
import android.view.ViewGroup;
import android.widget.RelativeLayout;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);


    TextView homeScreen = new TextView(this);
    homeScreen.setText("Welcome to Test App 001" + "\nThis TextView was created dynamically in Java!");
    homeScreen.setTextSize(24);
    homeScreen.setTextColor(Color.CYAN);
    homeScreen.setCursorVisible(true);
    homeScreen.setPadding(16,56,16,56);
    homeScreen.setBackgroundColor(Color.BLACK);
    homeScreen.setGravity(Gravity.CENTER);

    //dynamically set width to dp (converted to pixels ~600) and height to 'wrap content'
    // convert dp amount to pixels for size
    final float scale = getResources().getDisplayMetrics().density;
    int pixelWidth = (int) (2000 / scale + 0.5f);

    homeScreen.setLayoutParams(new ViewGroup.LayoutParams(pixelWidth , ViewGroup.LayoutParams.WRAP_CONTENT));

    RelativeLayout.LayoutParams params = (RelativeLayout.LayoutParams)homeScreen.getLayoutParams();
    params.addRule(RelativeLayout.CENTER_IN_PARENT);
    homeScreen.setLayoutParams(params);


    setContentView(homeScreen);
    }
}

我现在已经看过这样的帖子大约10次了，他们都有相同的解决方案，我似乎无法正确实现，它可能是我的代码的另一部分？我可能还会使用setLayoutParams设置宽度和高度吗？

Answer 1

您可以在构造函数上设置宽度和高度，然后使用它

Relative.LayoutParams(int width, int height)

所以你需要这样做：

homeScreen.setLayoutParams(width , height);

Answer 2

setContentView()调用应该用于设置全屏的布局。您当前在Activity代码中所做的只是将TextView设置为屏幕的完整视图，因此Activity没有引用您创建的XML布局。这就是为什么最后3行代码失败的原因，因为TextView正试图设置其LayoutParams的父级应如何放置和测量它，但是在这种情况下它没有父级。我建议做的是为XML中的id提供RelativeLayout属性，以便在Activity代码中获取对它的引用，如下所示：

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="home_screen_layout"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity"/>

然后在您的Activity代码中进行调整，以便使用XML文件的资源ID进行调用。如果我们假设它在资源目录的布局文件夹中被称为act_main.xml（例如在src/main/resources/layout/act_main.xml中），则会将setContentView(R.layout.act_main)称为onCreate()中的第一行在super()调用之后，框架有机会解析您的XML并对其进行充气（即实例化，对大小进行计算和确定其组件的位置等）。之后，使用findViewById(R.id.home_screen_layout)获取对RelativeLayout的引用，以便您可以创建新的TextView并将其添加到已经膨胀的布局中。

package com.example.android.testerapp1;

import android.graphics.Color;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.Gravity;
import android.view.ViewGroup;
import android.widget.RelativeLayout;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

    // make your view components private members as findViewById calls are expensive for the framework
    private RelativeLayout homeScreenLayout;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        // Have the activity inflate the XML file with your RelativeLayout
        setContentView(R.layout.act_main);

        // Now that it is inflated, get a reference to that parent
        homeScreenLayout = (RelativeLayout) findViewById(R.id.home_screen_layout);

        // Dynamically create a TextView associated with this Activity's context
        TextView homeScreen = new TextView(this);
        homeScreen.setText("Welcome to Test App 001" + "\nThis TextView was created dynamically in Java!");
        homeScreen.setTextSize(24);
        homeScreen.setTextColor(Color.CYAN);
        homeScreen.setCursorVisible(true);
        homeScreen.setPadding(16,56,16,56);
        homeScreen.setBackgroundColor(Color.BLACK);
        homeScreen.setGravity(Gravity.CENTER);

        //dynamically set width to dp (converted to pixels ~600) and height to 'wrap content'
        // convert dp amount to pixels for size
        final float scale = getResources().getDisplayMetrics().density;
        int pixelWidth = (int) (2000 / scale + 0.5f);

        // Adjust the placement in the parent
        RelativeLayout.LayoutParams params = new RelativeLayout.LayoutParams(pixelWidth , RelativeLayout.LayoutParams.WRAP_CONTENT)    
        params.addRule(RelativeLayout.CENTER_IN_PARENT, RelativeLayout.TRUE); // make sure to use the function which takes a boolean value for rules like CENTER_IN_PARENT
        homeScreen.setLayoutParams(params); // Add these parameters to the textview

        // Let the layout know about your newly created textview so that it can re-draw its canvas
        homeScreenLayout.addView(homeScreen);

    }
}

作为一个说明，我将补充说，您可以相对轻松地在XML中完成您所做的事情，但是由于您询问了如何以编程方式设置它，我不会详细介绍那个方面。但是，如果您对某些结构化资源感兴趣，我建议您查看Android Developer Guide，特别是关于XML layouts and how they interact with Activities的部分

编辑：请注意我对活动代码所做的更改。主要部分首先使用setContentView(int id)为空的RelativeLayout xml充气，然后将另一个TextView添加到给定的布局。我提供的有关CENTER_IN_PARENT行的代码存在轻微错误。根据[docs]（https://developer.android.com/reference/android/widget/RelativeLayout.LayoutParams.html#addRule(int，int）），在添加使用布尔值的规则时，必须使用函数的addRule(int, int)版本。

以编程方式将TextView设置为RelativeLayout.CENTER_OF_PARENT

2 个答案: