Home | History | Annotate | Download | only in requestduringlayout 
      1 /*
      2  * Copyright (C) 2013 The Android Open Source Project
      3  *
      4  * Licensed under the Apache License, Version 2.0 (the "License");
      5  * you may not use this file except in compliance with the License.
      6  * You may obtain a copy of the License at
      7  *
      8  *      http://www.apache.org/licenses/LICENSE-2.0
      9  *
     10  * Unless required by applicable law or agreed to in writing, software
     11  * distributed under the License is distributed on an "AS IS" BASIS,
     12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     13  * See the License for the specific language governing permissions and
     14  * limitations under the License.
     15  */
     16 
     17 package com.example.android.requestduringlayout;
     18 
     19 import com.android.requestduringlayout.R;
     20 
     21 import android.app.Activity;
     22 import android.content.Context;
     23 import android.os.Bundle;
     24 import android.util.AttributeSet;
     25 import android.view.View;
     26 import android.widget.Button;
     27 import android.widget.LinearLayout;
     28 
     29 /**
     30  * This example shows what horrible things can result from calling requestLayout() during
     31  * a layout pass. DON'T DO THIS.
     32  *
     33  * Watch the associated video for this demo on the DevBytes channel of developer.android.com
     34  * or on YouTube at https://www.youtube.com/watch?v=HbAeTGoKG6k.
     35  */
     36 public class RequestDuringLayout extends Activity {
     37 
     38     @Override
     39     public void onCreate(Bundle savedInstanceState) {
     40         super.onCreate(savedInstanceState);
     41         setContentView(R.layout.activity_request_during_layout);
     42 
     43         final MyLayout myLayout = (MyLayout) findViewById(R.id.container);
     44         Button addViewButton = (Button) findViewById(R.id.addView);
     45         Button removeViewButton = (Button) findViewById(R.id.removeView);
     46         Button forceLayoutButton = (Button) findViewById(R.id.forceLayout);
     47 
     48         addViewButton.setOnClickListener(new View.OnClickListener() {
     49             @Override
     50             public void onClick(View v) {
     51                 myLayout.mAddRequestPending = true;
     52                 myLayout.requestLayout();
     53             }
     54         });
     55 
     56         removeViewButton.setOnClickListener(new View.OnClickListener() {
     57             @Override
     58             public void onClick(View v) {
     59                 myLayout.mRemoveRequestPending = true;
     60                 myLayout.requestLayout();
     61             }
     62         });
     63 
     64         forceLayoutButton.setOnClickListener(new View.OnClickListener() {
     65             @Override
     66             public void onClick(View v) {
     67                 myLayout.requestLayout();
     68             }
     69         });
     70 
     71     }
     72 
     73     /**
     74      * Custom layout to enable the convoluted way of requesting-during-layout that we're
     75      * trying to show here. Yes, it's a hack. But it's a case that many apps hit (in much more
     76      * complicated and less demoable ways), so it's interesting to at least understand the
     77      * artifacts that come from this sequence of events.
     78      */
     79     static class MyLayout extends LinearLayout {
     80 
     81         int numButtons = 0;
     82         boolean mAddRequestPending = false;
     83         boolean mRemoveRequestPending = false;
     84 
     85         public MyLayout(Context context, AttributeSet attrs, int defStyle) {
     86             super(context, attrs, defStyle);
     87         }
     88 
     89         public MyLayout(Context context, AttributeSet attrs) {
     90             super(context, attrs);
     91         }
     92 
     93         public MyLayout(Context context) {
     94             super(context);
     95         }
     96 
     97         @Override
     98         protected void onLayout(boolean changed, int l, int t, int r, int b) {
     99             super.onLayout(changed, l, t, r, b);
    100             // Here is the root of the problem: we are adding/removing views during layout. This
    101             // means that this view and its container will be put into an uncertain state that
    102             // can be difficult to discover and recover from.
    103             // Better approach: just add/remove at a time when layout is not running, certainly not
    104             // in the middle of onLayout(), or other layout-associated logic.
    105             if (mRemoveRequestPending) {
    106                 removeButton();
    107                 mRemoveRequestPending = false;
    108             }
    109             if (mAddRequestPending) {
    110                 addButton();
    111                 mAddRequestPending = false;
    112             }
    113         }
    114 
    115         private void removeButton() {
    116             if (getChildCount() > 1) {
    117                 removeViewAt(1);
    118             }
    119         }
    120 
    121         private void addButton() {
    122             Button button = new Button(getContext());
    123             button.setLayoutParams(new LayoutParams(
    124                     LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT));
    125             button.setText("Button " + (numButtons++));
    126             addView(button);
    127         }
    128 
    129     }
    130 
    131 }
    132